EXPERIMENTAL 300 K MEASUREMENT OF THE RATE-CONSTANT OF THE REACTION OH-] PRODUCTS(BRO)

The results reported herein are believed to be the first experimental measurements of the rate constant for the reaction OH + BrO --> produc ts (eq 1), which was found to be (7.5 +/- 4.2) x 10(-11) cm(3) molecul e(-1) s(-1) (2 sigma) at 300 K and 1 Torr. The mean value is 7 times l arger than the estimate in the NASA stratospheric database, which curr ently finds widespread use to model the chemistry that controls strato spheric ozone concentrations. The reactant radicals were prepared in s eparate flow reactors and mixed in the main flow reactor, OH was prepa red by F + H2O --> OH + HF, and BrO was prepared by passing dilute mix tures of He/Br-2/O-2 through a microwave discharge. The composition of the gas mixture was adjusted empirically to minimize the effluent con centration of Br-2. Beam-sampling mass spectrometry supplemented by ch emical titration techniques was used to measure atom and radical conce ntrations. The rate constant for reaction 1 was obtained from a least- squares fit of the observed BrO concentrations as a function of time t o a numerical model of relevant reactions. Known values were used for all other rate constants while k(1) was fitted. Just three reactions s ignificantly affect the fitted value of k(1): OH + BrO --> Br + HO2 (e q 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 pro duct channel, with eq 1b hindered by a large activation energy for acc ess to the HBr + O-2 products. The magnitude of k(1), approximately on e-half of the gas kinetic limit, is attributed to the promotion of eff icient spin-orbit mixing of singlet and tripler surfaces in the [HOOBr ]# complex by the heavy Br atom.