Trajectory studies of O+H2 reactions on fitted a b i n i t i o surfaces. II. Singlet case

Abstract

Classical trajectory calculations of cross sections for the reaction O(1D)+H2(1Σg+;ν′,j) →OH(2Π;ν′,j′)+H(2S) have been performed for collision energies 0.25 kcal/mole ⩽E⩽5.0 kcal/mole using three analytic fits to a recently computed ab initio potential energy surface. The various potentials differ only slightly from each other in the long range region of the entrance channel and are equivalent in all other regions. The differences are of the same order as the assumed error in the ab initio surface. The three potentials have been probed to study the influence of these differences on the scattering results. The total reaction cross sections are sensitive to these changes, especially at very low collision energies, which is the energy regime that determines the total reaction rate constants at room temperature. Since the uncertainty in the available experimental rates is even larger, the validity of a theory–experiment comparison of reaction rate constants is questionable. The vibrational product distribution differs only slightly from the statistical prior distribution indicating that most of the trajectories via the deep well corresponding to the equilibrium geometry of H2O. With increasing energy, the barrier of almost 1 kcal/mole in the collinear geometry can be surmounted, so that gradually more reaction occurs by abstraction and the distribution becomes less statistical. This is even more pronounced, if no barrier is found in the potential surface. This result agrees qualitatively with the findings of Whitlock et al. but not with the study of Sorbie and Murrell where a highly inverted vibrational distribution was obtained using an empirical energy surface. The rotational product distribution is highly nonstatistical with a narrow peak at high rotational quantum number, as found previously in other trajectory studies. The product angular distribution is symmetric with respect to ϑ=90° as expected from reactive trajectories which proceed via a long-lived complex. With increasing energy the barrier to the collinear pathway can be surmounted, such that the angular distribution is slightly shifted towards the backward direction.