CC BY
25
UDC 544.176:547.39
N. А. Turovskij, E. V. Raksha, Yu. V. Berestneva, G. Е. Zaikov, Kh. S. Abzaldinov
NMR 1H AND 13C SPECTRA OF THE 1,1,3-TRIMETHYL-3-(4-METHYL-PHENYL)BUTYL HYDROPEROXIDE IN CHLOROFORM. EXPERIMENTAL VERSUS GIAO CALCULATED DATA
Keywords: NMR spectroscopy, 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide, chemical shift, magnetic shielding constant,
GIAO, molecular modeling.
NMR 1H and 13C spectra of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide in chloroform-d have been investigated. Calculation of magnetic shielding tensors and chemical shifts for 1H and 13C nuclei of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide molecule in the approximation of an isolated particle and considering the solvent influence in the framework of the continuum polarization model (PCM) was carried out. Comparative analysis of experimental and computer NMR spectroscopy results revealed that the GIAO method with B3LYP/6-31G (d,p) level of theory and the PCM approach can be used to estimate the NMR 1H and 13C spectra parameters of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide.
Ключевые слова: ЯМР-спектроскопия, гидропероксид 1,1,3-триметил-3- (4-метилфенил) бутила, химический сдвиг, константа магнитного экранирования, метод GIAO, молекулярное моделирование.
Исследованы 1H и 13C ЯМР-спектры гидропероксида 1,1,3-триметил-3-(4-метилфенил) бутила в хлороформе-d. В рамках модели поляризованного континуума (PCM) проведен расчет тензоров магнитного экранирования и химических сдвигов ядер 1H и 13С молекулы гидропероксида 1,1,3-триметил-3-(4-метилфенил) бутила приближенно к изолированной частице, а также с учетом влияния растворителя. Сравнительный анализ результатов экспериментальной и компьютерной ЯМР-спектроскопии показал, что метод GIAO с уровнем теории B3LYP/6-31G (d, р) и модель PCM могут быть использованы для оценки параметров и 13C ЯМР-спектров гидропероксида 1,1, 3-триметил-3- (4-метилфенил) бутила.
Introduction
Hydroperoxide compounds are widely used as chemical source of the active oxygen species. Variations in their structure allows purposefully create new initiating systems with a predetermined reactivity. Arylalkyl hydroperoxides are useful starting reagents in the synthesis of surface-active peroxide initiators for the preparation of polymeric colloidal systems with improved stability [1]. Thermolysis of arylalkyl hydroperoxides was studied in acetonitrile [2]. NMR 1H spectroscopy has been already used successfully for the experimental evidence of the a complex formation between a 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide and tetraalkylammonium bromides in acetonitrile [3-5] and chloroform solution [5].
Molecular modeling of the peroxide bond homolytic cleavage as well as processes of hydroperoxides association is an additional source of information on the structural effects that accompany these reactions. One of the criteria for the selection of quantum-chemical method for the study of the hydroperoxides reactivity may be reproduction with sufficient accuracy of their NMR :H and 13C spectra parameters. The aim of this work is a comprehensive study of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide (ROOH) by experimental 1H and 13C NMR spectroscopy and molecular modeling methods.
Experimental
The 1,1,3 -trimethyl-3 -(4-methylpheny l)buty l hydroperoxide (ROOH) was purified according to Ref. [1]. Its purity (99 %) was controlled by iodometry method. Experimental NMR :H and 13C spectra of the hydroperoxide solution were obtained by using the Bruker Avance II 400 spectrometer (NMR :H - 400
MHz, NMR 13C - 100 MHz) at 297 K. Solvent, chloroform-d (CDCl3) was Sigma-Aldrich reagent and was used without additional purification but was stored above molecular sieves before using. Tetramethylsilane (TMS) was internal standard. The hydroperoxide concentration in solution was 0.03 mol-dm-3.
1,1,3-Trimethyl-3-(4-methylphenyl)butyl hydroperoxide (4-CH3-C6H4-C(CH3)2-CH2-(CH3)2C-O-OH) NMR 1H (400 MHz, chloroform-d, 297 K, 5 ppm, J/Hz): 1.00 (s, 6 H, -C(CH3)2OOH), 1.39 (s, 6 H, -C6H4C(CH3)2-), 2.05 (s, 2 H, -CH2-), 2.32 (s, 3 H, CH3-C6H4-), 7.11 (d, J = 8.0, 2 H, H-aryl), 7.29 (d, J = 8.0, 2 H, H-aryl), 6.77 (s, 1 H, -COOH).
