2-硫代巴比妥酸互变异构的理论计算

Theoretical Studies in Tautomerism of 2-Thiobarbituric Acid

采用B3LYP/6-311G**方法对处于优势构象时的2-硫代巴比妥酸各互变异构体进行了几何全优化和频率计算,CPCM反应场溶剂模型用于水相的计算,得到它们的总能量、零点振动能和溶剂化能.计算结果表明,2-硫代巴比妥酸在气相中和水相中主要以(硫)酮式结构存在.采用相同方法计算并考察了(硫)醇式与(硫)酮式结构进行结构互变质子迁移有分子内质子迁移和水助质子迁移2种可能途径.计算结果表明,水助质子迁移所需要的活化能较小,氢键在降低反应活化能方面起着重要作用.

The preference conformations of 2-thiobarbiturie acid tautomers are calculated by the B3LYP/6-311G^＊＊ method, both in the gas and aqueous phases, with full geometry optimization. The CPCM solution theory model is employed for aqueous solution calculations. The total energy, zeropoint energy and energy of solvation are obtained. The calculations show that 2-thiobarbiturie acid ex- isting as the keto forms are the predominant isomers in the gas and aqueous phases. The two possible reaction pathways. （a） intramoleeular proton transfer; （b） water-assisted intermoleeular transfer are investigated. The calculated results show that the processes （b） has lower activation energies. It is likely that the hydrogen bonds formed in the complexes play an important role in proton transfer process in the later reaction pathway.