Vibrational Studies and DFT Calculations of Cytosine, Thiocytosine and Their Cations and Anions

Abstract

DFT calculations were carried out to compute the optimized molecular geometries, APT charges and fundamental vibrational frequencies along with their corresponding IR intensities, Raman activities and depolarization ratios of the Raman bands for the neutral Cyt and TCyt molecules and their cations and anions using the DFT/B3LYP method with the 6-311++G** basis set using the Gaussian-03 software. TCyt, Cyt+ and TCyt+ show planar structures and belong to Cs point group symmetry while Cyt, Cyt- and TCyt- possess non-planar structures with C1 point group symmetry. Conformational analysis was carried out to obtain the most stable configurations of these molecules. The normal modes of vibration for all the species have been assigned on the basis of PEDs obtained from normal coordinate analysis using the GAR2PED software. Information about the size, shape, charge density distribution and site of chemical reactivity of the molecule has been obtained by mapping the electron density isosurface with molecular electrostatic potential (MEP) surfaces. The energy gap from HOMO to LUMO of the Cyt is 5.2963 eV and that of TCyt is 5.0062 eV.