FT-IR and Raman spectra, ab initio and density functional computations of the vibrational spectra, molecular geometries and atomic charges of uracil and 5-methyluracil (thymine)

摘要

FT-IR (400-4000 cm(-1)) and Raman spectra (200-4000 cm(-1)) of uracil and 5-methyluracil (thymine) have been recorded and analyzed. The optimized molecular geometries, atomic polar tensor (APT) charges and vibrational characteristics have been studied theoretically using restricted Hartree-Fock (RHF) and density functional theory (DFT) methods. Using the Becke's exchange in conjunction with Lee-Yang-Parr's correlation functional and Becke's three-parameter hybrid method (B3LYP), the ab initio and DFT calculations were carried out to study the optimized molecular fundamental vibrational frequencies for uracil and 5-methyluracil (thymine) by employing Gaussian-03 program. The fundamental vibrational frequencies along with their corresponding intensities in IR and Raman activities and depolarization ratios of the Raman lines have also been calculated using the RHF and DFT methods employing different basis sets. In quantum chemical calculations, most of the B3LYP/6-311++G** vibrational frequencies are in excellent agreement with the available experimental assignments and helped to propose in the reassignments of some missing frequencies in experimental study. Assuming under the C-s point group for both molecules, the distribution of normal mode of vibrations between the two species as planar (a ') and non-planar (a '') for all 39 normal vibrational modes of 5-methyluracil are given by 26a ' + 13a '', of which 30 modes (21a ' + 9a '') correspond to the uracil moiety and 9 modes (5a ' + 4a '') to the CH3 group. Consistent assignments have been made for the internal modes of CH3 group, especially for the anti-symmetric CH3 stretching and bending modes. A possible explanation could be the planarity of pyrimidine ring and nonplanarity at carbon site of methyl group which might cause the splitting of frequencies including three components due to the substitution of CH3 group at the site of C-5 atom on pyrimidine ring of uracil. The three non-equivalent CH bonds of CH3 group a