Ab initio and DFT studies on tautomerism of 5-methyl cytosine in gaseous phase

Ab initio and DFT methods have been used to study the seven tautomeric forms of 5-methylcytosine molecule.The related tautomer in gas phase have been studied at HF/6-31G, HF/6-31G* and B3LYP/6-31G* levels oftheory. The structures,enthalpies,entropies,Gibbs free energies,relative tautomerization energies of tautomersand tautomeric equilibrium constants were compared and analyzed along with full geometry optimization. Thecalculations showed that the Oxo-amino(6), Oxo-imino(7) and Hydroxy-amino(4) tautomers are the most stablein the gas phase. The results are in a good agreement with the available experimental data. The entropy effecton the Gibbs free energy of the 5-methylcytosine bases is very small and it has a little significance on thetautomeric equilibria of pyrimidine bases.Therefore the enthalpic term is dominant in the determination of theequilibrium constants. 13C-NMR studies have been carried out for these tautomers and the results arediscussed.We have also evaluated the hybridation coefficient for bonds and hetero atom LPS in the aromaticring for the stable tautomers. Natural Bond Orbital Theory (NBO) calculation showed that the stable tautomersmust be considered aromatic.