Potential energy surfaces for the HBr+ + CO2 - Br + HOCO+ reaction in the HBr+2 Pi(3/2) and (2)Pi(1/2) spin-orbit states

摘要

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 (2)Pi(3/2) and (2)Pi(1/2) 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+((2)Pi(3/2)) + CO2 -> Br(P-2(3/2)) + 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 center dot center dot center dot 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 (2)Pi 3/2HBr+. This is also the case for the reactant region of the PES for (2)Pi(1/2) 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. (C) 2015 AIP Publishing LLC.