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

Abstract

The dynamics of the HBr + + CO 2 → 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 N 2) reactions of X − + CH 3 Y → CH 3 X + Y − type. In summary, this research has revealed interesting dynamics of the HBr + + CO 2 → 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.

Self-Reactions in the HBr+ (DBr+) + HBr System: A State-Selective Investigation of the Role of Rotation

“…Such a work will most suitably involve molecular dynamics calculations on an ab initio potential energy surface. 44 The rotational dependence on the cross section observed in the HBr + (DBr + ) + HBr reaction system is in overall agreement with the observation in previous studies of the reaction system HCl + (DCl + ) + HCl 26 although the mechanism leading to the minimum in σ as a function of E rot appears to be more complex in the HBr + (DBr + ) system. For the future, we plan to extend this research on hydrogen…”

“…The possible role of relative rotational velocities of reagents has been discussed, but a more sophisticated theory is required to fully understand the rotational effects observed. Such a work will most suitably involve molecular dynamics calculations on an ab initio potential energy surface …”

The Potential Energy Profile of the HBr+ + HCl Bimolecular Collision

“…It is important to note that this level of agreement of a computationally efficient method is excellent and consequently is as promising as previous, similar AIMD studies in revealing mechanistic details from experimental results (for more details, please see the Supporting Information). ,,,, …”

“…Therefore, with this 1:1 and 2:1 ratio of splitting between the ground SO and excited SO states, the spin-free energy is calculated, as shown in Figure S1 in the Supporting Information. This approach has been applied in previous studies and provided a fair comparison to experimental results. − The potential energy profile computed from CCSD­(T)/cc-pVDZ//CCSD­(T)/CBS serves as the benchmark for selecting a computationally efficient method for ab initio molecular dynamics simulations.…”

“…The performance of the AIMD candidate method is assessed by its ability to reproduce the benchmark potential energy profile and the computation cost. This protocol of selecting a quantum chemistry method for AIMD simulations has been widely used and has seen great success in elaborating experimental discoveries (please see the Supporting Information for examples). ,,,, …”