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

Abstract

Articles you may be interested inWe develop a full-dimensional global analytical potential energy surface (PES) for the F − + CH 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 −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 − + CH 3 F(v = 0) reaction using the new PES. Apart from low collision energies (E coll ), the S N 2 excitation function is nearly constant, the abstraction cross sections rapidly increase with E coll from a threshold of ∼40 kcal mol −1 , and retention trajectories via double inversion are found above E coll = ∼30 kcal mol −1 , and at E coll = ∼50 kcal mol −1 , the front-side attack cross sections start to increase very rapidly. At low E 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 coll , the S N 2 reaction mainly proceeds with direct rebound mechanism (backward scattering and hot product translation). C 2015 AIP Publishing LLC. [http://dx

High-level ab initio potential energy surface and dynamics of the F−+ CH3I SN2 and proton-transfer reactions

“…However, we found that one cannot distinguish between the two mechanisms based on simply the integration time, as we did in the case of the F – + CH 3 Cl, F – + CH 3 F, and F – + CHD 2 Cl reactions. 6,35,36 Fig. 3 also shows the cross sections for the induced inversions of the CH 3 I reactant, which are not followed by a substitution resulting in an inverted reactant.…”

An interacting quantum atom study of model SN2 reactions (X–···CH3X, X = F, Cl, Br, and I)

“…The noncovalent index (NCI) was also used to illustrate this S N 2 reaction in solution . S N 2 reactions in systems with different incoming and outgoing halogen atoms, and alternative mechanisms (front‐side attack, double inversion, and hydrogen abstraction) have also been investigated …”

Imaging Proton Transfer and Dihalide Formation Pathways in Reactions of F– + CH3I

“… 14 , 23 Also a hydrogen-bonded prereaction complex where the anion points to a C–H bond axis of the methyl halide has been established. 24 , 25 These alternative entrance channel geometries not only change the kinematics and dynamics of the reaction producing the mechanisms mentioned above but also can lead to completely different products competing with nucleophilic substitution in these ion–molecule reactive systems, e.g. : Reaction ( 2 ) consists of a proton abstraction mechanism induced by an XH-bonded prereaction complex, whereas reaction ( 3 ) produces a dihalide anion through a halogen abstraction mechanism.…”

“… 28 , 29 Theoretical studies have focused mainly on the reaction type ( 2 ), investigating the properties of the [X–HCH 2 Y] − prereaction complex. 24 , 25 , 30 The contribution of the proton transfer channel has been shown to strongly depend on the proton affinity of the nucleophile. 21 The halogen abstraction mechanism, which produces a dihalide anion, has only seen little attention up to now.…”