State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, Liaoning, China.
University of Chinese Academy of Sciences, Beijing 100049, China.
Nat Commun. 2017 Feb 22;8:14506. doi: 10.1038/ncomms14506.
Reactions occurring at a carbon atom through the Walden inversion mechanism are one of the most important and useful classes of reactions in chemistry. Here we report an accurate theoretical study of the simplest reaction of that type: the H+CH substitution reaction and its isotope analogues. It is found that the reaction threshold versus collision energy is considerably higher than the barrier height. The reaction exhibits a strong normal secondary isotope effect on the cross-sections measured above the reaction threshold, and a small but reverse secondary kinetic isotope effect at room temperature. Detailed analysis reveals that the reaction proceeds along a path with a higher barrier height instead of the minimum-energy path because the umbrella angle of the non-reacting methyl group cannot change synchronously with the other reaction coordinates during the reaction due to insufficient energy transfer from the translational motion to the umbrella mode.
瓦尔登反转机制作用于碳原子上的反应是化学中最重要、最有用的反应类型之一。在这里，我们报道了对该类型最简单反应的精确理论研究：质子取代反应及其同位素类似物。研究发现，与碰撞能相比，反应阈能高得多。反应在反应阈以上测量的截面表现出强烈的正常二级同位素效应，而在室温下则表现出较小但相反的二级动力学同位素效应。详细分析表明，反应沿一个具有更高势垒高度的路径进行，而不是沿最低能量路径进行，因为由于从平移运动到伞形模式的能量传递不足，反应过程中非反应甲基的伞形角度不能与其他反应坐标同步变化。
