基态丙酮酸单分子反应的机理

Mechanisms of Unimolecular Reactions for Ground-state Pyruvic Acid

运用MP2和B3LYP方法,对基态丙酮酸的8个可能的单分子反应的反应机理进行了详细的理论计算.研究发现,氢键相互作用及两个羰基的相对空间取向在丙酮酸异构体的稳定性中起着重要的作用.并且由最稳定的异构体Tc旋转为其它异构体的能垒都比较高,旋转异构反应在室温下是难于进行的.经由质子迁移过程可实现Ct和Tt异构体之间的转化,但反应能垒比内旋转过程高得多,说明质子迁移无法与内旋转过程相竞争.对基态丙酮酸脱羧和脱羰反应途径的研究均表明,脱羧反应是经由五中心过渡态首先生成甲基羟基卡宾中间体,然后中间体再进一步异构得到产物.四中心协同反应机理是不利的.

Eight possible unimolecular reacti on pathways for ground-state pyruvi c acid(PA)have been in-vestigated in this work at the MP2/cc-pVDZ and B3LYP /cc-pVDZ levels of th eory.The results indicate that hydrogen bond interaction and relative orientation of the two carbonyl g roups play an important role in the sta-bility of pyruvic acid rotamers.Mor eover,the barriers for Tc isomeriza tion to other isomers through intern al rotation are high,suggesting that t he isomerization reactions occur di fficultly at room temperature.The b arrier for transformation of Ct and Tt isomers t hrough proton migration is higher th an that through internal rotation,showing that the former can not compete with the later.Under the investig ation on decarboxylation and decar-bonylation reactions,it can be concluded that the decarboxylation reaction takes place through stepwise me cha-nism involved in a five-center transition state and produces methyl hydr oxycarbene firstly,and the concerted mechanism associated with a four-ce nter transition state is disadvanta geous.