CC BY
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UDC 544.176:547-39
N. А. Turovskij, E. V. Raksha, Yu. V. Berestneva, G. Е. Zaikov
NMR 13C SPECTRA OF THE 1,1,3-TRIMETHYL-3-(4-METHYLPHENYL)BUTYL HYDROPEROXIDE IN VARIOUS SOLVENTS: MOLECULAR MODELING
Keywords: NMR spectroscopy, 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide, chemical shift, magnetic shielding constant,
GIAO, molecular modeling.
GIAO-calculated NMR 13C chemical shifts as obtained at various computational levels are reported for the 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide. The data are compared with experimental solution data in chloroform-d, acetonitrile-d3, and DMSO-d6, focusing on the agreement with spectral patterns and spectral trends. Calculation of magnetic shielding tensors and chemical shifts for 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 MP2/6-31G(d, p) level of theory and the PCM approach can be used to estimate the NMR 13C chemical shifts of the 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide.
Ключевые слова: ЯМР-спектроскопия, 1,1,3-триметил-3-(4-метилфенил) бутил гидропероксид, химический сдвиг, константа
магнитного экранирования, ГИАО, молекулярное моделирование.
Представлены химические сдвиги l3C ЯМР рассчитанные методом ГИАО на различных вычислительных уровнях для 1,1,3-триметил-3-(4-метилфенил) бутил гидропероксида. Приводится сравнение полученных данных с экспериментальными в растворе хлороформа-d, ацетонитрила-d3, и ДМСО-d6, ориентируясь на соответствие со спектральными моделями и тенденциями. Приводится расчет тензоров магнитного экранирования и химических сдвигов для ядер 13С молекулы 1,1,3-триметил-3-(4-метилфенил) бутил гидропероксида в приближении изолированной частицы, а также с учетом влияния растворителя в рамках континуальной модели растворителя (PCM). Сравнительный анализ экспериментальных и расчетных результатов ЯМР-спектроскопии показал, что метод ГИАО с уровнем теории MP2/6-31G(d, р) и метод PCM могут быть использованы для оценки химических сдвигов 13C ЯМР 1,1,3 -триметил-3-(4-метилфенил) бутил гидропероксида.
Introduction
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]. The aim of this work is a comprehensive study of the 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide (ROOH) by experimental NMR 13C spectroscopy and molecular modeling methods.
Experimental
The 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide (ROOH) was purified according to Ref. [1]. Its purity (99 %) was controlled by iodometry method as well as by NMR spectroscopy. Experimental NMR 13C spectra of the hydroperoxide solutions were obtained by using the Bruker Avance II 400 spectrometer (NMR 1H - 400 MHz, NMR 13C - 100 MHz) at 297 K. Solvents, chloroform-d, acetonitrile-d3, and DMSO-d6, were Sigma-Aldrich reagents and were used without additional purification but were stored above molecular sieves before using. Tetramethylsilane (TMS) was internal standard. The hydroperoxide concentration in solutions was 0.03 mol-dm-3.
Molecular geometry and electronic structure parameters, as well as harmonic vibration 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) methods. 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 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, S, by si = xref - X, where both, xref and x, were taken from calculations at the same computational level. Table 1 illustrates x values for TMS molecule
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used for the hydroperoxide C nuclei chemical shifts calculations.
x 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]. x values for magnetically equivalent nuclei were averaged.
Inspecting the overall agreement between experimental and theoretical spectra RMS errors (ct) were used to consider the quality of the 13C nuclei chemical shifts calculations. Correlation coefficients (R) were calculated to estimate the agreement between spectral patterns and trends.
Table 1 - Magnetic shielding tensors for the TMS
13,
С nuclei of
Solvent MP2
1 2 3
- 207.54 199.71 199.37
Chloroform 207.86 200.13 199.79
Acetonitrile 208.01 200.32 199.99
DMSO 208.01 200.33 200.00
B3LYP
Solvent 1 2 3
- 191.80 184.13 183.72
Chloroform 192.08 184.53 184.13
Acetonitrile 192.19 184.70 184.30
DMSO 192.30 184.81 184.40
Results and Discussion
Experimental NMR 13C spectra of the 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide
Experimental NMR 13C spectra of the 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide (ROOH) were obtained from chloroform-d, acetonitrile-d3, and DMSO-d6 solutions.
Note: 1 - 6-31G(d, p); 2 311 ++G(d, p)
6-311G(d, p); 3 - 6-
The hydroperoxide concentration in all samples was 0.03 mol-dm-3. The experimental NMR 13C spectra of the ROOH are presented in Figure 1.
