GENERATION AND PROPERTIES OF INFRARED RADIATION Текст научной статьи по специальности «Физика»

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ЭНЕРГЕТИКА

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ЭНЕРГЕТИЧЕСКИЕ СИСТЕМЫ И КОМПЛЕКСЫ (ТЕХНИЧЕСКИЕ НАУКИ)

DOI: 10.33693/2313-223X-2019-6-2-101-137

GENERATION AND PROPERTIES OF INFRARED RADIATION

Rakhimov Rustam Khakimovich, doctor of technical Sciences, head of laboratory № 1. Institute of Materials Science «Physics-sun» Uzbekistan Academy of Sciences. Taschkent, Uzbekistan. E-mail: rustam-shsul@yandex.com

Annotation. The article presents the main basic laws of nature and modern theories of the nature of electromagnetic radiation, its generation, characteristics, and laws of reflection, absorption and scattering of light. The principle of transformation of the radiation spectrum of the primary source using the developed ceramic materials are shown, as well as experimental results of the interaction of IR radiation with matter and various mechanisms of influence on various objects and processes are described.

Key words: Electromagnetic radiation, quantum theory, electronic transitions, emitters, spectrum converters, ceramic materials, lasers, wavelength, temperature, frequency, pulse rise front, radiation flux.

Chapter 1

About light and color

1.1. The concepts of light and color.

History reference

For a long time, light and its character were considered as one of the most difficult mysteries of nature (at present, such concepts as gravity, space and time, as well as their interrelation) has many unanswered questions. Let's turn to the story.

In 1611, in Venice, a book appeared by the Italian monk of the Jesuit Order of Marco Antonio de Dominis, "About rainbow vision and light". The appearance of this treatise cost its author life - he was poisoned by his own "Order Brothers", and they took his body out of the grave and publicly burned it along with the book. However, neither burning books, nor persecution, and even the death of scientists discovering the new, could not stop the progress of science.

William Rayleigh, known for his studies in physics, argued that red is the color of blood and the color green of herbs makes up a secret which no one can penetrate. Less than a hundred years have passed, and it can be said that most of these secrets no longer exist. The fact that in the time of W. Rayleigh was considered "secret" it doesn't anymore. The works of such geniuses as Isaac Newton and Albert Einstein revealed the nature of light. The work of many generations of chemists has established a link between the structure of a substance and the presence of color in it. If Rayleigh cited chlorophyll and hemoglobin as an example of the most opposite colors, considering that these compounds have completely different structures, it is now established that animal blood and leaf greens contain similar structures. They are based on five-membered, so-called porphyrin cycles containing nitrogen. Four such cycles are bound with a metal ion: in the blood such ion is the ion

of iron, and in the plants it's an ion of magnesium and that was the core of the secret. In the first case, this structure provides the red color of hemoglobin in the blood, and in the second, the green color of the chlorophyll is the color of leaves.

The similarity of structures and the difference of ions creates opportunities for some living organisms that others do not have. Plants with chlorophyll can use light energy for a sufficiently long time to split water and release oxygen.

Magnesium changes the levels of electrons in the chemical structure of the chlorophyll molecule in such way that it becomes possible to use the energy of the incident sunlight to produce organic matter. In just one year under the effect of light on the Earth, according to approximate calculations, 6 • 1011 tons of organic substances are formed.

Blood-containing hemoglobin, which is iron-containing, serves primarily in the body as an oxygen carrier. The hemoglobin molecule consists of four iron ions in the oxidation state +2. Each of them is capable of combining with two atoms (that is, one molecule) of oxygen. The reaction with oxygen is reversible: it is absorbed in the place where its excess (in the lungs) is released and in tissues where there is little oxygen. When this happens a change in the color of the blood. Hemoglobin, which contains oxygen, dyes arterial blood in a bright red color, and hemoglobin, devoid of oxygen, gives the blood a dark red color. Most of the people believe that capture and transmittion of oxygen by hemobglobin occures becouse of the change in the valence of iron. In fact, this happens without a change in the valence of the iron ion: it is always in the same oxidation state +2. If the iron is oxidized to state +3, then the hemoglobin becomes brown in color (the form of agglomerated blood).

Electron

Fig. 1.1. The structure of the porphyrin cycle chlorophyll

Thus, in the case of hemoglobin, the state of the ion is determined only by the shades of color, and not the color itself. For structures of this type, to which hemoglobin and chlorophyll belong, in order to have a different color, a fundamental change is needed which means different ion. This is confirmed. The color of blood in some animals does not justify its name. For example, deep water sea cucumber's blood is not red, it is blue. It contains vanadium instead of iron. Similarly, algae growing in places where there is not enough oxygen and sunshine are blue or red color instead of green.

In 1668, the great English physicist Isaac Newton explained the rainbow colors and taught people to get it at will, passing the solar color through a three-sided prism. White light is a combination of rays of different colors, and its decomposition with the help of a prism gives a continuous spectrum, the colors in which gradually transform from one into another.

The untrained eye is not able to find even primary colors in the spectrum. Most of them think that there are six of them, and only two out of ten distinguish seven colors in a rainbow: red, orange, yellow, green, blue, indigo and purple. Even a special mnemonic for the colors of the rainbow was invented: "Richard Of York Gave Battle In Vain", in which words begin with the same letters as the colors in the rainbow. Each person perceives them in his own way. Even two people standing next to each other, see their own rainbow, more precisely, a rainbow formed by different droplets of water. Indeed, depending on the angle from which to look at the rainbow, you can see different parts of the spectrum. If the angle is 40°, then purple tones predominate in it, and at 42° - red. By changing the angle, it is possible to distinguish inside the part of the spectrum corresponding to the main color, the shades of one of the adjacent. For example, in yellow part it is orange or green. The color of one or another part of the rainbow, as well as the color of any colored substance, is determined by the wavelength, the energy of which prevails in a given radiation (Table 1.1).

The sunbeam contains all the colors of rainbow or the light waves of different length. Not all of them may easily pass through the substance. Some of them are retained by molecules or atoms of substance, while others pass easily.

If the energy of the light waves of the entire visible part of the spectrum is equally absorbed or reflected, then the substance appears white or colorless to our eyes. When all the components of the sun's ray falling on the body are absorbed to the same degree, but not completely, then the body appears

to our eye colored gray, if the absorption stronger then the color appears more black.

Fig. 1.2. White light is a combination of different colors

400

500

600

700

Fig. 1.3. Color is associated with the wavelength of radiation

Table 1.1

The color of compounds with one absorption band of the visible part of the spectrum

The impression of black is obtained if all the rays falling on the body are absorbed by it. Finally, bodies that absorb one of the incidents simple rays and scatter other ones, i.e. the substance transmits or reflects mainly the rays of certain wavelengths, which appear to our eyes as colored, depending on the wavelength of the radiation that has reached our eyes.

Thus color is a result of selective absorption of certain areas in the continuous spectrum of incident white light. For example, if the body absorbs red rays, it appears to be colored green; if the body absorbs bluish-greenish rays, it appears to our eye red. From what has been said it follows that when mixing dispersed rays with absorbed rays, when they are combined, white light should be produced. Therefore, the dispersed and absorbed rays complement each other in white light, so they are called reciprocal additional or just additional rays.

After returning to Earth, Yuri Gagarin made a statement that the planet is blue, and the sky is black. The message about the "black sky" was confirmed by German Titov, as he was the first among people who happened to see the sunrises and sunsets seventeen times in just one day. Both astronauts talked about how the color of the sky changes with altitude. At first it seems blue, then indigo, at a height of several tens of kilometers goes from violet-blue to dark-violet, and then there comes such colors that it makes it difficult to find the usual definitions.

From the Earth, sky seems blue in the afternoon and reddish-orange at sunset. Why does it change its color? After all, the Sun sends the same rays to the earth at any time. The color of the sky depends on how much of the daytime sunlight comes to our eyes. The fact is that light with different wavelengths is differently dispersed in in the atmosphere. The shortwave part of the spectrum (purple, indigo, blue rays) is dispersed much stronger than the longwave (orange and red).

Air consists of gases and water vapor. The molecules of these substances are also an obstacle to light, but an obstacle, of course, is incomparably much weaker than water. The sunbeam easily reaches the surface of the planet through the many kilometer layer of air, but as soon as a cloud appears, we find ourselves in its shadow. And yet the air does not just let the light through, but interacts with it. At the beginning, when the sun's ray only begins to penetrate the atmosphere, it does not meet any obstacles: the upper layers of the atmosphere are very rarefied and the distance between the individual gas

molecules is very large. The lower the beam pierces, it goes through more obstacles in its path, as the air becomes denser closer to the earth's surface. Colliding with gas molecules, the rays of light are partially reflected from them (as well as from water molecules) and dispersed. We perceive these dispersed rays as the blue color of the sky (due to the difference in the dispersing ability of the shortwave and longwave parts of the spectrum by the atmosphere). If we look directly at the Sun, then it seems yellowish to us and orange at sunset. These are the sun rays that broke through the entire thickness of the air. According to the color of the sunset, the old-timers are likely to even predict weather for tomorrow. There is nothing particularly surprising about this. After all, the heated air is less dense, and the rays of light pass through it with less losses. If the rain is close, then the atmosphere is saturated with water vapor, and they more strongly hold and reflect light. The color of the sunset sky will be different in each case: layers of cold and warm air differ in their density and so there will be different light transmission and reflection.

The long-wave (red and orange) rays of the visible spectrum are capable of bending around the molecules of the gases that make up the air. Therefore, they quite easily pass through the air, and at sunset we see the orange Sun and sky of almost the same color. Rays with a short wavelength (blue and indigo) are reflected from gas molecules and dispersed. We perceive them as the blue color of the sky, since the additional rays of the spectrum of that part past by our sight. And if you look directly at the Sun, it seems yellow. Such a distribution of colors in the spectrum is possible because even in that narrow wavelength interval that is accessible to the human eye, the energy of light quanta changes almost twice (from 149 to 299 kJ/mol of quanta). However, this is only the minor part of those radiations that are capable of impact on substance.

The practical application of this effect is used in dangerous areas. They are illuminated with control lamps of red or orange color, since in this case the location of the object can be determined much more precisely - red or orange light is almost not dispersed by the atmosphere.

It happens that the rays of light are reflected from some layers of air, as from a mirror. This is the secret of so called visions of fairy Morgana. Which usually happens in the big open spaces: deserts, steppes or in an open sea far from the coast. What is the reason for this? The air at the surface heated by the Sun is less dense than the layers above, so the rays of light from the sunlit objects on the surface of the earth reflect from the upper air layers, like a sunbeam from a mirror, and return to the surface many kilometers from where the sun illuminates subjects. Deceptive visions of salutary oases in the desert are reflections in the atmospheric "mirror" of distant real oases. Residents of the French Riviera sometimes see the mountains of the island of Corsica in the sky, although it is two hundred kilometers away by sea. Once a team of a Japanese ship, located on the eastern coast of Korea, saw the silhouette of a high mountain located on one of the islands of Japan, which was almost a thousand kilometers away.

Thus, ghostly visions are the result of two physicochemical processes: the interaction of light with molecules of substance and the effects of waves coming from a substance on the retina of the eye. The same sun beam gives us fiery-red sunsets, and the gentle iridescent colors of the morning dawns, and the azure sky.

Everyone knows that light is reflected from water, just remember the brilliance of water in a river, lake or sea on a sunny,

Absorption band width, nm Color of absorbed light Color of substance

400-435 Purple Yellowish-green

435-480 Blue Yellow

480-490 Greenish-blue Orange

490-500 Bluish-green Red

500-560 Green Magenta

560-580 Yellowish-green Purple

580-595 Yellow Blue

595-605 Orange Greenish-blue

605-750 Red Bluish-green

clear day. Water molecules merge into droplets, and droplets into water bodies, and already several tens of meters of water are enough, as twilight comes under water. Water partially reflects and partially absorbs light, and it weakens, making its way through the water column.

The apparent color of water is determined by two factors:

1. Reflects the blue sky.

2. Variously dispersion of the violet and red parts of the spectrum.

Our vision, although it is rather sensitive, is not capable of perceiving rays shorter than 400 and longer than 750 nm. The whole set of electromagnetic oscillations stretches quite broadly (see Fig. 1.3) - from radio waves to ultrashort x-ray or y-radiation. Due to the peculiarity of the human vision, to perceive electromagnetic oscillations only in a certain range, the entire vast spectrum from infrared radiation to hard X-rays is divided conditionally into three areas. Without going into a more detailed classification, these three areas can be denoted as follows: longer than 750 nm is infrared radiation, from 750 to 400 nm is the visible region, and all vibrations that have shorter wavelengths are UV light.

10-10 10-8 10-6 10-4 10-2 100 102 104 106 108 1010 J—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—I—1 CM

1 3 5 7 9

2 4 6 8

Fig. 1.4. Electromagnetic Radiation Scale:

1 - radiation; 2 - X-rays; 3 - ultraviolet region; 4 - visible area; 5 - infrared radiation; 6 - microwave region; 7 - ultrashort wave radiation; 8 - radio waves; 9 - alternating currents.

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We do not aim to talk about the entire range of electromagnetic waves, since we are interested in the infrared part of the spectrum and the possibility of its use in medicine. The author found it appropriate for a better understanding of the basic laws governing the electromagnetic waves of the infrared range to begin with radiation of the visible range, or light, as the most visual, since this range can be perceived by our senses, and then proceed to consider the properties of the infrared range.

Let's turn to some experiments that give certain characteristics of visible light.

Purkinje effect

Technique performance of the experiment. The figure shows two flowers - scarlet poppy and blue cornflower.

Fig. 1.6. Illustration of the Purkinje effect

Fig. 1.5. This figure shows the spectral characteristics for some light sources and our eyes

It would be desirable to note the difference in the concepts often confused in everyday life. For example, sound is oscillations, but it is not electromagnetic it is mechanical. Sound frequencies that go through wires, for example, telephone, are electrical oscillations of sound (often called low) frequency. The membrane of a telephone capsule, oscillating in accordance with the frequency of electrical oscillations, converts them into mechanical (acoustic) vibrations, which are perceived as sound. At the other end of the wire, the microphone converts mechanical vibrations into electrical ones. But if you apply these electrical oscillations to a coil, then an electromagnetic field is formed around it - electromagnetic oscillations (or waves) of low frequency.

Look at them under two different lighting conditions. First look at them under the sunlight or bright electric light. Which of the two flowers seems brighter? Take a look at the picture one more time, but now in low twilight lighting. Observe the flower color change. Now cornflower seems brighter.

Explanation of experiment. The dependence of human's vision from the condition of lighting when they perceive a multicolored object is confirmed by the phenomenon which first was discovered by scientist Purkinje. The fact is that for differently colored objects the ratio of their apparent brightness varies depending on the lighting. The maximum sensitivity with daylight lies at 556 nm, and with a weak night lighting it shifts towards the violet edge of the visible spectrum and is equal to 510 nm. Therefore, in weak light, the blue, indigo or purple colors win compared to red, orange or yellow. The end of the optic nerve of our eye consists of two types of photosensitive receivers - cones and rods (Fig. 1.7).

Waves of light radiation, perceived by them, cause a particular color sensation. When the light is bright vision uses only cones, and when light is weak it changes to rods.

Many processes in our body are different depending on the flow of energy (in our case, the flow of infrared radiation). The speed of many processes increases with the raise of energy flow. However, there are some processes in which this dependence quickly fades. Moreover, the effect can change to the opposite (for example, in bright light rods stop perceiving light - in the transition to low light conditions our vision takes time to adapt, which is associated with switching light (photochemical) reactions from cones to rods. According to the latest data, for complete adaptation to darkness takes

about 80 mins). This is due to many factors, which are functions of the intensity of irradiation. In particular, the reaction rate is determined not only by the flow of energy required for its conduct, but also by the presence of reagents entering the reaction: - if the reagents arrive slowly, the reaction rate will be determined no longer by the flow of energy, but

by the slowest stage of the process. In this example - the receipt of reagents. Consider this example. For example, there are two photochemical reactions:

A + B = AB; (1)

C + D = CD. (2)

Fig. 1.7. Scheme of the human eye according to modern concepts; rods and cones

We write down the equations of velocities of these reactions:

V1 = k1E [A][B]/[AB];

V2 = k2E [C][D]/[CD],

weer V1, V2 - the rates of the first and second reactions, respectively; kv k2 - their constants; [A], [B], [AB], [C], [D], [CD] -the concentration of the relevant substances.

It can often be found that at low energy flows the reaction rate, for example, (2) is higher than the reaction rate (1), and with a raise of the energy flow the reaction rate constant (1) increases significantly than the reaction rate constant (2). Even it may be that the reaction (1) does not go at all if the energy flow is below a certain threshold Q0 (Fig. 1.8). This allows using IR emitters on the organism to regulate certain processes without affecting others.

Qo

Qp

Radiation flux

Fig. 1.8. The dependence of the velocity constants of photochemical reactions from the intensity of the lighting

For example, the emitters ZB affect pathological tissue, particularly the collagen and while that the normal collagen remains unaffected.

In other words, by choosing the energy flow, it is possible to influence the processes in such way as to accelerate the process (1) with respect to the process (2) (energy flow above QR); accelerate the process (2) with respect to the process (1) (energy flow below QR); accelerate only the process (2) without affecting the process (1) (radiation flow below Q0).

Thus, choosing the wavelength of the IR radiation and its flow can in various ways, purposefully affect the processes in our body.

The Tyndall Effect

Technique performance of the experiment. In one part of the wooden box (with a barrier, it has a hole with a diameter of 1 cm), mount the electric lamp, and the stand for the glass -in the other one. In three high glasses with a capacity of 0.1 l, pour in advance: in the first - 70 ml of distilled water, in the second - 60 ml of distilled water and 10 ml of tap water, in the third glass pour 30 ml of distilled water and 40 ml of tap water. Add 1 ml of 1% silver nitrate solution to each glass. Put all three glasses on a sheet of black paper. In normal light between the first and second glasses there is no difference, in the third - there is a white opalescent silver chloride sol. Darken the room. Alternately, put the glasses on the table of the device, watching the Tindal cone in the second and third glasses. In the first glass there is no cone, if the water is clean enough.

Explanation of experiment. If a strong stream of light rays is passed through a colloid system, then as a result of light scattering (Fig. 1.9), a light cone (Tyndall cone) is seen by colloid particles, which is clearly visible in a dark room. The true solution seems to be "optically empty" under these conditions. It should be noted that this is the way how they prove the absence of colloidal particles in solutions. The Tyndall effect is also used in nephelometric analysis to determine the concentration of colloidal particles.

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Fig. 1.9. Tyndall effect. Reflection of light rays from colloidal particles, causing the appearance of a Tyndall cone

"The Tyndall cone" can be observed in the cinema. The viewers who are in the cinema hall, which is insufficiently cleared of dust, quite often see a ray stretching from the cinema installation to the screen. The same effect is observed from the light of a lamp, a searchlight or headlights.

It should be said that a living organism, to some extent, is a colloidal solution for light, therefore it will also observe the effect of light scattering. This, in many cases, allows the beam (including the infrared range), even very thin (for example, laser), to fall on a fairly wide area of our body.

Chapter 2

The nature of light

2.1. Particle or wave?

The concepts of particle or wave in the application to the light for a long time existed separately and even opposed to each other. In the beginning of the 20th century it came together, approving the concept of wave-particle duality of radiation.

The French philosopher and mathematician Rene Descartes (1596-1650) first tried to formulate general idea of light. From Descartes' ideas about the general picture of the world, it followed that light had the properties of corpuscles (particles).

The first scientifically and experimentally substantiated theory of light was, as already noted above created by Newton. During experiments on the decomposition of sunlight, he found out dependence of the position of a simple beam, selected by a prism, on its color, for the first time having received an objective sign of color.

Another feature of simple rays was revealed by passing them through a lens lying on a glass plate: around the point where the lens touched the glass, the correct alternating concentric rings appeared - black and the same color as the beam.

It turned out that the width of the rings depends on the color of the beam: for red it is the largest, for purple the smallest. In addition, each simple beam with a constant lens corresponds to a certain diameter of the first ring.

And finally, the practical conclusion of Newton on the distribution of light in a homogeneous medium in a straight line was supplemented by experiments on the ability of a narrow light beam to bend around small objects (diffraction).