Molecular geometry and electronic structure parameters, as well as harmonic vibrational frequencies of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide molecule were calculated after full geometry optimization in the framework of B3LYP/6-31G(d,p) and MP2/6-31G(d,p) calculations. The resulting equilibrium molecular geometry was used for total electronic energy calculations by the B3LYP/6-31G(d,p) and MP2/6-31G(d,p) methods. All calculations have been carried out using the Gaussian03
[6] program.
The magnetic shielding tensors (%, ppm) for :H and 13C nuclei of the hydroperoxide and the reference molecule were calculated with the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) equilibrium geometries by standard GIAO (Gauge-Independent Atomic Orbital) approach
[7]. The calculated magnetic isotropic shielding tensors, Xi, were transformed to chemical shifts relative to TMS molecule, 8, by 8 = Xref - X, where both, Xref and x, were taken from calculations at the same computational level. Table 1 illustrates obtained x values for TMS molecule used for the hydroperoxide :H and 13C nuclei
chemical shifts calculations. % values were also estimated in the framework of 6-311G(d,p) and 6-311++G(d,p) basis sets on the base of MP2/6-31G(d,p) and B3LYP/6-31G(d,p) equilibrium geometries. The solvent effect was considered in the PCM approximation [8, 9]. % values for magnetically equivalent nuclei were averaged.
Table 1 - GIAO-magnetic shielding tensors for JH and 13C nuclei of the TMS
Ten signals for the hydroperoxide carbon atoms are observed in the ROOH 13C NMR spectrum (Table 3). Signal of the carbon atom bonded with a hydroperoxide group shifts slightly to the stronger fields with the solvent polarity increasing, while the remaining signals are shifted to weak fields.
Table 3 - Experimental chemical shifts of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide NMR 13C spectra in chloroform-d
Nuclei MP2 B3LYP
1 2 3 1 2 3
The isolated particle approximation
*H 31.96 32.08 32.05 31.75 31.96 31.93
13c 207.54 199.71 199.37 191.80 184.13 183.72
Chloroform (PCM approximation)
1H 31.95 32.08 32.05 31.75 31.95 31.92
13c 207.86 200.13 199.79 192.08 184.53 184.13
Note: 1 - 6-31G(d,p); 2 - 6-311G(d,p); 3 - 6-311++G(d,p)
Carbon group S, ppm
C1 -CO-OH 83.93
C2 -CH2- 50.71
C3 -C(CH3)2OOH 25.98
C4 -CaH4C(CH3)2- 37.03
C5 -CRH4C(CH3)2- 30.91
C6 146.55
C7 C-aryl 125.81
C8 128.81
C9 135.01
C10 CH3-C6H4- 20.86
Inspecting the overall agreement between experimental and theoretical spectra RMS errors (ct) were used to consider the quality of the :H and 13C nuclei chemical shifts calculations. Correlation coefficients (R) were calculated to estimate the agreement between spectral patterns and trends.
Results and Discussions
Experimental NMR 1H u 13C spectra of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide
Experimental NMR :H and 13C spectra of the 1, 1,3 -trimethyl-3-(4-methylphenyl)butyl hydroperoxide were obtained from chloroform-d solution. The concentration of the hydroperoxide in sample was 0.03 mol-dm-3. Parameters of the experimental NMR :H and 13C spectra of the ROOH are listed in Tables 2 and 3 correspondingly.
y=\ CH3
To-OH N-' CH3 CH3
Comparing obtained results with those in acetonitrile-d3 [10], it should be noted that signals shift toward the strong field is observed in the spectrum of the hydroperoxide with increasing solvent polarity. On the other hand, the hydroperoxide moiety proton signal is observed in the weak field with the solvent polarity increasing. The -CO-OH group proton appears at 6.77 ppm in chloroform-d, and in the more polar acetonitrile-d3 it was found at 8.51 ppm.
