A quasiclassical trajectory study of H + CO₂ → OH + CO: Bulk reaction dynamics and the effect of van der Waals precursor formation

We present the results of a quasiclassical trajectory study of the H + CO₂ → OH + CO reaction, with particular emphasis on comparing the bulk reaction dynamics (isolated bimolecular collisions) with results for the van der Waals precursor photoinduced reason HBr·CO₂ + hv → OH + CO + Br. All calculations are based on a full-dimensional HCO₂ potential surface that was derived from ab initio calculations. The T-shaped HBr·CO₂ system was modeled by adding HBr, BrC, and BrO pair potentials to the HCO₂ potential. Our cross sections and OH product distributions for the bulk reaction are generally in good agreement with experiment over a wide range of collision energies. The agreement is especially good close to the effective threshold for reaction. The trajectory CO product distributions are in good agreement with recent measurements at 300 K, but there are important differences in the dependence of the distributions on initial rotational temperature. In modeling the van der Waals reaction, we use the experimental geometry for the heavy atoms to define initial conditions, with the orientation of the HBr relative to CO₂ treated as a variable. We find that reaction occurs efficiently over angular regions in which the H atom is directed into either HOCO or HCO₂ wells. Overall energy available to the OH + CO products is about 80% of the bulk value, which is consistent with the major component of the energy distribution that has been inferred from experiment. However, the trajectory calculations indicate that the energy in products is not reduced equally for all degrees of freedom. In particular, OH rotation is not necessarily colder than in the bulk, in contrast to the experimental result. In addition, the energy dependence of the reaction probability is found in the calculations to be stronger in the complexes than in the bulk, while it is about the same in the experimental results.