Theoretical dynamics studies of the CH3 + HBr → CH4 + Br reaction: effects of isotope substitution and vibrational excitation of CH3

Abstract

The rate coefficient for two deuterium substituted isotopologues of reaction CH₃ + HBr → CH₄ + Br has been determined using the quasiclassical trajectory (QCT) method. We used the analytical potential energy surface (PES) fitted to high-level ab initio points in earlier work. The PES exhibits a pre-reaction van der Waals complex and a submerged potential barrier. The rate coefficients of the deuterated isotopologue reactions, similarly to the pure-protium isotopologue, show significant deviation from the Arrhenius law, namely, the activation energy is negative below about 600 K and positive above it: k[CH₃ + DBr] = 1.35 × 10⁻¹¹ exp(− 2472/T) + 5.85 × 10⁻¹³ exp(335/T) and k[CD₃ + HBr] = 2.73 × 10⁻¹¹ exp(− 2739/T) + 1.46 × 10⁻¹² exp(363/T). The CH₃ + DBr reaction is slower by a factor of 1.8, whereas CD₃ + HBr isotopologue is faster by a factor of 1.4 compared to the HBr + CH₃ system across a wide temperature range. The isotope effects are interpreted in terms of the properties of various regions of the PES. Quantum state-resolved simulations revealed that the reaction of CH₃ with HBr becomes slower when any of the vibrational modes of the methyl radical is excited. This contradicts the assumption that vibrational excitation of methyl radicals enhances its reactivity, which is of historical importance: this assumption was used as an argument against the existence of negative activation energy in a decade-long controversy in the 1980s and 1990s.