Table 2 - Experimental chemical shifts of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide NMR 1H spectra in chloroform-d
A linear dependence between the :H and 13C chemical shifts values of the hydroperoxide (except for the -CO-OH group proton) is observed in the studied solvents (Fig. 1). This is consistent with authors [11], who showed linear correlation between the chemical shifts values in chloroform-d and dimethylsulphoxide-d6 for a large number of organic compounds of different classes.
s
7
6
lo 2 1
012345678
a
160 r
140 -
^ 120 -I
& 100 -| 8010 60 -40 -20 -
20 40 60 80 100 120 140 160
Proton group S, ppm
H1 -CH2- 2.05
H2 -C(CH3)2OOH 1.00
H3 -CRH4C(CH3)2- 1.39
H4 H-aryl 7 11
H5 7.29
H6 CH3-CRH4- 2.32
H7 -CO-OH 6.77
b
Fig. 1 - Acetonitrile-d3 versus chloroform-d experimental 1H (a) and 13C (b) chemical shifts (relative to TMS) of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide (except for the -CO-OH group proton)
Molecular modeling of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide structure and NMR 1H and13C spectra by MP2 andB3LYPmethods
The hydroperoxide molecule geometry optimization in the framework of MP2/6-31G(d,p) and B3LYP/6-31G(d,p) methods was carried out as the first step of the hydroperoxide NMR 1H and 13C spectra modeling. Initial hydroperoxide configuration chosen for calculations was those one obtained by semiempirical AMI method and used recently for the hydroperoxide O-O bond homolysis [2] as well as complexation with Et4NBr [4, 12] modeling. The main parameters of the hydroperoxide fragment molecular geometry obtained in the isolated particle approximation within the framework of MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory are presented in Table 4. Peroxide bond O-O is a reaction centre in this type of chemical initiators thus the main attention was focused on the geometry of -CO-OH fragment. The calculation results were compared with known experimental values for the feri-butyl hydroperoxide [13], and appropriate agreement between calculated and experimental parameters can be seen in the case of MP2/6-31G(d,p) method.
Table 4 - Molecular geometry parameters of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide -CO-OH moiety
Parameter MP2/6-31G(d,p) B3LYP/6-31G(d,p) Experiment*
lo-o, A 1.473 1.456 1.473
lc-o, A 1.459 1.465 1.443
lo-H, A 0.970 0.971 0.990
c-o-o, ° 108.6 110.0 109.6
O-O-H, ° 98.2 99.9 100.0
C-o-o-H, ° 112.4 109.1 114.0
*Note: experimental values are those for tert-butyl hydroperoxide [13]
Calculation of :H and 13C chemical shifts of the hydroperoxide was carried out by GIAO method in the approximation of an isolated particle as well as in chloroform within the PCM model, which takes into account the nonspecific solvation. Equilibrium hydroperoxide geometries obtained in the framework of MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels of theory for the isolated particle approximation were used in all cases (Fig. 2).
hydroperoxide structural model with corresponding atom numbering (MP2/6-31G(d,p) method)
The chemical shift values (8, ppm) for :H and 13C nuclei in the hydroperoxide molecule were evaluated on the base of calculated magnetic shielding constants (x, ppm). TMS was used as standard, for which the molecular geometry optimization and x calculation were performed using the same level of theory and basis set. Values of the :H and 13C chemical shifts were found as the difference of the magnetic shielding tensors of the corresponding TMS and hydroperoxide nuclei (Tables 5 and 6).
