Dynamics of H+O2 → OH+O at high collision energies

Abstract

Classical trajectory calculations of total and state-to-state cross sections for the reaction H(2S)+O2(3Σ−g) → OH(2Π)+O(3P) have been performed for collision energies E=28, 60, and 90 kcal/mol using an ab initio potential energy surface. The main conclusion is that with increasing collision energy the specifity of the initial conditions leading to reaction increases drastically. Thus, with increasing E, the range of impact parameters at which reactions occur becomes successively confined to a broad maximum at about half of the O2 equilibrium separation indicating a direct reaction mechanism. This results in a decrease of the total reaction cross section at high energies. The reaction specifity leads to increasingly nonstatistical rotational product distributions narrowly peaked at high rotational states. The vibrational product distributions are statistical at lower E and partly inverted at E=90 kcal/mol. At E=28 kcal/mol the angular product distribution is symmetric with respect to θ=90° as expected from reactive trajectories which proceed via a long-lived complex. At E=60 and 90 kcal/mol forward–backward scattering is observed with a slight preference for forward scattering. The bimodal angular distribution can be attributed to two essentially different types of trajectories. It is also observed at E=90 kcal/mol that trajectories leading to backward angles yield significantly enhanced product vibrational excitation.