The rate of the F + H2 reaction at very low temperatures.

The prototypical F + H2 â†’ HF + H reaction possesses a substantial energetic barrier (~800 K) and might therefore be expected to slow to a negligible rate at low temperatures. It is, however, the only source of interstellar HF, which has been detected in a wide range of cold (10-100 K) environments. In fact, the reaction does take place efficiently at low temperatures due to quantum-mechanical tunnelling. Rate constant measurements at such temperatures have essentially been limited to fast barrierless reactions, such as those between two radicals. Using uniform supersonic hydrogen flows we can now report direct experimental measurements of the rate of this reaction down to a temperature of 11 K, in remarkable agreement with state-of-the-art quantum reactive scattering calculations. The results will allow a stronger link to be made between observations of interstellar HF and the abundance of the most common interstellar molecule, H2, and hence a more accurate estimation of the total mass of astronomical objects.

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 thermodynamics of the elusive HO3 radical.

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 (Delta(f)H;{\overline{);\circ }}$$). Using a supersonic flow apparatus, we have observed by means of laser-induced fluorescence the decay of OH radicals in the presence of O2 at temperatures between 55.7 and 110.8 kelvin (K). Between 87.4 and 99.8 K, the OH concentration approached a nonzero value at long times, allowing equilibrium constants for the reaction with O2 to be calculated. Using expressions for the equilibrium constant from classical and statistical thermodynamics, and values of partition functions and standard entropies calculated from spectroscopic data, we derived values of D0 = (12.3 +/- 0.3) kilojoules per mole and Delta(f)H;{\overline{);\circ }}$$ (298 K) = (19.3 +/- 0.5) kilojoules per mole. The atmospheric implications of HO3 formation are therefore very slight.