Quantum chemical analysis of vibrational spectra of methylphenylcarbamate

Abstract

UDC 539.19 We present results of ab initio and DFT calculations of the structure, potential function of internal rotation of the methyl group, and vibrational frequencies and intensities in IR and Raman spectra of methylphenylcar- bamate. The calculations were carried out in different basis sets in the HF, MP2, and DFT/B3LYP approxi- mations with partial force field scaling. The influence of the phenyl substituent on structural and spectral characteristics of the urethane group has been analyzed. Calculated characteristics of vibrational spectra show satisfactory agreement with experimental values. Herein we present results of quantum-chemical ab initio and DFT calculations of the structure and IR and Raman vibrational spectra of MPC taking into account its internal rotation. We used our previous studies (3) to deter- mine the optimal ratio of approximations and bases for which calculations could adequately reproduce experimental structural and spectral-energy characteristics of methylcarbamate (MC) in a reasonable time. These approximations were used to calculate the corresponding parameters of MPC. Experimental. Structural, energy, and spectral characteristics of MPC were calculated using the US version (4) of the applied quantum-chemical set GAMESS (5) and the PC set GAMESS/Firefly (6). Results were plotted using the MacMolPlt program (7). Calculations were performed with the standard bases 6-31G, 6-311G, 6-311++G(2d, p), and cc-pVDZ (8) with the Hartree-Fock approximation and partial consideration of electron correlation using the methods MP2 (9) and DFT with the B3LYP hybrid functional (10). Discussion. Structure. Based on calculations of the MC structure (3), we selected the approximations B3LYP/6-311++G(2d, p) and B3LYP/cc-pVDZ to calculate the geometric parameters of MPC because ab initio calcu- lations using these approximations reproduce with high accuracy the experimental structural parameters of MC (3). The geometry from the B3LYP/6-311++G(2d, p) approximation was optimized assuming that the equilibrium structure of MPC has Cs symmetry. The geometry from the B3LYP/cc-pVDZ approximation was optimized for two molecular symmetries, Cs and C1. The calculated vibrational spectrum for the geometry optimized for Cs symmetry showed an imaginary frequency (for methyl torsion). Therefore, the final geometry was optimized for the structure with C1 sym- metry. Symmetry lowering produced insignificant (in the range 1-3 o ) deviations of the urethane atoms from the plane of the phenyl ring and also rotation of the methyl by ≈5 o relative to the symmetric position.