CC BY
84
УДК (54.422.3-74:51-7):547.53-39
N. А. Turovskij, E. V. Raksha, E. N. Pasternak,
Yu. V. Berestneva, I. A. Opeida, G. Е. Zaikov, S. Yu. Sofina
IR SPECTRA OF THE BENZOYL PEROXIDE SYMMETRICAL DERIVATIVES:
MOLECULAR MODELING
Keywords: benzoyl peroxide, diacyl peroxides, infrared spectra, molecular modeling, semiempirical methods, DFT-methods.
The infrared spectra of the benzoyl peroxide symmetrical derivatives (4-R-PhCOO)2 with R: NO2-, CF3-, CF3O-, I-, Br, Cl-, F-, H-, CH3-, CH3O- were studied by the semiempirical methods. There is a linear relationship between the frequencies of the normal vibrations of the experimental and calculated (PM6, PDDG and AM1) spectra for this series of peroxides. The effect of the DFT level on the normal vibrations frequencies of C=O group of benzoyl peroxide was estimated. The best reproduction of these frequencies is observed in the case of BLYP calculation method with 6-311G (d, p) basis set.
Ключевые слова: пероксид бензоила, диацилпероксиды, инфракрасные спектры, молекулярное моделирование,
полуэмпирические методы, DFT-методы.
Инфракрасные спектры симметричных производных пероксида бензоила (4-R-PhCOO)2 с R: NO2-, CF3-, CF3O-,
I-, Br-, Cl-, F-, H-, CH3-, CH3O- были изучены полуэмпирическими методами. Существует линейная зависимость между частотами нормальных колебаний экспериментальных и расчетных (РМ6, PDDG и AM1) спектров для этой серии пероксидов. Было оценено влияние уровня DFT на частоты нормальных колебаний С=O группы пероксида бензоила. Наилучшее воспроизведение этих частот наблюдается в случае метода расчета BLYP с базисным набором 6-311G (d, p).
Introduction
The methods of the molecular spectroscopy rightfully occupy one of the leading places in the study of the peroxide compounds structure. The IR spectroscopy is successfully used to establish the structure of the synthesized peroxides as well as for their identification and kinetic studies [1]. The IR spectra play the role of indicator of the oxidative stability and peroxide value in the foods oxidative modification [1].
An important feature of the IR spectroscopy is the absence of damaging effects of the infrared light quanta on the peroxide molecule. This method allows one to investigate peroxides in any aggregate state. Analytical capabilities of the infrared spectroscopy in the study of processes involving diacyl peroxides are possible due to the presence in peroxides extremely characteristic frequency of the intensity ratio and "doublet" form in the 1750 - 1820 cm"', which corresponds to the absorption of carbonyl group of the diacyl peroxide fragment (-C(O)-O-O-C(O)-).
There are two absorption bands belonging to the v(C=O) in the spectra of symmetric diacyl peroxides that explained by the resonant interaction of carbonyl groups
[2], as well as the in-phase (vs) and antiphase (vas) vibrations of the С=О groups [3]. The bands that belong to the group vibration absorption molecular fragments as a whole with the traditional interpretation of the assignment of bands in the IR spectrum were allocated for aliphatic peroxides - within 1132 - 1180 and 1045 -1076 cm-1, heterocyclic - 1265 ± 10 and 1070 ± 10 см-1 aromatic - 1048 - 1074 см-1. The largest differences are observed in assigning of the absorption bands of the peroxide fragment -O-O-, which most researchers refer to the strip in the area of 850 ± 15 cm-1 [4].
The development of quantum chemistry and the growth of computing power have led to the fact that modern semiempirical, DFT and ab initio methods of the quantum chemistry can significantly improve the speed
and accuracy of calculations of various physical and chemical characteristics of the objects or processes and in many cases allow to achieve precise agreement with the experimental data [5]. This makes it possible to predict the molecular force constants and frequencies of normal vibrations. The second derivative of the total energy of the molecule on the internal coordinates and on the electric field determines the intensity of the IR absorption [6]. Calculation of normal vibrations allows estimating the contributions of various fragments of the molecule in the distribution of potential energy at different frequencies. Comparison of experimental and calculated vibration spectra significantly simplifies the correct assignment of the bands to a certain type of vibrations. Calculated methodology successfully used to determine the characteristic frequencies of the peroxides with unknown individual IR spectra [7, 8].
The aim of this study is the assessment of opportunities of DFT and semiempirical methods for the reproduction and prediction of the diacyl peroxides IR spectra. The paper presents the results of IR spectra molecular modeling of the benzoyl peroxide symmetrical derivatives (4-R-PhCOO)2, R: NO2-, CF3-, CF3O-, I-, Br-, Cl-, F-, H-, CH3-, CH3O- by the semiempirical (AM1, PM6, PDDG) methods. The IR spectra of the benzoyl peroxide were investigated also at the DFT level.
