Theoretical Study of the Dynamics of the HBr⁺ + CO₂ → HOCO⁺ + Br Reaction

The dynamics of the HBr⁺ + CO₂ → HOCO⁺ + Br reaction was recently investigated with guided ion beam experiments under various excitations (collision energy of the reactants, rotational and spin-orbital states of HBr⁺, etc.), and their impacts were probed through the change of the cross section of the reaction. The potential energy profile of this reaction has also been accurately characterized by high-level ab initio methods such as CCSD(T)/CBS, and the UMP2/cc-pVDZ/lanl08d has been identified as an ideal method to study its dynamics. This manuscript reports the first ab initio molecular dynamics simulations of this reaction at two different collision energies, 8.1 kcal/mol and 19.6 kcal/mol. The cross sections measured from the simulations agree very well with the experiments measured with HBr⁺ in the ²∏_1/2 state. The simulations reveal three distinct mechanisms at both collision energies: direct rebound (DR), direct stripping (DS), and indirect (Ind) mechanisms. DS and Ind make up 97% of the total reaction. The dynamics of this reaction is also compared with nucleophilic substitution (S_N2) reactions of X + CH₃Y → CH₃X + Y type. In summary, this research has revealed interesting dynamics of the HBr⁺ + CO₂ → HOCO⁺ + Br reaction at different collision energies and has laid a solid foundation for using this reaction to probe the impact of rotational excitation of ion-molecule reactions in general.