Vibrational spectra of benzophenone conformers

Abstract

We have used direct quantum DFT methods to calculate the parameters of the adiabatic potential of four benzophenone conformers. According to the data obtained, the existence of only one conformer is possible (C 1 symmetry) for the free benzophenone molecule, for which torsional vibrations of the benzene moieties are reproduced and for which we calculated the vibrational spectrum in the anharmonic approximation of the theory of molecular vibrations.Key words: assignment of benzophenone vibrations, anharmonic resonance effects.Introduction. Benzophenone (diphenyl ketone: two benzene rings replace the methyl groups in the acetone molecule CO(CH 3 ) 2 ) is an aromatic ketone. It has a number of physical properties making it attractive for practical purposes. This especially involves a substantial piezoelectric effect, the ability to efficiently transfer excitation energy. So establishing a connection between the structure and spectra of benzophenone conformers in different phase states has not lost its timeliness. It is sufficient to refer to the review in [1], in which the spatial structure and electronic structure were calculated for the planar and twist conformers of benzophenone in the harmonic approximation of the theory of molecular vibrations [2]. To do this, the most promising of the many ab initio quantum methods was used: DFT/B3LIP/6-31G * [3]. However, benzophenone also allows for other spatial conformations, and the presence of a large number of hydrogen atoms requires analysis of the vibrational states of the molecule in the anharmonic approximation [4]. The same situation occurs for methyl-substituted and amino-substituted benzophenone. For example, in [5,6] agreement between calculated and experimental data was achieved with respect to the frequencies of the fundamental vibrations, using different schemes for the scaling procedure [7]. Thus the following question remains: does the procedure involve scaling by compensating for deficiencies of the density functional method DFT/B3LIP/6-31G * in calculating the fundamental vibrational states, or is this a method for taking into account the anharmonicity of the vibrations? For this purpose, we need to carry out calculations of the adiabatic potential for the studied compounds in the anharmonic approximation. In [1][2][3][4][5][6][7], all the calculations were performed in the harmonic approximation.In this paper, we present the results of model calculations of the adiabatic potential and the vibrational spectra for four conformers of benzophenone. The first two (C 2v symmetry) have already been mentioned. The remaining two were obtained from a planar model by rotation of one ring by the angles π/2 (C s symmetry) and π/4 (C 1 symmetry) about the C 1 -C 3 bond (the atoms in the molecule are numbered as follows: ring 1, C 3 C 4 C 5 C 6 C 7 C 8 ; ring 2, C 14 C 15 C 16 C 17 C 18 C 19; carbonyl group C 1 O 2 ; bridge C 1 C 3 -C 1 C 14 ).Discussion of Results. Detailed data on the geometric parameters for all the considered conformers of benzophenone are given in...