Ab initio study of intramolecular proton transfer reactions in cytosine

The activation energies of intramolecular proton transfer reactions of cytosine in the gas phase were computed through ab initio calculations. MP2/6-31G** and MP2/D95** energy calculations were performed over geometries obtained at the HF/3-21G, HF/6-31G(N8*) and HF/D95(N8*) level (d-type polarisation functions were added on the N8 atom). Corrections for the zero-point vibrational energy were included. It is suggested that not only thermodynamic but also kinetic factors may be responsible for the distribution of the tautomeric species in the gas phase. Results are discussed with reference to spectroscopic data in the literature.