%A Li Bao-Zong
%T Density Functional Theory Calculations of 6-thioxanthine Tautomers
%0 Journal Article
%D 2004
%J Acta Phys. -Chim. Sin.
%R 10.3866/PKU.WHXB20040512
%P 503-506
%V 20
%N 05
%U {http://www.whxb.pku.edu.cn/CN/abstract/article_25501.shtml}
%8 2004-05-15
%X Molecular structures of fourteen 6-thioxanthine tautomers were calculated by the B3LYP /6-311G** method, both in gas and aqueous phases, with full geometry optimization. The Onsager solvate theory model was employed for aqueous solution calculations. The structure, total energy, standard enthalpy, standard entropy and standard Gibbs free energy were obtained. The calculations show that 6-thioxanthine exists predominantly as the thione forms in the gas and aqueous phases. The thione-N7(H) isomer is more stable than the thione-N9(H) isomer in the gas and aqueous phase. The results are in good agreement with available experimental results. The entropy effect on the Gibbs free energies of 6-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 Gibbs free energies of solvation are well correlated with the dipole moments of 6-thioxanthine tautomers.