DFT study of molecular geometries and vibrational characteristics of uracil and its thio-derivatives and their radical cations

DFT calculations at the B3LYP/6-311++G** level have been carried out to study the vibrational characteristics of the neutral molecules, anionic and cationic radicals of uracil, 2-thiouracil and 4-thiouracil. In the U molecule, CC bond loses its double bond character and magnitude of the CC stretching frequency decreases significantly as a result of radicalization. Frequency for the in-plane deformation mode of CO increases when a sulfur atom is substituted for the oxygen atom at the site C2 in the uracil molecule but decreases when a sulfur atom is substituted for the oxygen atom at the site C4. The magnitude of both the N–H stretching frequencies decreases in all the radical cations as compared to their neutral molecules. Radicalization leads to significant changes in the magnitudes and intensities corresponding to some of the normal modes for all the three cases. Removal of an electron leads to decrease in the electronic charge mainly from the sulfur atom in the case of 2-TU and 4-TU, whereas it is distributed over the sites N1, C5, O8 and O10 in case of the U molecule.