Dominant Direct Mechanism at Low Collision Energy Induced by the Nucleophile

The indirect mechanism is typically dominant at room temperature for most S_N2 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 S_N2 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 S_N2 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 S_N2 reactions alters from indirect to direct, which implies the critical influence of the nucleophile and leaving group on the dominant mechanism of S_N2 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 S_N2 reactions.