The tautomeric equilibrium of benzimidazolone and benzimidazolthione have been studied by the density functional theory method using the CAM-B3LYP functional together with the 6-311G(d,p) basis set. Two separate mechanisms have been investigated: a direct intramolecular transfer using the polarizable continuum model and an indirect proton transfer assisted by a molecule of solvent (C6H12, H2O, CH3OH, and H2O2). In both cases, the results obtained indicate that ketone and thione are the most stable forms. However, the enhanced height of the activation barrier for the four-center mechanisms describing the tautomerism reaction as a direct intramolecular transfer implicates a relatively disadvantaged process. The participation of a polar protic solvent molecule allows the lowering of the activation energy barrier. Potential energy profiles of keto-enol and thio-enol tautomerism assisted by methanol and water are very different. The former one describes a concerted mechanism but the latter does not because it is associated with asynchronous processes that take place during the thio-enol tautomerism.

中文翻译：

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极性质子溶剂催化苯并咪唑酮和苯并咪唑硫酮互变异构过程中的质子转移

使用 CAM-B3LYP 泛函和 6-311G(d,p) 基组，通过密度泛函理论方法研究了苯并咪唑酮和苯并咪唑硫酮的互变异构平衡。已经研究了两种不同的机制：使用可极化连续介质模型的直接分子内转移和由溶剂分子（C6H12、H2O、CH3OH 和 H2O2）辅助的间接质子转移。在这两种情况下，获得的结果表明酮和硫酮是最稳定的形式。然而，将互变异构反应描述为直接分子内转移的四中心机制的激活势垒的高度增加意味着一个相对不利的过程。极性质子溶剂分子的参与允许降低活化能垒。由甲醇和水辅助的酮-烯醇和硫-烯醇互变异构的势能分布非常不同。前者描述了一种协调机制，而后者则不然，因为它与硫烯醇互变异构期间发生的异步过程有关。