Halving the Barrier to Gas-Phase Oxidation of Bromide by Ozone

Recent atmospheric measurements indicate that bromine and iodine may be responsible for up to 72% of halogen-induced ozone loss near the tropopause, yet there is ongoing uncertainty regarding the multiphase chemistry of bromide and iodide anions in ozone depletion. Here, we demonstrate the unequivocal ozone-dependence of the archetype ¹Br^- + ¹O₃ reaction, which proceeds with an experimental rate constant of 8.9 (±4.4) × 10^-15 cm³ molecule^-1 s^-1. The reaction mechanism is revised to proceed via a singlet transition state with a rate-limiting barrier of +22.1 kJ mol^-1 -half that of prior estimates -prior to facile spin-crossing to yield ¹BrO^- + ³O₂. Statistical rate modeling using this new barrier height predicts a rate constant of 5.7 × 10^-15 cm³ molecule^-1 s^-1, which is in excellent agreement with the experiment. This reconciliation of the kinetics for the intrinsic gas-phase reaction will enable systematic evaluation of temperature, pressure, and solvation effects on this ion-molecule chemistry and thus inform the impact of halide anion chemistry on atmospheric ozone.