QUANTUM-CHEMICAL CALCULATIONS OF THE STRUCTURE, VIBRATIONAL SPECTRA, AND TORSIONAL AND INVERSION POTENTIALS OF METHYLCARBAMATE M. B. Shundalov, a* G. A. Pitsevich, a M. A. Ksenofontov, b and D. S. Umreiko b UDC 539.19;535.34 We present results of ab initio and DFT calculations of the structure, potential functions of the methyl group internal rotation and the amino group inversion, and vibrational frequencies and intensities in IR and Raman spectra of methylcarbamate. The calculations were carried out using different basis sets in the HF, MP2, and DFT/B3LYP approximations. The influence of both the basis set size and the allowance for electronic corre- lation on peculiarities of the structure of the amino group in methylcarbamate has been analyzed. It is shown that the B3LYP/6-311++G(2d, p) and B3LYP/cc-pVDZ calculations reproduce highly accurately experimental geometric parameters of methylcarbamate. Parameters of torsional and inversion potentials and characteristics of vibrational spectra calculated in different approximations show satisfactory agreement with experimental values. Keywords: ab initio and DFT calculations, torsion potential, inversion potential, IR and Raman spectra, methylcar- bamate. Introduction. Polyurethanes are polymers that contain the urethane group [–OC(O)NH–] in the polymer chain. They are widely used in various branches of industry [1]. They are an essential element of polyurethane foams and foam plastics with high operating specifications [2]. In addition to the urethane with its amide [–C(O)NH–] and ester [–OC(O)–] groups, the polymer also contains hydrocarbon [–CH 3 , –CH 2 –], ether [–O–], aromatic [–C 6 H 4 –], and other groups. On one hand the variety of functional groups in the polyurethane chain gives rise to complicated vibrational spectra for these compounds; on the other, it generates intra- and intermolecular bonds of various nature that have a direct influence on the shape and position of spectral bands and lines. One of the urethanes that is often used to produce polyurethane foams is 4,4′-diphenylmethanebis(methyl)car- bamate (DPMC), which contains two urethanes, phenyls, and methyls. The structure, energy characteristics of various conformers, and vibrational spectra of DPMC have been studied only partially [3]. Quantum-chemical calculations of the structure and vibrational spectra of DPMC in the ab initio and DFT high-level approximations using expanded basis sets have not been performed. The DPMC molecule contains 41 atoms. Direct calculation of the geometric struc- ture of such an object without high symmetry could turn out to be an exceedingly expensive ordeal. It should be noted that the structure of the DPMC molecule consists of two methylphenylcarbamate molecules bonded through a methylene. Theoretical calculations of the structural parameters and spectral characteristics of simpler urethanes such as methylphenylcarbamate or methylcarbamate enable not only determination of the most suitable approximation and minimization of costs for labor and computer time to find the equilibrium configuration of the complicated polyure- thane but also solution of the more important problem of revealing trends in vibrational spectra of the urethane group as a function of its environment (from the simplest urethane methylcarbamate to the complicated polymer chain). We note that methylcarbamate is one of the simplest molecular systems in which two types of internal motion with large a Belarusian State University, 4 Nezavisimosti Ave., Minsk, 220030, Belarus; e-mail: shundalov@bsu.by, pitsevich@bsu.by; b A. N. Sevchenko Institute of Applied Physical Problems, Belarusian, State University, Minsk; e-mail: lab_dozator@mail.ru. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 76, No. 3, pp. 349–357, May– June, 2009. Original article submitted January 16, 2009. Journal of Applied Spectroscopy, Vol. 76, No. 3, 2009 0021-9037/09/7603-0325 ©2009 Springer Science+Business Media, Inc. 325 ∗ To whom correspondence should be addressed.