Reaction mechanism conversion induced by the contest of nucleophile and leaving group.

Direct dynamic simulations have been employed to investigate the OH - + CH 3 Cl reaction with the chosen B3LYP/aug-cc-pVDZ method. The calculated rate coefficient for the bimolecular nucleophilic substitution reaction (S N 2), 1.0 × 10 -9 cm 3 mol -1 s -1 at 300 K, agrees well with the experimental result of (1.3-1.6) × 10 -9 cm 3 mol -1 s -1 . The simulations reveal that the majority of the S N 2 reactions are temporarily trapped in the hydrogen-bonded complex at E coll = 0.89 kcal mol -1 . Importantly, the influences of the leaving group and nucleophile have been discussed by comparisons of X - + CH 3 Y (X = F, OH; Y = Cl, I) reactions. For the X = F - reactions, the reaction probability of S N 2 increases along the increased leaving group ability Cl < I, suggesting that the thermodynamic factor plays a key role. The indirect mechanisms were found to be dominant for both reactions. In contrast, for X = OH - , the fraction of S N 2 drops with the enhanced leaving group ability. In particular, a dramatic transition occurs for the dominant atomic reaction mechanisms, i.e. , from complex-mediated indirect to direct, implying an interesting contest between the leaving group and the nucleophile and the importance of the dynamic factors, i.e. , the dipole moment, steric hindrance, and electronegativity.