Disproportionation channel of self-reaction of hydroxyl radical, OH + OH → H₂O + O, studied by time-resolved oxygen atom trapping

The disproportionation channel of the self-reaction of hydroxyl radicals, OH + OH → H₂O + O (1a) was studied using pulsed laser photolysis coupled to transient UV-vis absorption spectroscopy over the 298-414 K temperature and 3-10 bar pressure ranges (bath gas He). To distinguish channel 1a from the recombination channel 1b, OH + OH → H₂O₂ (1b), time-resolved trapping of oxygen atoms, produced in channel 1a, was used. The ozone produced in the reaction of oxygen atoms with molecular oxygen was measured using strong UV absorption at 253.7 nm. The results of this study (k_1a = (1.38 ± 0.20) × 10^-12 (T/300) ^-0.76 confirm the IUPAC recommended value of Bedjanian et al. (J. Phys. Chem. A1999, 103, 7017-7025), as well as the negative temperature dependence over the temperature range studied, and do not confirm the ca. 1.8 higher value obtained in the most recent study of Bahng et al. (J. Phys. Chem. A2007, 111, 3850-3861). The V-shaped temperature dependence of k_1a based on combined current and previous studies in the temperature range of 233-2380 K is k_1a = (5.1 exp(-T/190 K) + 0.30(T/300 K) ^1.73) × 10^-12 cm³ molecule^-1 s^-1.