Fig. 2.1. Newton made a simple experiment, forcing light rays that fell into a wedge-shaped air gap between two glasses, to form dark and light rings

Fig. 2.2. The light from the hot rod, reflected by the mirrors of a complex optical device, passes through numerous lenses, prisms, diffraction gratings, and in the end is also divided into colored stripes

Analyzing the observed properties of light (straight line propagation, reflection of the beam from the mirror surface), Newton came to the conclusion that the light beam is a stream of the smallest particles of substance, corpuscles. It does not need an intermediate environment: the vacuum will not be the obstacle for the moving corpuscles in order to overcome the vast spaces of the Universe. At that time it was believed that the wave can propagate only in some kind of material environment. For example, to explain the passage of radio waves, a special "ether" was invented, which was not detected by the existing methods. Later it turned out that no ether is needed for an electromagnetic wave, but we still use the term "ether" when it comes to television or radio programs. Newton's delusions about the necessity of substance for the passage of radio waves made it possible to develop the corpuscular theory of light, and then the more general quantum theory, which gave startling discoveries and a deep understanding of the laws of Nature.

Point light source

Fig.2.3. Two tiny holes in the curtain, placed in the path of the rays, allow you to replace the glass in experiments on the interference of light

Newton reinforced his position with the regularity and uniformity of the motion of planets that did not meet with any resistance. He believed that the absence of a material medium between the Sun and the planets excludes the propagation of light by waves that cannot arise in a vacuum.

However, not all properties of light were explained by corpuscular theory. For example, the interaction of two rays in a lens could not be considered as a combination of particles. Some results of Newton's experiments were more accurately explained by another theory advanced by his contemporary Christian Huygens.

The Dutch scientist Christian Huygens (1629-1695) imagined light as elastic wave oscillations without moving particles. He argued that the light source creates elastic waves in a special medium - the "luminiferous ether" that fills world space. These waves propagate in the same way as waves from a stone falling into the water; any point to which the light wave reaches is itself a source of new waves.

Assuming the existence of the ether, it was necessary to believe that there is a matter with low density (so as not to interfere with the movement of real bodies) and high elasticity (to transmit a wave with tremendous speed, which by then was already known). Not one decade has gone on the search for evidence of the existence of ether, but instead they gradually accumulated arguments that disprove the existence of such an environment. So, when experimenting with the propagation of light in moving bodies, it was assumed that in a stationary mechanical medium it would be accompanied by a kind of "etheric wind" affecting the optical properties. Experience did not reveal such a "wind" and only increased doubts about the existence of the ether.

Color films of gasoline or oil on water, the iridescent surface of soap bubbles were interpreted from the positions of light waves as particular cases of interference. Light was represented as transverse vibrations. Scientists accurately calculated the length of the rays of different colors, their characteristics -

frequency - and gave a rigorous mathematical and physical explanation of color. According to the classical electromagnetic theory, light is a wave motion, the energy of which varies in proportion to the radiation intensity and does not depend on its frequency (v).

The works of Maxwell, Heinrich Hertz, and P.N. Lebedev introduced new view into wave theory. The electromagnetic nature of light explained its interaction with matter: the movement of electrically charged particles of matter had to generate electromagnetic waves, which cannot exist if there is an alternating electric field in space.

These indisputable confirmations of the wavelength pattern of light M. Plank and A. Einstein at the beginning of our century supplemented with evidence that light can be absorbed and emitted only by certain portions of energy - quanta or photons.

From October 19 to December 14, Max Planck was engaged in the theoretical substantiation of his formula, and then on December 14, 1900, he spoke at the University of Berlin with a conclusion about an elementary quantum of action. M. Planck, on the basis of his observations of the absorption of light by the substance and thermal radiation, came to the conclusion that the energy of light enters the irradiated substance in small portions. Planck offered to call such portions of energy quanta. It is easy for us to imagine that matter is capable of being transmitted by the smallest particles - atoms, so light can be transmitted and absorbed by no less than quanta. There can be neither a half-quantum, nor a quarter of a quantum - only a quantum entirely.

It is known that helium consists of light atoms, and uranium -of heavy ones. However, it is impossible to take a half-atom from either helium or uranium. These will be atoms of other elements: water in the case of helium and, for example, barium and krypton in the decay of uranium. Figuratively speaking, the "quantum" is the "atom" of light. It is accepted, as for a substance, to count the anergy per 1 mole of light quanta of a certain wavelength. Indeed, in ordinary experiments they deal with the flow of visible light incident on a huge (macroscopic) number of atoms. It is impossible to notice and isolate a separate quantum of light, but it is possible to calculate the mole of quanta of any radiation.

According to the Planck relation, the energy of the "atoms" is proportional to the oscillation frequency:

E = hv.

The number of quanta constituting 1 mol is equal to Avo-gadro's number NA = 6,02 • 1023 mol-1 (more precisely, then 6,022045 ± 0,000031 • 1023). Given the ratio between frequency and wavelength v = c/A, the Planck ratio can be written as:

E

■■ hcNA • 1/À.

After substituting the constant h = 6,6242 • 10-27 erg.s and the values for the speed of light with c = 3 • 1010 cm/s, we obtain the formula for calculating:

E = 1,197 • 108 • 1/A.

It is easy to verify that 1 mole of red light quanta carries about 160 kJ, and purple light about 280 kJ (per 1 mole of such quanta).

If you look at the table of colors of visible light (see Table 1.1), you can see that the shorter the wavelength, the more energy the beam transmits to the substance during a collision.

This was a serious blow to the classical theory, which was based on the idea of the continuity of change and the propagation of light as a wave. The views expressed by Planck seemed

so strange that he himself at first doubted to consider them as the basis of the new physics, and considered the formula he had received as a temporary assumption. However, experience has shown that Planck's views are fair. In 1805, a twenty-six-year-old employee of the Patent Office in Bern, Albert Einstein, studying the photoelectric effect, came to the conclusion that light, for some of its properties, is more like a stream of particles, which he called photons. The photon energy is defined by the formula E = mc2.

It was amazing discovery: mass and energy are manifestations of the same essence, i.e., two properties of a single matter. The coefficient of proportionality between them is the square of the speed of light in a vacuum. Each change in energy corresponds to a mass, expressed as E/c2. It is easy to guess that Planck's quanta and Einstein's photons are two different names for the same - the smallest portions of radiation energy. The energy E of each photon depends on the frequency of the radiation and does not depend on the intensity, since the frequency written above includes the frequency and does not include the intensity of the incident light (as would be supposed by the classical electromagnetic theory).

In any interaction of light with matter, its quantum nature is also manifested. For example, the phenomenon of fabric fading under the sun's rays is the result of the decomposition of dye molecules under the action of a light flux.

It is known that the fabric fades unevenly. To identify the reasons for this, it is necessary to determine the conditions in which discoloration of fabric under the effect of sun light would occur evenly. In order to reach even discoloration of the fabric complete identity of the chemical molecules of the dye and uniform distribution of light are required. Compliance with the first condition is confirmed by the laws of chemistry. To fulfill the second condition, the light flux must have the properties of a wave, and the surface spotting proves that energy has been unevenly distributed onto the fabric, in quantized portions.

To represent the concepts of radiation intensity and the quantum energy of light, the following analogy can be made. For example, balls of the same mass and size fall from a height of 1 meter. Each ball, being at a given height, has a certain potential energy, which is converted into kinetic energy when it falls. The number of falling balls will indicate the flow intensity, but not quantum energy - the quantum energy of all the balls is the same. Quantum energy will be associated with the speed of the falling ball, which, in turn, depends on the height from which the ball falls - the greater the height, the higher the quantum energy and vice versa. Thus, a drop of 10 balls from a height of 1 meter gives only an increase in intensity, but not speed, and, accordingly, quantum energy. One ball falling from a height of two meters has quantum energy twice as high as any ball falling from a height of 1 meter.

S.I. Vavilov proved the discontinuity of light in experiment using high sensitivity of the human eye.

Any light source, regardless of its intensity, generates a time-varying flux of quanta; it can be compared with the acoustic effect produced by the applauding audience: the claps of every person present, different in sound and different in sound, merging to form an even, monotonous noise.

Deviations from the average number of quanta are noticeable only at low-power sources, just as individual claps become perceptible when listening to radio transmissions through a single microphone. Consequently, by reducing the brightness of the light, it is possible to reach such limit when its oscillations

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(as a result, fluctuations in the number of quanta in the beam) will be noticeable to the naked eye. When the number of quanta is below the limit, the eye will not perceive the light, and when it becomes above the limit, the eye will feel a flash. In other words, the light source will be flashing.

The obstacle associated with the ability of the eye to maintain visual perception should be taken into account. This ability will smooth out the pulsation, preventing fixation of rapid changes in the intensity of the flow.

The finished circuit of the Vavilov's experiment contained a light source separated from the observer's eye by a rotating opaque disk with one hole.

Making one turnover per second, the disc opens the source to the eye only for a split second - while the beam is crossing the hole. Thus, the eye sees one flash per second. When the average number of quantum reached a critical point by decreasing brightness of the source, there were such periods when experimenter did not see the flash. This assumption is based on the quantum theory of light, and experiments have confirmed it: with a slow decrease in the brightness of the beam, the observer first saw continuous light, then blinking, with increasing dark intervals. Knowing the number of flashes and gaps, according to the laws of statistics, one can count the number of quanta that have passed into the hole in one flash. This number is individual for each participant in the experiment, because it characterizes the sensitivity of the eye; it varies from two to several dozen quanta. The subtlety of the experiment conducted by S.I. Vavilov is illustrated by the fact that the threshold of a noticeable visual sensation in the human eye, fixed by the installation, is at 4 • 10-17 W, 1018 times less than the power of a flashlight lamp.

The quantum nature of the luminous flux still remains inexplicable phenomena of diffraction and some other properties of light, which easily fit into the basic principles of the wave theory.

These and other similar facts led to the breakdown of classical ideas. It was no longer possible to consider light only as waves, but at the same time it was impossible to explain all light effects on the basis of one corpuscular theory. In both cases it's most important properties eluded consideration. Only one thing remained - to consider radiation as a phenomenon that possesses simultaneously both properties and corpuscles and waves; thus, the concept of wave-particle duality of radiation was established in physics. Light (visible or invisible) propagates as a wave motion, but its absorption by atoms of a substance occurs as an interaction of particles. However, since the absorption of energy by atoms occurs in portions, therefore, the energy of the atoms themselves does not change gradually, but also in portions, i.e., abruptly, and their energy state has a number of discontinuous values, or is said to be quantized.

So what changes when a quantum of light hits an atom? It was necessary to solve the same problem that A. Einstein solved, but from the other end. In order to reconcile the new quantum theory with the phenomena of diffraction and interference of light, A. Einstein suggested that the light waves are very weak ("ghost waves"). Their role was reduced to the transfer and distribution of quantum photons in space. Louis de Broglie, who tried to solve the problem from the opposite end, knowing the energy concentrated in each element of matter, needed to determine the properties of this wave. In other words, find out which wave is connected with any material particle of mass m.

Comparing Einstein's formula with Planck's formula (applied to the energy of the same material particle), Louis de Brog-lie used the relation v = c/X and found the wavelength he needed

hv = mc2; hc/X = mc2; X = h/mc or Xe = h/mv.

This formula (known as the de Broglie ratio) determines the length - the basic quantity that characterizes the waves of matter. Moreover, it characterizes, on the basis of the particle's mass and velocity, that is, on the basis of purely mechanical concepts. Thus, the unity of seemingly opposite things was achieved: the wave and corpuscular (quantum) theories of light.

The success of the calculation of Louis de Broglie marked the beginning of a new science - wave mechanics. The theory of quanta was included in the previously existing theory of waves, and, therefore, its position throughout this wave will be uncertain. Peculiarity of the quantum world is that the electron energy (momentum) and its exact position cannot be determined simultaneously. The wave process is a probabilistic process. A wave does not fix the position of an electron at one or another point in space, but characterizes its appearance with some probability, depending on the parameters of the wave. Let us explain this important principle in more detail. Suppose we decided to strictly fix the exact position of the electron. To observe it, it is necessary to use a radiation source, just like if we need to determine the position of an object in a dark room, a flashlight beam is needed, and to determine the position of the aircraft - a radar beam is used.

But in the compared situations there is a fundamental difference: neither the beam of light on an object in the room, nor the radar beam on the aircraft have a noticeable effect; the absorption of a quantum of light by an electron changes its speed and, consequently, its energy state (as if the beam of a flashlight shifted an object in a room).

The beam, reflected from the electron, will return to the observer and report the location, but not the speed of the electron (or, equivalently, the energy). It turns out strange at first glance picture.

If we ask ourselves to determine the energy of an electron, by measuring the frequency of its oscillations and calculating the speed using the formulas already familiar to us:

From here

X = c/v; X = h/mv.

c/v = h/mv; v = hv/mc

then in this case it will not be possible to determine the position of the electron more accurately than the wavelength. If we strive to determine the exact position of the electron using ultra-high-frequency (with an immeasurably small wavelength) quanta, then such quanta will change the electron's momentum and nothing can be said about its velocity, i.e. the electron energy state not fully defined.

Such paradoxical position is most clearly illustrated by the famous example of a pendulum. If we fix the positions of the pendulum, then we cannot say anything about the amplitude and frequency of its movement. To determine the characteristics of the motion of the pendulum, it is necessary that it oscillate. However, in this case, his position becomes completely uncertain over the entire amplitude of his oscillation.

The first, who realized that there was a limit to the experimental possibility of accurately determining the coordinates

of an electron and its impulse, was German physicist V. Heisenberg. He showed that there is always some uncertainty about the position of Ax and the impulse Ap, or, equivalently, the velocity v associated with the electron impulse by the relation p = mv. The product of these uncertainties cannot be less than a certain constant value h:

or puting

we have:

AxAp > h,

Ap = mAv

AxAv > h/m.

It follows that if the mass m of the particles is large, then the uncertainty is small. If m is very small, as is the case for an atomic-scale particle, then the uncertainty increases. V. Heisenberg believed that in atom particle cannot be considered with mathematical precision. Instead of that we should take into consideration a region of uncertainty in which a particle can stay, but with equal probability at all possible points. The properties of this area are as follows. The more precisely the deposition of an electron is determined in it then the less certainty can be given to its energy, and vice versa.

Here chemist researchers face a dilemma, which is more important for describing the state of an electron in an atom - its coordinates or energy. Since the chemical interaction of particles involved in the reaction is associated with a change in energy, the exact value of energy and its change when the electron passes from one state to another (or from one atom to another) should be recognized as more important for chemistry. We kind of "sacrifice" the accuracy of the electron coordinates (and, thus, the position of the electron in the atom is completely uncertain), but we can rather accurately get the value of its energy.

Thus, classical mechanics with its consideration of the trajectory of a point in space can no longer give a correct picture of the state of electrons in an atom. The manifestation of the properties of a wave by electrons indicates that their state can be interpreted on the basis of equations describing wave motion (for example, vibrations of a string or electromagnetic oscillations).

The meaning of this transition is illustrated in Fig. 2.4. If you try to determine the position of the electron in accordance with the laws of classical and quantum mechanics, the difference in this description will be big. In the classical representation, the probability of finding an electron for any point will be either one or zero. At all points except one, the probability will be equal to zero. The upper graph in the figure corresponds exactly to the case of exact fixing the position of electrons in classical mechanics.

As already mentioned, to describe the motion of a wave means to find the probability of its occurrence. According to wave mechanics, the probability can have intermediate values between 0 and 1. The most likely position of an electron coincides with the place that classical mechanics determines for an electron, but an electron can also be in other places. It is assumed that if the position of electron is measured not once, but many times, then the found points are located in accordance with the curve of the probability distribution. Strictly speaking, turning to wave motion, it is necessary to consider the constant attributes of this movement: amplitudes, phases of motion and their signs.

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t_í_I

Distance

Fig. 2.4. Atom and electron in the view of corpuscular (a) and wave (b) theories

2.2. How «orbital» better than «orbit»

As soon as scientists came to understanding that the world inside of atom doesn't obey the laws of classical physics, and requires other ideas for its description, as the shortcomings of quantum theory have already become visible Its internal contradiction was that to calculate the stationary state of an electron Niels Bohr introduced an orbit with a frequency of circulation v and energy E = hv. This orbit was calculated on the basis of the laws of classical physics in the concept of the electron as a moving particle.

According to Bohr, the orbit is the trajectory of the electron-particle, whose energy varies in steps, i.e. quantized.

The success of Bohr's quantum theory was associated with the calculation and prediction of the lines of the spectrum of the hydrogen atom. However, during the transition to other atoms, even to such a simple one as helium, no qualitative match was observed.

With the advent of the "pilot-wave" (Louis de Broglie), the uncertainty principle (V. Heisenberg) and the wave equation (E. Schrodinger), conditions were created for examining the state of an electron from the standpoint of wave mechanics. What in this case is a wave associated with the motion of an electron in an atom?

Electronic waves, in principle, can be considered as standing waves. Such waves can be seen and even created by ourselves. They occur if you bring in a wave-like movement of a rope tied to a wall at one end, or when you make the guitar string sound. In these examples, there is one fundamental similarity: both the rope and the string are fixed. The string is on the neck of the guitar, and one end of the rope is tied to the wall and the other end is in the hand. The electron is "fixed" in the atom by the action of field of the atomic nucleus.

Standing waves obey the sine wave equation, which describes a one-dimensional wave, i.e., distributing in one direction, along one axis of coordinates. The two-dimensional wave will spread along two axes, i.e. on surface. Such waves can be created by throwing pebbles into the water and watch the circles they form. The electron wave is three-dimensional and propagates in the volume of the atomic space. Something like this can be imagined if we recall how radio waves propagate.

The electron in this case will appear to us in the form of a "cloud". The difference with radio waves, besides other things, is also in the fact that the bulk electron waves propagate in three directions of Cartesian coordinates differently.

For a long time scientists couldn't find appropriate way to describe three-dimensional electron waves. In 1926, Erwin Schrodinger suggested an equation for this, which was called "wave". However, it turned out, that it was rather difficult to obtain an exact solution of the Schrodinger wave equation. Even now, with an accuracy that coincides with the experimental one, it is strictly decided only for a hydrogen atom. This, however, does not reduce its value.

The electronic structure of molecules is the subject of quantum chemical research. According to the adiabatic approximation, the motion of electrons in chemical systems is viewed at fixed positions of the nuclei and is described by an electron wave function depending on the coordinates of the electrons; from the nuclear coordinates, this wave function depends from the parameters. From incomplete information about the form of this function, it is possible to derive a qualitative interpretation of the physical properties of molecules and their spectra, whereas the calculation of more accurate functions makes it possible to obtain quantitative results.

The foundations of the theory of many-electron systems were laid by the work of V. Heisenberg on the helium atom (1926), as well as the studies of the hydrogen molecule by V. Heitler and F. London (1927). They showed that the properties of these systems cannot be explained within the framework of classical representations. The very fact of the existence and stability of even the simplest chemical object, the H2 molecule, is a characteristic quantum phenomenon. Subsequent studies have developed methods for determining the electron wave functions for more complex molecules, for example, the method of valence schemes, and the method of molecular orbitals. These methods are variously simplified versions of a more general configurational interaction method, which, in principle, makes it possible to calculate fairly "reliable" wave functions of molecules. Finding and using even the simplest wave functions is associated with very time-consuming computations. In the early quantum chemical studies, almost exclusively approximate semi-empirical methods were used. In conjunction with perturbation theory, they developed as a way to make qualitative predictions practically without computation, based on intuition and analogies. Thus, the original concepts of the theory of chemical bonding and intermolecular interactions were introduced, the theoretical foundations of molecular spectroscopy were developed, and a qualitative theory of the structure and reactivity of conjugated organic molecules was created.

The development of computing in 1960 of XX century changed the style and direction of quantum-chemical studies. Nonempirical methods for calculating molecules and quantitative variants of semi-empirical methods began to develop intensively. The computation of the electronic structure of medium-sized molecules (20-30 electrons) accomplished with accuracy which in many cases, is sufficient to predict the geometric structure, physical properties, and spectra of such molecules. Quantum-chemical methods are especially important when studying short-lived active particles and activated complexes that are not amenable to experimental recording; theoretical calculation turns out to be the only tool for their direct research.

At the present stage of quantum chemistry, along with traditional calculations of electron wave functions, new problems and methods are being developed. The quantum

a

b

b

c

theory of nuclear motion in chemical systems is developing. In the transition from static systems to systems that change over time, in particular as a result of chemical reactions, photoexcitation and decay, new theoretical methods which were developed in quantum mechanics and statistical physics were required, so that quantum chemistry can rightly be regarded as a branch of theoretical physics. Objects of quantum chemistry are becoming increasingly diverse: from elementary processes in chemical lasers and the electrical conductivity of molecular crystals to the complex mechanisms of the functioning of biological systems.

