Experimental 300 K Measurement of the Rate Constant of the Reaction OH + BrO f Products Denis J. Bogan, †,‡,§ R. Peyton Thorn, †,‡ Fred L. Nesbitt, †,| and Louis J. Stief* ,† Laboratory for Extraterrestrial Physics (Code 690), NASA/Goddard Space Flight Center, Greenbelt, Maryland 20771, Department of Chemistry, The Catholic UniVersity of America, Washington, DC 20064, and Department of Natural Sciences, Coppin State College, Baltimore, Maryland 21216 ReceiVed: March 15, 1996; In Final Form: June 5, 1996 X The results reported herein are believed to be the first experimental measurements of the rate constant for the reaction OH + BrO f products (eq 1), which was found to be (7.5 ( 4.2) × 10 -11 cm 3 molecule -1 s -1 (2σ) 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 + H 2 O f 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 f Br + HO 2 (eq 1a), OH + Br 2 f HOBr + Br (eq 2), and BrO + BrO f products (eq 6). The mechanism of reaction 1 is believed to be OH + BrO f [HOOBr] # f Br + HO 2 , ΔH R )-10 kcal mol -1 (eq 1a) and OH + BrO f [HOOBr] # f HBr + O 2 , ΔH 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 triplet surfaces in the [HOOBr] # complex by the heavy Br atom. 1. Introduction On a per atom basis, bromine is considerably more destructive of stratospheric ozone than is chlorine. 1 Formation of reactive BrO x radicals within the stratosphere is initiated by photo- dissociation of methyl bromide (CH 3 Br) and the halons CF 3 Br, CF 2 ClBr, and CF 2 BrCF 2 Br. 2 The major route for participation of the BrO radical in the chemistry of the lower stratosphere, as originally proposed by Yung et al., 3 is through its synergistic relationship with ClO in the catalytic cycle: It is now known 4 that only about 8% of the overall BrO + ClO reaction yields BrCl, which is immediately photodissociated in the stratosphere to Br + Cl. The major process yields Br + OClO/ClOO. The other key BrO cycle in this region involves reaction with HO 2 : 5 Above 25 km altitude, the reaction with atomic oxygen becomes the dominant BrO-catalyzed O 3 destruction cycle. 6 The reaction is potentially important in the partitioning of stratospheric bromine. The reaction could also affect HO x partitioning, but bromine concentrations in the stratosphere are probably too low for the effect to be significant. 2 There have been no previously * Author to whom correspondence should be addressed: e-mail, u1ljs@lepvax.gsfc.nasa.gov. † NASA/Goddard Space Flight Center. ‡ The Catholic University of America. § NAS/NRC Senior Research Associate. | Coppin State College. X Abstract published in AdVance ACS Abstracts, August 1, 1996. BrO + ClO f Br + Cl + O 2 Br + O 3 f BrO + O 2 Cl + O 3 f ClO + O 2 Net: 2O 3 f 3O 2 BrO + HO 2 f HOBr + O 2 HOBr + hν f OH + Br OH + O 3 f HO 2 + O 2 Br + O 3 f BrO + O 2 Net: 2O 3 f 3O 2 BrO + O f Br + O 2 Br + O 3 f BrO + O 2 Net: O + O 3 f 2O 2 OH + BrO f products (1) 14383 J. Phys. Chem. 1996, 100, 14383-14389 S0022-3654(96)00807-6 CCC: $12.00 © 1996 American Chemical Society