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 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 CH4* is internally excited close to its dissociation threshold (E-vib approximate to 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 CH3, 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 approximate to 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 + CH3 = CH4* under conditions of practical relevance to combustion. Finally, the accuracy of an effective temperature model for estimating k* from k is quantified. Published under license by AIP Publishing.