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

Abstract

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 k 1,298K ∼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, k 1,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 k 1,298K. 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 k 1,298K 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.

Accurate Kinetic Studies of OH + HO2 Radical–Radical Reaction through Direct Measurement of Precursor and Radical Concentrations with High-Resolution Time-Resolved Dual-Comb Spectroscopy

“…In addition, by simultaneous analysis of the OH and HO 2 time traces obtained under different experimental conditions, the uncertainty of k OH+HOd 2 caused by the errors of the line strengths of OH (7%) and HO 2 (4%), spectral analysis (4%) as well as the effective absorption path (7%) was evaluated to be ∼9%. Considering all possible errors, the overall uncertainty of k OH+HOd 2 was estimated to be ∼11% and the rate coefficient k OH+HOd 2 was hence obtained to be (1.10 ± 0.12) × 10 −10 cm 3 molecule −1 s −1 at 296 K, which is in excellent agreement with the result reported by Assaf et al, 6 but it is ∼10 times larger than the most recent experimental value from Speak et al 7 Moreover, experiments in the presence of water were also conducted in this work to explore the possible enhancement effect on the rate coefficient of the reaction OH + HO 2 . By employing rotationally resolved infrared absorption spectra, the concentrations of both H 2 O 2 and H 2 O in the reactor were calibrated and measured directly.…”

“…Figure 5 displays the comparison of the rate coefficient k OH+HOd 2 obtained from different experiments at near room temperature, 6−14 and the detailed descriptions of these experiments are summarized in TableS2. The rate coefficients k OH+HOd 2 are displayed in the range of (0.1−1.2) × 10 −10 cm 3 molecule −1 s −1 , and the maximum discrepancy of 13.5σ for the k OH+HOd 2 is observed between the two more recent experiments which were respectively conducted by Assaf et al6 and Speak et al7 Owing to the high reactivity of OH radicals and indirect measurements of HO 2 radicals, many studies expended considerable effort on calibration and estimation of radical concentrations through various methods and employed multiple reactions to perform cross comparisons. Each experiment would have its systematic errors, and the additional uncertainty could be increased through the indirect determination based on kinetics of multiple reactions, because each reaction might be carried out using different systems in different groups.In conclusion, we have accomplished the accurate measurement of the kinetics of the OH + HO 2 radical−radical reaction through direct determination of precursor and radical concentrations in the reaction system of the 248 nm photolysis of hydrogen peroxide.…”

“…S2. The rate coefficients k OH+HOd 2 are displayed in the range of (0.1−1.2) × 10 −10 cm 3 molecule −1 s −1 , and the maximum discrepancy of 13.5σ for the k OH+HOd 2 is observed between the two more recent experiments which were respectively conducted by Assaf et al 6 and Speak et al 7 Owing to the high reactivity of OH radicals and indirect measurements of HO 2 radicals, many studies expended considerable effort on calibration and estimation of radical concentrations through various methods and employed multiple reactions to perform cross comparisons. Each experiment would have its systematic errors, and the additional uncertainty could be increased through the indirect determination based on kinetics of multiple reactions, because each reaction might be carried out using different systems in different groups.…”

“…A brief description of the previous studies of the reaction OH + HO 2 is shown in Note S1. Although numerous investigations on the kinetics of the reaction OH + HO 2 have been conducted during the past couple of decades, − the reported rate coefficients k OH+HO 2 from both experimental and theoretical studies are scattered in a wide range of (0.1–1.7) × 10 –10 cm 3 molecule –1 s –1 at around room temperature, as shown in Figure S1. In particular, the most recent measurements of the rate coefficient carried out by Assaf et al and Speak et al show a significant discrepancy between each other by a factor of 10.…”

“…Although numerous investigations on the kinetics of the reaction OH + HO 2 have been conducted during the past couple of decades, − the reported rate coefficients k OH+HO 2 from both experimental and theoretical studies are scattered in a wide range of (0.1–1.7) × 10 –10 cm 3 molecule –1 s –1 at around room temperature, as shown in Figure S1. In particular, the most recent measurements of the rate coefficient carried out by Assaf et al and Speak et al show a significant discrepancy between each other by a factor of 10. Assaf et al employed laser-induced fluorescence (LIF) and near-infrared cavity ring down spectroscopy (CRDS) to respectively monitor the OH and HO 2 radicals generated upon laser flash photolysis (LFP) of the mixture of H 2 O 2 /(COCl) 2 /CH 3 OH/O 2 to investigate the kinetics of the reaction OH + HO 2 .…”