Through complex calculations, based on simple mathematical hypotheses, E. Schrodinger derived from his equation the results of N. Bohr and Louis de Broglie for an atom. Besides that in his seven major and fundamental works which he published during one year, he obtained in general terms all the known results of the "old" quantum theory. He found, without any additional hypotheses, almost all the amendments that gradually supplemented the previous theory, in order to better align it with the facts.

Let us consider the essence of the new description of the state of an electron in an atom. The three-dimensionality of the electron wave requires the introduction (according to the number of coordinate axes) of three constant numbers -three quantum numbers. You may argue that there are four quantum numbers. Yes, indeed, this is true, but the fourth -"spin" - characterizes not the wave, but the corpuscular qualities of an electron. Thus, according to a simple arithmetic balance, an electron is a three-quarters wave, and one-fourth is a particle. The properties of such microparticles as an electron, proton, etc., can be fully taken into account only when they are simultaneously described from the point of view of two theories - wave and corpuscular. When using only one theory important characteristics are overlooked.

All quantum numbers received their special names and corresponding visual interpretation. However, it should always be remembered that they appeared and were introduced as constants. These numbers are necessary to solve the quantum wave equation indicating the electron energy "smeared" around the nucleus due to uncertainty in the coordinate. Each of the wave quantum numbers has an integer value, and the corpuscular - fraction, equal to ±1/2.

The change in the density distribution of the electron cloud with the distance from the nucleus of an atom is depicted in the three dimensional coordinate system in the form of peculiar volume figures. Such figures include a certain part of an atom. Their shape and size depend on the electron energy, that is, on the characteristic that we seek to determine with the utmost care. However, the more accurately the energy state of the electron is estimated, the more uncertain is its position in space. After all, we can only talk about a certain area of the atom, where with a certain degree of probability there can be an electron.

The region of atomic space, where an electron with a given energy is found with a probability of 90%, is called the atomic electron orbital. The word "orbital" is similar to the word "orbit", but think about what the fundamental difference is. The orbit is a line - the trajectory of a point or, in this case, a particle - an electron. The orbital is a three-dimensional region of the atomic space, resulting in the complete rejection of the consideration of the coordinates of the electron. We fully admit that the electron somehow moves in this area and may even leave it, but we cannot say how it happens The concept

of "trajectory of motion" in the wave mechanics is completely absent

Theoretically, the atom has no boundaries. However, the electron must be in the nucleus field at each moment of time. Consequently, the total probability of its being in the perinuclear zone is always equal to one, and it must be somewhere there. The probability of the electron being far from the nucleus is rather small, although it exists. So, in principle, the electron orbital can go to infinity. Therefore, in quantum chemistry, visual representation is more preferable - the boundary surface, which is the shape of the orbitals. It is carried out in such way that the probability of finding an electron inside is 90%. Moreover, the electron is most likely to be located directly at the boundary surface and less likely to be in depth or outside. Quantum numbers help to find this region and thereby determine the electron orbital corresponding to its energy.

2.3. Quantum numbers

By 1930, it turned out that physicists, relying on the achievements of mathematics, had in their hands such a description of the properties of an atom that made it possible to quite accurately determine the state of electrons and predict the individual characteristics of elements. The spectral characteristics of a substance are largely determined by the way electrons interact with light quanta. And this, in turn, depends on the energy state of the electrons.

Quantum-mechanical description of the atom today is generally accepted. It is based on a complete rejection of the laws of classical physics in the description of the internal structure of the atom. Strictly speaking, one should generally consider an electron as some amount of negative charge and mass, which is appropriately distributed around the nucleus. Moreover, depending on the energy, this distribution will be different each time.

From mathematics and physics we know that the problem of three moving bodies does not have an exact solution. If you send a rocket to Mars, it is impossible to calculate immediately where the messenger will be at one time or another. His location is determined with some probability. That is why constant measurements of the orbital elements of satellites and spacecraft and additional corrections of the trajectory of their movement are necessary. The parameters by which one can judge the state of electrons in the field of action of the nucleus and other electrons are the quantum numbers. However, here the exact solution of the problem is possible only for the atom of the hydrogen, where there are only two moving bodies: the nucleus (proton) and the only electron. By exciting an electron through the transfer of a quantum of energy, we transfer it to higher energy states - in other words, it moves from one orbitals to another. It turns out the whole system of potential electron orbitals. It is assumed that the same orbitals exist in other more complex atoms. Placing electrons on them, we obtain electron configurations of atoms of all chemical elements of the periodic table. There are four quantum numbers characterizing electron.

The principal quantum number (n). It characterizes the average energy of an electron, its distance from the nucleus. The visual interpretation of the principal quantum number is the energy layers or shells on which the electron is located. It has values from 1 to 7, which corresponds to the numbers of periods in the periodic table. In each new period, a new energy envelope is filled with electrons, which coincides in number with

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the period. The main quantum number is determined by the size of the orbitals. The number of electrons is related to the number n by the simple relation Z = 2n2. The resulting sequence - 2, 8, 18, 32 - corresponds exactly to the number of elements in the periods of the periodic table.

Secondary quantum number (/). Its physical meaning is that the electrons of one energy layer-level mutually repel each other and tend to occupy the position at which their approximation to each other is minimal. The energy level is divided into several sublevels. The side quantum number has a value from 0 to (n - 1). For clarity, it is considered to be responsible for the shape of atomic orbitals (Fig. 2.5), which are designated by letters in the manner used in atomic spectroscopy:

Value l 0 1 2 3 4 5 6

Letter designation s P d F g h i

Fig. 2.5. Forms of s- and p-electron clouds and orientation of p-orbitals (boundary surfaces of electron clouds).

The first four notations have been formed historically from the names of the lines of the spectrum, and then they go in order.

When recording the energy states of electrons, the principal quantum number is denoted by a digit, followed by the letter designation of the secondary (orbital) quantum number. The recording order is simple: the number of subshells in this level is equal to its number. Using this, one can write down all the allowed energy states in any of the periods (Table 2.1).

Table 2.1

Allowed energy states of electrons in atoms by period

Period Orbitals

1 Is

2 Is 2s 2p

3 Is 2s 2p 3s 3p 3d

4 Is 2s 2p 3s 3p 4d 4s 4p 4d 4f

5 Is 2s 2p 3s 3p 3d 3s 4p 4d 4f 5s 5p 5d 5f 5g

6 Is 2s 2p 3s 3p 3d 4s 4p 4d 4f 5s 5p 5d 5f 5g 6s 6p 6d 6f 6g

7 6h Is 2s 2p 3s 3p3d 4s 4p 4d 4f 5s 5p5d 5f 5g 6s 6p 6d 6f 6g 6h 7s 7p 7d 7f 7g 7h 7i

Actually, the principal and secondary numbers would be sufficient to denote all possible energy states, if there hasn't been magnetic properties of the atom and the electron, i.e. magnetic fields that emerge as the result of the movement of charged particles - the nucleus and electrons.

Magnetic quantum number (m/). With any movement of electric charges, a magnetic field appears which begins to interact with an external magnetic field. These fields affect the state of the electron. The calculation of this effect is made using a magnetic quantum number. Usually it is considered (see Fig. 2.5) as a projection of the electron orbital to some direction (x, y, z). For this, in the three-dimensional coordinate system, the orbitals of the same form are represented, but oriented along different axes depending on the magnetic quantum number (see Fig. 2.5).

The field affects the electron in such a way that within the same sublevel it becomes possible to have a finer gradation of energy states. In the terminology of quantum chemistry, they say that orbitals have energy cells. At the suggestion of F. Gund, they are depicted in the preparation of the electronic formula of an atom by squares. The number of cells for a particular orbital is determined by the formula mt = 2l + 1. With l = 0 (s-orbital) there is one such cell, with l = 1 (p-orbital) there are three similar cells, for l = 2 (d-orbital) there are already five of them, and when l = 3 (/-orbital) -seven. The numeric values corresponding to each of the cells are "aligned" in the sequence from-l through 0 to +l. Thus, for l = 1, the magnetic quantum number is capable of taking three values (mz = 2l + 1 = 3), equal to -1, 0, and +1. The null appears in the case when the orbital is oriented in such a way that the projection of the magnetic moment on the direction of the magnetic field is zero. In Fig. 2.5, the direction of the magnetic field coincides with the x axis, and the px orbital has a magnetic quantum number value equal to zero. For a sphere (s-orbital) the value is l = 0. Its projection onto any chosen direction gives zero, since the projection of one half of the ball compensates for the projection of the other. In this case, the only (mz = 2 • 0 + 1 = 1) value of the magnetic quantum number is zero.

Spin quantum number (ms). Characterizes the intrinsic magnetic moment of the electron. Its presence, which follows from the experimental data, is explained as follows.

When three quantum numbers are defined, the wave function describes a particular electron with a certain amount of energy, and is usually called the atomic orbital in this case. However, besides these three quantum numbers, there is one more that is not derived from the solution of the wave equation. It was introduced in 1925. J. Uhlenbeck and S. Goudsmit. Based on the study of numerous atomic spectra, it was concluded that some features of the spectra can be explained only if we introduce one more additional characteristic for the electron. They postulated that the electron has a rotation around an axis like a top, and from the English word spining, the term "spin of the electron" came into use. This is a tribute to the corpuscular properties of the electron.

The top model allows you to find the magnetic field due to the angular momentum of the spin. The magnitude of this moment is also measured in units of h/2n and is determined by the expression Vs (s +1). However, the result of the calculation was twice as large as the value observed in the experiment. The reason, as it is believed, lies in the nonidentity of each other of the distribution of the mass and charge of the electron, and this cannot be taken into account according to the classical

P

P

P

x

particle theory. In order for the calculations to coincide with the experiment, it is necessary to divide the calculation results by two. Hence, the value / appears which essentially represents the coefficient of divergence between the experiment and the simple calculation for the relation of the moment to the field. However, the details for chemistry are not significant. What is important for us is that magnetic measurements (for example, by electron paramagnetic resonance - EPR - spectrometry is one of the main methods used to study radicals and free-radical reactions) allowing us to find dependencies between the spectra and electron characteristics.

A feature of this fourth quantum number ms is the ability to accept only two possible values of +1/2 and -V2. If we assume that the spin of the electron is a consequence of its corpuscular properties, then the visual interpretation of this number is also introduced from the same positions. According to classical mechanics, an electron is a small ball with a negative charge. It can rotate around its own axis either in one direction (say, by the clock arrow) or in the opposite direction. Thus, depending on the direction of rotation (Fig. 2.6), it is attributed to ms = +1/2 or ms = -1/2. The electron spin creates its own magnetic moment, the direction of which coincides with the direction of the vector.

Fig. 2.6. Electronic top.

Visual representation of the spin quantum number as the rotation of an electron around its own axis

How is the electron spin reflected in the spectrum of an atom or molecule? If electrons have the same three quantum numbers n, l and ml, then they are on the same atomic orbital. If there are two electrons in this orbital, whose spins have inverse values (ms = +1/2 and ms = -1/2), then their magnetic moments are oppositely directed and should mutually compensate each other. However, an electron that cannot be counterposited by another electron with an opposite spin, contributes to the magnetic moment of an atom or molecule. This, in particular, explains the magnetic properties of oxygen, which in a liquid state can deviate in a magnetic field.

The above interpretation should not be understood literally, because it does not agree with quantum-mechanical concepts, and also cannot be used for rigorous quantitative calculations. It is only useful for a qualitative description. If one strictly follows wave mechanics, it is not at all necessary to consider that the electron actually experiences physical rotation. Simply an additional degree of freedom capable of taking one of two possible values is attributed to an electron. Due to this, atomic orbitals with the same n, l, and ml each contain two electrons with the same energy.

2.4. Electron orbitals in an atom

Clear boundaries of the nucleus, as well as clear boundaries of the atom do not exist. The study of the scattering of fast charged particles has shown that, for the overwhelming majority

of nuclei, its shape can be taken as a sphere with a radius of r = 1.1 • 10-13 cm (100 000 times smaller than the radius of atoms ~10-8 cm). The radius of 10-13 cm is equal to the distance from the center of the nucleus to the point where the density has decreased by half compared with the density in the center.

The masses of all nuclei are assumed to be integer, and the charges are equal to the number of protons that coincides with the ordinal number of the element. In addition to protons, there are neutrons in the nucleus connected in pairs with protons into a single particle (p + n) - nucleon. A positively charged nucleus creates a fairly significant electromagnetic field in which the movement of electrons takes place. It is clear that the greater the charge of the nucleus, the stronger the interaction of electrons with it. However, when there are many electrons, the nuclear charge is shielded from the electrons located on the far orbitals, those that are closer to the nucleus. How effectively such a screen operates depends on the number of internal electrons and on the structure of the atomic orbital itself, on which the electron is located.

The concepts of one-electron approximation are usually used to consider electronic states. And although this is not the latest achievement of the theory, made on the basis of studying the spectra of atoms and molecules, it is quite enough to explain most of the applications about the electron configuration of the atom. The last term implies the distribution of electrons in different orbitals. The one-electron approximation is based on two assumptions:

1. Electrons in a multielectron atom should be placed in a system of orbitals, formally the same as in the orbitals of the hydrogen atom. In other words, the wave functions obtained by solving the wave equation for a hydrogen atom, for different values of quantum numbers, are also preserved for other atoms.

2. Assuming that any atom has a set of orbitals corresponding to a hydrogen one, in fact it is assumed that a single electron moves in an averaged spherically symmetric field created by the nucleus and all other electrons.

As a result of these two assumptions, it is assumed that the electrons of "large" atoms are occupied by atomic orbitals, analogous to the orbitals of hydrogen, but with the proviso that the energy of these orbitals can differ quite significantly from the energies corresponding to the simplest case. The data obtained from the atomic spectra, as well as direct measurements of the ionization energy expended to remove an electron from a given orbital, show that the relative order of the energy states changes depending on the nuclear charge of the atom. Such an order can be depicted in the form of a scheme of energy levels.

2.5. Electrons are stocking up the energy

Consider some of the laws of absorption of light by atoms or molecules. The quantum mechanical theory has established that these particles can receive and give energy in separate portions - quanta. In photochemistry and in chemistry of colored substances, the energy that quanta of light possess corresponds to the so-called electronic transitions. As a result of such transitions, the energy state of atoms and molecules can take a well-defined series of individual values.

Let us consider any chemical compound consisting of several atoms. For example, let's take greenish-yellow gas - molecule of chlorine. This molecule consists of two atoms and contains two nuclei and 34 electrons. The covalent bond between atoms is carried out by two electrons: external electron of each atom

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of chlorine on the 3p-orbital. If we dispose external electron by its energy, then these two atoms will have the most energy.

Usually the energy of the molecules is depicted by the molecular level diagrams (Fig. 2.7). The energy of the electrons of the last filled orbit is assumed to be zero. Higher than this level so uncompleted molecular levels s^ s2 located. These levels are free in normal state, but could be filled if the electron is informed by additional energy (stirred up) and transmitted from zero level of molecule to a higher one. The absorption of a quantum of light with an energy E = hv, equal to the difference between Es1 - Es0, will lead to the transition of one electron to the level sr If the energy of a quantum of light is greater than and equal to Es2 - Es0, then the electron moves to the level s2.

44

Fig. 2.7. The molecular level diagram and the possible transition of an electron from the occupied level s0 to free

A pair of electorns at the level of s0 is identical in all its quantum characteristics, except for one - the spin, that is when electrons rotate around their own axes in the opposite directions.

In the state s0 the total spin is equal to 0 (since + J/2 + (-V2) = 0). Such a condition is denoted by the term "singlet". If, during excitation, the electron passes to another level, and its spin does not change, then the total spin in the excited state is also equal to zero (the singlet excited state s*). However, it may occur that when an electron is excited, and it passes to another state, it changes its spin value and then the spins of electrons at the ground and excited level are parallel and the total spin will be equal to unity:

+1/, (+1/,) = 1, -1/, + (-1/,) :

-1.

22

This state is called triplet. It is more stable than the singlet one, and the time it takes for molecules to exist in it is from 10-3 to several seconds.

The triplet state is characterized by less energy than the singlet excited state (Fig. 2.8).

44

Fig. 2.8. The explanation of the phenomena of phosphorescence, luminescence and fluorescence.

The transition of electrons from an excited singlet level to a triplet level is accompanied by the emission of the energy of quanta of visible or ultraviolet light, manifested in the form of colorful color effects

After some time, the electron begins to give up energy and spontaneously passes from the singlet excited level to the triplet level (see Fig. 2.8), which is accompanied by a change in the color of the substance and various sometimes very colorful light phenomena: fluorescence, phosphorescence, luminescence.

It is these processes that cause the glow of various substances in the dark. You have probably seen the pictures painted with such paints which start to shine when you hold them against bright sunlight or electric light. It is the electrons in the molecules of the phosphorescent substances, which stored the energy from the light rays, and then began to give it away in the dark. However, there are some exceptions.

The peculiar nature of oxygen is reflected in the fact that the standard for other molecules state in which all electrons are paired, as we have already specified, is singlet, for oxygen it is not the most stable, passive, but on the contrary, activated, aggressive state. The usual molecular oxygen 3O2 can be converted into active JO2 by using light in the presence of sensitizers. Due to the fact that two-electron reactions are banned activity of JO2 increases. A change in the state of oxygen leads to a change in its properties in the human body. Singlet oxygen can, in principle, do everything the same as ordinary oxygen, and a lot more. In particular, it is very easy to attach to multiple bonds of organic molecules. Such as lipids or carotenoids. It has been shown that the body's photosensitivity can be reduced by a simple method - carotene ingestion, therefore beta-carotene, the coloring matter contained in carrots and the skin of ripe tomatoes, can serve as the best (but not sufficient) protection against its action. This method today is considered the most effective in the treatment of various, even very severe porphyria (photodynamic diseases).

Singlet oxygen can also react with saturated molecules if they contain sulfide groups or ammonia residues. It reacts in particular, with amino acids and proteins. Moreover, its negative effect is sharply enhanced by so-called xenobiotics - substances that enter the body from the environment. The xenobiotics include dry distillation products of tobacco, ethyl alcohol, and some chlorine-containing compounds. This circumstance is another argument against the benevolent attitude towards smoking and drinking, which continues to persist among many ignorant people. In an excited singlet state, the molecule can be very short-lived, approximately 10-8 - 10-9 s. During this time, it can emit a quantum of light - this is what causes the phenomenon of fluorescence. It can nonradiatively squander the excitation energy, with an increase in the vibrational energy of the molecule itself and the molecules of the medium. Possible accompanied by radiation or nonradiative transition of the molecule into a triplet state. In addition, the molecule can lose the excitation energy in collisions with other molecules - quenchers. And finally, an excited molecule can enter into a photochemical reaction. Which of these paths will the process take? This largely depends on the ratio of the rate constants of the respective reactions and the conditions of the photochemical process.

So the quant of light effects electrons molecules and transits them in different state. If the electron from the entire visible spectrum is "sensitive" to some particular type of rays, then the color of the substance appears. Thus, the electrons that bind atoms in a chlorine molecule are susceptible to the greenish-yellow part of the spectrum; in metallic sodium - to red.

When the quantum energy is high enough, then a stronger interaction of the photon and the electron is possible. In this case, the electron goes to a higher energy level and some time later emits the stored energy. This is the essence of the previously considered phosphorescence and fluorescence. The waves emitted by an electron are usually of a different length. Therefore, gray or white zinc sulfide becomes blue, orange or indigo color and glows in the dark.

S

s

s

u

Ti

T

S

Electronically excited molecules are essentially, new molecules characterized by their electron density distribution, structure, and their chemical properties. The presence of excess energy makes the excited molecule more chemically active than the original molecule. Thus, a substance can acquire color not only under the action of visible light, but also as a result of interaction with electromagnetic oscillations in a wider range. It interacts with those that are at the invisible ends of the rainbow - infrared and ultraviolet rays.

2.6. Relationship of the color

of a substance with the position of elements in the periodic system

It is known that there are s-, p, d- and /-elements. Each of these types has its own characteristics in the formation of compounds. Appearing products do not always have color; in some cases they are colorless or white.

