Molecular Structure and Vibrational IR Spectra of Cytosine and Its Thio and Seleno Analogues by Density Functional Theory and Conventional ab Initio Calculations

Full geometry optimizations (without any constraints on the planarity of the systems) were carried out for six tautomers of cytosine and their thio and seleno analogues by means of the density functional theory with the combined Becke3-LYP exchange-correlation energy functional (DFT(B3-LYP)) and conventional ab initio Hartree−Fock (HF) method. The valence double-ζ basis set augmented by polarization functions on all atoms was used in both approaches. At these geometries, the vibrational IR spectra of the tautomers were calculated at the DFT(B3-LYP) and HF levels with the same basis set. Additionally at the HF geometries of the tautomers, their dipole moments were computed at the MP2(fc) level and electronic energies at the MP2(fc), MP3(fc), and MP4(fc)(SDQ) levels to estimate relative tautomeric energies of the isolated tautomeric forms. The calculation results were compared with the corresponding experimental data when available. Calculations show that (a) the DFT(B3-LYP) method predicts better rotational constants than the HF calculations, (b) the DFT(B3-LYP)-calculated vibrational IR spectra of cytosine and thiocytosine agree well with the available recorded IR spectra, and they show marked improvement over the IR spectra predicted at the HF level, and (c) the relative internal energies at 0 K of the tautomeric species seem to be better predicted by conventional ab initio calculations than by the DFT(B3-LYP) method.