New Measurements and Calculations on the Kinetics of an Old Reaction: OH + HO₂ → H₂O + O₂

Literature rate coefficients for the prototypical radical-radical reaction at 298 K vary by close to an order of magnitude; such variations challenge our understanding of fundamental reaction kinetics. We have studied the title reaction at room temperature via the use of laser flash photolysis to generate OH and HO₂ radicals, monitoring OH by laser-induced fluorescence using two different approaches, looking at the direct reaction and also the perturbation of the slow OH + H₂O₂ reaction with radical concentration, and over a wide range of pressures. Both approaches give a consistent measurement of k₁,₂₉₈K ∼1 × 10⁻¹¹ cm3 molecule⁻¹ s⁻¹, at the lowest limit of previous determinations. We observe, experimentally, for the first time, a significant enhancement in the rate coefficient in the presence of water, k1,H₂O, 298K = (2.17 ± 0.09) × 10⁻²⁸ cm⁶ molecule⁻² s⁻¹, where the error is statistical at the 1σ level. This result is consistent with previous theoretical calculations, and the effect goes some way to explaining some, but not all, of the variation in previous determinations of k₁,₂₉₈K. Supporting master equation calculations, using calculated potential energy surfaces at the RCCSD(T)-F12b/CBS//RCCSD/aug-cc-pVTZ and UCCSD(T)/CBS//UCCSD/aug-cc-pVTZ levels, are in agreement with our experimental observations. However, realistic variations in barrier heights and transition state frequencies give a wide range of calculated rate coefficients showing that the current precision and accuracy of calculations are insufficient to resolve the experimental discrepancies. The lower value of k₁,₂₉₈K is consistent with experimental observations of the rate coefficient of the related reaction, Cl + HO₂ → HCl + O₂. The implications of these results in atmospheric models are discussed.