2-巯基苯并咪唑及其类似物互变异构的理论研究

采用B3LYP/6-311G**方法, 计算了2-巯基苯并咪唑及其类似物(2-巯基苯并噁唑、2-巯基苯并噻唑、2-羟基苯并咪唑、2-羟基苯并噁唑、2-羟基苯并噻唑以及2-巯基咪唑、2-巯基噁唑、2-巯基噻唑、2-羟基咪唑、2-羟基噁唑、2-羟基噻唑)的(硫)醇式与(硫)酮式结构进行质子迁移的3种可能途径: (a)分子内质子迁移; (b)水助质子迁移; (c)甲醇助质子迁移．结果表明, 途经b和c所需要的活化能较小, 氢键在降低反应活化能方面起重要作用．采用PCM方法研究了反应体系的溶剂化效应．结果表明孤立分子、一水合物和一甲醇合物的最稳定异构体相同, 都为(硫)酮式, 与气相结论一致．溶剂化效应对异构化能垒的影响较小.

A Theoretical Study on Tautomerism of 2-Mercaptobenzimidazole and Its Analogues

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 in- vestigated 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 intermo- lecular transfer were investigated. The calculated results showed that the processes (b) and (c) had lower ac- tivation 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.