Accurate ab initio potential energy surface, thermochemistry, and dynamics of the F- + CH3F SN2 and proton-abstraction reactions

István Szabó, Hajnalka Telekes, G. Czakó

Research output: Contribution to journalArticle

21 Citations (Scopus)

Abstract

We develop a full-dimensional global analytical potential energy surface (PES) for the F- + CH3F reaction by fitting about 50 000 energy points obtained by an explicitly correlated composite method based on the second-order Møller-Plesset perturbation-F12 and coupled-cluster singles, doubles, and perturbative triples-F12a methods and the cc-pVnZ-F12 [n = D, T] basis sets. The PES accurately describes the (a) back-side attack Walden inversion mechanism involving the pre- and post-reaction (b) ion-dipole and (c) hydrogen-bonded complexes, the configuration-retaining (d) front-side attack and (e) double-inversion substitution pathways, as well as (f) the proton-abstraction channel. The benchmark quality relative energies of all the important stationary points are computed using the focal-point analysis (FPA) approach considering electron correlation up to coupled-cluster singles, doubles, triples, and perturbative quadruples method, extrapolation to the complete basis set limit, core-valence correlation, and scalar relativistic effects. The FPA classical(adiabatic) barrier heights of (a), (d), and (e) are -0.45(-0.61), 46.07(45.16), and 29.18(26.07) kcal mol-1, respectively, the dissociation energies of (b) and (c) are 13.81(13.56) and 13.73(13.52) kcal mol-1, respectively, and the endothermicity of (f) is 42.54(38.11) kcal mol-1. Quasiclassical trajectory computations of cross sections, scattering (θ) and initial attack (α) angle distributions, as well as translational and internal energy distributions are performed for the F- + CH3F(v = 0) reaction using the new PES. Apart from low collision energies (Ecoll), the SN2 excitation function is nearly constant, the abstraction cross sections rapidly increase with Ecoll from a threshold of ∼40 kcal mol-1, and retention trajectories via double inversion are found above Ecoll = ∼ 30 kcal mol-1, and at Ecoll = ∼ 50 kcal mol-1, the front-side attack cross sections start to increase very rapidly. At low Ecoll, the indirect mechanism dominates (mainly isotropic backward-forward symmetric θ distribution and translationally cold products) and significant long-range orientation effects (isotropic α distribution) and barrier recrossings are found. At higher Ecoll, the SN2 reaction mainly proceeds with direct rebound mechanism (backward scattering and hot product translation).

Original languageEnglish
Article number244301
JournalThe Journal of Chemical Physics
Volume142
Issue number24
DOIs
Publication statusPublished - Jun 28 2015

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Thermochemistry
Potential energy surfaces
thermochemistry
Protons
potential energy
attack
protons
Trajectories
Scattering
Electron correlations
inversions
Extrapolation
energy
trajectories
Hydrogen
Substitution reactions
Ions
cross sections
relativistic effects
products

ASJC Scopus subject areas

  • Physics and Astronomy(all)
  • Physical and Theoretical Chemistry

Cite this

Accurate ab initio potential energy surface, thermochemistry, and dynamics of the F- + CH3F SN2 and proton-abstraction reactions. / Szabó, István; Telekes, Hajnalka; Czakó, G.

In: The Journal of Chemical Physics, Vol. 142, No. 24, 244301, 28.06.2015.

Research output: Contribution to journalArticle

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