Accurate ab initio potential energy surface, thermochemistry, and dynamics of the F{sup −} + CH{sub 3}F S{sub N}2 and proton-abstraction reactions

Journal Article · Sun Jun 28 00:00:00 EDT 2015 · Journal of Chemical Physics

DOI:https://doi.org/10.1063/1.4922616· OSTI ID:22490837

Szabó, István; Telekes, Hajnalka; Czakó, Gábor ^[1]

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1. Laboratory of Molecular Structure and Dynamics, Institute of Chemistry, Eötvös University, H-1518 Budapest 112, P.O. Box 32 (Hungary)

We develop a full-dimensional global analytical potential energy surface (PES) for the F{sup −} + CH{sub 3}F 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{sup −1}, respectively, the dissociation energies of (b) and (c) are 13.81(13.56) and 13.73(13.52) kcal mol{sup −1}, respectively, and the endothermicity of (f) is 42.54(38.11) kcal mol{sup −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{sup −} + CH{sub 3}F(v = 0) reaction using the new PES. Apart from low collision energies (E{sub coll}), the S{sub N}2 excitation function is nearly constant, the abstraction cross sections rapidly increase with E{sub coll} from a threshold of ∼40 kcal mol{sup −1}, and retention trajectories via double inversion are found above E{sub coll} = ∼ 30 kcal mol{sup −1}, and at E{sub coll} = ∼ 50 kcal mol{sup −1}, the front-side attack cross sections start to increase very rapidly. At low E{sub coll}, 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 E{sub coll}, the S{sub N}2 reaction mainly proceeds with direct rebound mechanism (backward scattering and hot product translation)

• OSTI ID: 22490837
• Journal Information: Journal of Chemical Physics, Vol. 142, Issue 24; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
CALCULATION METHODS
CHEMICAL REACTIONS
COLLISIONS
DIPOLES
DISSOCIATION ENERGY
DISTRIBUTION
ELECTRON CORRELATION
ENERGY SPECTRA
EXCITATION FUNCTIONS
HYDROGEN
METHYL FLUORIDE
PERTURBATION THEORY
POTENTIAL ENERGY
PROTONS
RELATIVISTIC RANGE
SCATTERING
SURFACES