Temperature dependence of the OH^- + CH₃I reaction kinetics. experimental and simulation studies and atomic-level dynamics

Direct dynamics simulations and selected ion flow tube (SIFT) experiments were performed to study the kinetics and dynamics of the OH^- + CH₃I reaction versus temperature. This work complements previous direct dynamics simulation and molecular beam ion imaging experiments of this reaction versus reaction collision energy (Xie et al. J. Phys. Chem. A 2013, 117, 7162). The simulations and experiments are in quite good agreement. Both identify the S_N2, OH^- + CH₃I → CH ₃OH + I^-, and proton transfer, OH^- + CH ₃I → CH₂I^- + H₂O, reactions as having nearly equal importance. In the experiments, the S_N2 pathway constitutes 0.64 ± 0.05, 0.56 ± 0.05, 0.51 ± 0.05, and 0.46 ± 0.05 of the total reaction at 210, 300, 400, and 500 K, respectively. For the simulations this fraction is 0.56 ± 0.06, 0.55 ± 0.04, and 0.50 ± 0.05 at 300, 400, and 500 K, respectively. The experimental total reaction rate constant is (2.3 ± 0.6) × 10^-9, (1.7 ± 0.4) × 10^-9, (1.9 ± 0.5) × 10 ^-9, and (1.8 ± 0.5) × 10^-9 cm³ s^-1 at 210, 300, 400, and 500 K, respectively, which is approximately 25% smaller than the collision capture value. The simulation values for this rate constant are (1.7 ± 0.2) × 10^-9, (1.8 ± 0.1) × 10^-9, and (1.6 ± 0.1) × 10^-9 cm ³s^-1 at 300, 400, and 500 K. From the simulations, direct rebound and stripping mechanisms as well as multiple indirect mechanisms are identified as the atomic-level reaction mechanisms for both the S_N2 and proton-transfer pathways. For the S_N2 reaction the direct and indirect mechanisms have nearly equal probabilities; the direct mechanisms are slightly more probable, and direct rebound is more important than direct stripping. For the proton-transfer pathway the indirect mechanisms are more important than the direct mechanisms, and stripping is significantly more important than rebound for the latter. Calculations were performed with the OH^- quantum number J equal to 0, 3, and 6 to investigate the effect of OH^- rotational excitation on the OH^- + CH₃I reaction dynamics. The overall reaction probability and the probabilities for the S_N2 and proton-transfer pathways have little dependence on J. Possible effects on the atomistic mechanisms were investigated for the S _N2 pathway and the probability of the direct rebound mechanism increased with J. However, the other atomistic mechanisms were not appreciably affected by J.