Potential energy surfaces for the HBr+ + CO2 → Br + HOCO+ reaction in the HBr+ 2Π3/2 and 2Π1/2 spin-orbit states

Quantum mechanical (QM) + molecular mechanics (MM) models are developed to represent\udpotential energy surfaces (PESs) for the HBr+ + CO2 → Br + HOCO+ reaction with HBr+ in the\ud2Π3/2 and 2Π1/2 spin-orbit states. The QM component is the spin-free PES and spin-orbit coupling\udfor each state is represented by a MM-like analytic potential fit to spin-orbit electronic structure\udcalculations. Coupled-cluster single double and perturbative triple excitation (CCSD(T)) calculations\udare performed to obtain “benchmark” reaction energies without spin-orbit coupling. With zero-point\udenergies removed, the “experimental” reaction energy is 44 ± 5 meV for HBr+(2Π3/2) + CO2 →\udBr(2P3/2) + HOCO+, while the CCSD(T) value with spin-orbit effects included is 87 meV. Electronic\udstructure calculations were performed to determine properties of the BrHOCO+ reaction intermediate\udand [HBr· · ·OCO]+ van der Waals intermediate. The results of different electronic structure methods\udwere compared with those obtained with CCSD(T), and UMP2/cc-pVTZ/PP was found to be a\udpractical and accurate QM method to use in QM/MM direct dynamics simulations. The spin-orbit\udcoupling calculations show that the spin-free QM PES gives a quite good representation of the shape\udof the PES originated by 2Π3/2HBr+. This is also the case for the reactant region of the PES for\ud2Π1/2 HBr+, but spin-orbit coupling effects are important for the exit-channel region of this PES. A\udMM model was developed to represent these effects, which were combined with the spin-free QM\udPES