Meryem Tizniti

The thermodynamics of the elusive HO3 radical.

The Weakness of HO3 OH radicals play a critical role in the chemistry of Earths atmosphere. Understanding atmospheric reaction networks thus requires an accurate knowledge of OH sources and sinks. One vexing question has been whether or not a significant pool of OH binds temporarily with oxygen to form HO3. Le Picard et al. (p. 1258) have succeeded in measuring the equilibrium constant for this reaction using sensitive fluorescent tracking of OH in a laboratory apparatus. This measurement was then used to quantify the strength of the O2–OH bond, which was found to be too weak for the complexation to play a major role in the atmosphere. Molecular kinetics shows that hydrogen superoxide is too unstable to play a major role in atmospheric chemistry. The role of HO3 as a temporary reservoir of atmospheric OH radicals remains an open question largely because of the considerable uncertainty in the value of the dissociation energy of the HO−O2 bond (D0) or, equivalently, the standard enthalpy of formation of HO3 (ΔfH° ). Using a supersonic flow apparatus, we have observed by means of laser-induced fluorescence the decay of OH radicals in the presence of O2 at temperatures between 55.7 and 110.8 kelvin (K). Between 87.4 and 99.8 K, the OH concentration approached a nonzero value at long times, allowing equilibrium constants for the reaction with O2 to be calculated. Using expressions for the equilibrium constant from classical and statistical thermodynamics, and values of partition functions and standard entropies calculated from spectroscopic data, we derived values of D0 = (12.3 ± 0.3) kilojoules per mole and ΔfH° (298 K) = (19.3 ± 0.5) kilojoules per mole. The atmospheric implications of HO3 formation are therefore very slight.

Low temperature kinetics: the association of OH radicals with O2

We report the first measurements of rate constants for the reaction in which OH radicals associate with O(2) to form HO(3). Our recent measurements (Science, 2010, 328, 1258) have shown that the HO-O(2) bond dissociation energy is only (12.3 ± 0.3) kJ mol(-1). Consequently, above ca. 90 K under attainable experimental conditions, the rate of the reverse dissociation of HO(3) becomes comparable to, and then greater than, the rate of the forward association reaction. We have used the CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme) method to access low temperatures and have explored the kinetics of OH + O(2) + M → HO(3) + M in two series of experiments. At temperatures between 55.9 and 79.2 K, the OH radicals, created by pulsed laser photolysis of H(2)O(2) and observed by laser-induced fluorescence, decayed by pseudo-first-order kinetics to effectively zero concentration at longer times. The third-order rate constants derived from these experiments fit the expression: k(3rd)(o) (T) = (4.2 ± 1.9) × 10(-34) (T/298 K)(-(3.5 ± 0.3)) cm(6) molecule(-2) s(-1). At temperatures between 87.4 and 99.8 K, rate constants for the association reaction were determined allowing for the significant occurrence of the reverse dissociation reaction. The values of the derived rate constants are consistent with those obtained in the lower temperature range, though the errors are larger. The experimental values of k(3rd)(o) (T) are compared with (a) those for other association reactions involving species of similar complexity, and (b) values of k(3rd)(o) (T) estimated according to both the energy transfer (ET) and the radical-complex (RC) mechanisms. We conclude that the RC mechanism probably makes the major contribution to the association of OH + O(2) at the low temperatures of our experiments.

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, ΔfHo298(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 ΔfHo298(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 ΔfHo298(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.