Mode-Specific Quantum and Quasi-Classical Trajectory Dynamics of the F^- + CH₃I → I^- + CH₃F S_N2 Reaction

Bimolecular nucleophilic substitution (S_N2) reactions in the gas phase are of great importance in chemistry, whose microscopic mechanisms are, however, not comprehensively understood. In this work, the mode-specific quantum dynamics of the prototypical S_N2 reaction F^- + CH₃I → I^- + CH₃F are investigated using reduced six-dimensional quantum wave packet and full-dimensional quasi-classical trajectory methods. The reaction probabilities are calculated for both the ground and excited vibrational states of the reactant CH₃I at collision energies up to 2.3 eV. The results show that the CH stretching vibration slightly suppresses the reaction with collision energies below 0.4 eV, becomes a spectator mode in the collision energy range of 0.4-1.5 eV, and then enhances the reaction at higher collision energies.