Inorganic substances whose molecules are formed by s-and p-elements and have ions with filled shells of electrons have no color: alkali metal and alkaline earth metal cations, nonmetal anions of the first three periods. They are joined by compounds (mainly oxides) of elements located in the D.I. Mendeleev periodical system at the conditional metal - nonmetal boundary: antimony, bismuth, lead, and aluminum. Of the subgroups, the compounds of the group IV elements (transition metals): titanium and zirconium have white color. Moreover, zirconium, as a more metallic element, is included in the composition of substances only in the form of the Zr4* cation, and titan as a cation, and as part of the anion. Titanic acid salts of magnesium, calcium, barium and some other elements are widely used as white pigments. Due to the composition of these compounds, oxygen and cations of elements of group II cannot transfer an electron from the ground state to the excited state, since there are no free orbitals to which electrons could save their energy from the light quantum. In titanium and zirconium, the difference in energy values between the filled sublevels and vacant ones is too large. Quanta of visible light simply do not have enough energy to excite electrons.

Ions with incomplete shells form colored compounds in most cases. Moreover, if the anion is not capable of strong polarization, then the color of the substance is determined by the cation and corresponds to the color of the cation in the aqueous solution: iron - yellow, copper - blue etc.

One of the remarkable features is the presence of colored compounds in all transition metals. The dependence of color on the presence of free d-orbitals at the pre-outer level of metal atoms can be explained as follows. As is known, there are five orbitals in the d-level. They have different, but perfectly defined positions in space. On each of these five orbitals, there can be two electrons in accordance with the Pauli principle. Moreover, if an atom (or ion) has five or fewer electrons on the d-sublevel, then each of them tries to occupy a separate orbital. In this case, their energy is the smallest of all possible. If electrons become more than five, then pairing occurs, accompanied by electron transitions. The energy of such electron transitions corresponds to the energy of visible light quanta. The absorption of such quanta from solar white light determines the color of Cu2, Cu2+, Fe2+, Fe3+, Со2+, Ni2+, Cr3+, Mn3+, Mn4+, Mn6+, Mn7+ other colored ions of transition elements.

Half-filled and less-filled internal electron orbitals provide room for electron transitions.

The color of the compounds of d-elements of the IV period is determined by the transitions of electrons from one d-orbital to another and charge transfer to the metal ion. Sucking the electrons from the anion orbitals to the vacant orbitals of their atoms, the cations of chromium, manganese, iron, cobalt, nickel and some other metals give the corresponding color to all their compounds. That explains the color of a number of oxides of elements with transitional properties (metals).

However, it should be noted that the possibility of transition determined by the impact of atoms with which the atom of a given d-element comes in contact. Five d-orbitals occupy a slightly different position in the molecule than in a free atom. The difference in the energies of these orbitals corresponds exactly to the quantum energy of the visible part of the electromagnetic radiation and determines the color of the substance containing the ions Cr3+, Fe2+, Fe3+, Co2+, Ni2+, Mn4+, Mn7+. The color of some substances, for example, iron (III) oxide Fe2O3 and iron (III) hydroxide Fe (OH)3, is immediately determined by two circumstances: electronic transitions from one d-orbital to another and charge transfer from anion to cation.

The potential of charge transition depends on interatomic, inter-ion, internuclear distances. Consequently, the deform-ability of the cation and anion also plays an important role in the compounds of d-elements.

Elements of large periods, located at the bottom of groups of elements, are easily deformed. Especially if they have many internal unfinished layers or 18-electron shells. This applies to both metal cations and non-metal anions. An example confirming this behavior is the mutual influence of lead ions Pb2+ and iodine r. Both of them are colorless in an aqueous solution and the solution of lead iodide also has no color.

When the precipitate of this compound starts to extract from the solution, the ions approach each other and a beautiful golden-yellow precipitate of PbI2 crystals falls out. Here both the cation and anion are easily deformed and mutual polarization occurs. If the ion strongly deforms the neighbor's shell, then we speak of its strong polarizing effect.

The increase in deformability is promoted by an increase in the radius of the ion and a decrease in the positive nuclear charge. Since these values are predictable on the basis of the periodic law of D.I. Mendeleev, it is in principle possible to predict the presence of color in one or another compound, composed of any specific anions and cations.

The occurrence of color in the oxide element and the absence of color in fluoride is possible because the oxygen ion is polarized more easily than the fluoride ion, since it has less positive nuclear charge and more radius. The sulfur anion is deformed even more easily, because it has more internal electron layers and there are (even though completely empty) d-orbitals, which it uses in the formation of chemical bonds. However, the zinc cations Zn2+, aluminum Al3+ and silicon Si4+, despite the rather large radii, are not capable of deformation, since they have a large ion charge.

Most inorganic substances with color are somehow connected with metal ions, and the metals themselves represent one of the types of simple substances that have color.

In the periodic system, starting from the II period, the metals are located in all groups from the first to the eighth. Naturally, the nature of the members of these groups varies from one group to another and from period to period. However, despite the large variety of properties, metals have qualities inherent in all metallic substances without exception.

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2.7. Electron's condition in molecules. The state of electrons in molecules

Light falling on a molecule causes a change in the energy state of both the molecule as a whole and the individual components of its atomic groups. The color of the substance is mainly result of the transition of electrons from one state to another. The change of all types of energy components during the transition of a molecule from the ground state to the excited state characterizes both the color of the substance and its entire absorption spectrum.

If electrons in a molecule of a substance are able to absorb quanta of certain energy, then the color of the substance will be due to this fact. This applies only to external electrons. So what is the difference between external electrons state from the state of internal electrons? The fact is that the external electrons, when they combine individual atoms into a single molecule are socialized, while the internal ones remain firmly fixed, each behind "its own" atom. The socialization of electrons occurs by overlapping and mutual penetration of atomic orbitals. After the formation of a molecule, individual atomic orbitals cease to exist: the combined orbitals became common and cover the entire molecule. The electrons that existed before the creation of the molecule are transferred now to molecular orbitals, of which there are as many as there were atomic orbitals before combining.

So each hydrogen atom has one /s-orbital with a single electron. With the approach of two atoms and the formation of a molecule, two molecular orbitals appear, which differ significantly in energy.

On each of them, there can be only two electrons in accordance with the Pauli principle, differing in spin quantum number. In this case, both atoms in the sum have only two electrons. Therefore, one of the two molecular orbitals will be filled, and the other will be empty.

From the above consideration follows a fundamentally important conclusion. If atoms have n unpaired electrons, when combined in pairs with n such atoms in the resulting molecule, half of the molecular orbitals will be occupied by pairs of electrons, and half will be free. In typical metals and non-metals, unpaired electrons of the s- and p-orbitals are generalized. In transitional elements, internal incomplete d-orbitals also participate in the creation of a system of molecular orbitals. In accordance with this, the possibilities of these elements are expanded.

If s-elements capable of only giving up the electrons during the formation of molecules, and p-elements basically accept them, then the transition ones is able of doing both. When donating electrons from their outer orbitals, d-elements act as electron donors, and taking them to their incomplete internal orbitals acts as acceptors. The state of electrons in common molecular orbitals varies compared with the initial atoms, and for their excitation, other visible light quanta are required. Since in any case the number of electrons participating in the bond is less than the number of molecular orbitals that have arisen, some of the energy cells are filled and some are empty.

In metals that unite many identical atoms (Fig. 2.9), whole bands are formed, corresponding to s-, p-, and other electron states. Neighboring energy levels will be located very closely; the difference between them is of the order of 10-22 - 10-23 eV (whereas in atoms there are a few electron-volts, eV). In this case, the sublevels and stripes merge into a common area. Where there are electrons, it is called the "filled zone", while the higher one (there are no electrons in it) is called the "conduction zone".

Sigma bond of p-electrons

iL iL

tlfpl I » * * É

tT

Fig. 2.9. The overlapping of the orbitals (a) and the formation of (b) metal bonding

The physical picture of such state corresponds to the "socialization" of electrons. This means that electrons from one atom, using free levels, can move on to another. Thus, in metallic substances in the solid and liquid states, there are unfilled zones of allowed energies common to all atoms are formed. Metals are elements containing no more than three electrons at external energy levels (exception: Sn, Pb, Bi, Po), therefore, in the condensed state, they have large unfilled zones of allowed energies. This causes their high electrical conductivity, the photoelectric effect (knocking electrons out of a metal under the action of light) and, of course, the reflective ability and color of the metal (Fig. 2.10).

100

Red

O. Y.

Green B.

700

600

500

400

A, nm

Fig. 2.10. Metal reflection spectra

The unification of the sublevels of valence electrons into energy bands is a consequence of the interaction between metal atoms. Common conduction zones are located inside the crystal and do not go beyond it. The outer surface of the crystal consists of metal ions, connected to each other by common energy zones. The occurrence of conduction bands in a metal crystal does not mean that electrons are completely free in them. They cannot spontaneously leave the metal, since for this it is necessary to overcome the potential barrier on its surface. In order to snatch an electron from a metal, it is necessary to expend energy.

In such active metals as cesium and rubidium, this can be done by using visible light.

The energy of light quanta is enough to pull electrons out of these metals. In most cases, visible light, falling on the metal and interacting with electrons, is only able to transfer them from the filled zone to one or another level of the conduction band. And besides for each particular e/ement strict/y defined

x

X

0

energy quanta are required. The remaining rays are simply reflected from the surface. And since the reflected radiation lacks the rays spent on the transfer of electrons, the additional colors of the spectrum are observed, which determines the color of the metal.

What are these examples for? The fact is that in our body there are also processes that can be influenced by electromagnetic radiation of a certain quantum energy and intensity. At lower levels of these parameters, the process does not occur. This gives us the opportunity to stop pathological processes (as in the case of knocking out electrons from certain metals with a photoelectric effect) without affecting other metabolic processes. Conversion of pathological collagen in a soluble state (gelatin), without affecting our own collagen also is an example of same separation process. The same applies to the possibility of destroying viruses in our body without affecting our own DNA and RNA. In general, the principles of resonance therapy are based on the fact that the impact is aimed at normalizing the processes, the basis of which is the selectivity of the impact on these or other processes.

2.8. How does color appear?

A molecule that has absorbed a quantum of light cannot remain in an excited state for a long time. During millions of fractions per second, it tends, in one way or another, to use up its energy, and the excited electron returns abruptly to its former stable state. The recoil of electrons energy can occur in several ways, one of which is called resonant radiation. Getting to know it helps to better understand how color appears.

If we take two pitchforks placed next to each other and make one pitchfork sound and then stop the sound, we could clearly hear how the second pitchfork is sounding even though nobody have touched it. The pitchfork not only absorbs the waves of certain frequency, but also reflects them. Molecules of a substance possessing color, like tiny pitchforks, absorbing light quanta of a certain frequency, begin to resonate - emit radiation quanta, which are perceived as the color of a substance. The difference with the pitchfork is that the substance absorbs some quanta, and gives away different quanta. For a short instant of existence of an excited state, a part of the energy perceived by the electron from the light quantum has time to dissipate. Therefore, the quantum emitted by the dye is smaller than the one that was absorbed, and its wavelength is longer. The emission spectrum of the dye is always shifted to the long-wave side compared with the absorption spectrum.

Thus, color is caused by two radiation fluxes. The first is the one that passed through the substance or was reflected from it In this flow there are no quanta of the resonant frequency that is absorbed by the substance. This phenomenon we perceive as the appearance of color The second stream arises as a result of the fact that the excited electron returns abruptly to its previous state, and the excess energy is displayed in the form of quanta of secondary radiation - fluorescence.

The spectrum of the secondary radiation of the dye is a curve that is a mirror image of the absorption curve. Absorption and emission curves partially overlap.

The color of a substance is a cumulative effect from absorption and emission spectra. Knowing that color is the result of a combination of two components, you can use it to turn invisible light into visible. By illuminating with a layer of matter (phosphor) that can absorb ultrashort quanta by invisible ultraviolet rays, we can cause a luminescence to appear in the visible part of the spectrum. This is the principle by which

fluorescent lamps work. By selecting phosphors, the emission spectrum of such lamps is approached to the spectral composition of daylight.

The transformation of longer-wave radiation into shortwave radiation would seem to contradict the law of energy conservation. In fact, it is possible to transform longer wavelengths - infrared radiation - into visible color. For this the overlapping areas of absorption and emission is used. If we choose light, the quanta of which lie in the general region (absorption and emission), then the color of the substance will be reproduced, as usual, within the entire luminescence band. In this part of the quanta of radiation, will be more shortwave than the light absorbed. Quanta are generated with more energy than the original, and this contradicts the law of energy conservation. However, it just seems to be contradicting the law of conversation. Although some of the radiation does contain quanta of higher frequency, but the total energy output is less than the absorbed energy. The principle of night vision devices is based on this effect Due to these devices it is possible in the dark to distinguish objects on the infrared radiation that is invisible to the eye, which these devices transform into visible.

Based on a combination of absorption and emission spectra in the color of the substance, so-called self-emitting dyes work. They are created by adding to conventional dyes substances that can convert into visible color radiation from the ranges adjacent to this part of the spectrum - ultraviolet and infrared. They find the most diverse use from banners, advertisements to covering aircraft fuselages. The secret of their unusual brightness is that they not only reflect the visible rays (like ordinary dyes), but also convert part of the absorbed short-wave rays from sunlight into visible light. When using ultraviolet illumination invisible to the eye, self-luminous paints give unusually bright and contrasting images.

Such dyes can glow for a while in the dark. This phenomenon was called phosphorescence, since its external manifestation is similar to the luminescence of phosphorus (however, in reality, phosphorus glows for other reasons). The essence of phosphorescence of dyes is that the electron, while it returns to the main level, can linger on some intermediate one. Such delay happens due to fact that the electron has already had time to waste some energy before its return. It got into the debt energy hole. In order to make a jump to the initial state, it must first gain energy (get out of the "debt pit"), rise again to the exited level, and then abruptly return to the initial state. All this takes time. If the temperature is high enough, it is easy to gain energy due to thermal movement. If the temperature is low and the state of the dye is solid or glassy, the process is slowed down and phosphorescence can last for a long time. And due to this fact it's possible to prepare solid or glassy (often polymeric) compounds that can glow in the dark for a long time.

2.9. Mechanism of color emergence in a matter

The energy impacting on a molecule when ultraviolet, visible or infrared radiation hits it several processes are expended.

• First, on the movement of the molecule as a whole, mainly on its rotation.

• Secondly, to increase the vibrational energy of individual fragments of the molecule.

• Third, the bulk of the energy of the incident quanta is spent on the transfer of electrons from their normal energy level (basic) to a higher (excited) level.

05.14.01

First of all, it is necessary to clarify which of the electrons are able to interact with visible light and only then find out how the state of such electrons changes with a change in the structure of molecules.

Exact devices are able to show us that quantum of light that is absorbed by matter. Recall that the ratio of energy E and frequency v or wavelength K of the incident light is as follows:

E = hv; v = c/K; E = hc/K.

Therefore, knowing what the wavelength K of an absorbed quantum is, it is easy to determine what energy is required to excite a given molecule (or for 1 mole - NA. Molecule);

E = hcNA/\ --

(28 000/X) • 4,2 kJ/mol.

An example of such a molecule is benzene. Of course, one must keep in mind that the probability of a flow of n-electrons in one direction is equal to the probability of flowing in the opposite direction. Under normal conditions, the n-electron current does not detect itself. But, by placing the molecule in a magnetic field, it is possible to facilitate the movement of electrons in one direction and to impede their movement in the opposite direction. Indeed, benzene in a magnetic field detects unusual magnetic properties - increased diamagnetism. These and other similar experimental data prove the mobility of the n-electrons in the molecules of benzene and other compounds.

Internal electrons in atoms are tightly bound. For their excitation, hard X-rays carrying 103 - 106 kJ/mol are required. Consequently, the energy of visible light (450-290 kJ/mol) is too small to make at least some noticeable effect on these electrons.

External electrons (participating in chemical bonding) more easily change their state. For their excitement, much smaller portions of energy are required. Its value is determined by the nature of the chemical bond in which they participate.

There are two main types of compounds of carbon atoms in organic molecules: a- u n-bond. When the first of them is formed, the electron orbitals of two atoms, overlapping, form a single cloud of two electrons, concentrated along the straight line connecting the nuclei of neighboring atoms. The bond is covalent, durable, more than 2500 kJ/mol is required for its rupture, and 760 kJ/mol energy is required for excitation. This corresponds to the absorption of radiation with a wavelength of less than 200 nm, that is, in the region of a hard ultraviolet. Now it is clear why UV can lead to undesirable effects in our body.

Another thing is the n-link. In this case, electron clouds (Fig. 2.11) are located perpendicular to the axis connecting the centers of the atoms. Wave electron orbitals have the form of "eights", are oriented parallel to each other and overlap with "sides." The strength of such a bond is one and a half time less than that of the a-bond, but then the electrons entering into it are more mobile. Clouds of n-electrons of one atom can overlap with two adjacent ones, located on the sides. Then systems are obtained within which the n-electrons are able to move relatively freely from atom to atom. If the system turns out to be closed, then electrons in such a molecular system circulate, as in a closed conductor (Fig. 2.12).

Fig. 2.12. Conjugation effect in benzene and n-electron currents

The existence in many organic molecules of whole systems of n-electrons belonging simultaneously to several atoms leaves a unique imprint on the chemical and optical properties of these molecules.

First, n-bond electrons are excited much easier than a-electrons.

Secondly, n-bonds overlap in the case when the atoms are separated by one a-bond, i.e. there is an alternation of double and single bonds. In this case, the "conjugation effect" occurs, which has already been discussed. Since this is the most important condition of the mechanism for the occurrence of color, then we consider this in more detail using butadiene as an example. In butadiene, two bonds belonging to extreme atoms (1 and 4) contain n-electrons, but due to the proximity of these bonds, n-clouds "overlap" and a single conjugate system of n-electrons are obtained:

2 o 1 2

--CH = CH,

o

3 4

Fig. 2.11. Electronic structure of benzene molecules;

a - a-bonds; b - n-bonds

This is marked with a dotted line between atoms 2 or 3. Therefore, in the case of "overlapping", not only the charges on the atoms change, but also the electron density on each bond. Atoms 2 and 3 are connected by a bond, which is somewhat similar to a double one - a cloud of n-electrons evens out the differences between the bonds (if between atoms 1 and 4 there is a chain from the CH2 group then clouds of electrons do not overlap and the conjugation is broken). The location of n-electrons in benzene and other aromatic hydrocarbons lead to the fact that conventional formulas with double bonds are meaningless. Therefore, the nucleus of benzene is designat-

b

a

ed either as a hexagon with a ring in the middle part, or bind atoms with an additional dotted line. There is another feature of the system of conjugated bonds: the longer the chain of such bonds, the easier it is for the n-electrons to shift. With sufficient elongation, the photon of visible light transmits the n-electrons into the excited state (Fig. 2.13).

b

Fig. 2.13. With the effect of conjugation, energy in complex molecules, as in connected pendulums, can be transferred from one bond to another, for example, from a bond 2-3 (a) to a bond 1-2 (b)

For example, the coloring matter of tomatoes - lycopene -is red-orange in color, since the chain of alternating double and single bonds present in it is excited by indigo-blue rays with a wavelength of 480-510 nm:

CH3 CH3

I 3 I

CH3-C=CH=CH2(-C=CH-CH=CH-

CH3

lycopene

CH3

tive absorption of light energy quanta with wavelengths already in the interval of the visible part of the spectrum.

For benzene, aniline and nitrobenzene, the quanta they absorb are in the ultraviolet region:

Benzene

Aniline

Nitrobenzene

These compounds appear to us colorless, although they have a cycle with conjugated bonds and auxochromes: —NH2 u —NO2.

NH,

-NO,

The first of the auxochromes is capable of shifting its lone pair of nitrogen electrons to the side of the benzene ring (Fig. 2.14), and the second is delaying the n-electrons of the cycle in its direction. With the joint presence of these groups in the para-nitroaniline compound, the electron system is shifted from the amino to the nitro group. As a result, the substance becomes capable of absorbing big amounts of the rays of the visible part of the spectrum and becomes yellow color.

0 -0,03

-(-CH=C-CH=CH-)3=CH=C-CH3

What has been said about the mobility of n-electrons between two carbon atoms also applies to those structures where there are n-bonds between carbon and nitrogen, two nitrogen atoms, etc. By joining the system of conjugated bonds, they also contribute to the mobility of n-electrons and lengthen the conjugation circuit.

The presence of a mobile system of electrons facilitates the transfer of the molecule to the excited state. Molecules of the same substance in an excited and unexcited state differ significantly from each other.

