Self‐Reaction of HO₂ and DO₂: Negative temperature dependence and pressure effects

Abstract

The negative temperature dependence, pressure dependence, and isotope effects of the self-reaction of HO z are modeled~using RRKM theory, by assuming that the reac· tion proceeds via a cyclic, hydrogen-bonded intermediate. The negative temperature dependence is due to a tight transition state, with a negative threshold energy relative to reactants, for decomposition of the intermediate to products. A symmetric structure for this transition state reproduces the observed isotope effect. The weak pressure dependence for DO z self-reaction is due to the approach to the high-pressure limit. Addition of a polar collision partner, such as ammonia or water vapor, enhances the rate by forming an adduct that reacts to produce deexcited intermediate. A detailed model is presented to fit the data for these effects. Large ammonia concentrations should make it possible to reach the high·pressure limit of the self-reaction of HO z .

The effect of methanol on the self reaction of HO2 radicals

“…The data obtained in this work are consistent with the mechanism suggested by Mozurkewich and Benson [7] for the self reaction of HO,. The calculated energies for the complexes of HOz with methanol, water, and ammonia are all comparable with the energy of a normal hydrogen bond.…”

“…The most recent work is by Mozurkewich and Benson [7], 1985, who suggest a mechanism involving doubly hydrogen bonded cyclic dimers of H 0 2 as intermediates. They provide a plausible explanation of the observed kinetics, including the effect of water and ammonia.…”

Kinetics of HO2 + HO2 → H2O2 + O2: Implications for Stratospheric H2O2

“…For methanol, the scheme can be described as where k 4 > k 1 . The temperature dependence of k ″ can be shown to result from the enthalpy change due to equilibrium Mozurkewich and Benson [1985]. The measured E a / R for k ″ in the our experiment was equivalent to (−9.08 ± 0.48) kcal mol −1 , which is consistent with the stabilization energy of a strongly hydrogen‐bonded complex (L. E. Christensen et al, manuscript in preparation, 2002B).…”

Equilibrium constant of the HO2‐H2O complex formation and kinetics of HO2 + HO2‐H2O: Implications for tropospheric chemistry

“…Experimental results from Aloisio et al [2000] are denoted by triangles. Theoretical estimations reported by Mozurkewich and Benson [1985] (solid line), Aloisio and Francisco [1998] (dotted line), Lendvay [2001] (dashed line), and Zhu and Lin [2003] (dash‐dotted line) are also shown.…”