MINDO/3 and MNDO studies of some thio analogues of nucleic acid bases

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, C=C bond loses its double bond character and magnitude of the C=C stretching frequency decreases significantly as a result of radicalization. Frequency for the in-plane deformation mode of C=O increases when a sulfur atom is substituted for the oxygen atom at the site C(2) in the uracil molecule but decreases when a sulfur atom is substituted for the oxygen atom at the site C(4). 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 N(1), C(5), O(8) and O(10) in case of the U molecule.

Vibrational frequencies and structure of 2-thiouracil by Hartree-Fock, post-Hartree-Fock and density functional methods

Vibrational study of the biomolecule 2-thiouracil was carried out. Ab initio and density functional calculations were performed to assign the experimental spectra. A comparison with the uracil molecule was made, and specific scale factors were deduced and employed in the predicted frequencies of 2-thiouracil. Several scaling procedures were used. The geometry structure of the molecule was determined. The effect of sulfur substitution at C2 position in the uracil molecule, on the N1-H and N3-H frequencies and intensities reflects changes in proton donor abilities of these groups. Calculations with the 6-31 G** basis set with HF and DFT methods appear in general to be useful for interpretation of the general features of the IR and Raman spectra of the molecule. Using specific scale factors a very small error was obtained. The use of these specific scale factors resolve and correct some of the controversial assignments in the literature.