Nonstatistical effects and detailed balance in quasiclassical trajectory calculations of the thermal rate coefficient for O+OH→O2+H

Abstract

A detailed theoretical investigation of dynamical effects in the thermal rate coefficient for the title reaction is reported. Quasiclassical trajectory (QCT) calculations in both the forward (+1) and reverse (−1) directions are employed in the analysis. The results are compared with each other, with the variational transition state theory (VTST) calculations of Rai and Truhlar, with previous trajectory results for the reverse reaction, and with experiment. It is found that nonstatistical, or non-RRKM, ‘‘recrossing’’ effects play an important role in determining the thermal rate coefficient (the nonstatistical correction is almost a factor of 4 at 2500 K and nearly a factor of 2 even at 200 K). The results from the two trajectory methods satisfy detailed balance at high temperature, but at low temperature (250–300 K) the (−1) trajectories (through the equilibrium constant) predict a rate coefficient for the title reaction that is higher than that predicted by the (+1) method by about a factor of 2. This discrepancy is attributed to the crude handling of quantum effects near threshold in the (−1) trajectory method. The predictions of the (+1) method are in excellent agreement with experiment over the entire temperature range where experimental results are available (250–2500 K). At high temperatures (T>500 K) the VTST results of Rai and Truhlar predict accurately the rate of formation of HO*2 complexes, but at lower temperatures the VTST and trajectory results are in disagreement. The VTST calculations are unable to predict the thermal rate coefficient of the title reaction at any temperature for a combination of reasons.