The transition to the excited state is associated with the redistribution of electron density: in some places the electron molecules become smaller (they leave these points), and in others bigger. According to this, at those points from where the electrons left, a positive charge appears which the molecule did not have in unexcited state. In those places where the electron density increased, a negative charge appears. Thus, the polarization of the molecule happens.

The intensity of light absorption and the emergence of a particular color in a substance depend mainly on the ease with which n-electrons shift when interacting with light quanta. Ease increases if the molecule as yet in a normal unexcited state has some displacement of electrons from one atom to another. In other words, there has been already some polarization of the molecule. As a result, the difference in energy levels between the ground and excited states decreases. The energy required to excite becomes smaller. The facilitation of the transition of the molecule to the excited state determines the selec-

-0,01 <v

0 -0,03

Fig. 2.14. Distribution of electron density in aniline molecule. The numbers of the atoms are the fractions of the charge of the electron, inside the cycle the lengths of the bonds in nanometers

Thus, the polarizing ability increases, if there are chains of bonds in a molecule, along which electrons can easily move. This ability increases especially strongly if at the ends of the chains of atoms —CH = CH—CH = CH— there are substitutes that facilitate the shift of the electrons. The tendency to polarization under the action of electromagnetic radiation is a common property of all substances. The presence of color and intensity of it depend on the ease of polarization of the molecules. However, most of them require high energy quanta -ultraviolet light. Among organic substances enough of color has those molecules which are able to polarize under the action of small quanta, which carry visible light, and their energy is enough to excite electrons.

This means that all those structural changes that, without destroying the planar structure of the molecule, will contribute to the shift of the n-electron system of the molecule, its polarization and the appearance of a constant distribution of positive and negative charges in it. It eases the shift to exited state under the action of quanta of visible light, i.e., cause the appearance of color.

A= 255 nm Amax = 282 nm Amax = 268 nm

a

2.10. Spatial structure of molecules

In the previous sections, when discussing the dependence of color on the state of the system of electrons, it was assumed that the shift of n-electrons in all cases occurs without difficulty along the entire system of conjugated bonds in the molecule. In reality, this is possible only in the case when all the bonds in a molecule are more or less in one plane and it itself has a planar structure. If the molecule for some reason acquires a different form (Fig. 2.15), some of its fragments are rotated or out of the plane, then the interaction of n-electron clouds is broken. This leads to a partial or complete disconnection of the conjugation circuit.

a b

Fig. 2.15. Spatial configuration of molecules:

a - ethane; b - benzene

Of the two molecules shown below, one is colorless and the other is not:

AA, /V\ /V\

w w w

AA aA

W W

M

w

05.14.01

Introduce volumetric isoprophyle group

-NO,

Yellow 1

Colorless 2

CH,

then the color will almost disappear, only a slight yellowness will remain (Kmax = 420 nm). Being located near the benzene ring, the introduced —CH(CH3)2 group presses the —N(CH3)2 group and "squeezes" it out of the plane. The nonstandard electrons of the nitrogen atom no longer interact so much with the benzene ring system. Therefore, the shift of electrons from nitrogen is attenuated and the entire polarization in the molecule is disrupted.

The color changes associated with the need for the molecule to be entirely in one plane are explained by the peculiarities of the n-electrons. Their electron clouds are symmetrical with respect to the axes connecting the centers of the atom, i.e. they lie at the same distance "above" and "below" this axis. Clouds of n-electrons are oriented parallel to each other; their greatest overlap is obtained if the entire molecule is flat. Then n-electron clouds, like water on a flat surface, cover the entire molecule with a uniform, inseparable layer (see Fig. 2.15). The distortion of the plane of the molecule violates the parallelism of the "eights" and reduces the degree of their mutual overlap. The interaction of n-electrons changes and the possibility of their displacement along a chain of conjugated bonds decreases. The color of the compound increases, i.e. the substance absorbs shorter waves.

In the case, when change of molecule shape happens without significant violation of its planar configuration; the color does not disappear and may even become deeper. Such situation is possible when the angels between the directions of atomic bonds change. At the same, the mutual overlap of the n-electrons is not substantially infringed since the axes of their electron clouds remain parallel. The deepening of the color becomes possible because (as the angles between the bonds change) a voltage appears in the molecule and the energy level of the ground state approaches the level of the excited state. The difference in levels decreases. Consequently, to transform a molecule into excitation, quanta with lower energy are required, the ones that are close to the red edge of spectrum.

N

H3C—CH

Due to the fact that in the second structure there is the possibility of free rotation around the a-CONNECTION connecting two naphthalene rings, the system of conjugated bonds is disturbed. The halves of the molecule have a chain that is two times shorter, and for the excitation of n-electrons, photoviolet radiation is required.

Sometimes breakdown of the system of conjugated bonds does not occur completely, but some large groups introduced into the composition somehow force it to bend and twist. For example, if into the dye molecule

H3C. H3C'

-/\-N=N-/\-

Amax = 475 nm

-NO,

Chapter 3

Color of inorganic and organic substances

3.1. Introduction

Unified color theory does not exist. However, the main laws which relate to the connection of color of substance with the structure of molecules are firmly established. The main thing is clarified: the color is related to the mobility of electrons in atomic orbitals in a substance molecule and with the "mobility" of electrons, i.e. with their possibility to absorb the energy of a quantum of light, to transfer to free energy levels, but not in an atom, in a substance molecule.

The mechanism of color appearance in metals, non-metals, inorganic compounds and in organic molecules is quite different. Indeed, in all cases, color appears as a result of the interaction of light quanta with electrons in substance molecules. However,

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the state of electrons in metals and non-metals, in organic and inorganic compounds is different, and therefore the mechanism of the appearance of the color of these substances is not the same.

For metals, the correctness of the crystal lattice and the relative freedom of electrons throughout the piece of metal are important for color. For most inorganic compounds, the color is due to electron transitions and, accordingly, charge transfer from the atom of one element to the atom of another in the molecule. In this case, the main, decisive role is played by the valence states of the elements, the structure of their outer electron shell.

Not all organic substances have a color in the visible range, but those substances that have a color have an extremely important similarity in the structure of molecules. For the occurrence of color, it is not the electrons of individual atoms that matter, but the state of a system of electrons that encompasses the entire molecule as a whole. Organic substances with the ability of color have molecules consisting of ten atoms. The mobility of such a system, its ability to easily change its state under a small influence of light quanta and causes the selective absorption of certain waves from the set of visible light.

In order to understand the dependence of chromaticity on the structure, it is necessary to consider what the features of the energy state of electrons of one or another type of molecules are.

3.2. Absorption spectra and color of inorganic substances

A particular color of a substance means that from the entire 400-700 nm interval of the wavelengths of visible light, they absorb some specific quanta (Table 4), whose energy is generally small.

From this, in turn, it follows that in the molecules of the colored substances the energy levels of electrons are close to each other. If the AE difference is large, then other quanta that carry more energy, such as ultraviolet, are used. Such substances as nitrogen, hydrogen, fluorine, noble gases, seem to us colorless. Quanta of visible light are not absorbed by them, since they cannot bring electrons to a higher excited level. If our eyes were capable of perceiving ultraviolet rays, then in such ultraviolet light both hydrogen, and nitrogen, and inert gases would appear colored to us.

The more electrons in an atom, the closer the electron levels are to each other. It is especially good if there are orbits unoccupied by electrons in an atom. In this case, to transfer an electron from one state to another, quanta of light are required with a lower energy, which is not visible in the visible part of the spectrum. Such multielectronic halogens, such as chlorine, bromine, iodine, are already colored. Oxides of nitrogen NO2, N 2O3 and co-valent compounds, for example CuCl2, AlI3, are colored. The color of molecules consisting of several atoms depends on a number of factors. If the effect of these factors is such that they bring the electronic levels closer together, then this contributes to the appearance or deepening of color.

So a closer interaction of atoms in the transition from a gaseous to a liquid and further solid state can contribute to the emergence or deepening of color, especially in those cases when the atoms have orbitals unoccupied by electrons.

The difference in the nature of the interaction affects the spectra. The absorption spectra of the simplest molecular compounds - gases and substances in the gaseous state -consist of several series of narrow bands (lines). This means

that from the entire flux of white light they select only some photons, whose energy is just equal to the difference between the ground and "excited" states of the electrons. In the liquid and especially solid states, the spectrum becomes essentially continuous, because due to the strong interaction of closely spaced atoms many new energy levels of electrons appear and, consequently, the possibilities of new electronic transitions increase, the number of energy levels of molecules and ions grows. The spectrum includes a large number of broad bands extending to several tens of nanometers (a nanometer, denoted by nm, is one billionth of a meter). The intensity of the bands and their various superimposition on each other determine the final color of the substance. Indeed, with various combinations of primary colors: red, blue, green or red, yellow, blue, all other colors of the spectrum are obtained.

Fig. 3.1. The color of nitrogen oxides disappears when cooled

As a rule, the absorption bands of inorganic substances begin in the visible region, and end in the ultraviolet. The position of the most intense absorption bands determines the color (Table 3.1).

Table 3.1

The wavelength of absorption of some inorganic substances

Substancee Absorption wavelength, nm Color

PbO Lead oxide 440 Yellowish orange

Cu20 Copper oxide (I) 500 Red

Cu(OH)2 Hydroxide copper (II) 670 Blue

The color of the substance consists of the sum of the reflected waves (or passing the substance without delay), and the intensity of those or other waves may be different. Therefore, even if the spectrum consists of the same waves, according to their relative share in the spectrum is changed, then we see substances of different colors. The rays, combined in the spectrum with each other, will give different colors. Here is an example. Cadmium and mercury are elements of the same subgroup II of the periodic system. Their atoms differ from each other in the number of internal electrons. Their sulfides HgS and CdS strongly absorb the rays of the violet part of the spectrum and much weaker - red-orange (Fig. 3.2). As a result, it would

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seem that an insignificant difference in reflection results in a different combination.

Red Or Yell G B I

HgS - orange CdS- yellow

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Fig. 3.2. The difference in the absorption spectra of compounds determines the color of the substance, cadmium sulfide turns yellow, and mercury sulfide turns orange

Fig. 3.3. The difference in the color of sulphides of arsenic (stone), cadmium (yellow) and lead (black)

The diagram shows the spectra of several substances, in which the intensity of reflection of waves of different sections of visible light is different. Fig. 3.4 shows at what proportions we see one or another color. In the event that the curves intersect, the colors mutually "destroy" each other and we see only the color that remains. When the color with a wavelength of 480 nm is reflected, the substance is bluer, since the red and green colors are mutually destroyed. At 500 nm - green, above 600 nm - red with a yellowish tinge. Color sensation from different colors (obtained, for example, by imposing three electron beams on the TV) is perceived by us as total.

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Fig. 3.4. The explanation of the color image on the TV screen: the color is the result of the imposition of three rays with a specific wavelength corresponding to each of the three nerve centers of the eye, creating a feeling of red, green and blue

3.2.1. Features of the solid state

A feature of crystalline bodies is the ordered arrangement of many hundreds and thousands of atoms, ions or molecules. They differ both in the type of particles entering the solid body and in the type of chemical bonds between them. In order to understand their structure, we consider the peculiarities of various aggregative states.

In the gaseous state, substances are characterized by considerable distances between particles and small forces of interaction between them. They are able to occupy any given volume, and their properties are mainly determined by the behavior of individual particles.

In the liquid state, the particles of substances are close to each other, commensurate with their size, the interaction forces between the particles are significant. Particles of a substance are combined into large aggregates, in which their mutual arrangement is ordered and the movement is oscillatory in nature (short range order). At considerable distances from the centers of the aggregates (long-range order), this refinement is broken. The strength of the bonds between aggregates of particles in a liquid is small, so in the liquid state the substance takes up a certain volume, but it is able to change the shape under the action of gravity. The behavior of substances in this state is determined by both the properties of the particles and their aggregates, and the interactions between them.

In the solid state, an ordered arrangement of particles occurs in both the short-range and long-range orders. Solid matter can not only maintain a certain volume, but also the invariance of form under the action of gravity. The properties of a substance are determined both by its elemental composition and structure. The mutual arrangement of particles in a solid substance is characterized by the distance between the centers around which they perform rotor movements.

If a substance has a repetitive spatial arrangement of many hundreds and thousands of atoms, it is called a crystal. Depending on the direction chosen, the arrangement of particles in the crystal may be different, but necessarily repetitive. An ordered arrangement of particles, repeated many times along any straight line, is called a crystal lattice.

The following types of crystal lattices exist: hexagonal closest packing. (Fig. 3.5, a), face-centered cubic (Fig. 3.5, b), cubic body-centered (Fig. 3.5, c).

The majority of chemical elements crystallize in the formation of simple substances in one of the three indicated structures. However, the formation of more complex crystalline bodies is also possible. Thus, a diamond structure (Fig. 3.5, d), in which germanium and tin (gray) also crystallize, can be thought of as two intersecting face-centered cells. Some substances, depending on the conditions, can form different types of lattices. So, gray tin can, by changing its color and properties, turn into white.

Such a transition is carried out if the tin is heated above 13 °C. At the same time, a body-centered structure with metallic properties appears.

The following symbols: a, P, y, 6 etc are introduced in order to distinguish different crystalline formations of the same element, for each crystalline modification of a substance. Gray tin is called a-modification, and white is considered P-tin. If there is a sharp difference in color, then the various allotropic substances formed by the same element are called like this: for example, white, yellow, red, or black phosphorus. Carbon forms dramatically different structures that they have different names: diamond, graphite, carbin.

Fig. 3.5. The main types of crystal lattices

Let us point out one feature that researchers encounter when studying crystals: in real structures, the location of particles is far from the ideal that we have depicted. Because of this properties of substances changes dramatically. So, tests of strength showed that it is hundreds or even thousands of times less than theoretically calculated. This proves that there are significant interaction forces between particles in the crystals. These forces not only affect the strength of the crystals, but also determine its optical properties: brilliance and color.

As you know, in 1915 Max Born created the theory of crystals, which well explained a large number of optical, electrical and other properties of crystalline bodies. The validity of the theory was confirmed by a large number of experiments, however, experiments related to the mechanical properties of crystals were in conflict with the proposed theory - the measured strength of the crystals turned out to be hundreds of times less than those calculated theoretically. An explanation of the observed phenomena was given by A.A. Griffiths. Two fundamental ideas underlie his theory: the destruction of a body should be considered as a result of the energy absorbed by this body and in the real solid body there are already microscopic cracks exists which is the nuclei of future large cracks. Because of them, the divergence of theory and experiment was observed. Based on the foregoing, we consider the situation in which a crack of size l appeared in a solid. According to the first idea of Griffiths, the energy released in the volume is proportional to l3, and the area of the newly formed surface will be proportional to only l2. The unit of volume due to applied loads is, say, equal to a. Then the released energy in the volume will be al3. To form a unit of surface area in the crack zone, energy is needed b. Hence, the formation of cracks will require energy bl2. If with the growth of a crack, more energy is released than spent on increasing the surface area of the body, then, considering that al3 > bl2, we will observe a spontaneous growth of the crack. Conclusion: if the crack is large enough, it is energetically favorable to grow, and if it is smaller than the critical size, i.e., l < b/a, then it is stable. It should also be said about the P.A. Rebinder effect (he was one of my teachers. May he rest in peace) which was discovered in 1928. In the interaction of molecules of ionic crystals, electric

forces play a crucial role. If a fluid with dielectric constant e falls into a crack that is smaller than the critical size, then the interaction forces between charges will weaken by a factor of e, which will reduce the energy b required for a surface unit, and this in turn will lead to the propagation of cracks for those stresses at which they were stable without liquid.

The properties of solids depend not only on what type of crystal lattice they have. Substances similar to the type of elementary crystalline cells may have a different character. A brilliant silver-white titanium, for example, can crystallize in a hexagonal structure resembling black and gray graffiti, and in volume-centered, like pinkish sodium.

An important role is also played by the type of bond between the components constituting a particular crystal, as well as the structure of their atoms. The forces of interaction between particles in lattices affect the physicochemical properties of a solid. By the nature of the interaction between particles in the lattices, crystalline substances can be divided into several groups.

Atomic crystals. In the lattice sites, the neutral atoms of the elements are connected due to the socialization of valence electrons (for example, diamond).

Metallic crystals. In the lattice sites there are ions of the same metal that are interconnected due to semi-free electrons located in the conduction band common to all ions.

Ionic crystals. In the lattice sites, oppositely charged ions are located, whose electrostatic attraction determines the nature of the solid (for example, KC1, NaCI).

Semiconductor crystals. By character of the bonds it occupies an intermediate position between atomic and ionic (for example, Cu2O).

Molecular crystals. In the lattice sites there are neutral molecules that form a lattice due to intermolecular interaction forces (for example, metallic gallium OR solid CO2).

However, not only crystalline substances have color. Liquids and gases extract the rays from the sent light and signals about their presence to them. For example, an analysis of the composition of the atmosphere of distant planets is carried out such way. To understand the differences in the occurrence of colors of metals, non-metals and other inorganic substances, it is necessary to understand the difference in the state of electrons in the atoms and molecules that make up these substances.

3.2.2. Color and polarization

Atoms and ions in the crystal lattice continuously make oscillatory motions. In this case, the distance between adjacent particles becomes smaller and bigger than the equilibrium. This causes either a stronger or a less strong interaction of them with each other, since the nuclei of the atoms converge, then move away from each other. The impact of neighbors on an atom or ion leads to a violation of the distribution of positive and negative charge in it. Two poles appear in the molecule (Fig. 3.6), i.e. it is polarized.

If the influence of a polar particle is sufficiently strong, then the neighboring atom or ion begins to deform, i.e., it acquires a constant non-uniform electron density distribution around the nucleus. When there are a lot of neighbors, then as a result of this it experiences a multi-sided polarization, leading to a multilateral deformation. It is greatly exaggerated is shown in Fig. 3.7. The arising additional forces between the ions affect the interaction of the atoms that make up the crystal lattice. This influence changes the color of a substance if it forms several types of crystals.

Fig. 3.6. The polarization of molecules under the action of neighboring molecules

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between particles with such oscillations change, causing respectively a change in the distribution of charges - polarization (Fig. 3.9, b). If the polarizing effect of the neighbors and the intrinsic deformability of ions or atoms are sufficient, then this will affect the state of electrons who will already perceive the quanta of visible light.

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Fig. 3.7. Multilateral deformation

Sulfur can have a different color - from light yellow to dark brown, depending on its crystalline structure. Various allotropic modifications of phosphorus: white, yellow, red, brown, purple, black, and a number of others (11 in total) have different physical and chemical properties. After all, these qualities, as well as color, depend on the state of electrons. The same atoms, located in space in a different way, can create a substance like dielectric or possessing electrical conductivity. The structure of such black phosphorus is shown in Fig. 3.8. According to its properties, it resembles graphite by its color, hardness, resistance to air and some other signs. However, black phosphorus conducts electrical current to a much lesser extent than graphite. Carbon is another striking example of a change in color and properties depending on the allotropic structure. It can be a transparent and brilliant diamond or graphite that can be turned into a diamond by artificial means.

Fig. 3.9. Influence of polarization on the position of the atom in the crystal lattice

If the vibrational motions are large or amplified, for example, by heating, then the resulting deformation increases the attraction of the ions and the regular nature of the vibrational motion is disturbed (Fig. 3.9, c). There is a further convergence, and this causes a restructuring of the crystal structure of the substance. As a result of such a restructuring, it may turn out that the ion is surrounded by neighbors which are located at a really close distance. And sometimes their number changes: some of the neighbors become closer (three of the four cations in Fig. 3.9, c), and others farther than they were before.

Examples of the formation of such compounds having different colors are the yellow and orange forms of lead oxyde PbO. The first of them corresponds to the rhombic configuration, and the second is tetragonal.

The effect of structure on color is also manifested in more complex compounds. Thus, lead chromate PbCr04 can be both dark yellow (monoclinic crystal lattice) and light yellow (orthor-hombic structure). Consequently, the change in spatial arrangement leading to a change in color can also occur with a large group of atoms. In lead chromate, this concerns a molecule of six atoms.

3.2. 3. Color of polar molecules

When cations fall into the field of action of anions, there is a mutual influence (Fig. 3.10). The results depend on the ability of electron shells of ions to deformation. This ability is due to the nature of the ion and the force with which the ion can affect the neighbors shell. As a rule, ions of small radius and large positive charge deform weakly: very strong in this case, a positive nucleus attracts electrons. The deformability and the associated polarization are also small if the outer electron shell of the ion is similar to the inert gas shell which means the ion shell filled with electrons is completed.

