title: Low temperature kinetics: the association of OH radicals with O2.
abstract: We report the first measurements of rate constants for the reaction in which OH radicals associate with O(2) to form HO(3). Our recent measurements (Science, 2010, 328, 1258) have shown that the HO-O(2) bond dissociation energy is only (12.3 ± 0.3) kJ mol(-1). Consequently, above ca. 90 K under attainable experimental conditions, the rate of the reverse dissociation of HO(3) becomes comparable to, and then greater than, the rate of the forward association reaction. We have used the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method to access low temperatures and have explored the kinetics of OH + O(2) + M → HO(3) + M in two series of experiments. At temperatures between 55.9 and 79.2 K, the OH radicals, created by pulsed laser photolysis of H(2)O(2) and observed by laser-induced fluorescence, decayed by pseudo-first-order kinetics to effectively zero concentration at longer times. The third-order rate constants derived from these experiments fit the expression: k(3rd)(o) (T) = (4.2 ± 1.9) × 10(-34) (T/298 K)(-(3.5 ± 0.3)) cm(6) molecule(-2) s(-1). At temperatures between 87.4 and 99.8 K, rate constants for the association reaction were determined allowing for the significant occurrence of the reverse dissociation reaction. The values of the derived rate constants are consistent with those obtained in the lower temperature range, though the errors are larger. The experimental values of k(3rd)(o) (T) are compared with (a) those for other association reactions involving species of similar complexity, and (b) values of k(3rd)(o) (T) estimated according to both the energy transfer (ET) and the radical-complex (RC) mechanisms. We conclude that the RC mechanism probably makes the major contribution to the association of OH + O(2) at the low temperatures of our experiments.
authors.affiliation: Institut de Physique de Rennes, Equipe: Astrochimie Expérimentale, UMR 6251 du CNRS, Bât. 11c-Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France.
authors.forename: Meryem
authors.initials: M
authors.lastname: Tizniti
authors.forename: Sébastien D
authors.initials: SD
authors.lastname: Le Picard
authors.forename: André
authors.initials: A
authors.lastname: Canosa
authors.forename: Ian R
authors.initials: IR
authors.lastname: Sims
authors.forename: Ian W M
authors.initials: IW
authors.lastname: Smith