A statistical model for the product energy distribution in reactions leading to prompt dissociation

Abstract

Direct dynamics calculations have been performed for three reactions: C3H8 + H → i-C3H7 + H2, C3H8 + H → n-C3H7 + H2, and C2H3 + O2 → HCO + CH2O. The fraction of the population for the radical products that promptly dissociates is computed. The results for C3H8 + H are qualitatively similar to previous results for C3H8 + OH, but the new results exhibit a slightly higher branching fraction for prompt dissociation products, owing to the fact that a greater fraction of the internal energy in the transition state ends up in the radical. For C2H3 + O2 → HCO + CH2O, the fraction of HCO that promptly dissociates is in excess of 99%. Consequently, the main product for C2H3 + O2 at lower temperatures should be written as H + CO + CH2O and not HCO + CH2O. These results are then compared with four previous systems: CH2O + H → HCO + H2, CH2O + OH → HCO + H2O, C3H8 + OH → i-C3H7 + H2O, and C3H8 + OH → n-C3H7 + H2O. Based upon these seven system, several statistical models are presented. The goal of these statistical models is to predict the fraction of the transition state energy that ends up in the rovibrationally excited radical. On average, these statistical models provide an excellent prediction of product energy distribution. Consequently, these models can be used instead of costly trajectory simulations for predicting prompt radical dissociation for larger species.