Dominant Direct Mechanism at Low Collision Energy Induced by the Nucleophile

The indirect mechanism is typically dominant at room temperature for most SN2 reactions X- + CH3Y (X = F, Cl; Y = Cl, Br, I) due to a dynamic bottleneck formed by the prereaction minimum, which prevents the direct collisions. However, recent studies have found that when X = OH, the SN2 reaction shifts to be dominated by the direct mechanism. In this work, direct dynamic simulations with the B3LYP/ECP/d method were employed to thoroughly investigate the cause of this transition by exploring the OH- + CH3Br reaction. And the dynamic properties of X- + CH3Y (X = OH, F; Y = Cl, Br, I) reactions were further compared to investigate the mechanism-changing regularities of this series of SN2 reactions. For the X = F reactions, the indirect mechanisms were found to be dominant, although their proportion decreases along the leaving group changing from Cl to I. In contrast, a reaction mechanism conversion was found for X = OH. When the leaving group changes from Cl to Br or I, the dominant mechanism of the SN2 reactions alters from indirect to direct, which implies the critical influence of the nucleophile and leaving group on the dominant mechanism of SN2 reaction and the importance of the kinetic and dynamic factors, i.e., the energy barrier, electronegativity, and dipole moment. This work clearly summarizes the dynamical behaviors in the seemingly similar X- + CH3Y (X = F, OH; Y = Cl, Br, I) reactions and provides insights into the impact of the leaving group and the nucleophile on the dominant mechanism of SN2 reactions.