The Quest for the Hydroxyl-Peroxy Radical

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 stibility that is values of D-0(HO-O-2) the HO-O-2 bond dissociation energy or equivalently Delta H-f(298)o(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 + O-2 + M -> HO3 + M and (ii) equilibrium constants from which values of D-0(HO-O-2) and Delta H-f(298)o(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-O-2) and Delta H-f(298)o(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 tractions of OH will be bound with O-2 under the conditions found at all levels of the Earth s atmosphere