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.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry