Trajectory study of O+H2 reactions on fitted a b i n i t i o surfaces I: Triplet case

Abstract

Classical trajectory calculations of cross sections for the reaction 0(3P)+H2(1Σ+g; ν, j) →OH(2Π; ν′, j′)+H(2S) have been performed for collision energies 1 kcal/mol⩽E⩽40 kcal/mol using an analytical fit to a recent ab inito potential energy surface. Three initial vibrational states of H2, ν=0, 1, and 2, are considered in order to study the influence of vibrational reactant energy on the OH production. With increasing vibrational quantum number, (a) the threshold shifts to lower energies, and (b) the cross sections rise more steeply with collision energy. Rotational excitation of H2 enhances the total reaction cross section for each vibrational state over the range of H2(ν,j) states studied. The cross sections have been used to calculate reaction rate constants for temperatures 300°K⩽T⩽1000°K and the three lowest vibrational states. The ratio k (T,ν=0):k (T,ν=1):k (T,ν=2) is found to be 1:1.13×104:1.42×106 at 300°K and 1:2.12×101:1.24×102 at 1000°K, demonstrating that vibrational energy strongly enhances the reaction rate. At low temperatures, T≲500°K, the calculated rate constants for ν=0 are considerably smaller than the experimental results indicating that the barrier in the potential surface is probably too high. Agreement with experiments is improved for intermediate temperatures and is best for T≳1500°K where threshold effects are least important. The present calculations are in very good agreement with a recent rate measurement at T∼302°K by Light for reactions of vibrationally excited H2(ν=1) molecules.