The unusual effect of reagent vibrational excitation on the rates of endothermic and exothermic elementary combustion reactions

Abstract

Surprisal analyses are carried out for the reactions H + O 2( v) → OH + O, O + H 2( v) → OH + H, and OH + H 2( v) → H 2O + H. The rate constants are taken from published quasiclassical trajectory calculations covering the temperature range 300–4000 K. The dichotomy expected when reagent vibrational energy converts an endoergic reaction into an exothermic one is observed. However, contrary to the usual trend observed for the X + HY( v) → Y + HX reaction (X, Y = halogen), here vibrational energy detracts from the rate of the endothermic reaction, and enhances the rate of the exothermic reaction when compared to statistical prior expectations. There are indications that at sufficiently high temperatures there is a role reversal for reagent vibrational energy. Figures are also provided which relate the slopes of surprisal plots for the general reaction A + BC( v) → AB + C to the surprisal parameter, λ, for the state-to-state reaction, A + BC( v) → AB( v′) + C. The procedure as applied to the H + O 2 reaction reveals large discrepancies with experimental measurements of state-to-state rate constants. All published data are reconciled, however, by means of a non-linear surprisal analysis.