Quantum mechanical (QM) + molecular mechanics (MM) models are developed to represent potential energy surfaces (PESs) for the HBr+ + CO2 → Br + HOCO+ reaction with HBr+ in the 2Π3/2 and 2Π1/2 spin-orbit states. The QM component is the spin-free PES and spin-orbit coupling for each state is represented by a MM-like analytic potential fit to spin-orbit electronic structure calculations. Coupled-cluster single double and perturbative triple excitation (CCSD(T)) calculations are performed to obtain “benchmark” reaction energies without spin-orbit coupling. With zero-point energies removed, the “experimental” reaction energy is 44 ± 5 meV for HBr+(2Π3/2) + CO2 → Br(2P3/2) + HOCO+, while the CCSD(T) value with spin-orbit effects included is 87 meV. Electronic structure calculations were performed to determine properties of the BrHOCO+ reaction intermediate and [HBr⋯OCO]+ van der Waals intermediate. The results of different electronic structure methods were compared with those obtained with CCSD(T), and UMP2/cc-pVTZ/PP was found to be a practical and accurate QM method to use in QM/MM direct dynamics simulations. The spin-orbit coupling calculations show that the spin-free QM PES gives a quite good representation of the shape of the PES originated by 2Π3/2HBr+. This is also the case for the reactant region of the PES for 2Π1/2 HBr+, but spin-orbit coupling effects are important for the exit-channel region of this PES. A MM model was developed to represent these effects, which were combined with the spin-free QM PES.
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spin-orbit interactions, potential energy surfaces, electronic-structure theory, zero point energy, coupled-cluster methods, density functional theory, molecular mechanics, computer simulation, correlation-consistent basis sets, electronic structure methods
Sun, Rui, Giovanni Granucci, Amit K. Paul, Matthew Siebert, Hongliang J. Liang, Grace Cheong, William L. Hase, and Maurizio Persico. "Potential energy surfaces for the HBr++ CO2→ Br+ HOCO+ reaction in the HBr+ 2Π3/2 and 2Π1/2 spin-orbit states." The Journal of chemical physics 142, no. 10 (2015): 104302.
The Journal of chemical physics