Literature rate coefficients
for the prototypical radical–radical
reactionat 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 HO2radicals, monitoring OH by laser-induced fluorescence using two
different approaches, looking at the direct reaction and also the
perturbation of the slow OH + H2O2reaction
with radical concentration, and over a wide range of pressures. Both
approaches give a consistent measurement ofk1,298K∼1 × 10–11cm3molecule–1s–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) × 10–28cm6molecule–2s–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
ofk1,298K. 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 ofk1,298Kis 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.