Abstract: Molecular structures of fourteen 2-thioxanthine tautomers were calculated by the B3LYP /6-311G** method, both in the gas and aqueous phases, with full geometry optimization. The Onsager solvate theory model was employed for aqueous solution calculations. The structures, total energy, standard enthalpy, standard entropy and standard Gibbs free energy were obtained. The calculations show that 2-thioxanthine existing as the thione(keto) forms is the predominant tautomer in the gas and aqueous phases. The thione(keto)-N(7)(H) tautomer is more stable than the thione(keto)-N(9)(H) tautomer in the gas and aqueous phases. The results are in good agreement with available experimental results. The entropy effect on the Gibbs free energy of 2-thioxanthine base is very small and there is little significance for the tautomeric equilibria of the base.The enthalpic term is dominant in the determination of tautomeric equilibria. The free energy of solvation is well correlated with the dipole moments of 2-thioxanthine tautomer. Additionally, the relative stabilities of 2-thioxanthine tautomers and 6-thioxan thine ones were examined.

Key words: 2-thioxanthine, 　Tautomer, 　Density functional theory, 　Self-consistent reaction field method, 　Dipole moment