Experimental 300 K measurement of the rate constant of the reaction OH+BrO-> products

The results reported herein are believed to be the first experimental measurements of the rate constant for the reaction OH + BrO --> products (eq 1), which was found to be (7.5 +/- 4.2) x 10(-11) cm(3) molecule(-1) s(-1) (2 sigma) at 300 K and 1 Torr. The mean value is 7 times larger than the estimate in the NASA stratospheric database, which currently finds widespread use to model the chemistry that controls stratospheric ozone concentrations. The reactant radicals were prepared in separate flow reactors and mixed in the main flow reactor, OH was prepared by F + H2O --> OH + HF, and BrO was prepared by passing dilute mixtures of He/Br-2/O-2 through a microwave discharge. The composition of the gas mixture was adjusted empirically to minimize the effluent concentration of Br-2. Beam-sampling mass spectrometry supplemented by chemical titration techniques was used to measure atom and radical concentrations. The rate constant for reaction 1 was obtained from a least-squares fit of the observed BrO concentrations as a function of time to a numerical model of relevant reactions. Known values were used for all other rate constants while k(1) was fitted. Just three reactions significantly affect the fitted value of k(1): OH + BrO --> Br + HO2 (eq 1a), OH + Br-2 --> HOBr + Br (eq 2), and BrO + BrO --> products (eq 6). The mechanism of reaction 1 is believed to be OH + BrO --> [HOOBr](#) --> Br + HO2, Delta H-R = -10 kcal mol(-1) (eq 1a) and OH + BrO --> [HOOBr](#) --> HBr + O-2, Delta H-R = -48 kcal mol(-1) (1b), where [HOOBr](#) denotes a short-lived vibrationally excited addition complex. It is argued that eq 1a is the predominant and perhaps exclusive product channel, with eq 1b hindered by a large activation energy for access to the HBr + O-2 products. The magnitude of k(1), approximately one-half of the gas kinetic limit, is attributed to the promotion of efficient spin-orbit mixing of singlet and tripler surfaces in the [HOOBr]# complex by the heavy Br atom.