Proton transfer tautomerization mechanisms of 2-thioxanthine

Mechanisms of the intramolecular proton transfer reactions of 2-thioxanthine in the gas phase were investigated with ab initio methods. Geometries of the reactants, transition states, intermediates and products were optimized with B3LYP/6-311+G(d,p). Reaction pathways were confirmed with the intrinsic reaction coordinates. Thermochemistry properties were calculated with the accurate model chemistry method of G3MP2 combined with the standard statistical thermodynamics. Reaction paths were analyzed with respect to the Gibbs free energies. Three paths of the proton transfers in the imidazole ring were discussed in detail, nine necessary energy barriers of the proton transfers in the pyrimidine ring were examined and the tautomerizations between the thiol–enol and keto–thione isomers were investigated. The results show that the highest energy barrier for the imidazole associated paths are 209.9, 306.2 and 224.9 kJ/mol, corresponding to a C8 sp3-type hybrid intermediate, a C8 carbene type intermediate and two intermediates of tetrahedral carbons C(4) and C(5). The necessary energy barriers of the pyrimidine ring associated reactions are within 218.3–295.0 kJ/mol. The reactions in producing thiol–enol and keto–thiol isomers need smaller energy barriers.