Molecular dynamics simulation of partially diffusion-controlled reaction between mono- and diatomic molecules

Abstract

Molecular dynamics simulations were performed for model partially diffusion-controlled reactions between mono- and diatomic molecules at liquid density with energetical and geometrical restrictions on the reactivity. Since the reorientational motion of the diatomic molecule was fast, a spherical reaction surface approximation could be applied in the analysis of the short-time transient effect on the rate constant. In this approximation, the two types of restrictions could be treated as different profiles of reactant velocity distributions at the reaction surface. The transient rate constants for the two types of restrictions obtained by the simulations were virtually identical if the initial rate constants were the same, though the reaction probability per collision and the spatial population distribution of reactants were different. The simulation results were compared with three theories: two based on the Fokker–Planck–Kramers equation and one on the diffusion equation. The three theories, which assumed different functional forms of velocity distribution, reasonably well explained the time dependences of the rate constants obtained by the simulations and their insensitivity on the detailed profile of reactant velocity distribution.