title: Potential energy surfaces for the HBr(+) + CO2 → Br + HOCO(+) reaction in the HBr(+)  (2)Π3/2 and (2)Π1/2 spin-orbit states.
abstract: 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)Π3/2 and (2)Π1/2 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)Π3/2) + CO2 → Br((2)P3/2) + 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 (2)Π3/2HBr(+). This is also the case for the reactant region of the PES for (2)Π1/2 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.
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, USA.
authors.forename: Rui
authors.initials: R
authors.lastname: Sun
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, University of Pisa, Pisa, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Granucci
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, USA.
authors.forename: Amit K
authors.initials: AK
authors.lastname: Paul
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, USA.
authors.forename: Matthew
authors.initials: M
authors.lastname: Siebert
authors.affiliation: Department of Engineering, Swarthmore College, Swarthmore, Pennsylvania 19081, USA.
authors.forename: Hongliang J
authors.initials: HJ
authors.lastname: Liang
authors.affiliation: Department of Chemistry, Haverford College, Haverford, Pennsylvania 19041, USA.
authors.forename: Grace
authors.initials: G
authors.lastname: Cheong
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, USA.
authors.forename: William L
authors.initials: WL
authors.lastname: Hase
authors.affiliation: Dipartimento di Chimica e Chimica Industriale, University of Pisa, Pisa, Italy.
authors.forename: Maurizio
authors.initials: M
authors.lastname: Persico