半经验MNDO和MNDOC方法对化学反应过渡态研究的应用

本文对15个简单有机分子反应进行MNDO和MNDOC计算,得到与从头算相似的势能面特征和过渡态数目。半经验过渡态的优化结构与从头算过渡态的优化结构吻合较好,而半经验活化能与从头算活化能偏离较大。其中,MNDOC结果稍好于MNDO结果。计算表明用半经验方法作为反应势能面的快速扫描工具以确定其定性特征,对从头算难以实现的大分子体系尤为适用。对于中等大小分子的过渡态研究,用半经验过渡态作为从头算自动优化的初始点,则能大大提高从头算定位鞍点的成功率。

Application of Semiempirical Methods MNDO and MNDOC to the Studies of Transition States in Chemical Reactions

MNDO and MNDOC calculations are performed for 15 reactions of simple organic molecules, resulting in the features of potential surfaces and the number of transition states similar to ab initio calculations. A better agreement is found in the geometry between semiempirical and ab initio transition states, and a greater deviation in the activation energy, in which. MNDOC results are slightly better than MNDO's. It follows that the semiempirical methods, as a tool for fast initial scans of potential energy surfaces to assess their qualitative features, seem to be particularly suitable for the large molecules where ab initio calculations can not be easily effectuated, and for the transition state researches of medium-size molecules, by taking the semiempirical transition states as the starting points of automatical ab initio optimizations, it will increase greatly the successful chances for locating saddle points with ab initio method.