Disproportionation Channel of the Self-reaction of Hydroxyl Radical, OH + OH → H2O + O, Revisited

The rate constant of the disproportionation channel 1a of the self-reaction of hydroxyl radicals OH + OH → H2O + O (1a) was measured at ambient temperature as well as over an extended temperature range to resolve the discrepancy between the IUPAC recommended value (k 1a = 1.48 × 10–12 cm3 molecule–1 s–1, discharge flow system, Bedjanian et al. J. Phys. Chem. A 1999, 103, 7017 ) and a factor of ca. 1.8 higher value by pulsed laser photolysis (2.7 × 10–12 cm3 molecule–1 s–1, Bahng et al. J. Phys. Chem. A 2007, 111, 3850 , and 2.52 × 10–12 cm3 molecule–1 s–1, Altinay et al. J. Phys. Chem. A 2014, 118, 38 ). To resolve this discrepancy, the rate constant of the title reaction was remeasured in three laboratories using two different experimental techniques, namely, laser-pulsed photolysis–transient UV absorption and fast discharge flow system coupled with mass spectrometry. Two different precursors were used to generate OH radicals in the laser-pulsed photolysis experiments. The experiments confirmed the low value of the rate constant at ambient temperature (k 1a = (1.4 ± 0.2) × 10–12 cm3 molecule–1 s–1 at 295 K) as well as the V-shaped temperature dependence, negative at low temperatures and positive at high temperatures, with a turning point at 427 K: k 1a = 8.38 × 10–14 × (T/300)1.99 × exp­(855/T) cm3 molecule–1 s–1 (220–950 K). Recommended expression over the 220–2384 K temperature range: k 1a = 2.68 × 10–14 × (T/300)2.75 × exp­(1165/T) cm3 molecule–1 s–1 (220–2384 K).