Direct chemical dynamics simulations of the CH₃O^- + CH₃I reaction: substitution vs. proton transfer

Comprehending the kinetics of bimolecular nucleophilic substitution (S_N2) reactions involving bigger nucleophiles is crucial for linking fundamental model investigations to more intricate organic systems. In this work, classical trajectory simulations were performed for the CH₃O^- + CH₃I reaction across collision energies ranging from 0.4 to 1.6 eV, corresponding to conditions recently explored in velocity-map imaging experiments. Two major reactive pathways were identified: the substitution channel forming I^- + CH₃OCH₃ products and the proton transfer channel producing CH₂I^- and CH₃OH. In the experimental investigation, the proton transfer channel progresses from indirect to direct scattering as collision energy increases, while the S_N2 channel has more complex dynamics, characterized by predominantly direct scattering throughout the assessed collision energy range. Both the forward scattering caused by direct stripping and the backward scattering caused by the direct rebound mechanism were observed. The simulations align closely with the experiments on proton-transfer channels, demonstrating a distinct shift towards direct dynamics, as evidenced by an increased proportion of forward-scattered products. In contrast, the S_N2 channel predominantly proceeds through a direct rebound mechanism at all collision energies. The scattering angles and energy distributions of the products were calculated, and comprehensive atomic-level reaction mechanisms are presented.