Relative stability of cytosine tautomers with the coupled cluster method and first-order correlation orbitals

Three tautomeric forms of cytosine molecule have been studied by the coupled cluster method with single, double, and triple excitations using first-order correlation Orbitals and Gaussian DZP basic sets. The zero-point nuclear energy has been estimated by the SCF 3-21G method with analytical first and second derivatives of the total energy. The hydroxy-amino tautomer has been predicted to predominate in the gas phase in agreement with recent experimental data. The oxo-amino and oxo-imino forms are less stable than the hydroxy-amino tautomer by 4.2 and 6.4 kJ mol^-1, respectively. The pertinence of the present results to the interpretation of the experimental IR spectra is discussed.