The H + C2H2 (+M) ⇄ C2H3 (+M) and H + C2H2 (+M) ⇄ C2H5 (+M) reactions: Electronic structure, variational transition-state theory, and solutions to a two-dimensional master equation

Abstract

In this article we investigate the kinetics of the H + C₂H₂ and H + C₂H₄ reactions, as well as their reverse dissociations, in some detail. High level electronic structure calculations are used to characterize the potential energy surfaces, and these results are not adjusted to obtain good agreement with experiment in the subsequent kinetic analysis. An approximate two-dimensional master equation is used to determine phenomenological rate coefficients, k(T,p). The effects of angular momentum conservation, tunneling, and the use of variational transition-state theory (as opposed to conventional transition-state theory) to compute microcanonical rate coefficients are investigated in detail. For both reactions, the low-pressure limit is approached very slowly, because reaction just above threshold must occur strictly by tunneling. Assuming a single-exponential-down model for P(E,E′), we deduce from experiment values of 〈ΔE_d〉, the average energy transferred in a deactivating collision, as a function of temperature for both C₂H₃ and C₂H₅ in baths of He, Ar, and N₂. Our results support the idea that 〈ΔE_d〉 increases roughly linearly with temperature, at least for weak colliders. The agreement between theory and experiment is remarkably good for both reactions. Values of k(T,p) for the two reactions are given in the Troe format for use in modeling.