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QUANTUM-CHEMICAL INVESTIGATION OF METAL-ORGANIC COMPLEXES PARTICIPATING IN DEHYDROPOLYCONDENSATION REACTION
N.V. KHOKHRIAKOV, V.I. KODOLOV, O.YU. BOLDENKOV
Basic Research-High Educational Center of Chemical Physics and Mesoscopy, Udmurt Scientific Center, Ural Division, Russian Academy of Sciences 222, Gorky St., 426000, Russia; e-mail: kodol@istu.udm.ru
ABSTRACT. Quantum-chemical investigations of the mechanism of dehydropolycondensation reaction of aromatic hydrocarbons in the presence of metals and their salts are presented. The role of metals as reaction stimulators is discussed and the comparative analysis of processes in media containing ions of different transition metals is carried out.
INTRODUCTION
After carbon nanotubes synthesis in 1991, the interest to these objects is constantly-increasing. However, till recently high-temperature evaporation of graphite in arc charge has been the only method of nanotubes synthesis. Significant success has been achieved in the frameworks of this method, but there still is the necessity to develop low-temperature synthesis methods with the possibility to control the chemical process, to modify the structure of already prepared tubes and to obtain heterojunctions on their basis.
Thereupon the method, the essence of which consists in the use of fine clusters of transition metals and their salts as stimulators of dehydropolycondensation reaction of aromatic hydrocarbons [1], seems to be perspective. At the same time, phenanthrene and anthracene are used as initial products for the following polymerization, and the reaction proceeds at relatively low temperatures.
The main target of quantum-chemical investigations given in the paper is to reveal the reaction mechanism, stimulators role and to make the comparative analysis of the influence of different metals and their salts on the process. Cu, Ni, Co, Fe, Mn and Cr are considered as stimulators.
CALCULATION METHODS AND THEORETICAL BASIS
The calculations are done in the limits of ab initio Hartree-Fock method [2] in TZV basis [3] using GAMESS program complex [4]. Additional calculations in enhanced TZV bases have not revealed qualitative changes in the results. As ab initio calculations require considerable calculation resources, the investigations are carried out on the simplest model systems including one benzene ring and transition metal ion.
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quantum-chemical investigation of metal-organic complexes participating in
dehydropolycondensation reaction
The parameters of metal / benzene ring complexes optimized by energy are given in the tables. The lengths of bonds of optimized structures for benzene and its compounds with the metals of the row considered are shown in Table 1. In the Table the fraction bar divides non-equivalent bond lengths formed while decreasing benzene ring symmetry. In the frameworks of calculations done it is found that ion Cu(+) does not distort the benzene ring geometrical structure (see Fig. lb), but there is considerable distortion of benzene structure followed by the decrease of its symmetry under the influence of Co, Ni, Mn ions. Thus, complexes with Co and Ni have symmetry C2V, at the same time two opposite carbon atoms move outside the ring plane, as a result the ring acquires "bath" conformation (see Fig. lc). Manganese complex symmetry - C3V. The ring plane distortion is not observed, long and short C-C bonds alternate in the structure obtained.
The influence of metal ions on the strength of C-H bond in benzene and the interaction energy of metal ion with benzene ring are revealed. Besides, the exponents and lengths of chemical bonds in the compounds considered, and atomic charges are calculated.
C-H bond energy is calculated as the difference of complex energy in an equilibrium configuration and its energy without a proton (the proton energy is assumed to be zero):
Ec-h = E (C6H6Me)+n - E (C6H6Me)+n
<=> <=>
Fig. 1. a) ring structure of benzene, b) complex metal / benzene ring with Cu, c) complex metal / benzene ring with Co and Ni
Table 1. Bonds lengths in metal - organic complexes (in angstroms)
C-C C-H Me-C
c6H6 1,387 1,072
(CuC6H6)+l 1,396 1,071 2,579
(CoC6H6)+l 1,400/1,396 1,071/1,070 2,521/2,494
(MnC6H6)+l 1,385/1,410 1,07 2,521
(NiC6H6)+2 1,414/1,396 1,072/1,073 2,370/2,298
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To estimate the interaction energy of metal cation with aromatic ring the following formula is applied:
EMe-ring= [E(C6H6) + E(Me+n)] - E(C6H6Me)+n The calculation results are given in Table 2.
