Theoretical investigation of the temperature dependence of the O+O2 exchange reaction

Abstract

The exchange reaction O16+18O2→16O18O+18O and, in particular, its dependence on the transition state region is investigated by classical trajectories on three potential energy surfaces, all based on high-level electronic structure calculations. The first one is the original potential recently constructed by Siebert, Schinke, and Bittererová [Phys. Chem. Chem. Phys. 3, 1795 (2001)]; it has a very small barrier above the O+O2 asymptote. The second potential is a modification of the first one in that the transition state region is adjusted according to new electronic structure calculations on higher levels of theory; it has a small barrier below the asymptote. The third potential is obtained by artificially removing this barrier. The variation of the exchange reaction cross section with collision energy and the magnitude of the thermal rate constant at and below room temperature depend drastically on the shape of the potential at intermediate distances. The second potential, which is believed to represent the transition state structure of the true ground-state potential of ozone best, yields a reaction rate for room temperature that is about a factor of three smaller than the experimental rate. The neglect of nonadiabatic transitions between the several electronic states in the entrance channel may explain this discrepancy. The very slight negative temperature dependence found in the calculations is caused by the strong decrease of the reaction cross section with the initial rotational excitation of the oxygen molecule. Statistical calculations give poor agreement with the classical energy and initial-state dependent cross sections. Nevertheless, the statistical thermal rates are in fair agreement with the classical ones, because the overestimation of the cross sections for low j’s and the underestimation for high j’s partly compensate.