Wu Xiangyu, Ying Fei, Wang Hongyi, Yang Li, Zhang Jiaxu, Xie Jing
Key Laboratory of Cluster Science of Ministry of Education, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100081, China.
State Key Laboratory of Urban Water Resource and Environment, MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China.
ACS Phys Chem Au. 2024 Sep 19;4(6):581-592. doi: 10.1021/acsphyschemau.4c00061. eCollection 2024 Nov 27.
Roundabout (RA) is an important indirect mechanism for gas-phase X + CHY → XCH + Y S2 reactions at a high collision energy. It refers to the rotation of the CH-group by half or multiple circles upon the collision of incoming nucleophiles before substitution takes place. The RA mechanism was first discovered in the Cl + CHI S2 reaction to explain the energy transfer observed in crossed molecular beam imaging experiments in 2008. Since then, the RA mechanism and its variants have been observed not only in multiple C-centered S2 reactions, but also in N-centered S2 reactions, proton transfer reactions, and elimination reactions. This work reviewed recent studies on the RA mechanism and summarized the characteristics of RA mechanisms in terms of variant types, product energy partitioning, and product velocity scattering angle distribution. RA mechanisms usually happen at small impact parameters and tend to couple with other mechanisms at relatively low collision energy, and the available energy of roundabout trajectories is primarily partitioned to internal energy. Factors that affect the importance of the RA mechanism were analyzed, including the type of leaving group and nucleophile, collision energy, and microsolvation. A massive leaving group and relatively high collision energy are prerequisite for the occurrence of the roundabout mechanism. Interestingly, when reacting with CHI, the importance of RA mechanisms follows an order of Cl > HO > F, and such a nucleophile dependence was attributed to the difference in proton affinity and size of the nucleophile.
迂回（RA）是气相中X + CHY → XCH + Y S2反应在高碰撞能量下的一种重要间接机制。它指的是在亲核试剂碰撞后、取代发生之前，CH基团旋转半圈或多圈。RA机制最早在Cl + CHI S2反应中被发现，用于解释2008年交叉分子束成像实验中观察到的能量转移。从那时起，RA机制及其变体不仅在多个以碳为中心的S2反应中被观察到，也在以氮为中心的S2反应、质子转移反应和消除反应中被观察到。这项工作回顾了关于RA机制的近期研究，并从变体类型、产物能量分配和产物速度散射角分布等方面总结了RA机制的特点。RA机制通常发生在小碰撞参数下，并且在相对较低的碰撞能量下倾向于与其他机制耦合，迂回轨迹的可用能量主要分配给内能。分析了影响RA机制重要性的因素，包括离去基团和亲核试剂的类型、碰撞能量和微溶剂化。大量的离去基团和相对较高的碰撞能量是迂回机制发生的先决条件。有趣的是，当与CHI反应时，RA机制的重要性顺序为Cl > HO > F，这种对亲核试剂的依赖性归因于亲核试剂的质子亲和力和大小的差异。
