Monte Carlo trajectories: The reaction H + Br2 → HBr + Br

Abstract

A quasiclassical trajectory study of the electronically adiabatic exothermic reaction H + Br2 → HBr + Br using 13 different semiempirical and empirical potential energy surfaces has been conducted. Detailed results are reported for 8035 reactive trajectories using Monte Carlo selection of initial conditions drawn from six different sets of distributions. For all the surfaces examined, the reaction is dominated by sideways collisions. The differential cross section for unit plane angle is sideways peaked but the differential cross section per unit solid angle peaks for molecular scattering angles (measured relative to the direction of the incident atom) in the range 156°–180°, i.e., backward scattering. There is a good correlation of scattering angle and initial orientation of the intermolecular axis, but it was not possible to use this correlation to guess a potential energy surface which yields a differential cross section in quantitative agreement with experimental results. The trajectory results, however, do indicate that a surface which has a low barrier or no barrier for collinear collisions yields too much forward scattering as compared to experiment. We find the computed angular distribution is not a strong function of atomic temperature in the range 300° to 5600°K or molecular temperature in the range 100° to 900°K. Under 300°K conditions the final state internal energy distributions are in good agreement with chemiluminescence results and at an atomic temperature of 2800°K the final translational energy distribution is in good agreement with molecular beam results. The reaction cross section is calculated to be a decreasing function of translational energy, in agrement with White and Su's interpretation of the hot atom studies; and the room temperature energy of activation is calculated to be only 0.2 kcal/mole which agrees with experiment within experimental error. The absolute values of the cross section and the room temperature rate constant are about a factor of 3 larger than experimental values. Many other details of the results are given and these may serve as a model study for the important mass combination where a light atom reacts with a heavy homonuclear diatomic molecule.