Fig. 3.8. Structure of conductive black phosphorus

The color change caused by a change in the state of the electrons and associated with the restructuring of the structure can be explained by oscillatory motion in the crystal. Assume that the particles in the crystal are firmly fixed. In this case, each of them would experience a strictly symmetrical effect (Fig. 3.9, a). The deformation from different neighbors would compensate each other. In reality, however, oscillatory motions occur continuously in a crystal. The distances

a b

Fig. 3.10. The occurrence of polarization effects (a) and the increasing polarization of ions (b)

If the molecule consists of ions with filled electron shells (MgO, Zno), then the possibility of electron transition is practically excluded, since it is simply nowhere to go. Then, from the entire spectrum of visible light, the molecule does not prefer any site. Such molecules have no color. In solution, they are colorless, and in the solid state white. To this type of coloring substances belongs zinc oxide, magnesium oxide, phosphate and zinc sulfide, barium sulfate. As you can see, these are all the compounds of the elements of group II of the periodic system with fully completed internal electronic shells.

Such compounds can directly serve as inorganic dyes, pigments. As dyes are used such individual compounds, for example like, white - zinc oxide or titanium oxide (IV); black - is one of the allotropic States of carbon - soot. A color can appear only in the case that the cation with the sublevels are filled by electrons associated with anion, capable of considerable polarization, for example with heavy ions of Halogens, such as Br- or I-, some oxygen-containing anions PO|-, AsO^- and a number of others. Salts and oxides of metals having atoms with unfilled shells, for the most part have color. Metal ions have about the same color, which is inherent in them in aqueous solution: Cu2+ - blue, SG3+ -green, etc. There are numerous anions, capable of giving color to ions, especially if it is metal ions of side subgroups. For example, the yellow anion CgO^- affects the colorless silver cation Ag+, so in the result of the reaction it produces a red precipitate of silver chromate.

3.2.4. Molecules are colorless and the substance is colored

And yet, in some cases, the color of the same substance does not depend on the structure. To be more exact, it depends not from the type of crystal lattice. There are no perfect structure substances in nature. Man tries to correct this natural "defect" and grows crystals close to ideal. Modern optics is unthinkable without such crystals. However, natural crystals amaze with a variety of colors and shades. This can be seen if you look at the crystals such simple like ground table salt or carbonates.

In the suburbs of the Polish town of Wieliczka there are salt mines, where extensive corridors and huge halls, carved in layers of rock salt, the gallery stretches for tens of kilometers. In the niches on the sides of the gallery you can see figures made of salt and crystals of amazing shape. Dimly lit, they make a fantastic impression. Sometimes they are painted blue or purple. Where does this color in a giant mass of colorless salt come from? The color of crystals constructed of colorless ions and atoms appears as a result of violations of the ideal crystal lattice. There are several types of imperfections.

Firstly, because of the improper arrangement of the atoms that make up the crystal lattice (Fig. 3.12). Atoms are absent where they should be - in the nodes of the crystal lattice; unoccupied places becomes available i.e. vacant. Displaced atoms can appear in the intervals between those that maintain their normal position. The crystal imperfections include major disturbances of the order. Most crystalline bodies have a mosaic or block structure. Between such blocks (grains) the correct location in many cases is broken.

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Fig. 3.11. Formation of precipitate of chromate of silver from colorless silver ions and from yellow ions of chromate

In a similar reaction, a colorless Hg2+ mercury ion forms an orange compound called HgCrO4. However, the ion of lead -metal of main group IV, connecting with CgO42- doesn't change the yellow chromate of lead PbCrO4.

Mutual influence of cations and anions allows varying shades of color.

Therefore, the most commonly used compounds of variable composition: yellow crown - a mixture of chromium and lead sulfate PbCrO4 • nPbSO4, emerald-chromium hydroxide of varying composition of Cr2O3 • nH2O (n = 1,5 - 2,5), cobalt light-violet and violet - cobalt phosphate, hydrated with water CO3(-PO4) • 2.8H2O or CoNH4PO4.

Thus, the color of the polar molecule depends on the presence of free electron sublevels in the cation, and to the ability of the cation to polarize the anion and therefore to the ability of this anion to polarization.

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Fig. 3.12. Defects in the crystal structure: the formation of voids and the appearance of an atom between the nodes of the crystal lattice

The sizes of blocks are usually from 1000 to 10 000 atomic diameters (10 000 • 10-8 cm = 10-4 cm), and on their borders an area with the wrong arrangement of atoms is formed. Such imperfections cause the presence of color centers in the crystal (they are of two types: F- and V-centers) due to the fact that in these areas the normal interaction of the electromagnetic field is disturbed, which is created by ions and electrons with the electromagnetic incident flux of quanta. This type of colored compounds is widespread in nature.

Secondly, the color of colorless substances determines the presence of atoms of foreign elements and random impurities. Foreign atoms can be scattered throughout the crystal or grouped together. And in this and in another case they distort the crystal lattice. Indigo or purple color of colorless rock salt occurs due to the release of metallic sodium under the influence of radioactive radiation. Sometimes, along with sodium chloride, it contains particles of other salts, which violate the structure as well as metallic sodium.

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The new Athos cave is striking in its size. In the halls, which reaches the height of 100 m, from the ceiling hangs a huge stalactites. Stalagmites rise from the bottom of the cave in the form of columns to meet them. Sometimes both are joined together, forming a bizarre column. Where does this fantasy of colors come from?

the intensity of color increases. It should be taken into account that there is a closer interaction of the cation C2+ with ammonia-a-complex ion [Cu(NH3)4]2+ is formed. The known reaction to polyatomic alcohols is also based on the enhancement of the color intensity of Si2+. Blue precipitate si(OH)2 becomes deep indigo color while the formation of copper glycerate. The organic molecule is easily deformed by the action of the copper ion. In the case of copper, the deformation apparently affects the stability of one of the d-electrons of copper. It becomes able, absorbing already long-wave quanta, to pass into an excited state. If the easily deformable anion is forced out of the cation environment less polarizable, the color may disappear altogether. For example, PbI2 in solid form is Golden yellow, and in solution it is colorless. Upon dissolution and subsequent dissociation, the ion I-surrounding the lead in the solid compound is replaced by more difficult deformable water molecules. And if there is no deformation, then the color disappears.

Fig. 3.13. Stalactites

After all, the main component of stalactites and stalagmites is calcite, which is one of the two crystalline colorless forms of calcium carbonate CaCO3. The color of the crystals is caused by inclusions of foreign molecules and ions, some of which have their own color. Ions and atoms sodium and potassium give underground decorations blue, indigo or violet connotation; rubidium and cesium - red or orange. Different combinations of these elements form the whole colorful polygamy crystals, forming stalactites, stalagmites.

3.2.5. Color of inorganic substances in solution

In the crystals of a solid substances atom or ion affected by few closest neighbors. The state of ions in the solution is influenced by the external field of solvent molecules.

Cations, anions in solution are surrounded by solvent shell. The layer of such molecules, directly adjacent to the ion, called solvate shell (from the word solver - dissolve). The number of molecules entering here is difficult to determine. However, we are interested in a different effect of solvation.

In solutions, ions can affect not only each other, but also the surrounding solvent molecules, and those in turn affect ions. While dissolved and as a result of solvation previously colorless ion becomes colored. For example, anhydrous CuF2 and CiSO4 are white, and their solutions are dyed in blue color. It is the color of the hydrated copper ion. Its immediate environment includes at least six molecules of water. Four of them are connected with it firmly, and two - loosely. The replacement of the ion environment from poorly deformable ions F - and SO^- by easily polarized water molecules leads to the appearance of color. The removal of water (e.g. by evaporation) leads to the loss of crystallohydrates of the same color. After all, they contain water molecules. SO, in the crystallohydrate of honey sulfate CiSO4• 6H2O four of the five molecules are placed around the copper ion, and the fifth occupies an intermediate position and is associated with both C2+ and SO^- group.

Replacement of water molecules with ammonia (complex compounds, which we have already talked about) deepens the color. Ammonia molecules are deformed more easily and

Fig. 3.14. Scheme of structure of hydrated copper sulfate

The replacement of one solvent with another could have even more influence on color of the compound. The indigo solution of CoCl2 in ethyl alcohol becomes pink when diluted with water. Instead of the usual blue color of hydrated copper ions, green appears if the white powder of the anhydrous CuCi2 salt is dissolved not in water, but in ethyl alcohol.

The reason for the change in color is the different deformability of solvent molecules and cations, which in turn experience a polarizing effect from water or ethanol molecules. Mobile, easily excited the electron becomes capable of absorbing additional quanta of visible color. Cobalt ion in water is less polarized and its "colored" electrons require shorter beams. It transmits or reflects the long wavelength, causing its aqueous solution appears pink. In the alcohol solution of copper in the reflected rays, the proportion of blue rays decreases, and the alcohol solution becomes green.

When replacing the solvent, the coloring may even disappear altogether. The color ion becomes invisible: the Golden-yellow color of Pb2 disappears without a trace in the water. The disappearance is due to the fact that the substance breaks down into separate ions, each of which is colorless, being together in the sediment, they cause color. Exactly the same occurs with dimernye Al2Cl6 molecules that have the blue color in ethanol and in the water becomes colorless because the dissociation of water separates the cations Al3+ and the anion C1-.

Sometimes the disappearance of color occurs without the decay of matter into ions.

The bright red salt of HgI2 mercury iodide becomes completely colorless when dissolved in ether. Special studies have found that the molecules state in solution in an undissociated form. The reason for the disappearance of color is believed to be a decrease in the deformation of ions. Solvate complexes of [HgI2 (ether)J type are formed in the ether. The number of particles on which the Hg2+ cation has its polarizing effect increases: in fact, along with two easily deformable ions I several ether molecules appear. The force field of the cation is split between the particles. Its action is not enough to cause polarization of all particles at once. The deformation of each of them is small, and the anions I - becomes significantly less than in the solid state. The consequence of this change in interaction is the disappearance of color. It is only necessary to reduce by half the effect of the two-charge mercury cation on the iodine anion to reduce their deformation to such an extent that the molecule becomes undyed, even if its size increases. This is the case when excess potassium iodide solution is added to the brightly colored Pb2 or HgI2 precipitate. The resulting [PbI4]2- and [HgI4]2- ions have no color in visible light.

It is known that crystalline iodine is practically insoluble in water. In 100% H2SO4, a pink solution is formed, and in 30% oleum 0.5 M iodine solution has a brown color, the same as in ethyl alcohol. Solvents change the state of molecules and ions. In the medium of concentrated sulfuric acid there are complexes and ions; pink I+, blue I+, brown I+.

3.2. 6. Color of inorganic substances depending on the oxidation state of ions

It is known that the color of most inorganic compounds is determined by the oxidation state of its ions. This is widely used in analytical chemistry. The possibilities of changing the color are due to the different state of the electrons depending on the oxidation state, as well as the change in the polarizing action of these ions (see Fig. 3.9).

Manganese ion MP2+ has no color in aqueous solution. The removal of two electrons from the 4s-orbital does not affect the state of the internal d-electrons, of which manganese has just five and each occupies one of the five possible state. However, a higher degree of oxidation has a strong influence on these electrons.

MnS04 or MnC03 crystals are colorless (sometimes MnC03 is light pink), but MnO oxide is gray-green, MnCl2 and Mn(N03)2 are pink. If the sea water creates an increased concentration of manganese, it affects the formation of corals, pearlescent and pearls. In Japan, there are special underwater plantations, breeding of pearl oysters - bivalves. These organisms on the inner surface of the shells deposited plate layers of aragonite -one of the crystal forms of calcium carbonate (the second - cal-cite - mentioned in connection with stalactites). If these layers get manganese ions, the layers begin to acquire a pink hue and it becomes pink pearl. Adding other ions imparts a yellowish tint, and very rarely the pearls are even black. Since the composition of pearls is calcium carbonate, it can also be formed in underground caves. In caves in New Athos was discovered such pearls in quite significant amounts.

Oxidation level of manganese +3 corresponds to Mn203 brown or in Mn304 black-brown. However, the last compound contains not only Mn3+, but also Mn4+, which deepens the color. Under normal conditions MnO2 are black crystals. Ion MP6+ can be present only in the composition of the anion Mn042- painted in green color. H2Mn04, acid corresponding to this anion is not

isolated in free form, but formed from salts during acidification of solutions of manganates:

K2Mn04 + 2CH3COOH ^ H2Mn04 + 2CH3COOK

spontaneously decomposes into dark brown MnO2 and permanganate KMnO4:

3K2Mn04 + 4CH3COOH ^ ^ MnO2 + KMnO4 + 4CH3COOK + 2H2O.

Anion Mn04-, where the oxidation state of manganese is the highest +7, corresponds to a different-purple-crimson color. Everyone must have seen it - the color of the solution "potassium permanganate". Such a variety of colors of manganese compounds of different oxidation states and their simultaneous combination in the solution allowed K. Scheele (he is known for being the first to discover nitrogen in the air) to call K2Mn04 mineral chameleon. In 1774, this researcher received potassium manganate by fusion:

Mn02 + 2KOH + KN03 ^ K2Mn04 + KN02 + H20.

The reaction product gave a green solution with water, but gradually, when standing in the air (under the action of oxygen), it began to turn first into indigo, then into purple and finally became crimson (the color of Mn04-).

Fig. 3.15. The color of the solution of manganese compounds varies from dark green to red-purple while transformation of the manganate ion into a permanganate

This variety of colors is explained by the change in the nature of manganese ions. The higher the oxidation state, the greater the polarizing effect of manganese. It comes to the fact that Mn6+ and Mn7+ are among the strongest oxidizers. They take the oxygen ion from the water, creating an environment of four O2- ions. The difference in the state of only one electron determines the color - green or purple-crimson, and in addition - oxidizing ability.

Something similar happens with chromium ions. Hydrated chromium ion Cr2+ is blue color. This is one of the strongest reducing agents. It is unstable neither in solution nor in the composition of the solid. One of its relatively stable (in the absence of air) compounds - acetate Cr(CH300)2. The polarizing effect of the Cr2+ ion is such that the acetate acquires a red color. The Cr2+ ion tends to go to Cr3+, which has a different color in the solution - green, and some of its compounds - purple

05.14.01

(for example, CrC13). By oxidation with sodium peroxide, chromium can be converted to its highest oxidation state +6:

2NaCrO2 (greenish) + 3Na202 + 2H20 = = 2Na2Cr04 (yellow) + 4NaOH.

Such ion Cr6+ can enter the structure of the anion of the two acids: chrome - H2Cr04 and bichromate - H2Cr207. Each of them has its own color: the first - yellow, and the second - orange. It may transfer from one form to another by adding acid or alkali:

2Cr042- + 2H+ = H20 + Cr2072-

Cr202- + 20H-

:2Cr04 + H20.

Certainly, chromium, which is highly oxidized, is a strong oxidizer:

K2Cr207 + 3C2H5OH + 4H2S04 = = 3CH3CHO + Cr2(S04)3 + K2S04 + 7H20.

The oxidation state is determined by the valence electrons. Each oxidation state has its own color and character. From the blue unstable ion +2 with reducing properties to Cr6+ -oxidant passes a whole range of colors. Changing the properties of an ion and change of color have the same basis - a certain state of electrons. The transition from one oxidation state to another makes the electronic system sensitive to light quanta of a strictly defined energy corresponding to the difference of energy d-sublevels. The variety of colors of the ionic states of the same element proves that this difference is quite subtle.

Such color schemes have other transitional elements. As an example, let's consider the color change of solid compounds -oxides and solutions of vanadium halides (Table 3.2).

Table 3 .2

The color spectrum of vanadium compounds of various oxidation states

State of oxidation Solid substance powder Liquid solution

Compound formula Color Compound formula Color

V1+ V2O Light grey - -

V2+ VO Grey VCl2 VBr2 VI2. Green Brown Red

V3+ V2°3 Black VCl3 VBr3 VI3. Purple Black Black

V4+ VO2 Dark indigo VCI4 VI4 Red brownish Red

V5+ V2°5 Orange hvo3 Pale yellow

Change of the solutions color in accordance with the degree of oxidation characteristic of the nonmetals. Thus, iodine in the free state has a purple color. In 100% sulfuric acid iodine solution has a pink color, it corresponds to the complex ion I3+. This complex consists of a molecule of iodine and adsorbed on it cation I+. When adding oxidizer:

2I3+ + HIO3 + 8H2S04 = 7I+ + 3H30+ + 8HS04-

the color of the solution, where there is mainly ion I+, becomes indigo dark.

3. 2.7. Optical properties of metals

From the whole set of possible electromagnetic oscillations, waves with short wavelength easily pass through the metal and long-wave waves are reflected. Ultrashort wave and y-rays are able in considerable degree to penetrate through metal, and it is used at flaw detection - with detection of hidden metal cracks and voids. The ability of metals to reflect radio waves is used in radar.

Metals are opaque - this means the waves of vibrations of the visible part of the spectrum, as well as radio waves, are not able to pass through the metal. When light hits the surface of the metal, we see first of all the metallic luster peculiar to all metals. However, through the gloss we can distinguish the color of the metal: pinkish-red copper, yellow gold, bluish lead, etc.

The optical properties of metals are determined by the state of their electrons in the crystal lattice. It is impossible to distinguish and separate individual molecules, although the existence of dimer molecules of sodium, copper, gold and other metals is known in pairs.

In the gaseous state, most metals are monatomic and characterized by low thermal conductivity. Opacity, electrical conductivity, thermal conductivity and a number of other properties of metals are associated with the transition from gaseous to liquid and solid state. While cooling substances there comes a moment when the increased forces of interaction cause the contact of their atoms. There is a liquid phase emerges, and by further cooling solid crystalline phase appears. The convergence of atoms to contact leads to the mutual overlap of their free or partially filled energy sublevels. In this case, the sublevels of the electrons of individual atoms are combined into a common conduction band for the entire crystal. The differences between the neighboring levels in it are about 10-22 eV.

Individual energy states in the conduction band are subject to Pauli's prohibition principle. Therefore, the maximum number of electrons in the sublevels s, p, d and f of the corresponding zones cannot be greater than 2n, 6n, 10n, 14n, where n - is the number of atoms forming the crystal. In general, the sublevels are separated by an energy barrier - areas of energy in which the electrons cannot be. In some cases, the formation of crystals is accompanied by overlapping energy bands of individual atoms, and then formed a common, so-called hybrid, zone, which includes various sub-levels (s and p, p and d, etc.). The merger of the sublevels into a common conduction band and the overlapping of these zones in metals in the condensed state lead to a decrease in the energy required to separate electrons from atoms. Thus, the ionization of the copper atom is 7.70 eV, and to remove the electron from the copper crystal requires only 4.3 eV. The work of the separation of the electron from the crystal of matter is called the work of the electron output.

If the zone is partially filled and a number of energy bands are free, then under the influence of an external field or heat electrons are able to move from one sublevel of the band to another free level. If the energy bands in the crystal are completely occupied by electrons, then the electrons inside them do not move. An atomic crystal of the insulator or semiconductor is formed. The whole piece of metal can be considered one giant molecule. The state of electrons in such a supermolecule is called "electron gas" (electronic medium). Such gas cannot escape outside of the crystal lattice, the field charges of ion-atoms firmly holds it. To get out of this field, you need help from the outside. The effect of knocking electrons out of a piece of metal is used to create solar cells. Thus, electrons, obeying the laws of quantum theory, move in the energy zone formed by overlapping external electron orbitals of atoms.

When electromagnetic waves hit the metal, electrons begin to interact with the incident quantum beam. This absorbs the radiation energy, which makes the metal opaque.

After all, if the radiation passes through a material, it seems to us transparent (for example, water in a glass).

Quanta of light, getting into the electromagnetic field of "electron gas", excite electrons and transfer them to a higher energy state. When returning to the lowest level of energy, electrons emit quanta, which determine the color of the metal. The white or gray color of most metals indicates that the electron medium absorbs more or less equally all the rays of the visible part of the spectrum.

