The Quest for the Hydroxyl-Peroxy Radical

Abstract

The hydroxyl-peroxy radical, HO₃, has been the subject of numerous theoretical and experimental studies, not least because of its potential importance in atmospheric chemistry. Nevertheless, it has proved difficult to establish its stability: that is, values of D₀(HO-O₂), the HO-O₂ bond dissociation energy or, equivalently, Δ_fH^o₂₉₈(HO₃), its standard enthalpy of formation. In this short article, a review is given of the results of experiments and calculations on HO₃, culminating in experiments performed at low temperatures (55.9–99.8 K) in a CRESU apparatus, which establish both (i) rate constants for the formation of HO₃ in the three-body reaction, OH + O₂ + M → HO₃ + M, and (ii) equilibrium constants from which values of D₀(HO-O₂) and Δ_fH^o₂₉₈(HO₃) can be derived – making use of spectroscopic data for HO₃, from which partition functions and standard entropies can be calculated. It is shown that these absolute experimental values for D₀(HO-O₂) and Δ_fH^o₂₉₈(HO₃) are: (i) in agreement with the limiting values determined in experiments by Lester and co-workers, and (ii) in fair agreement with the most recent theoretical results. Furthermore, these thermodynamic data make it clear that only very small fractions of OH will be bound with O₂ under the conditions found at all levels of the Earth’s atmosphere.