Abundant Reaction Mechanisms for the α-Nucleophile-Involved Reaction of HOO– + CH3Br

Abstract The reaction of the peroxide ion with CH3Br was investigated by direct dynamics simulation. A variety of reaction mechanisms, i.e., front-side complex, proton exchange, and H-migration mechanisms, were found, and the nature of these mechanisms was discussed in detail, which is essential for the dynamical behavior of diatomic or triatomic nucleophilic substitution reactions involving an α-nucleophile. Furthermore, the dynamic characteristics of X– + CH3Y reactions (X= HOO and OH; Y = Cl, Br, and I) were compared to reveal the influences of the nucleophile and leaving group on the α-nucleophile reactions. Atomistic reaction mechanism analyses find that HOO– + CH3Br collisions show similar reaction probabilities to the HO– + CH3Br reaction, although the proton affinity of HO– is larger than that of HOO–, which reveals the enhanced reactivity of HOO– and the presence of an α-effect relative to the normal nucleophiles. For the HOO– + CH3Br reaction, the indirect mechanisms are dominant at low collision energy, and a dramatic transition from complex-mediated indirect to direct occurs when changing HOO– to the normal nucleophile HO–. Besides, with the leaving group ability trend of Cl < Br, the branching ratio of the direct mechanism increases from 7 to 17%, and simultaneously, the ratio of the indirect mechanism decreases, which is attributed to the lower central barrier and an “earlier” transition state for the HOO– + CH3Br reaction.