The colors of the metals depend on the length of the waves they reflect. Thus, the white luster of silver is due to the uniform formation of almost the entire set of visible rays. Gold is reddish-yellow because it reflects almost completely the long-wave part of the visible light and absorbs blue, indigo and purple rays. But tantalum and lead better absorb long-wave rays, so they seem bluish. The silvery-white color of bismuth and cobalt is mixed with a pink hue due to the difference in the absorption of short and long rays; as you can see from the picture, the reflection gradually decreases from long to short waves. As already mentioned, convincing examples of the interaction of light with electrons, in which there is a transfer of them to a higher level and even a complete separation, are semiconductors and solar cells. In the first case, the action of the rays can cause the movement of electrons and the appearance of current, and in the second it pulls them out of the metal (Fig. 3.16).

Ray of light

Electrons

O O O nCLO O Q° O

\

Metal plate

À

pH7 (indigo) pH11 (greenich) pH13 (red)

Fig. 3.16. Scheme of the photoelectric effect

The photoelectric effect is a direct proof of the quantum nature of the interaction between the metal and the light beam. It is most easily found in alkali metals. After all, the weaker the connection of an external electron with an atom, the easier it is to tear it off. In the case of rubidium or cesium, the energy of the quantum of the visible part of the spectrum is enough to not only transfer the electron to the excited state, but also to pull it out of the metal. This property has found application in solar cells.

The plate covered with cesium, under the action of light emits electrons (Fig. 3.17). There is a so-called photocurrent; the flow of electrons falls on a metal ring placed in the focus of a curved mirror of metal covered with cesium, and closes the electrical circuit.

3.2.9. Color of semiconductors and dielectrics

Most inorganic pigments and dyes are semiconductors. The color of these compounds is determined by other reasons than the color of metals. In the illumination of metals, reflection prevails, and in the illumination of semiconductors and dielectrics - refraction and selective absorption of light prevails.

b

Fig. 3.17. The effect of temperature (a) (1 - 30, 2 - 90 °C) and acidity of the medium (b) on the state of the dye in the solution

Semiconductors have a gap between the filled zone and the conduction band. For the transfer of an electron from one zone to another it is necessary to spend a relatively large energy. Illumination of the semiconductor surface (or heating) allows electrons to obtain the necessary energy and move to the conduction band. For colored semiconductors, the photon absorption of visible light is sufficient to fill the energy gap between the zones. There is an excitation of the electron and its transition to a higher level in the conduction band. In the future, most often there is a movement of the electron, a change in the distribution of electron density in the molecule and the appearance of color. Semiconductors - simple substances include silicon, germanium, gallium, and indium.

Overlapping of different but filled zones in the crystal can lead to the formation of a filled hybrid zone. In this case there is an atomic crystal insulator or semiconductor appears as well. For example, the formation of diamond occurs due to 2sp3-hy-brid bonds. The zone can contain 2-4n electrons. The diamond area is full. The next energy conduction band belongs to the third level and is separated from the band by the energy barrier - the forbidden zone of 5,3 eV. This value is almost insurmountable, so that's why diamond is an insulator. In other elements of the same subgroup (silicon, germanium, tin), the band gap is reduced to 1.12; 0.78 and 0.1 eV, respectively, which leads to the appearance of their semiconductor qualities.

Complex substances - semiconductors - oxides and sulfides of transition metals have a variety of colors: from yellow to black. And of course, semiconductors include a significant part of organic substances that have color.

In the future, we will have a detailed acquaintance with the fundamentals of the theory of the structure of colored organic substances. Attention will be paid to the connection of the structure of organic molecules with the presence of color. And now we will examine the mechanism of electron behavior in an organic semiconductor. Without going into the details of the structure, we will consider the organic dye as a macromolecule of enormous size with socialized electrons.

a

À

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Semiconductors are divided into two types according to the mechanism of the occurrence of electrical conductivity in them under the action of light or heat. In the first type, which is said to have electronic conductivity, the absorption of photon transfers an electron to a higher empty level in the "zone" of conductivity, being in which the electron moves around the crystal freely, as if the latter was a metal. From this level, the electron can be completely removed from the semiconductor, and moreover more easily than from the initial level. This type of semiconductor corresponds to photo-oxidizing dyes that donate an electron when illuminated with wavelengths within the visible absorption band. By analogy with electron semiconductors, it can be assumed that in the photoactive state the electron is less tightly bound in the molecule and has the ability to move to its "periphery". The remaining electrons of the dye, unaffected by light, remain firmly bound and do not migrate in this sense.

In the second type of semiconductor, photon absorption also transfers the electron to a higher energy level, but the electron in this state continues to be firmly fixed, freeing the vacant position at the initial, lower level. The ability to migrate in a crystal is now possessed by an electron which was not displaced by light, and its empty space or "hole" in the electron sheath, has an excess positive charge. The conduction mechanism in such defective semiconductor is that the vacant orbit is filled with an electron from another nearby lower level (or orbit), and the resulting new empty space is filled in turn from an adjacent level and so on, causing a relatively free movement of an electron defect through a crystal.

This type of semiconductor is undoubtedly similar to photoreduced dyes. It is not the electron that has mobility in them, but the electronic "defect" that reaches the periphery of the electron shell of the dye during the photo-activated state and is neutralized by an outside electron.

In dielectrics, the amount of energy required to transfer an electron to the conduction band is so large that almost no single electron can overcome this barrier. Therefore, most dielectrics do not have or have a color due to the action of the environment (for example, a solution) or ion polarization.

If the crystals of a dielectric substance are formed by atoms of different elements, the zones of the valence electrons of which are energetically different, due to this the electrons from the higher energy zones will transfer to the lower energy zone.

For example, the energy of the 3s-zone of sodium is significantly higher than the 3p-zone of chlorine. Therefore, the formation of NaCl crystal as the Na and C1 atoms approach each other is accompanied by the transition of electrons from the 3s-zone of sodium to the 3p-zone of chlorine. An NaCI ionic crystal is formed with 2p- and 3s-bands filled respectively. Such crystal cannot have electronic conductivity (the passage of current through the NaCI melt is due to its transfer by ions of Na+ and C1-) and does not have color.

3.3. ??????????????

3.3.1. Basic theory of the color of organic molecules

Attempts to link the color of organic matter with its structure have been made exclusively for a long time. About a hundred years ago, the first theory was put forward, combining color with the presence of certain groups of atoms in a molecule.

Particular importance for the structure of a colored compound has a chain of C atoms linked to each other by alternating double and single bonds:

—CH = CH—CH = CH—, etc.

In such chains, the conjugation effect is manifested. There is a sort of alignment of double and single bonds:

The overlapping of the orbitals on which the-electrons are located is such way that it becomes possible to form an additional bond, as it were, between those carbon atoms that are connected by a single bond; all atoms are encompassed by single molecular orbitals. Electron gets the opportunity to move around the molecule as a whole.

We encounter a similar conjugation effect when studying the properties of benzene, in which it is impossible to distinguish between individual double and single bonds; yes, they are not in the C6H6 molecule and all connections are equal.

However, the formation of such delocalized n-bonds imposes a restriction on the structure of the molecule: in order for the electron orbitals to overlap, the atoms in the molecule must lie at least approximately in one plane.

Empirically, even before the laws of the electronic structure and its change during the interaction of a substance molecule with a beam of light were discovered, it was possible to notice the most important by the influence of structural fragments of molecules on the color of the compounds. So it turned out that the elongation of the chain of conjugated double bonds leads to a transition from colorless or slightly colored to dark colors:

C6H5—(CH=CH)—C6H5 (stilben) - transparent

C6H5—(CH=CH)3—C6H5

(diphenylhexatriene) - yellow

C6H5—(CH=CH)6—C6H5

(diphenyldodeichexaene) - brownish orange

If instead of simple aromatic nuclei (such as benzene) appears condensed (like naphthalene), then this causes a deepening of the color.

Red

C=O groups linked to each other and cause a deeper connection color:

CO

NH

CO Colorless

C=O CH

NH

Orange

CH2=CH-CH=CH2

O

C

More stronger and closer bond between carbon-atoms, belonging to separate parts of a molecule, leads to more intense and deeper coloring:

O

Colorless

O

Orange

Besides, chains of conjugation, other groups of atoms are responsible for color, among which there are also unsaturated bonds. Such groups, due to which the possibility of occurrence of color in the substance arises are called chromophores from the Greek words ""chrome" - color and "foreo" - carry, in other words - "non-inherent color". Here are few examples of such groups:

—N=N— Azo group >C=N— Azomethine —Nx Nitro group >C=NH Carbimine -N=O Nitrosogroup

Substances containing chromophores are called chromo -genes. By themselves, these substances do not considered as dyes, because they are not bright or pure color. This is explained by the fact that, even though in such molecules a redistribution of electrons and their energy occurs, but it is not enough to absorb quanta of light of only one specific wavelength selectively and in large quantities. Such a possibility appears only after groups are introduced into the compound molecule, which differ either by a pronounced affinity for the electron, or by giving their electrons to a considerable degree to common use. In a word, such groups, which dramatically change the state of electrons in chromophore groups.

The groups that enhance the coloration of substances are called auxochromes (from Greek, the word "auxo" -increase). There are two types of such groups: • Electron donor

—OH, —NH2, —SH, —OCH3, —NHCH3, —N(CHJ,

Electronophilic

-NO,, —NO,

-COCH,

Only after the introduction of auxochromes, the color of the compound becomes clear (the selective absorption of rays of a certain wavelength begins) and sufficiently intense (the incident light easily shifts electrons in the molecule). The greatest effect is achieved when both electron-donating and electron-ophilic groups of atoms are simultaneously present in the compound molecule. Some of them give, while others respectively attract electrons of the overall electron system of the molecule.

So, from the structural features of organic molecules for the appearance of color in a substance matter the following:

• a chain of alternating single and double bonds (in this case, double bonds can participate in such a chain not only between carbon atoms);

• the presence of groups or atoms strongly attracting or, contrariwise, easily donating their electrons to the total electron system of molecules;

• the atoms in the molecule must lie in the same plane (or very close to this state).

All this is subject to a common target - the ease of exposure of quanta of visible light to the electron system of molecules and its translation into an excited state.

Now we have to take the next step - to associate the above features of the structure of organic color substances with the presence of their color.

Let's consider the mechanism of color in organic substances. Eventually, it was only after the discovery of electrons and the appearance of the quantum mechanical theory of chemical bonds that it became possible to explain the structural features that determine the color of a substance. The absorption of a portion of the light energy by a molecule from a stream of light falling on it causes its transition to an excited state. The energy of the incident beam is converted into electron energy, or, more simply, electrons are stored with energy.

3.3.2. Color of organic matter in solution

Let us see the color of the organic matter in solution on the example of organic dyes. All substances related to organic dyes have complex structures.

Thiazine dyes, to which methyl blue is referred, it represents salts in which the dye itself is in a cationic form, and chlorine is the anion. The dye can exist in the form of a one-dimensional (Mr+) or in the form of a dimeric (Mr2+) cation with the following maxima in the absorption spectra:

• Total spectrum 620 and 667 nm

• Monomer 665 nm

• Dimer 605 and 680 nm

The binding energy of the monomer cations in the methylene blue dimer molecule is 2.5 kJ/mol.

The ratio of the monomeric and dimeric forms of the dye depends on a number of factors: temperature, concentration of the dye, dispersing ability of the solvent, composition of the ionic medium. The dependence of the relative height of the monomeric and dimeric maxima on temperature (see Fig. 3.17) indicates a reversible thermal dissociation of the dimeric ion. The intermolecular bond between them is carried out by dispersive forces, which for polyatomic dye molecules can reach a large size. There are good reasons to believe that hydrogen bonds are created between molecular ions in a dimer.

Dimerization begins with a dye concentration of 10-4 mol/l. The maximum of the dimer ion is shifted by 50 nm in the direction of short waves from the maximum of the monomer. For the thiazine colors of the dimeric ion, two maxima are observed, located on both sides of the monomer maximum. In the general spectrum (see Fig. 3.17), all three maxima (one monomeric and two dimeric) are superimposed in such a way that either new maxima or inflection points are obtained. The equilibrium shift 2Mr+ O (Mr2)2+ is influenced by the concentration of the dye (in accordance with the principle of Le Chatelier) and the ionic composition of the medium. With increasing concentration of the dye in the aqueous solution, further aggregation of the dye into trimers and larger aggregates occurs. In the case of alkali additions, the intensity of the maxima in the spectrum of the dye decreases (see Fig. 3.17). At the same time, both the ratio of aggregate ensembles changes as well as its color. Moreover, the color changes in the direction of absorption of long waves. The ratio of the dye depends not only on the ionic medium (i.e. the content of electrolytes in the solution), but also on the properties of the solvent itself.

05.14.01

To determine the state of certain dyes, it is important to know not only their electronic configuration (in other words, the distribution of the electron density in their structure) and the influence of solvents on the distribution of the electron charge in the dye molecule. Here the most important property of the solvent is the dispersing ability of the solvent. Molecular bonding is carried out by dispersion forces, which for dyes such as methylene blue can reach a fairly large value due to the large size of the dye cation. If the separating dispersing ability of the solvent is high, it reduces the adhesion of dye ions and, consequently, affects the equilibrium shift, for example, the ratio of monomers and dimers in dye solutions (Fig. 3.18).

Blue form

400

480

560

640

720 nm

of its color to the color of a fuchsia flower. For 140 years, thanks to the rapid development of chemical science and industry, researchers have learned to purposefully carry out the synthesis of dyes and create dyes with previously known qualities.

The color of an organic compound is due to the absorption of visible light and depends on the ease of transition to the excited state of the n-electrons of the system of conjugate bonds. Consequently, any change in the molecule that impedes the state of such electrons affects the color. Here is a guide to how create a substance with color.

A colorless molecule containing a conjugation chain can be transformed into a color one if it is ionized. So by adding a base to a colorless para - nitro phenol, you can get a compound colored yellow.

O,N-

✓ A

-OH-

O,N-

✓ A

-O- + H2O

The high-quality reaction to the aluminum cation is well known - obtaining a compound of red color with alizarin. This method of determining the A13+ + cation in solution is based on the ionization of alizarin:

O

OH

+ 2Al(OH)3 ^ Al2

O Yellow

O

O

Red

Fig. 3.18. Spectra of blue and red forms of methylene blue in solvents:

1 - water; 2 - acetone; 3 - ethanol; 4 - dioxane;

5 - pipiridine; 7 - CCl4 methanol; 8 - butanol

The conducted researches allowed to establish quantitatively the influence of such solvents as pyridine and ethanol on the state of the dye. At room temperature in ethyl alcohol there is no dimerization of methylene blue dye. The maximum corresponding to the dimer practically disappears: it has turned into an inflection point. The color of dye changes also.

Consequently, same factors have effect on the state of the dye in the solution, as in the case of inorganic molecules with color. The temperature determines the crystalline state and aggregation of molecules (see Fig. 3.18).

The acidity of the medium, i.e. the content of hydrogen cations, affects the state of the molecules in the solution, since the distribution of the electron density in the molecule changes. Depending on the polarity of the solvent, the interaction between it and the dye changes, which affects the color of the solution (see Fig. 3.18).

3. 3. 3. How to control the color of organic matter

There are few dozen of natural organic colors are known. The rest are artificially obtained, and currently there are a few thousand of them. The first of them were obtained in 1856 by the English chemist U. Perkin by oxidizing aniline mixture and almost at the same time with him in Warsaw, Professor J. Naban-sons chemically isolated a red dye called fuchsin for the similarity

Most organic dye molecules have a structure that allows us to call them internal salts. This ability is inherent in the structure and amino acids.

Remember that they have two groups with opposite qualities: the amino group NH2 - with the properties of the base and the ability to attach to itself the cation H+ (as do the bases) and the acid carboxyl group - soon, cleaving H+. In the case if the amino acid molecule is ionized

H2N—CH2—C^ ^ H2N—CH2—C^ + H+ ^OH ^O-

the separated hydrogen cation can join the amino group. It turns out to be "internal salt"

+

NH3 —CH2—COO-

in which there are opposite electric charges that are in different parts of the same molecule. This is the same feature of the structure for the most dyes:

N=N-

-NH3+

Thus, the ionization of a molecule, carried out in any way, leads to an increase of color, and if as a result of ionization, the displacement of electrons in the system of conjugated bonds is enhanced.

Methods of ionization of molecules can be very diverse. The most common seems to be a change in the acidity of the medium. This method is used for dyeing fabrics by changing the medium, transferring the dye from one form to another. In chemical practice, the use of indicators is widespread,

+OH

O

3

+ 6H2O

C

C

3

the color of which varies depending on the environment. Fig. 3.19 shows the color change of the indicator depending on the pH of the solution.

pH

Fig. 3.19. Change of the color of the indicator paper, depending on the acidity of the medium

Another effective way to change the color of the organic dye is salt formation. If a metal cation replaces hydrogen in the group —OH, then the color deepens, for example, in quinazarine from red to violet

O

II OH

C

O

II ONa

C

C 1 C <

C oh

O O

Red Purple

If salt formation occurs due to the addition to the amino group and its conversion to NH3, the color rises and may even disappear completely.

Interaction with metal ions can lead to the formation of a stable complex compound. Most often such ions are ions of transition elements. The metal enters the molecule so that with one of its atoms it forms a normal valence bond, and with the other - coordination. This type of bond is that the metal ion provides an empty d-opbital for the bond, and an atom (usually N or O) provides its lonely pair of electrons.

Such type of coordination arises, in which the lone pair of electrons from the system of the molecule is "drawn" into the electron shell of the atom. If this significantly changes the state of the electrons of the molecule, then the color changes accordingly. Yellow colored Alizaryn gives different color complexes:

Ion Color

Al3+ Red

Cr Brown

Fe Purple

If the formation of a complex compound does not substantially change the electron shell of atoms belonging to the system of conjugated bonds, then this does not change the color.

So, azo dye yellow:

OH

NO

HO-C=O'

^Me

after exposure to metal salts, remains yellow. The lone electrons of the oxygen atom of the carboxyl group, due to which the coordination bond of the molecule with the metal ion originated, do not participate in the displacement of the n-electrons in the conjugated system.

3.3.4. Temperature-Sensitive Compounds

Temperature-sensitive compounds have the ability to change their color depending on temperature. Both inorganic and organic chemical compounds can form their basis.

The simplest reversible heat-sensitive pigments are crystalline hydrates. Remember the copper sulfate. Anhydrous CuSO4 is white, and as it absorbs water, its color becomes blue, for example, when CuSO4• 5H2O is formed. To get rid of this compound from water, it must be heated to about 250°C. Consequently, when heated, the blue vitriol changed its color from blue to white; it means that the temperature reached 250 °C.

The simplest of the well-known thermo-reversible pigments are the double salts of hydriodic acid such as HgI2• 2AgI or HgI2 • 2CuI. In the previous sections, it has already been said that the color of the crystalline compound depends on the structure. This is exactly what is happening in this case. When heated, the salt structure changes: it goes from one crystalline form to another. When cooled, the color quickly becomes the same, since at a certain temperature point the double salt acquires a structure that is stable at lower temperatures.

For the compounds which could reversibly change their color, the original color is restored more slowly. The cobalt and nickel halides forming compounds with the hexamethylene tetramine NiBr2• 2C6H12N4• 10H2O and CoI2• 2C6H12N4• 10H2O restore the same color in 2-4 hours. They lose ten times the water (per molecule of salt) when heated to certain temperature. When cooled, they again absorb moisture from the air, and it takes much longer to change than to change the structure. Due to the fact that the color change of crystalline hydrates with evaporation and absorption of water, they serve as color indicators at a temperature of about 100 °C. For higher temperature ranges, irreversible heat-sensitive pigments are used.

Fundamentally, their action is based on the formation, under the influence of the temperature of a new compound, which is different in color from the original. For example, it is known that when heated, hydroxides, carbonates, basic carbonates of a number of metals turn into oxides having a different color.

A new compound with a color different from the original can be obtained as a result of chemical interactions of not one, but several substances. In this case, a mixture of compounds is used, which react with each other when heated. Thus, red lead with thiourea gives black lead sulfide, and the mixture of lead sulfide with barium peroxide turns it into white color under the temperature action:

PbS +4Ba02 = PbS04 + 4BaO.

Thus, the mixture of three substances of red lead, thiourea and barium peroxide when heated changes color twice: orange-black-white.

Aluminum powder treated sequentially with tannin, oxalic acid and some basic dye also undergoes a multi-stage temperature change in color. But in this case, the resulting pigment will be reversibly temperature-sensitive.

