Document Type : Research Paper
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 of
theory. The structures,enthalpies,entropies,Gibbs free energies,relative tautomerization energies of tautomers
and tautomeric equilibrium constants were compared and analyzed along with full geometry optimization. The
calculations showed that the Oxo-amino(6), Oxo-imino(7) and Hydroxy-amino(4) tautomers are the most stable
in the gas phase. The results are in a good agreement with the available experimental data. The entropy effect
on the Gibbs free energy of the 5-methylcytosine bases is very small and it has a little significance on the
tautomeric equilibria of pyrimidine bases.Therefore the enthalpic term is dominant in the determination of the
equilibrium constants. 13C-NMR studies have been carried out for these tautomers and the results are
discussed.We have also evaluated the hybridation coefficient for bonds and hetero atom LPS in the aromatic
ring for the stable tautomers. Natural Bond Orbital Theory (NBO) calculation showed that the stable tautomers
must be considered aromatic.