化学反应的内禀反应坐标法——Ⅲ.乙炔与氢化锂二聚体加成反应的反应路径解析

The IRC Method in Chemical Reactions Ⅲ. Reaction Ergodography for the Addition of LiH Dimer to Acetylene

本文利用量子化学从头计算法(内禀反应坐标法)在3-21G基组上对乙炔与氢化锂二聚体的加成反应作了反应路径解析。确定了过渡态的结构、反应势能曲线、活化能、反应热以及沿着IRC的一些物理量的变化。研究结果表明:在乙炔与氢化锂二聚体的加成反应途径上,靠近孤立反应物存在一个准稳定的分子复合物状态,并且这个分子复合物经单分子重排生成产物的基元过程构成了该反应的定速步骤。根据RRKM理论还估算出相应于单分子重排过程的频率因子和活化熵值。对反应过渡态的前线分子轨道分析表明:在过渡态的形成过程中,反应物系分子之间的HOMO-LUMO和HOMO-HOMO相互作用都起了重要作用。通过比较乙炔与氢化锂单体和二聚体两个加成反应的活化能、活化熵和反应频率因子,指出了这两个反应发生相互竞争的可能性。

The ab initio calculation has been performed on the addition of LiH dimer to acetylene at RHF/3-21G basis set. The geometries and energies of the isolated reactant, molecular complex, transition state and product have been determined on the singlet potential energy surface of the ground state. Our results indicate that there is a metastable molecular complex near the isolated reactant system in the reaction pathway. The process from isolated reactant to molecular complex is a non-bonding-exchanging reaction process, and the process from the molecular complex to product is the ratecontrolling step of the reaction. We also estimate the activated entropy and the reaction frequency factor by using the RRKM theory. The FMO analysis for the transition state reveals the transition state HOMO to be formed from both HOMO-LUMO and HOMO-HOMO interactions. By comparing the activared energy and activated entropy or the reaction frequency factor of this reaction with that of the addition of LiH monomer to acetylene, we point out the possibility that these two reactions happen in a compoting way in an actual system.