Concerning the spectral pattern of protons, inspection of Table 5 data reveals that the patterns of :H spectra of the hydroperoxide are correctly reproduced at all computational levels used in the study. For MP2 method the expansion of the basis set leads to slightly worse reproducing of protons chemical shifts values (except for hydroperoxide group proton) in the case of an isolated particle approximation. But as for CO-OH group proton the best reproduction of the experimental 8 value is observed when 6-311++G(d,p) basis set is used (6.77 ppm). For results obtained at other computational level, we note that with respect to spectral patterns and trends, results obtained with the computationally less expensive B3LYP optimized geometry are very similar to those obtained with the MP2 calculations. When passing to the calculations in the PCM mode solvation accounting leads to more correct results for the MP2 and B3LYP methods. The lowest ct value is obtained for 6-31G(d,p) basis set
Table 5 - *H NMR GIAO chemical shifts (8, ppm) of the 1,1,3-trimethyl-3- (4-methylphenyl)butyl hydroperoxide
Nuclei MP2
1 2 3 4
The isolated particle approximation
H1 1.60 1.63 1.65 2.05
H2 1.44 1.44 1.47 1.00
H3 1.46 1.47 1.50 1.39
H4 7.54 7.58 7.64 7.11
H5 7.42 7.57 7.63 7.29
H6 2.25 2.39 2.45 2.32
H7 6.68 6.56 6.77 6.77
a 0.09 0.10 0.12 -
R 0.995 0.995 0.995 -
Chloroform (PCM approximation)
H1 1.56 1.60 1.62 2.05
H2 1.47 1.47 1.51 1.00
H3 1.49 1.50 1.53 1.39
H4 7.62 7.67 7.74 7.11
H5 7.50 7.67 7.73 7.29
H6 2.29 2.43 2.49 2.32
H7 7.14 7.03 7.24 6.77
a 0.13 0.14 0.19 -
R 0.995 0.995 0.995 -
nuclei B3LYP
1 2 3 4
The isolated particle approximation
End table 5
H1 1.57 1.56 1.50 2.05
H2 1.40 1.40 1.42 1.00
H3 1.40 1.38 1.41 1.39
H4 7.32 7.33 7.34 7.11
H5 7.16 7.30 7.37 7.29
H6 2.18 2.33 2.36 2.32
H7 5.89 5.76 5.90 6.77
a 0.18 0.21 0.19 -
R 0.989 0.986 0.988 -
Chloroform (PCM approximation)
H1 1.53 1.52 1.47 2.05
H2 1.43 1.43 1.46 1.00
H3 1.42 1.40 1.43 1.39
H4 7.39 7.42 7.43 7.11
H5 7.23 7.38 7.47 7.29
H6 2.22 2.37 2.40 2.32
H7 6.35 6.22 6.37 6.77
a 0.10 0.12 0.12 -
R 0.993 0.995 0.984 -
There is a linear correlation between the experimental and calculated with solvent influence accounting 8 values for the hydroperoxide :H nuclei in chloroform (see Fig. 3). The correlation coefficients (R) corresponding to obtained dependences are shown in Table 5. The best R values for MP2 and B3LYP methods are observed for 6-311G(d,p) basis set and further basis set extension leads to slightly worse values. Joint consideration of ct and R values indicates B3LYP/6-311G(d,p) method is the best among all used combinations. Nevertheless is should be noted that the B3LYP with 6-31G(d,p) basis set yields the similar results.
The correct spectral pattern for the hydroperoxide NMR 13C spectrum was obtained for all methods and basis sets used within the isolated molecule approximation as well as solvation accounting (See Table 6). Exceptions are aromatic hydrocarbons C8 and C9, which signals are interchanged for all calculations.
Note: 1 - 6-31G(d,p); 2 - 6-311G(d,p); 3 - 6-311++G(d,p); 4 - experiment in chloroform-d
MP2/6-31G(d,p)
MP2/6-311G(d,p)
2 3 4 5
Scalc
8 7 6 5
§•4
CO
3 2 1 0
MP2/6-311++G(d,p)
4 5
Scaic
Fig. 3 - Experimental (8exp) versus GIAO calculated (8ca|c) 1H chemical shifts (relative to TMS) of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide in chloroform
8
S
The best reproduced experimental chemical shift value for the carbon atom of the CO-OH group (83.93 ppm) is observed in the case of MP2/6-31G(d,p) (83.61 and 84.29 ppm), the B3LYP with the same basis set gives slightly worse value (85.77 and 86.44 ppm). Basis set extension to 6-311++G(d,p) leads to a deterioration of the calculation results.
Linear relationships between the experimental parameters of the NMR 13C spectrum and the calculated
values 8caic for the hydroperoxide 13C nuclei (see Fig. 4) have been obtained for both methods and all basis sets. Sufficiently high values of correlation coefficients (see Table 6) correspond to these dependences. Joint account of ct and R values indicates possibility of B3LYP method with 6-31G(d,p) basis set using for the calculation of the hydroperoxide 13C nuclei chemical shifts.
