Dynamics of the F^- + CH₃I → HF + CH₂I^- Proton Transfer Reaction

Direct chemical dynamics simulations, at collision energies E_rel of 0.32 and 1.53 eV, were performed to obtain an atomistic understanding of the F^- + CH₃I reaction dynamics. There is only the F^- + CH₃I → CH₃F + I^- bimolecular nucleophilic substitution S_N2 product channel at 0.32 eV. Increasing E_rel to 1.53 eV opens the endothermic F^- + CH₃I → HF + CH₂I^- proton transfer reaction, which is less competitive than the S_N2 reaction. The simulations reveal proton transfer occurs by two direct atomic-level mechanisms, rebound and stripping, and indirect mechanisms, involving formation of the F^-···HCH₂I complex and the roundabout. For the indirect trajectories all of the CH₂I^- is formed with zero-point energy (ZPE), while for the direct trajectories 50% form CH₂I^- without ZPE. Without a ZPE constraint for CH₂I^-, the reaction cross sections for the rebound, stripping, and indirect mechanisms are 0.2 ± 0.1, 1.2 ± 0.4, and 0.7 ± 0.2 Ų, respectively. Discarding trajectories that do not form CH₂I^- with ZPE reduces the rebound and stripping cross sections to 0.1 ± 0.1 and 0.7 ± 0.5 Ų. The HF product is formed rotationally and vibrationally unexcited. The average value of J is 2.6 and with histogram binning n = 0. CH₂I^- is formed rotationally excited. The partitioning between CH₂I^- vibration and HF + CH₂I^- relative translation energy depends on the treatment of CH₂I^- ZPE. Without a CH₂I^- ZPE constraint the energy partitioning is primarily to relative translation with little CH₂I^- vibration. With a ZPE constraint, energy partitioning to CH₂I^- rotation, CH₂I^- vibration, and relative translation are statistically the same. The overall F^- + CH₃I rate constant at E_rel of both 0.32 and 1.53 eV is in good agreement with experiment and negligibly affected by the treatment of CH₂I^- ZPE, since the S_N2 reaction is the major contributor to the total reaction rate constant. The potential energy surface and reaction dynamics for F^- + CH₃I proton transfer are compared with those reported previously (J. Phys. Chem. A 2013, 117, 7162-7178) for the isoelectronic OH^- + CH₃I reaction.