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.


Chemistry and Biochemistry

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© 2015 AIP Publishing LLC


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

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The Journal of chemical physics