Kinetic and mechanistic studies of the recombination of OH with NO ₂: Vibrational deactivation, isotopic scrambling and product isomer branching ratios

The kinetics and mechanism of the three-body recombination of OH with NO₂ were studied using a pulsed laser photolysis - pulsed laser induced fluorescence technique. The rate coefficients for deactivation of vibrationally excited OH (v = 1-5) by NO₂ were found to be independent of vibrational level with a value of (6.4 ± 0.3) × 10^-11 cm³ molecule^-1 s^-1 at 298 K. The rate coefficient for reaction of ¹⁸OH with NO₂ was measured and found to be much faster than for unlabeled OH with a "zero pressure" rate of 1 × 10^-11 cm³ molecule ^-1 s^-1 at 298 K and 273 K. Observation of temporal profiles of ¹⁶OH and ¹⁸OH suggest that isotopic scrambling in the initially formed [H¹⁸ON¹⁶O₂] complex is complete on the microsecond time scale of our experiments. The rate coefficient for reaction of unlabeled OH with NO₂ was measured at 413 K in 400 Torr of He. Biexponential temporal profiles were obtained and are consistent with a 10 ± 3% yield of the weakly bound HOONO isomer.