In accordance with the Table data it is possible to conclude that metal ion influence on chemical bond in benzene ring increases in the row Cu, Co, Mn, Cr, Fe, Ni. In general, metal ions considerably facilitate the proton breaking-off from benzene ring accelerating dehydropolycondensation process. In parallel the calculations of the same complexes have been done in the frameworks of INT AS project by chemists team under Professor Molina's supervision (Spain) [5]. The calculations have been done using program product GAUSSIAN-98. They have also used ab initio Hartree-Fock method, but basis set 6-311+g*. Their results are mainly in accordance with the results presented in this paper.
Table 2. Energies of metal ions and aromatic ring interactions and of C-H bond. The comparison of calculations (in Hartee) in TZV bases and enhanced TZV
Me+C6H6 proton breaking-off
TZV TZV* TZV TZV*
CÔHÔ 0,674
CuC6H6+ -0,052 -0,054 0,501 0,497
NiCélV4 -0,222 -0,231 0,305 0,297
CoC6H6+ -0,056 -0,060 0,498 0,492
FeC6H6" -0,696 0,310
MnCeH/ -0,054 -0,058 0,498 0,493
CrCeH*" -0,032 0,380
Table 3. The orders of non-equivalent bonds in different metal/aromatic ring systems
CôHé CuC6H6+ CoC6H6+ MnC6H6+
C-C 1,525 1,436 1,40/1,430 1,505/1,327
C-H 0,896 0,861 0,861 0,861
Me-C 0,127 0,136/0,170 0,166
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Table 4. Charges and free valences of non-equivalent atoms in different metal/aromatic ring systems
CÔHÔ CuC6H6* CoC6H6+ MnC6H6+
Charge according to Mulliken
c -0,21 -0,209 -0.168/-0.214 -0,198/-0,202
H 0,21 0,279 0,277 0,276
Me 0,58 0,531 0,538
Charge according to Lowdin
C -0,135 -0,09 -0,057/-0,063 -0,058/-0,059
H 0,135 0,165 0,168 0,169
Me 0,548976 0,355756 0,339225
Bond valence
C 4,046 3,962 3,962 3,95
H 0,914 0,88 0,88 0,885
Me 0,757 0,956 1,018
Also, the general mechanism of dehydropolycondensation reaction is investigated. RESULTS AND THEIR DISCUSSION
The graphs for potential energies of interaction between benzene ring and copper clusters (white squares in Fig. 2) and copper chloride (black squares) are obtained from results of semi-empirical model MNDO [6]. Copper is chosen as an interaction ion, for CuC6H6 interaction is the weakest. While considering the interaction of copper chloride and benzene ring, chloride is located along the ring axis and copper ion - closer to the ring. While preparing the graphs the distance between the complexes changes but complexes geometry does not distort, copper and chlorine ions are located on the ring axis.
Besides, the dependence graphs of proton interaction energy with complex MeCeHs on the distance between the complex and proton are drawn. In this case Cu and Co are considered. Both graphs are drawn in relative axes, i.e. the energy of optimized complex metal-benzene ring as zero energy is chosen. Also we supposed that the ring geometry is not change and metal ion is always on the ring axis. When the distances from the proton to the complex are different, the energy is not minimized additionally. The curves obtained are given in Fig. 3. The graph for the complex with Cu is shown by the solid line, the graph for
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300 -|
200 -
100
o o i
E Û
s 3 -100
JÉ
L1J -200 -
-300
-400 -
-500 -
Fig. 2. Interaction of benzene ring with copper atom (white squares) and copper ion (black squares). Axis Y - energy (kcal/mol), axis X - distance from cluster to ring plane
I
S
LU
c-H distance
Fig. 3. Proton interaction energy with complex MeC6H5 as the function of the proton distance away from its balanced position in benzene ring. CuC6Hô - solid line, CoC^Hé -triangles
Co - by triangles. The comparative analysis of the graphs is carried out to estimate the entropy contribution to proton-complex interaction non-zero temperature. It is seen from the Figure that in the region considered the curves are identical to the shift.
