The processes of the proton transfer between the thione (keto) tautomer and the thiol (enol) tautomer of 2-mercaptobenzimidazole and its analogues (2-mercaptobenzoxazole, 2-mercaptobenzothiazole, 2-hydroxybenzimidazole, 2-hydroxybenzoxazole, 2-hydroxybenzothiazole, 2-mercaptoimidazole, 2-mer- captooxazole, 2-mercaptothiazole, 2-hydroxyimidazole, 2-hydroxyoxazole and 2-hydroxythiazole) were investigated by density functional theory at B3LYP/6-311G** level. The three possible reaction pathways: (a) intramolecular proton transfer; (b) water-assisted intermolecular transfer and (c) methanol-assisted intermolecular transfer were investigated. The calculated results showed that the processes (b) and (c) had lower activation energies because of the formation of hydrogen-bonded complexes in (b) and (c). It is likely that the hydrogen bonds formed in the complexes play an important role in the proton transfer processes in the later two reaction pathways. The mechanisms of the tautomerization in water solution and methanol solution have been investigated at the same level by SCRF calculations using the PCM model. Calculated results indicate that the thione (keto) tautomer is always dominant both in the gas phase and aqueous or methanol solution. The solvent effects on the tautomerization reactions of 2-mercaptobenzimidazole and its analogues observed by the PCM model did not significantly alter the barrier height.

Key words: 2-mercaptobenzimidazole, 2-mercaptoimidazole, tautomerization, density functional theory method, self-consistent reaction field