Theoretical Study of the Potential Energy Profile of the HBr+ + CO2 → HOCO+ + Br· Reaction.

Recent guided ion beam experiments have revealed interesting reaction dynamics of the HBr+ + CO2 → HOCO+ + Br· reaction under different conditions. The hypothesis is that the predominant reaction mechanism depends on the collision energy between two reactants, the angular momentum of HBr+, and the spin-orbit coupling state of the system. The potential energy profile of the HBr+ + CO2 → HOCO+ + Br· reaction is studied in this research to lay the groundwork for an ab initio molecular dynamics simulation. First, a benchmark potential energy profile of this reaction is identified using coupled-cluster theory extrapolated to the complete basis set limit. A transition state connecting the previously reported intermediates is found, making the potential energy surface of the HBr+ + CO2 → HOCO+ + Br· reaction double-welled. Second, various single reference ab initio methods are compared with the benchmark potential energy profile to search for the most suitable ab initio method for the dynamics simulation. Two combinations of double-ζ basis sets (with effective core potentials) with MP2 and density functional theory have been identified to accurately represent the potential energy profile of this reaction.