Nowadays, pigments that can change their color several times when heated as a result of chemical reactions at different temperatures are being used widely. Such compositions are used in signaling devices, applied to the surfaces of machines and friction parts of machines, if the temperature should not exceed a certain limit. Unusual way to use such multi-colored compositions proposed one of the American companies. They patented a disposable medical thermometer. It is a thin transparent plate with microcapsules embedded in it, filled

NO2 HO-C=O

with heat-sensitive substances. When it comes into contact with a heat source, for example, with a human body, it quickly reacts to small temperature fluctuations. The color changes dramatically and irreversibly. It takes 15 seconds to measure the temperature. The color remains unchanged, and such thermometer, if necessary, can be attached to the medical history of the patient.

Table 3 . 3

Temperature Sensitive Pigments

Compounds The color change temperature Original color Color after exposure to temperature

Reversible

CoCl2-2C6Hi2N4- 10H2O 35 Pink Blue

CoBry 2C6H12N4 • IOH2O 40 Pink Blue

MgI2-2AgI 45 Dark yellow Dark brown

Co^ 2C6H^ IOH2O 50 Pink Green

CoSO4-2C6H^ 9H2O 60 Pink Purple

NiC^ 2C6H^ IOH2O 60 Light green Yellow

NiBry 2C6H12N4- 10H2O 60 Light green Blue

HgI2-2CuI 65 Carmine-red Chocolate

Co(NO3)2-2C6H12N4 • 10H2O 75 Pink Magenta

Irreversible

NiNH4PO4-6H2O 120 Light green Greyish green

Co3(PO4)2-8H2O 140 Pink Blue

CoNH4PO4-H2O 140 Magenta-red Dark blue

Pb(OH)2 c 4.5% H2O 145 White Yellow

NH4VO3 150 White Brown

(NH4)3PO4- 12MoO3 160 Yellow Black

(NH4)2V2O7 200 Yellow Grey

Cd(OH)2 200 White Yellow

7CuO • 2SO3-6H20 220 Blue Brown

CoC03-nCo(OH)2 250 Pink Black

FeO-OH 280 Yellow Red brown

2PbOT3-Pb(OH)2 285 White Yellow

PbC03 290 White Yellow

CdCO3 310 White Brown

CoC13 400 Pink Dark brown

CuC03 400 Light green Dark brown

NH4MnP2O7 400 Purple White

Cu(OH)2 • ^04)2 650 Grey Green

05.14.01

Usually, with the help of temperature-sensitive compositions, the temperature in refrigerators, boilers, engines, internal combustion engines are controlled. Such compositions are also used in those places where direct control of the equipment is not available. Some brands of heat-sensitive irreversible pigments are suitable for measurements up to 1000 °C. Compositions are available either in the form of a set of pencils for a specific set of temperatures, or in the form of a powder containing a resin or a polymer that is soluble in alcohol or acetone. The powder is stirred before use in a solvent (ethanol or acetone), the resin and pigment are transferred into the solution. The mixture is applied on the surface. However, such compositions have two significant drawbacks. They do not have high durability at high temperatures. The resin burns out and the pigment begins to crumble. In addition, they are gradually polluted and the change of color becomes poorly noticeable.

3.3.5. Dyes in the medicine field

A number of dyes have healing properties. In the first-aid kit of each house there is a "Zelenka" - an alcohol solution of brilliant green dye (green antiseptic). Methylene blue helps from carbon monoxide poisoning, malaria, and stomatitis, and for this purpose it is specially produced in the form of finished products. Lapis lazuli was used as the remedy for wart in ancient times. Yellow sandalwood back then recommended for heartache, and verdigris was advised to apply to a wound for quick healing. In the latter case, the recipe is questionable or requires clarified recommendations, since the verdigris itself is a poison.

A number of dyes are used as antidotes to prevent the consequences of severe poisoning. Among them there are dyes: rivanol, trypaflavin and the already mentioned methylene blue. The same dye helps in cases of poisoning with such substances as potassium cyanide, hydrocyanic acid, carbon dioxide (II), carbon monoxide and hydrogen sulfide. In the form of a weak alcoholic solution, it is injected into a vein of a person affected by one of the poisons listed.

Methylene blue is similar to one of the enzymes -the biological catalyst involved in the respiratory system of the body. When the chain that supplies oxygen to the cells of the body is blocked by the action of poisons such as carbon monoxide or hydrogen sulfide, methylene blue can help. It has the ability to reversibly reduce and oxidize, taking and giving up two hydrogen atoms. Therefore, it can serve as a carrier of hydrogen. Taking hydrogen from the internal biochemical systems of the body, it transfers oxygen to it, and ultimately it turns into water.

Methylene blue dye can be considered as a model of one of these biocatalysts - dehydrase. In its structure and in the energies of the electron orbitals, it is close to some of the structures (riboflavins) that make up the designs of "biochemical machines". In addition, this dye itself can directly be included in the work of the respiratory chain, acting as a savior - an antidote. Of course, it is important to deeply understand the mechanism of the dye-drug. In connection with our topic, this process is interesting because the dye in a sequential chain of chemical reactions changes its color several times from the original blue to the reduced colorless form. The color changes in the following order:

blue-green-yellow-colorless

It was possible to study the structure of each of the multicolored forms. They can also be distinguished by the absorption spectra, since the positions of the maxima in the spectra are different.

Methylene blue refers to thiazine dyes. Its usual blue form is a positively charged ion, and anion is chlorine:

(HaC)2N

N(H3C)2

This form is stable in acidic and weakly acidic medium, and blood is just such a medium. Taking the hydrogen from the reducing agent, the dye in several stages turns into its colorless leuco form. Then, under the action of oxygen, which oxidizes the leuco form, removing hydrogen from it, the methylene blue returns to its normal state and becomes blue again.

The difference in the chemical composition of the blue and colorless forms is two hydrogen atoms. How do they move from donor-matter to dye? In the form of hydrogen atoms, hydrogen molecules or in parts: 2 electrons + 2 protons? The course of the reaction was studied by electron paramagnetic resonance (EPR). The essence of the method is that it allows you to capture the presence of an unpaired electron in a molecule. The obtained experimental data on the shape of the EPR signals and the nature of their changes are consistent with the following scheme of the mechanism of the process and color change.

For dyes that have conjugated bonds in a molecule, the addition of hydrogen atoms leads to the disappearance of the color. The colorless substance of reaction of the reduction of the dye for this reason is called leukoform, i.e. colorless form. For dyes of the type of thiazine, the hydrogen atoms in the leukoform are not firmly bound and can be split off by reaction with air oxygen, accompanied by reversible dye regeneration.

One of the common views on the process of hydrogen transfer in body systems is as follows. It is believed that the biochemical chain from the level of one molecule to the unfilled level of another passes an electron, gradually wasting its supply of energy. At the last stage, it combines with a proton from the medium. Thus, a whole hydrogen atom reacts with oxygen.

The dye molecule, acting as an antidote, is a "lifeboat" where an electron and a proton can be loaded, and this rescuer has a double capacity, since it can receive two electrons and, accordingly, two protons. Thus, the dye transfers oxygen to a single hydrogen molecule.

The known healing properties of the dyestuffs led to the idea, whilst dyeing the fabrics, to give them healing properties. The first step in this direction was the creation of antiseptic bandages dyed with copper salts. Such bandages stop blood more quickly, since copper salts in small quantities contribute to blood clotting. Hemostatic, i.e. blood stopping gauze faithfully serves surgeons in operations. Scientists went further and decided to combine two stages in one process: dyeing and antimicrobial treatment.

Usually, dyes applied to fabric are retained on it due to the special functional groups present in their molecules. So, dyes for wool have sulfo group for these purposes. Methylene blue uses its amino group, etc. Consequently, the task was to impart biocidal properties to dyes and to retain their ability to cling to fabric. The fungicide yellow is used as the basis of biocidal dyes for wool.

The protection of cotton fabrics is more difficult, since it is more difficult to introduce dyes that frighten bacteria off and make the fabric inedible for them into more complex dyes for such fabrics. In this case, the researchers went a different way: they began to modernize the fiber itself. The basis of such

tissues is still cellulose, but chemically modified. The essence of this fiber change is as follows. It can be flax or cotton to natural cellulose - or even various groups are chemically attached to an artificial fiber, due to which material acquires the necessary properties.

If the fabric is impregnated with special disinfecting compounds, then such fabric is suitable only for single use (say, for dressings on wounds). After washing, it may lose its bactericidal properties. The bactericidal groups included in the cellulose molecules will retain these qualities forever. Externally, this fabric looks like a normal chintz of slightly creamy color. All properties of cotton fabric are preserved: softness, hygroscopicity, ability to withstand washing many times. The fabric has proven itself as therapeutic underwear for patients with skin diseases.

It is irreplaceable for workers of some professions, for example steelworkers. As you know, high temperature and high humidity contribute to skin irritation. This is where the protective properties of the new fabric are used. Miners, tunnellers of underground tunnels, divers, and there are many more professions where underwear made of such fabrics will keep a person healthy and provide normal working conditions.

3.3.6. Glowing paint

In the shot down fascist airplanes in Great World War II, instruments and maps of the area, dyed with colors capable of glowing in the dark were found. At night, in the darkness of the cabin, it was enough to turn on the source of invisible ultraviolet rays, as the instrument pointers immediately began to glow. Now such bright colors are not unusual thing. Nowadays they are widely used to create bright advertisements, determine surface defects, color fabrics and also added to cosmetics and drinks. The basis of such dyes is the phenomenon of fluorescence.

Molecules of a substance, absorbing quanta of one energy, "flash" rays with a different wavelength. The surface covered with a fluorescent substance, under the action of ultraviolet rays, begins to glow, as the molecules emit quanta of the visible part of the spectrum.

On the basis of fluorescent dyes prepare fluorescent paints. A dye or mixture of dyes is incorporated into the coloring composition. Together with synthetic resins they give enamel paints. Apply them to road signs, caution signs, day and evening black advertising. Typically, these dyes absorb short waves and are yellow. Selection of several fluorescent dyes can produce any color of radiation. As a result of successive transformations with two or three phosphors, it is possible to obtain bright colors in the longest-wave part of the spectrum - in the region of orange and red rays. These colors are used for coloring objects to facilitate their identification (rescue inflatable boats, cosmonauts' suits, polar aviation aircraft). Airplanes marked with fluorescent colors, can be recognizable up to 20 km.

The use of phosphor paints along with the usual allows you to create two paintings on the same canvas. One of them is visible under normal lighting, and the other under ultraviolet. Basically, such method allows for example, in the theater to go without changing the decorations, limited only to the change in lighting.

In order to bleach, colorless phosphors are also added to laundry detergent. They absorb the ultraviolet rays of the solar spectrum and give the fabric a bluish fluorescence. The blue color is complementary to yellow, mixed with it and gives a pure white color. It is no longer necessary to blue fabrics washed with optical bleach.

Cl

3.3.7. Mysteries of scarlet polymers

Imagine that in your hands elegant scarlet and cool bearings, bushings and gears. Made of polyamides (one of the most well-known representatives of polyamides are capron and polyester), they are light and durable. It is possible that there is only bewilderment: why should we paint in such bright and elegant colors the parts working in the depths of the machine and constantly in contact with the lubricant? Probably, you will be surprised a lot when you find out that the "non-colored" bearings, bushings and other parts from the same polymer have a much shorter life.

The dyes contained in the polymer are not for decorative purposes, they help the polymer to more successfully cope with the load. The essence of such cooperation is that the dye makes the structure of high-molecular compounds more regular and correct. After all, during polymerization and subsequent crystallization, the long chains of polymer molecules are stacked in bundles.

Connections arise between the molecules. The more orderly and better oriented macromolecules in the "pack", the stronger they are connected with each other. There is a continuous transition from individual contacts of molecules to large regions with an ordered crystalline structure. Then such packs are rolled up, forming a kind of a long thread. The shape of the coil is close to the shape of an ellipsoid and is called "spherulite". The size of the molecules exceeds their transverse size by hundreds and thousands of times, and in the collapsed state, the length exceeds the width of the spherulite by no more than 10 times, i.e. the molecule is folded a thousand times.

Dyes behave quite differently in polymer. They have a number of advantages over inert additives. First, they ideally dissolve in the monomer.

During the formation of packs, they do not create an uneven structure and do not interfere with the macromolecules "stitching" into packs. Secondly, when the formation of spherulite tangles begins, the dyes, being centers of crystallization, create ellipses that are almost ideally the same. The difference in size is only three microns (from 12 to 15). Such promotion of dyes favorably affects the properties of the product from colored polymer. Its density is uniform throughout the product. Since there are fewer internal cavities (remember also the dependence of strength on the size of cracks), shrinkage is sharply reduced. Consequently, it is possible to make castings with an accuracy of much more than the former. If shrinkage has decreased, then internal stresses, which are the main cause of premature "breakdowns", have also become less. The service life of parts from such models has increased.

Research and technological tests have shown that the most suitable for adding to polymers is a scarlet dye, which is called "caprosol scarlet C" and blue - "phthalocyanine blue". These dyes are one of the most widely used ones.

3.3.8. Spectral analysis

When smelting steel, obtaining magnesium, copper, aluminum and other alloys, sorting and controlling various materials, one of the exact and relatively fastest methods of analysis is spectral. There are two types of spectra: absorption and emission (of course, also reflection). The first was the subject of a rather detailed discussion in previous chapters. Recall that its essence is that the quanta of the absorbed light transfer the electrons of the molecule from the main energy level to another - excited, i.e. the energy of the incident radiation is converted into particle energy.

05.14.01

Separate lines in the spectra of various elements may coincide with each other, but at the same time other lines of the spectra of these elements are different. The quantitative spectral analysis is based on the ratio between the intensity of the spectral lines and the concentration: with an increase in the concentration of the determined element in the sample, the intensity of its spectral lines increases. However, the intensity of the lines also depends on the flame temperature, excitation conditions, etc. In order to eliminate as far as possible or minimize the influence of these factors, the concentration of the element being analyzed with all methods of analysis is determined not by the absolute intensity of its lines, but by relative intensity relative to one of the lines of the so-called internal standard. Each pair of lines, i.e., the line of the analyzed element and the line of comparison, is chosen so that the relative intensity of both lines is as independent as possible from the measurement conditions.

In most cases, the element of comparison is the main elements of the sample, for example, iron in the case of analysis of steels, magnesium in the analysis of magnesium alloys, etc. Due to the poverty of the spectra, the lines for some elements (for example, magnesium, aluminum) are sometimes the internal standard is copper, iron, or other elements used as permanent electrodes when creating a discharge.

The emission spectrum contains information not only about the state of the electrons, but also about the vibrations of atoms and molecules. For example, the flame color of calcium (brick-red) and strontium (carmine-red) compounds is due to CaO and SrO molecules, which withstand high temperatures. Along with electronic transitions, vibrational and rotational transitions contribute to it. Therefore, it is not the lines that are obtained in the spectrum, it is the bands, and the spectrum itself acquires a complex structure.

Despite the complexity of the emission spectra and the difficulty of deciphering them, spectral analysis has great merit to chemistry and science in general. Through this analysis, we know the approximate composition of the sun, planets and stars. In 1868, lines were found in the spectrum of the Sun that does not correspond to any of the known substances on Earth. These lines attributed a new element - helium. Only 27 years later, helium was found in the gases evolved when the mineral clevent was heated. Due to spectral analysis, such elements were discovered that were found as small impurities in the minerals of other elements. So, one of the creators of the spectral analysis R. Bunsen (the other was K. Kirchhoff) discovered cesium (1860) and rubidium (1861) in the waters of salt sources.

The ions of each of the elements of the periodic system have their own characteristic emission spectrum. Therefore, the presence of one or another element can be identified in the flame, for example, open-hearth furnaces. Quantitative rapid analysis is carried out at metallurgical plants in a few minutes. This makes it possible to make adjustments to the chemical composition of the alloy during the smelting process. Experienced steelmakers are able to judge the proximity of the steel-making process to completion by the color of the flame in the furnace. After all, the elements-impurities, congruently, give the color of the flame a peculiar shade. As their burnout flames seem to be cleared. A steelmaker thus performs a spectral analysis. Such a visual analysis is suitable for determining the presence of elements giving intense lines in the visible part of the spectrum.

A more accurate result (with a sensitivity of 10-3 - 10-5%) gives a definition with the help of instruments that can

capture both the visible and infrared and ultraviolet parts of the spectrum. Using instruments that have the ability to distinguish the fine structure of spectra, one can follow either the changes in the entire spectrum or a particular line or band. These devices - spectroscopes or spectrophotometers - allow

Reference list

1. Patent RF №2053029, MPK V 06 V 1/20, opubl. 27.01.96 g.

2. Rakhimov R.H., Ermakov V.P., Rakhimov M.R. application of functional ceramics in complex formation // Chemical technology. 2015. № 1.

3. Rakhimov R.Kh., Yermakov V.P., Rakhimov M.R., Yuldashev N.H. et al. Features of synthesis of functional ceramics with a complex of the set properties by a radiation method. Part 3 // Comp. nano-technol. 2018. № 2. Рр. 76-82.

4. Rakhimov R.Kh. Development of highly efficient equipment based on functional ceramics synthesized in a solar furnace with a capacity of 1 mw // Computational nanotechnology. 2018. № 3. Рр. 91-100.

us to solve two problems. Spectroscopes help to determine how much and what specific substances will be formed in the course of reactions. These devices provide invaluable assistance to the researches in determining the presence of a particular element in the mixture.

5. Mamatkosimov М.А. et al. Applications of big solar furnace // XIV International scientific-practical conference «The Strategies of Modern Science Development». USA. 2018. 8-9 February. Pp. 39-43.

6. Rakhimov R.Kh. et al. Nanomaterials synthesized in the BSP in the production of functional products Conference materials "The use of renewable energy sources: new research, technologies and innovative approaches." Tashkent, September 25-26, 2018. P. 61-64.

7. Rakhimov R.Kh. Development of ceramic coatings and application of their infrared radiation // Материалы конференции: «Использование возобновляемых источников энергии: новые исследования, технологии и инновационные подходы». Ташкент, 25-26 сентября, 2018. С. 234-240.

DOI: 10.33693/2313-223X-2019-6-2-101-137

ГЕНЕРАЦИЯ И СВОЙСТВА ИНФРАКРАСНОГО ИЗЛУЧЕНИЯ

Рахимов Рустам Хакимович, доктор технических наук, зав. лабораторией №1. Институт Материаловедения Научно-производственное объединение «Физика-Солнце» Академии наук Республики Узбекистан. Ташкент, Узбекистан. E-mail: rustam-shsul@yandex.com

Аннотация. В статье приводятся основные базовые законы природы и современные теории природы электромагнитного излучения, его генерации, характеристики, законы отражения, поглощения и рассеивания света. Показан принцип преобразования спектра излучения первичного источника с помощью разработанных керамических материалов, экспериментальные результаты по изучению взаимодействия ИК-излучения с веществом и различные механизмы воздействия на различные объекты и процессы.

Ключевые слова: Электромагнитное излучение, квантовая теория, электронные переходы, излучатели, преобразователи спектра, керамические материалы, лазеры, длина волны, температура, частота, фронт нарастания импульса, поток излучения.

Литература

1. Патент РФ №2053029, МПК В 06 В 1/20, опубл. 27.01.96 г.

2. Rakhimov R.H., Ermakov V.P., Rakhimov M.R. Application of functional ceramics in complex formation // Chemical technology. 2015. № 1.

3. Rakhimov R.Kh., Yermakov V.P., Rakhimov M.R., Yuldashev N.H. et al. Features of synthesis of functional ceramics with a complex of the set properties by a radiation method. Part 3 // Comp. nano-technol. 2018. № 2. Рр. 76-82.

4. Rakhimov R.Kh. Development of highly efficient equipment based on functional ceramics synthesized in a solar furnace with a capacity of 1 mw // Computational nanotechnology. 2018. № 3. Рр. 91-100.

5. Mamatkosimov М.А. et al. Applications of big solar furnace // XIV International scientific-practical conference "The Strategies of Modern Science Development". USA. 2018. 8-9 February. Pp. 39-43.

6. Рахимов Р.Х. и др. Наноматериалы, синтезированные на БСП в производстве функциональных продуктов Материалы конференции: «Использование возобновляемых источников энергии: новые исследования, технологии и инновационные подходы». Ташкент, 25-26 сентября, 2018. С. 61-64.

7. Rakhimov R.Kh. Development of ceramic coatings and application of theirinfrared radiation // Материалы конференции: «Использование возобновляемых источников энергии: новые исследования, технологии и инновационные подходы». Ташкент, 25-26 сентября, 2018. С. 234-240.