Fig. 1 - The relationship between the experimental NMR 13C chemical shifts (relative to TMS) of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide in different solvents
Ten signals for the hydroperoxide carbon atoms are observed in the ROOH 13C NMR spectrum. 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. A linear dependences between the 13C chemical shifts values of the hydroperoxide are 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. Equations corresponded to the obtained relationships (Fig. 1) are listed below. <W/v = 0-02 ± 0.23 K 1006 ± 0.002 8cda3
sDMSO-d6 = ("0.46 ± 0.41 K 0-999 ± 0.004 8CDa3
^dmso-d6 = "0.48 ± 0.24 K 0.993 ± 0.003 Sc^ov
Molecular modeling of the 1,1,3-trimethyl-3-1(34-methylphenyl) butyl hydroperoxide NMR 1 C spectra by MP2 and B3LYP methods
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 13C spectra modeling. Initial hydroperoxide configuration chosen for calculations was those one obtained by semiempirical AM1 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) (Fig. 2) and B3LYP/6-31G(d, p) levels of theory are presented in Table 2. 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 tert-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 2 - Molecular geometry parameters of the 1,1,3-trimethyl-3-(4-methylphenyl) butyl
hydroperoxide -CO-OH moiety
parameter МР2/6-31G(d, p) B3LYP/6-31G(d, p) Experi ment*
1o-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 from [13]
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Calculation of C chemical shifts of the hydroperoxide was carried out by GIAO method in the approximation of an isolated particle as well as in
studied solvents 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.
«4 »
Fig. 2 - The 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide structural model (MP2/6-31G(d, p) method)
The chemical shift values (8, ppm) for 13C nuclei in the hydroperoxide molecule were evaluated on the base of calculated magnetic shielding constants (%, ppm). TMS was used as standard, for which the molecular geometry optimization and % calculation were performed using the same level of theory and basis set. Values of the 3C chemical shifts were found as the difference of the magnetic shielding tensors of the corresponding TMS and hydroperoxide nuclei (Tabl. 3 and 4).
Table 3 - NMR 13C chemical shifts (8, ppm) of the 1,1,3-trimethyl-3-(4-methylphenyl)butyl hydroperoxide (the isolated particle approximation)
nuclei MP2
1 2 3
C1 83.61 86.87 88.24
C2 53.50 57.48 57.42
C3 26.46 26.71 26.62
C4 37.04 40.23 40.37
C5 30.10 30.93 31.03
C6 141.39 153.47 153.93
C7 116.91 125.90 126.29
C8 127.07 137.37 137.97
C9 121.55 130.79 131.41
C10 22.98 23.82 23.75
nuclei B3LYP
1 2 3
C1 85.77 90.90 92.04
C2 53.23 57.70 57.00
C3 24.62 25.27 24.93
C4 41.14 44.66 44.49
C5 28.57 29.78 29.75
C6 144.46 158.38 158.37
C7 119.80 130.58 131.03
C8 130.75 142.40 143.32
C9 123.68 134.44 135.07
C10 21.84 23.25 22.84
Note: 1 - 6-31G(d, p); 2 - 6-311G(d, p); 3 - 6-311 ++G(d, P)
The correct spectral pattern for the hydroperoxide NMR 13C spectrum was obtained for all methods and basis sets used within the isolated molecule approximation (See Table 3) as well as solvation accounting (See Table 4). Exceptions are aromatic C8 and C9 carbons, which signals are interchanged for all calculations.
The best reproduced experimental chemical shift value for the carbon atom of the CO-OH group is observed in the case of MP2/6-31G(d, p) approximation in all used solvents whereas B3LYP with the same basis set gives slightly worse values. Basis set extension to 6-311++G(d, p) leads to a deterioration of the calculation results. Calculated value for the carbon of CO-OH group (83.61 ppm) within the isolated molecule approximation is closest to experimental one in
acetonitrile (83.74 ppm). When passing to the calculations in the PCM mode solvation accounting leads to more correct results for the MP2 and B3LYP methods. The lowest ст values for all solvents are obtained with 6-31G(d, p) basis set. Linear relationships between the experimental NMR 13C chemical shifts and the calculated values 5calc for the hydroperoxide 13C nuclei (see Fig. 3) have been obtained for both methods and all basis sets. The correlation coefficients (R) corresponding to obtained dependences are shown in Table 4. Joint account of ст and R values indicates possibility of MP2 method with 6-31G(d, p) basis set using for the calculation of the hydroperoxide 13C nuclei chemical shifts.
