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

Jiaxu Zhang, JING XIE, William L. Hase

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Direct chemical dynamics simulations, at collision energies Erel 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 SN2 product channel at 0.32 eV. Increasing Erel to 1.53 eV opens the endothermic F- + CH3I → HF + CH2I- proton transfer reaction, which is less competitive than the SN2 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 Å2, 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 Å2. 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 Erel 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 SN2 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.

Original languageEnglish (US)
Pages (from-to)12517-12525
Number of pages9
JournalJournal of Physical Chemistry A
Issue number50
StatePublished - Dec 17 2015

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