Which tautomeric form of 5-methylcytosine predominates in vacuum?

An application of high-level ab initio quantum chemistry methods to study the tautomeric equilibria of 5-methylcytosine in the gas-phase is described. Calculations for the hydroxy-amino, oxo-amino and oxo-imino forms have been performed at the second-order level of the many body perturbation theory (MBPT(2)) with a double-zeta basis set augmented with polarization functions. The optimized geometry and vibrational modes were obtained with the SCF/3-21G method. The calculations clearly show that the most stable tautomeric form in vacuum is hydroxy-amino. The remaining two forms, i.e. oxo-amino and oxo-imino, are each estimated to be less stable than the main form by about 15 kJ mol^-1.