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

Jiaxu Zhang; Jing Xie; William L Hase

doi:10.1021/acs.jpca.5b08167

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

J Phys Chem A. 2015 Dec 17;119(50):12517-25. doi: 10.1021/acs.jpca.5b08167. Epub 2015 Nov 2.

Authors

Jiaxu Zhang¹, Jing Xie², William L Hase³

Affiliations

¹ Institute of Theoretical and Simulation Chemistry, Academy of Fundamental and Interdisciplinary Sciences, Harbin Institute of Technology , Harbin 150080, P. R. China.
² Department of Chemistry, University of Minnesota , Minneapolis, Minnesota 55455, United States.
³ Department of Chemistry and Biochemistry, Texas Tech University , Lubbock, Texas 79409, United States.

PMID: 26473337
DOI: 10.1021/acs.jpca.5b08167

Abstract

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 Å(2), 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 Å(2). 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.