UDC 539.19We present results of ab initio and DFT calculations of the structure and IR vibrational spectra of the monomer and dimers of N,N-dimethylformamide (DMF). The calculations were carried out in the B3LYP/cc-pVDZ approximation with subsequent force-field scaling. The calculated characteristics of the vibrational spectra of DMF show satisfactory agreement with experimental values, allowing them to be used in spectral and structural analysis.Introduction. Dimethylformamide (DMF) is one of the simplest amides and has several interesting properties. DMF does not contain an NH group, which is responsible for the nature of the intermolecular structure formed by this compound in the condensed state. The methyl and aldehyde groups act as proton donors in forming dimeric and polymeric structures of DMF. Therefore, in contrast with the closest analogs of DMF, formamide (FA) and methylformamide (MFA), the molecular clusters of DMF that are formed in the liquid phase have comparatively weak binding energies and vibrational spectra that are unperturbed by the effect of strong H-bonds. Thus, DMF is a convenient subject for studying the structural and spectral characteristics of molecular systems that are important from biological and technical viewpoints. Furthermore, liquid DMF is an important aprotic dipolar solvent. DMF often fulfills the role of an organic ligand in complexes with ions and compounds of various metals owing to the presence in it of the carbonyl.The molecular structure and vibrational spectra of DMF were studied several times both experimentally [1-9] and theoretically [7,8,[10][11][12]. However, the question of whether the molecular skeleton is planar or non-planar has not yet been answered. Also, the low symmetry of the molecule is responsible for significant interactions between vibrational modes. This causes definite difficulties in assigning vibrational bands and lines. Therefore, vibrational spectra of monomeric and dimeric forms of DMF have also not been conclusively interpreted.Herein results of quantum-chemical calculations of the structure and energy and spectral characteristics of DMF monomer and dimers are presented. Experimental IR absorption spectra are interpreted. Vibrational spectra of DMF are modeled in the gas and liquid phases.Experimental. IR absorption spectra of DMF were recorded in the range 400-4000 cm -1 on a Bruker Vertex 70 spectrophotometer. The first sample was prepared as a solution of DMF in CCl 4 (0.05 M). The second sample was pure DMF. IR spectra of the samples were recoded in a KBr cuvette (0.05 mm).Calculations. Structural and spectral characteristics of DMF monomer and dimers were calculated using the applied quantum-chemical program GAMESS [13,14]. The results were visualized using the MacMolPlt program [15]. The standard basis sets cc-pVDZ and cc-pVTZ [16], DFT methods, and the hybrid functional B3LYP [17][18][19] were utilized to optimize the equilibrium structure and calculate the force field and harmonic vibrational eigenfrequencies and intensities in IR spec...

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We present theoretical models and results of calculations of the energies of torsional states of dihydroxybenzenes: flexible molecules with two non-coaxial internal tops of low symmetry.Introduction. Dihydroxybenzenes (dihydric phenols), containing a six-membered ring in their molecules, are aromatic compounds widely distributed in nature and used for practical purposes. Thus dihydroxybenzenes are included as a basic structural unit in gas-filled polymers (foam plastics) which, owing to their unique combination of low density and high strength with exceptionally good soundproofing and thermal insulation properties, have been widely used in various areas of human activity [1].The properties of dihydroxybenzenes vary depending on the relative positions of the OH groups (1,2-, 1,3-, or 1,4-substitution), which also affects the properties of the final product (the polymer). The relative positions of the OH groups also has a substantial effect on the torsional spectra of the dihydroxybenzenes resulting from internal rotations of the hydroxyl groups relative to the core of the molecule (the benzene ring).Spectral structural analysis of dihydroxybenzenes and their derivatives has mainly been carried out with respect to the fundamental vibrational frequencies [2]. Until recently, there was virtually no proper attention paid to the problem of calculating and interpreting the high-resolution torsional spectra, lying in the far IR and microwave regions, for flexible molecules with two non-coaxial internal tops. The major approaches making it possible to calculate the energy states for multi-dimensional torsional motion do not differ in principle from the one-dimensional case [3,4]. However, their application at the moment is limited to compounds containing highly symmetric internal tops (with a three-fold symmetry axis) [5], because the mathematical apparatus needed for the calculation becomes considerably more complicated when going to a two-dimensional model.In this paper, we present the results of a theoretical calculation of the energies of the torsional states of 1,2-, 1,3-, and 1,4-dihydroxybenzene molecules, taking into account interaction between the hydroxyl groups.Procedure. The rotational-torsional Hamiltonian (in cm -1 ) of a flexible molecule with two internal tops has the form [4]:

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We have obtained an analytical expression for the two-dimensional potential energy function for internal rotation in 1,2-dihydroxybenzenes, allowing us to use perturbation theory methods to calculate and interpret the torsional spectra of these compounds.