Temperature dependence of fast neutral–neutral reactions: a triatomic model study

Abstract

Some neutral–neutral reactions are known to proceed rapidly at low temperatures as a consequence of strong inverse temperature power-law dependences of the rate constants. Previous calculations, based on capture approximations, failed to account for these experimental data. In this article, short-range effects (subsequent to capture) are investigated using a simple planar atom–diatom toy model based on pairwise atomic interactions. Reaction rate constants have been estimated in the temperature range of 25–300 K using a quasi-classical trajectory Monte Carlo approach. It is shown that a small short-range barrier in the entrance valley may significantly influence the reactivity. In particular, our crude triatomic model can reproduce a strong inverse temperature dependence of the rate constant in good agreement with experimental evidence for more complex systems. These predictions are interpreted using a 3-D representation of the effective potential surfaces, illustrating the crucial importance of vector correlations between partial angular momenta. Thus, the strong inverse power-law temperature dependence of the rate constant may be attributed to the population of higher rotational states of the reactants with increasing temperature.