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It is seen from the tables that metal weakens C-H bond in benzene molecule and facilitates hydrogen atom removal process form hydrocarbon substance. In order to investigate the following polymerization of aromatic hydrocarbons, the computer modeling of anthracene molecule interaction and several benzene molecules is done. The part of hydrogen atoms of the molecules mentioned is removed. The result of energy optimization of the complex mentioned within the frameworks of semi-empirical method MNDO is shown in Fig. 4.
It is seen from the Figure that a highly defective branched structure is formed. Though the primary hybridization degree of carbon atoms is sp2, atoms with sp3- and sp-hybridization are presented in the complex. Not all stopped bonds of carbon atoms close on other stopped bonds. In many cases the system stabilizes due to the increase of C-C bond order.
During the formation of carbon shells the metal clusters and their salts can stimulate the orientation. This interaction is shown in Fig. 3 and Table 1. Some energy must be expended for carbon net bending to form a carbon tube out of a graphite sheet. The calculation results show that for tubes of different diameters obtained experimentally, the interaction energy of metal ion with benzene ring exceeds the bending energy of carbon net even with copper.
In the frameworks of semi-empirical tight - binding model in Gudwin parameterization [7] the modeling of polyaromatic carbon strip with metal particle is carried out. The metal particle is imitated by cylindrical field, in which carbon atoms are present. The field potential is estimated out of the curves shown in Fig. 3. At the start time the strip is given the rate directed to the cylinder axis, initially the band is located at an angle to the axis. The investigations are carried out using own program complex, which allows making quantum-chemical molecular-dynamic investigations of carbon systems. As a result of molecular-dynamic experiment the strip "winds around" the cylinder, its ends close forming covalent bond between carbon atoms, as it is shown in Fig. 5.
Fig. 4. Complex optimized by energy containing phenanthrene molecule and six benzene molecules. Hydrogen atoms are partly removed
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Fig. 5. The formation of carbon shell on the metal particle surface CONCLUSION
The following conclusions can be made from the results obtained. A metal weakens C-H bond in benzene ring stimulating proton breaking-off and chemical bond formation between different rings, in other words facilitating dehydropolycondensation process. Besides, the role of metal salts consists in absorption of free protons by chlorine anions. In turn, metal particles attract hydrocarbon atoms, playing an orienting part in during their dehydropolycondensation.
Quantum-chemical investigations of metal-organic complexes and model systems point out the formation possibility of defective carbon shells with metals in the frameworks of the reaction considered. This fact is also confirmed by experimental investigations, in particular, microphotographs and diffractograms of the samples obtained, which demonstrate the presence of cylindrical graphite-like shells with metal inside.
REFERENCES
1. V.I. Kodolov, I.N. Shabanova, A.P. Kuznetsov, O.A. Nikolaeva et al. Anal, and Control. 4, 1999.
2. N.F. Stepanov, V.I. Pupyshev. Quantum mechanics of molecules and quantum chemistry. Moscow. MGU, 1991, 384 p.
3. T.H. Dunning, J. Chem. Phys. 55 (1971) 716-723. A.D. McLean, G.S. Chandler, J. Chem. Phys. 72, 5639-5648, (1980). A.K. Rappe, T.A. Smedley and W.A. Goddard III, J. Phys. Chem. 85,(1981), 2607-2611.
4. M.W. Schmidt et al. J. Comput. Chem. 14, 1347-1363, (1993).
5. S. Melchor Ferrer. Private communication.
6. J. Sadlej. Semi-empirical methods of quantum chemistry. Ellis Horwood limited. 1985.
7. Goodwin, L. J. Phys.: Condens. Matter. 1991, 3, 3869.
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