Mechanism and kinetics of Br + HO₂ → HBr + O₂ and Br + H₂O₂ → products over the temperature range 260-390 K

A discharge flow system employing simultaneous, direct detection of HO₂ and Br using laser magnetic resonance and resonance fluorescence, respectively, is used to study the kinetics of the title reactions. Over the temperature range 260 to 390 K, decays of HO₂ in excess Br yield the rate constant k₁ = (1.4 ± 0.2) × 10^-11 exp[(-590 ± 140)/T] cm³ molecule^-1 s^-1 for Br + HO₂ → HBr + O₂. Experiments were carried out at 0.8 to 2 Torr total pressure in helium and in argon with multiple Br sources. The cited uncertainties include an estimate of the systematic errors at the 95% confidence level. Additional studies place an upper limit of 5 × 10^-16 cm³ molecule^-1 s^-1 on the overall rate constant and product channels forming OH and HO₂ for the reaction Br + H₂O₂ → products from 298 to 378 K. These results, in conjunction with data from other HO₂ reactions, suggest that X + HO₂ → HX + O₂ reactions (where X = F, Cl, Br, and OH) proceed by direct attack on the H-O σ bond, rather than by radical addition to form a (HOOX)* intermediate followed by rearrangement and elimination of HX and O₂ from a modified four-center transition state. The implications of these studies for stratospheric chemistry are discussed.