Theoretical study of the chemical reaction mechanism and rate of SFn− + H2O (n = 3–6) under discharge

Both the chemical reaction mechanism and rate can largely explain the formation mechanism of the by-products of SF6. By understanding this mechanism, we can develop and improve models of the chemical kinetics of SF6 decomposition under discharge. Using quantum chemistry, this study compares the reaction mechanisms and rates of SF6−, SF5−, SF4−, and SF3− and H2O under discharge at 298 K–12 000 K and reveals the formation mechanism of their anionic by-products, i.e., SOF4−, SOF3−, SOF2−, and SOF−. The key parameters such as the reaction equation and reaction rate to improve the chemical kinetic models under partial and arc discharges were then obtained. In this study, the structural optimizations, vibrational frequencies, and zero-point energies of the reactants, products, complexes, intermediates, and transition states were calculated at the B3LYP/6-311G(d,p) level. The single-point energies of all species were calculated at the CCSD(T)/aug-cc-PVTZ level. The strengths and sites of weak interactions were determined from the electrostatic potential of the molecular surface, and the reaction rates were obtained using transition state theory. It has been found that SF6−, SF5−, SF4−, and SF3− combined with H2O to form weak-interaction complexes dominated by hydrogen bonding, thus providing the initial conditions for R1, R2, R3, and R4, respectively. All four reactions were composed of multiple elementary reactions with the first step being the rate-determining step. Moreover, compared to their corresponding reactions of SF5, SF4, SF3, and SF2 with H2O, they achieved lower potential energy barriers and higher reaction rates. Note that the reaction rates decreased in the following order: R3 > R2 > R1 > R4.