{"title":"Roundabout Mechanism of Ion–Molecule Nucleophilic Substitution Reactions","authors":"Xiangyu Wu, Fei Ying, Hongyi Wang, Li Yang, Jiaxu Zhang, Jing Xie","doi":"10.1021/acsphyschemau.4c00061","DOIUrl":null,"url":null,"abstract":"Roundabout (RA) is an important indirect mechanism for gas-phase X<sup>–</sup> + CH<sub>3</sub>Y → XCH<sub>3</sub> + Y<sup>–</sup> S<sub>N</sub>2 reactions at a high collision energy. It refers to the rotation of the CH<sub>3</sub>-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<sup>–</sup> + CH<sub>3</sub>I S<sub>N</sub>2 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 S<sub>N</sub>2 reactions, but also in N-centered S<sub>N</sub>2 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 CH<sub>3</sub>I, the importance of RA mechanisms follows an order of Cl<sup>–</sup> > HO<sup>–</sup> > F<sup>–</sup>, and such a nucleophile dependence was attributed to the difference in proton affinity and size of the nucleophile.","PeriodicalId":29796,"journal":{"name":"ACS Physical Chemistry Au","volume":null,"pages":null},"PeriodicalIF":3.7000,"publicationDate":"2024-09-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":null,"platform":"Semanticscholar","paperid":null,"PeriodicalName":"ACS Physical Chemistry Au","FirstCategoryId":"1085","ListUrlMain":"https://doi.org/10.1021/acsphyschemau.4c00061","RegionNum":0,"RegionCategory":null,"ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q2","JCRName":"CHEMISTRY, PHYSICAL","Score":null,"Total":0}
引用次数: 0
Abstract
Roundabout (RA) is an important indirect mechanism for gas-phase X– + CH3Y → XCH3 + Y– SN2 reactions at a high collision energy. It refers to the rotation of the CH3-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– + CH3I SN2 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 SN2 reactions, but also in N-centered SN2 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 CH3I, 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.
期刊介绍:
ACS Physical Chemistry Au is an open access journal which publishes original fundamental and applied research on all aspects of physical chemistry. The journal publishes new and original experimental computational and theoretical research of interest to physical chemists biophysical chemists chemical physicists physicists material scientists and engineers. An essential criterion for acceptance is that the manuscript provides new physical insight or develops new tools and methods of general interest. Some major topical areas include:Molecules Clusters and Aerosols; Biophysics Biomaterials Liquids and Soft Matter; Energy Materials and Catalysis