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

Dynamics and mechanisms of the F^- + CH₃Br(v = 0) → Br^- + CH₃F (S_N2 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 S_N2 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-point energy violation.