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

Abstract

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.