FT-IR and Raman spectra, ab initio and density functional computations of the vibrational spectra, molecular geometries and atomic charges of uracil and 5-halogenated uracils (5-X-uracils; X = F, Cl, Br, I)

详细信息    查看全文

• 作者：J.S. Singh
• 关键词：Infrared spectrum ; Raman spectrum ; Ab initio ; Density functional theory ; Geometry optimization ; Atomic polar tensor ; Vibrational assignment
• 刊名：Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
• 出版年：3 January, 2014
• 年：2014
• 卷：117
• 期：Complete
• 页码：502-518
• 全文大小：1656 K

文摘

Raman (200-4000 cm^鈭?) and FT-IR (400-4000 cm^鈭?) spectra of uracil and 5-halogenated uracils (5-X-uracils; X = F, Cl, Br, I) have been recorded and analyzed in the range 200-4000 cm^鈭?. 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. Ab initio and DFT calculations [using Becke鈥檚 exchange in conjunction with Lee-Yang-Parr鈥檚 correlation functional and Becke鈥檚 three-parameter hybrid method (B3LYP)] were carried out to study the optimized molecular fundamental vibrational frequencies for uracil and 5-halogenated uracils by employing Gaussian-03 program. Gauss View software was used to make the vibrational analysis. Raman and IR spectra have been computed theoretically for the uracil and 5-halogenated molecules. 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. Quantum chemical calculations helped in the reassignments of some fundamental vibrational modes. Most of the B3LYP/6-311++G^鈭椻垪 vibrational frequencies are in excellent agreement with available experimental assignments. The ring breathing and kekule stretching modes are found to lower magnitudes compared to those for uracil which could be due to mass effect of halogen atom in place of the hydrogen atom. The C-X (X = F, Cl, Br, I) stretching frequency is distinctly separated from the CH/NH ring stretching frequencies on the pyrimidine ring. All other bands have also been assigned different fundamentals/overtones/combinations.