Experimental
Parameters of the molecular geometry,
electronic structure and thermodynamic properties of the benzoyl peroxide (BPO) molecule and its symmetrical derivatives were calculated by the GAUSSIAN09 [9]. The molecular geometry
optimization of all objects was carried out at the first stage of the work. The calculation of harmonic frequencies of vibrations and thermodynamic
parameters were performed after that. The stationary
points obtained after the molecular geometry optimization were identified as minima, as there were no negative values of analytic harmonic vibration frequencies for them. The reaction center of the peroxide compounds is a peroxide bond -O-O-. Therefore, selection criterion for the quantum chemical calculation method was the best reproduction of the peroxide moiety molecular geometry.
To solve the problem of choosing the optimal method for molecular geometry optimization and calculation of infrared spectra of BPO and its symmetrical derivatives the evaluation parameters of the structure of these compounds and the strength of peroxide bond were performed by semiempirical (AM1 [10], PM6 [11], PDDG [12]) and DFT methods (BLYP [13], B1LYP [14], B3LYP [12], B971 [15], B972 [16], BHandHLYP [17], M06HF [18], O3LYP [19], PBE1PBE [20], PBEhlPBE [21], X3LYP [22]). The solvent effect was taken into account in the CPCM approximation [23]. Visualization of the calculated IR spectra was carried out using Chemcraft 1.6 [24].
The experimental IR spectra of the benzoyl peroxide (Fig. 1) and its derivatives in CH2Cl2 used in the study were taken from [4]. The following symmetric diacyl peroxides: (4-NO2-C6H4-COO)2, (4-CF3-C6H4-COO)2, (4-CF3O-C6H4-COO)2, (4-I-C6H4-COOH (4-Br-C6H4-COO)2, (4-Cl-C6H4-COO)2, (C6Ha-COO)2, (4-CH3-C6H4-COO)2, (4-CH3-O-C6H4-COO)2 have
been investigated.
frequencies in the case of PM6 calculation method application.
Table 1 - Parameters of the benzoyl peroxide molecular geometry in CH2Cl2
Parameters Experi ment [4, 25, 26] PM6/ CPCM AM1/ CPCM PDDG/ CPCM
c=o, A 1.190 1.205 1.230 1.216
o-o, A 1.460 1.423 1.292 1.440
c-o, A 1.380 1.408 1.406 1.367
o-o-c, ° 111.0 109.8 114.5 113.0
c-o-o-c, ° 91.0 101.5 91.5 105.9
M, D 1.60 1.53 1.71 1.88
Vas(C=0), cm'1 1767 1800 2065 1993
Vs(C=0 ), cm'1 1790 1808 2076 2004
Liner relationships between the experimental (vexp) and calculated (vcaic) values of the normal vibrations are observed for the BPO IR spectra (Fig.
2). These relationships are described by equations (1) -
(3) for PM6, PDDG and AM1 quantum chemical calculation methods respectively. v^ = (195 ± 26) + (0.79±0.02)-vcak,
n = 19, R = 0.995
% = (119 ± 29) + (0.82± 0.02) • vcalc, n =19, R = 0.995
V, = (195 ± 26) + (0.79 ± 0.02) •vcalc, n = 19, R = 0.995
(2)
(3)
Fig. 1 - Structural model of the benzoyl peroxide (PM6 method) and its experimental IR spectrum in CH2Cl2, [ROOR] = 0.14 mol/dm3, thickness 0.058 mm (according to the Ref. [4])
Results and discussion
The equilibrium configuration of the benzoyl peroxide molecule was obtained by the PM6, AM1 and PDDG semiempirical methods using the CPCM solvation model (Table 1). The equilibrium configuration geometry of the benzoyl peroxide was used in the calculation of IR spectra.
Calculation results listed in Table 1 show the best reproduction of the C=O group normal vibrations
Vcalc, cm'1
Fig. 2 - Relationships between the experimental (vexp) and calculated (vcalc) normal vibrations frequencies of the benzoyl peroxide IR spectrum in CH2Cl2 (CPCM solvation model)
The influence of the structure of the benzoyl peroxide symmetrical derivatives (R-PhC(O)-O-O-C(O)Ph-R) on the position of the carbonyl groups absorption band in the IR spectra has been also investigated. Calculations were carried out in the approximation of AM1 , PM6 and PDDG semiempirical methods. The best correspondence
between the experimental and calculated parameters is observed in the case of PM6 method (Table 2). Introduction of electron-donor substituents into the structure of benzoyl peroxide leads to a shift of the normal vibrations mode of C=O groups towards the field of short waves, while shift of the mode towards the region of long waves is observed in the case of electron-acceptor substituents in peroxide molecule.
