Nonthermal rate constants for CH 4 * + X → CH 3 + HX, X = H, O, OH, and O 2 .

Quasiclassical trajectories are used to compute nonthermal rate constants, k ^* , for abstraction reactions involving highly-excited methane CH ₄ ^* and the radicals H, O, OH, and O ₂ . Several temperatures and internal energies of methane, E _vib , are considered, and significant nonthermal rate enhancements for large E _vib are found. Specifically, when CH ₄ ^* is internally excited close to its dissociation threshold (E _vib ≈ D ₀ = 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 CH ₃ , with k ^* > 10 ^-10 cm ³ 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 ≈ D ₀ /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 + CH ₃ = CH ₄ ^* under conditions of practical relevance to combustion. Finally, the accuracy of an effective temperature model for estimating k ^* from k is quantified.