The Quest for the Hydroxyl-Peroxy Radical

Abstract

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 D 0(HO-O2), the HO-O2 bond dissociation energy or, equivalently, Δ fH o 298(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 o 298(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 D 0(HO-O2) and Δ fH o 298(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.