Trajectory study of reactions in HBr–Br2 systems

A large number of trajectories have been calculated employing a semiempirical potential energy surface for H₂Br₂. Primary attention has focussed on reactions of H with HBr and Br₂ and the isotopically substituted analogs. Cross sections have been determined for a large number of different relative velocities and initial vibration‐rotation states of the diatomic reactant. Center‐of‐mass angular scattering distributions and energy partitioning distributions were determined and compared with experimental data from molecular beam and infrared chemiluminescence data. Good agreement is found in the case of energy partitioning with about 80% of the available energy appearing as internal energy. Agreement is excellent in the case of angular scattering—the trajectory calculations predict a small amount of forward scattering, which increases and becomes fairly uniform in the backward hemisphere. Rate constants were computed from cross sections for both thermal and hot atom experiments. Absolute thermal rate coefficients for H + Br₂ and H + HBr are generally to high. The ratio of these rates compares favorably with experimental results. The thermal rate of abstraction $(\mathrm{D+HBr}\to \mathrm{DH+Br})$ versus exchange $(\mathrm{D+HBr}\to \mathrm{DBr+H})$ has been computed and is in poor agreement with experimental results. For hot atom reactions occuring in the ${\mathrm{HBr–Br}}_2$ and ${\mathrm{DBr–Br}}_2$ systems, agreement is satisfactory.