Detailed quasiclassical dynamics of the F- + CH3Br reaction on an ab initio analytical potential energy surface.

Abstract

Dynamics and mechanisms of the F- + CH3Br(v = 0) → Br- + CH3F (SN2 via Walden inversion, front-side attack, and double inversion), F- + inverted-CH3Br (induced inversion), HF + CH2Br- (proton abstraction), and FH⋯Br- + 1CH2 reactions are investigated using a high-level global ab initio potential energy surface, the quasiclassical trajectory method, as well as non-standard configuration- and mode-specific analysis techniques. A vector-projection method is used to identify inversion and retention trajectories; then, a transition-state-attack-angle-based approach unambiguously separates the front-side attack and the double-inversion retention pathways. The Walden-inversion SN2 channel becomes direct rebound dominated with increasing collision energy as indicated by backward scattering, initial back-side attack preference, and the redshifting of product internal energy peaks in accord with CF stretching populations. In the minor retention and induced-inversion pathways, almost the entire available energy transfers into product rotation-vibration, and retention mainly proceeds with indirect, slow double inversion following induced inversion with about 50% probability. Proton abstraction is dominated by direct stripping (evidenced by forward scattering) with CH3-side initial attack preference, providing mainly vibrationally ground state products with significant zero-pointenergy violation.