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Fig. 3 - Experimental (Sexp) versus GIAO calculated C chemical shifts (relative to TMS) of the 1,1,3-trimethyl-3-(4-methylphenyl) butyl hydroperoxide
Table 4 - NMR 13C chemical shifts (8, ppm) of the 1,1,3-trimethyl-3-(4-methylphenyl )butyl
hydroperoxide in different solvents
nuclei MP2 4
1 | 2 | 3
Chloroform
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 4
1 1 2 | 3
Chloroform
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
nuclei MP2 4
1 1 2 | 3
Acetonitrile
C1 84.58 88.13 89.73 83.74
C2 53.80 57.88 57.78 51.15
C3 26.70 27.13 27.02 26.13
C4 37.64 40.99 41.18 37.65
C5 30.22 31.19 31.29 31.37
C6 142.50 154.84 155.37 148.03
C7 117.52 126.80 127.20 126.89
C8 128.29 138.90 139.49 129.47
C9 122.08 131.63 132.17 132.66
C10 23.03 24.04 23.96 20.84
a 24.571 22.325 25.741
R 0.998 0.998 0.998
nuclei B3LYP 4
1 1 2 | 3
Acetonitrile
C1 86.72 92.27 93.61 83.74
C2 53.44 57.98 57.27 51.15
C3 24.77 25.63 25.28 26.13
C4 41.71 45.43 45.32 37.65
C5 28.59 29.95 29.93 31.37
C6 145.42 159.62 159.66 148.03
C7 120.24 131.37 131.84 126.89
C8 131.88 143.85 144.75 129.47
C9 123.94 135.00 135.52 132.66
C10 21.80 23.41 22.97 20.84
a 17.395 55.560 60.243
R 0.997 0.997 0.996
nuclei MP2 4
1 1 2 | 3
DMSO
C1 84.60 88.15 89.75 81.79
C2 53.80 57.88 57.78 50.18
C3 26.70 27.13 27.03 25.76
C4 37.65 41.00 41.19 36.64
C5 30.22 31.19 31.30 30.85
C6 142.52 154.86 155.39 146.73
C7 117.53 126.81 127.22 125.65
C8 128.31 138.92 139.52 128.43
C9 122.09 131.64 132.18 133.98
C10 23.03 24.04 23.96 20.45
a 25.496 31.656 35.949
R 0.997 0.997 0.997
nuclei B3LYP 4
1 1 2 | 3
DMSO
C1 86.84 92.38 93.72 81.79
C2 53.54 58.09 57.37 50.18
C3 24.87 25.74 25.37 25.76
C4 41.82 45.54 45.43 36.64
C5 28.69 30.06 30.03 30.85
C6 145.54 159.74 159.77 146.73
C7 120.35 131.48 131.94 125.65
C8 131.99 143.97 144.86 128.43
C9 124.05 135.11 135.62 133.98
C10 21.90 23.51 23.06 20.45
a 21.206 70.994 76.092
R 0.995 0.996 0.995
Note: 1 - 6-31G(d, p); 2 - 6-311G(d, p); 3 - 6-311 ++G(d, p); 4 - experimental data
Conclusions
A comprehensive study of the 1,1,3-trimethyl-3-(4-methyl-phenyl) butyl hydroperoxide by experimental NMR 13C spectroscopy and molecular modeling methods was performed. A comparative assessment of the 13C nuclei chemical shifts calculated
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by GIAO in various approximations. For NMR C spectra of the hydroperoxide in different solvents 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 correlations between the calculated and experimental values of the 13C chemical shifts for the studied hydroperoxide molecule were obtained for all solvents studied. In al cases, the MP method combined with 6-31G(d, p) basis set allows to get a better agreement between the calculated and experimental data as compared to the B3LYP results.
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© N. A. Turovskij - Ph.D., Associate Professor of Physical Chemistry Department, Donetsk National University, Donetsk, Ukraine, email: [email protected], E. V. Raksha - Ph.D., L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, Donetsk, Ukraine, e-mail: [email protected], Yu. V. Berestneva - L.M. Litvinenko Institute of Physical Organic and Coal Chemistry, Donetsk, Ukraine, G. E. Zaikov - Doctor of Chemistry, Full Professor of Plastics Technology Department, Kazan National Research Technological University, Kazan, Russia.
© Н. А. Туровский - кандидат химических наук, доцент кафедры Физической химии, Донецкий национальный университет, Донецк, Украина, [email protected]; Е. В. Ракша - кандидат химических наук, Институт физико-органической химии и углехимии им. Л.М. Литвиненко, Донецк, [email protected]; Ю. В. Берестнева - Институт физико-органической химии и углехимии им. Л.М. Литвиненко, Донецк, Г. Е. Заиков - доктор химических наук, профессор кафедры Технологии пластических масс, Казанский национальный исследовательский технологический университет, Казань, Россия.