Table 2 - The normal vibrations frequencies of the C=O groups (v(C=O), cm-1) of the BPO derivatives R-PhC(O)-O-O-C(O)Ph-R
R- Experiment [4] PM6 AM1 PDDG
vas vs vas vs vas vs vas vs
NO2- 1780 1801 1851 1847 2101 2090 2029 2021
CF3- 1776 1798 1849 1845 2100 2090 2027 2020
CF3O- 1771 1795 1846 1843 2098 2087 2025 2018
I- 1771 1792 1846 1842 2099 2088 2024 2017
Br- 1772 1792 1846 1842 2099 2088 2024 2017
Cl- 1765 1793 1845 1841 2098 2088 2023 2016
F- 1767 1790 1843 1841 2097 2087 2023 2016
H- 1767 1790 1844 1840 2097 2087 2023 2016
CH3- 1762 1785 1841 1838 2097 2086 2022 2015
CH3O- 1759 1781 1837 1834 2096 2086 2018 2012
There is a linear dependence between experimental (vexp) and calculated by the PM6 method (vcaic) values of the normal vibrations frequencies of carbonyl groups of the benzoyl peroxide derivatives (Fig.
3). These linear dependences (Fig. 3) in the linear regression analysis are described by the following equations:
v^ = (-1072 ± 297) + (1.54 ± 0.16) -vcalc,
exp
n = 10, R = 0.959
v„.
= (-1141 ± 187) + (1.59 ± 0.10) -vci
n = 10, R = 0.984
(4)
(5)
Fig. 3 - Structural model of (4-CF3O-C6H4-COO)2 (PM6 method) and dependence of the normal antisymmetric (a) and symmetric (b) vibration frequencies of carbonyl groups obtained experimentally (vexp) and calculated by the PM6 method (vcalc) for the benzoyl peroxide derivatives
Thus, among of the used semiempirical methods only PM6 provides a mathematical model for predicting of the normal vibrations frequencies position of the
carbonyl group in the IR spectrum of the benzoyl peroxide symmetric derivatives.
The positions of the normal vibrations mode of carbonyl groups can be used as an analytical signal of qualitative and quantitative analysis of this class of peroxides by IR-spectroscopy. DFT methods are promising for computer structural chemistry of peroxide compounds because they with sufficient accuracy reproduce parameters of the molecular geometry and electronic structure of peroxides [5]. The opportunity of different DFT methods to reproduce the values of the normal vibrations frequencies of the benzoyl peroxide C=O group was estimated. The calculations were performed using the 6-311G (d, p) basis set in all cases. The results are listed in Table 3.
Table 3 - The effect of the DFT level on the normal vibrations frequencies of the benzoyl peroxide C=O group
Method vas, cm-1 vs, cm-1 Av, cm-1 Avas , cm-1 Avs , cm-1
Experim ent [4] 1767 1790 23 0 0
BLYP 1739 1762 23 28 28
B1LYP 1839 1864 25 72 74
B3LYP 1827 1852 25 60 62
O3LYP 1832 1853 21 65 63
X3LYP 1834 1859 25 67 69
B971 1837 1862 25 70 72
B972 1857 1888 31 90 98
PBEH1PBE 1868 1894 26 101 104
PBE1PBE 1868 1894 26 101 104
BH and HLYP 1926 1953 27 159 163
M06HF 1938 1966 28 171 176
*Av=v-v **AV =vcalc -vexp
as as as
*** Av =vcalc-vexp
s s s
An appropriate correspondence of the calculated and experimental values of * Av = vs -vas is observed for all
DFT methods under consideration. The best reproduction of the normal vibrations frequencies of C=O group is observed in the case of the BLYP method.
Conclusions
IR spectra of the benzoyl peroxide and its symmetrical derivatives (4-R-PhcOo)2, R: NO2-, CF3-, CF3O-, I-, Br-, Cl-, F-, H-, CH3-, CH3O- have been calculated by the semiempirical and DFT methods. There is a linear dependence between the frequencies of normal vibrations experimentally obtained and calculated (PM6, PDDG and AM1) spectra of the benzoyl peroxide. Among the used semiempirical methods, only PM6 provides an appropriate mathematical model for predicting of the position of the carbonyl group normal vibrations frequencies in the IR spectra of the benzoyl peroxide symmetrical derivatives. The values of the normal vibrations frequencies of C=O group of the benzoyl
peroxide were calculated on the different DFT levels. It was found that the best reproduction of the experimental values was observed in the case of BLYP calculation method with the 6-311G (d, p) basis set.
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© N. A. Turovskij - Donetsk National University, [email protected]; E. V. Raksha - Donetsk National University; E. N. Pasternak - Donetsk National University; Yu. V. Berestneva - Donetsk National University; I. A. Opeida - L.M. Litvinenko Institute of Physical Organic and Coal Chemistry National Academy of Sciences of Ukraine; G. E. Zaikov - Institute of Biochemical Physics, Russian Academy of Sciences; [email protected]; S. Yu. Sofina - Kazan National Research Technological University.