Table 6 - 13C NMR GIAO chemical shifts (8, ppm) of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl
hydroperoxide
Nuclei MP2
1 2 3 4
The isolated particle approximation
C1 83.61 86.87 88.24 83.93
C2 53.50 57.48 57.42 50.71
C3 26.46 26.71 26.62 25.98
C4 37.04 40.23 40.37 37.03
C5 30.10 30.93 31.03 30.91
C6 141.39 153.47 153.93 146.55
C7 116.91 125.90 126.29 125.81
C8 127.07 137.37 137.97 128.81
C9 121.55 130.79 131.41 135.01
C10 22.98 23.82 23.75 20.86
a 23.49 13.13 15.39 -
R 0.997 0.997 0.997 -
Chloroform (PCM approximation)
C1 84.29 87.74 89.27 83.93
C2 53.69 57.73 57.64 50.71
C3 26.62 26.99 26.89 25.98
C4 37.46 40.74 40.92 37.03
C5 30.18 31.10 31.20 30.91
C6 142.14 154.40 154.90 146.55
C7 117.33 126.51 126.91 125.81
C8 127.92 138.43 139.03 128.81
C9 121.90 131.35 131.91 135.01
C10 23.01 23.96 23.88 20.86
a 27.86 25.63 28.66 -
R 0.997 0.997 0.997 -
nuclei B3LYP
1 2 3 4
The isolated particle approximation
C1 85.77 90.90 92.04 83.93
C2 53.23 57.70 57.00 50.71
C3 24.62 25.27 24.93 25.98
C4 41.14 44.66 44.49 37.03
C5 28.57 29.78 29.75 30.91
C6 144.46 158.38 158.37 146.55
C7 119.80 130.58 131.03 125.81
C8 130.75 142.40 143.32 128.81
C9 123.68 134.44 135.07 135.01
C10 21.84 23.25 22.84 20.86
a 11.99 41.22 44.24 -
R 0.996 0.996 0.996 -
Chloroform (PCM approximation)
C1 86.44 91.86 93.14 83.93
C2 53.37 57.89 57.17 50.71
C3 24.73 25.52 25.17 25.98
C4 41.54 45.20 45.07 37.03
C5 28.59 29.90 29.88 30.91
C6 145.12 159.23 159.25 146.55
C7 120.11 131.14 131.60 125.81
C8 131.56 143.43 144.34 128.81
C9 123.85 134.81 135.37 135.01
C10 21.81 23.36 22.93 20.86
a 20.82 59.16 63.32 -
R 0.996 0.996 0.996 -
Fig. 4 - Experimental (8exp) versus GIAO calculated (8calc) 13C chemical shifts (relative to TMS) of the 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide in chloroform
Note: 1 - 6-31G(d,p); 2 - 6-311G(d,p); - experiment in chloroform-d
3 - 6-311++G(d,p); 4
llLYnilll^ij)
aiLVPii-]ii++u(i,j)
30 to ID 10 100 130 110 110 110
End fig. 4
Conclusions
A comprehensive study of the 1,1,3-trimethyl-3-(4-methyl-phenyl)butyl hydroperoxide by experimental NMR :H and 13C spectroscopy and molecular modeling methods was performed. A comparative assessment of the :H and 13C nuclei chemical shifts calculated by GIAO in various approximations. For NMR :H and 13C spectra of the hydroperoxide in chloroform MP2 and B3LYP methods approximations with 6-31G(d,p), 6-311G(d,p), and 6-311++G(d,p) basis sets allow to obtain the correct spectral pattern. A linear correlation between the calculated and experimental values of the :H and 13C chemical shifts for the studied hydroperoxide molecule. In both cases, the B3LYP combined with 6-31G(d,p) basis set allows to get a better agreement between the calculated and experimental data as compared to the MP2 results.
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© N. А. Тигоуэкт] - Ph.D., Associate Professor of Physical Chemistry Department, Donetsk National University, Donetsk, Ukraine, [email protected], E. V. Raksha - Ph.D., Associate Professor of Physical Chemistry Department, Donetsk National University, Donetsk, Ukraine, Yu. V. Berestneva - Post-Graduate Student of Physical Chemistry Department, Donetsk National University, Donetsk, Ukraine, G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, Kh. S. Abzaldinov - Ph.D., Associate Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia, [email protected].
© Н. А. Туровский - канд. хим. наук, доц. каф. физической химии, Донецкий национальный университет, Донецк, Украина, [email protected], Е. В. Ракша - канд. хим. наук, доц. той же кафедры, Ю. В. Берестнева - аспирант той же кафедры, Г. Е. Заиков - д-р хим. наук, проф. каф. технологии пластических масс КНИТУ; Х. С. Абзальдинов - канд. хим. наук, доц. той же кафедры, [email protected].
