Nonthermal rate constants for CH4* + X → CH3 + HX, X = H, O, OH, and O2.

Quasiclassical trajectories are used to compute nonthermal rate constants, k^*, for abstraction reactions involving highly-excited methane CH₄^* and the radicals H, O, OH, and O₂. Several temperatures and internal energies of methane, E_vib, are considered, and significant nonthermal rate enhancements for large E_vib are found. Specifically, when CH₄^* is internally excited close to its dissociation threshold (E_vib ≈ D₀ = 104 kcal/mol), its reactivity with H, O, and OH is shown to be collision-rate-limited and to approach that of comparably-sized radicals, such as CH₃, with k^* > 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹. Rate constants this large are more typically associated with barrierless reactions, and at 1000 K, this represents a nonthermal rate enhancement, k^*/k, of more than two orders of magnitude relative to thermal rate constants k. We show that large nonthermal rate constants persist even after significant internal cooling, with k^*/k > 10 down to E_vib ≈ D₀/4. The competition between collisional cooling and nonthermal reactivity is studied using a simple model, and nonthermal reactions are shown to account for up to 35%–50% of the fate of the products of H + CH₃ = CH₄^* under conditions of practical relevance to combustion. Finally, the accuracy of an effective temperature model for estimating k^* from k is quantified. [ABSTRACT FROM AUTHOR]