Potential energy surfaces for the HBr+ + CO2 → Br + HOCO+ reaction in the HBr+ 2Π3/2 and 2Π1/2 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.

Self-reactions in the HCl+ (DCl+) + HCl system: a state-selective investigation of the role of rotation

“…of the type reported by Hase and coworkers, 36,37 are required for a better understanding of the new results presented here. of the type reported by Hase and coworkers, 36,37 are required for a better understanding of the new results presented here.…”

“…36 Here, the generation of the appropriate multi-dimensional potential energy surface represents a formidable challenge. In fact very recently Hase and coworkers reported such a PES for the reaction system HBr + + CO 2 , 37 for which the state-selective cross sections for proton transfer have been reported by Paetow et al 12,13 Based on this PES MD calculations can now be performed as the next step. 38 The ab initio calculations providing the one dimensional reaction path presented in a previous section are a helpful input to more extensive multi-dimensional PES calculations.…”

“…Clearly, more sophisticated dynamics calculations, e.g. of the type reported by Hase and coworkers, 36,37 are required for a better understanding of the new results presented here. We hope to stimulate such calculations by this report.…”

Theoretical Study of the Dynamics of the HBr+ + CO2 → HOCO+ + Br Reaction

“…As stated in the previous section, the potential energy profile (shown in Figure ) of the HBr + + CO 2 → HOCO + + Br reaction has been characterized by Sun et al and Shoji et al In Sun et al, the potential energy profile, including the reaction energy, the H-bonded intermediates (IM), and the van der Waals intermediates (v-IM), is characterized by the coupled cluster theory with complete basis set limit (CCSD­(T)/CBS , ). In a more recent study by Shoji et al, a transition state (TS) that connects the aforementioned intermediates has been identified at a similar level of theory, making the reaction potential energy profile double-welled.…”

“…Quantum chemistry calculation has been an effective method of studying the energy profile of gas-phase reaction since its development in the early 1950s. , There have been several ab initio calculations to unravel the potential energy profile of the reaction, including the original manuscript of Paetow et al, in which the relative potential energies of the products and the hydrogen bond intermediate, IM ([BrH···OCO] + ), were calculated with CCSD­(T)/TZ2P from the structures optimized with MP2/TZ2P. Later, Sun et al studied the potential energy profile of this reaction with the CCSD­(T) geometry optimization and reported another van der Waals intermediate ([HBr···OCO] + ), v-IM, which lies about 0.13 eV above the IM. Recently, Shoji et al have identified a transition state (TS) that connects the aforementioned IM and v-IM.…”