巴比妥酸互变异构的理论计算

采用B3LYP/6-311G**方法对处于优势构象时的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,巴比妥酸在气相中和水相中主要以酮式结构存在.采用相同方法计算并考察了烯醇式与酮式结构进行结构互变质子迁移的2种可能途径:(a)分子内质子迁移;(b)水助质子迁移.结果还表明,途经(b)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.

Theoretical studies on tautomerism of barbituric acid

The preference conformations of barbituric acid tautomers were calculated by the B3LYP/6-311G method, both in the gas and aqueous phases, with full geometry optimization. The CPCM solution theory model was employed for aqueous solution calculations. The total energy, zero-point energy and energy of solvation were obtained. The calculations show that barbituric acids existing as the keto forms are the predominant isomers in the gas and aqueous phases. The two possible reaction pathways: (a) intramolecular proton transfer and (b) water-assisted intermolecular transfer were investigated. The calculated results showed that the process (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.