Quantum Wave Packet Study of the H + Br₂ → HBr + Br Reaction on a New Ab Initio Potential Energy Surface

An accurate global potential energy surface (PES) for the HBr₂ system has been constructed using the fundamental invariant neural network fitting method based upon 11 698 ab initio energies at the UCCSD(T)/CBS level of theory, with the spin-orbit coupling of the ²P_3/2 orbit of the Br atom properly included. The time-dependent wave packet calculations have been performed to study the H + Br₂ → HBr + Br reaction on the new PES. The total reaction probabilities for total angular momentum J = 0 for the ground initial state show no threshold due to the submerged barrier height (-0.351 kcal/mol) of the PES. The total integral cross sections (ICS) for reactant Br₂ in ro-vibrational states (v₀ = 0, j₀ = 0, 10, 20, 30; v₀ = 1-5, j₀ = 0) were calculated for collision energy of up to 0.5 eV. It is found that the initial rotational excitation has a negligible effect on the ICS, and the initial vibrational excitation depresses the reactivity to some extent. The thermal rate constants for the title reaction in the temperature range of 100-1000 K were calculated from the Boltzmann averaging of the v₀ = 0-5 rate constants, which overestimated the experimental results to some extent.