Effects of Reagent Rotation and Vibration on H + OH (υ, j)→ O + H2

Abstract

The dynamics of the reaction H + OH → O ((3)P) + H2 have been studied in a series of quasi-classical trajectory (QCT) calculations and transition state theory (TST) methods using high quality (3)A' and (3)A″ potential energy surfaces (PESs). Accurate OH (υ, j) state resolved cross sections and rate constants on both potential energy surfaces are presented and fitted for OH at (υ = 0, j = 0-16) and (υ = 1, j = 0-6). The cross sections were calculated for different collisional energies (Ec), ranging from the threshold energy at each specific rovibrational state up to 1.0 eV with step sizes of 0.1 eV or less. They increase steeply with collision energy when the barrier to reaction can be overcome, after which the cross sections stay nearly constant with energy. State resolved rate constants in the temperature range 200-2500 K are presented based on the cross sections. Total thermal rate constants were calculated by summing the rates for reaction on the (3)A' and (3)A″ potential energy surfaces weighted by 1/3 and taking into account the thermal populations of the rovibrational states of the OH molecules. The currently calculated thermal rate constants generally agree well with previous indirectly obtained rate constants by Tsang et al. (Tsang, W.; Hampson, R. F. Chemical Kinetic Data Base for Combustion Chemistry. Part I. Methane and Related Compounds. J. Phys. Chem. Ref. Data 1986, 15, 1087-1279). It is shown that the improved canonical variational transition (CVT) treatments with the approximation of zero-curvature tunneling (ZCT) or small-curvature tunneling (SCT) produce results more in accord with the QCT results than the TST and CVT methods. The reactions are governed by the direct reaction mechanism. The rate constants for OH in excited vibrational and rotational states are orders of magnitude larger than the thermal rate constants, which needs to be taken into account in astrochemical models.