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Mechanistic Study of the CH3O2• + HO2• → CH3O2H + O2 Reaction in the Gas Phase. Computational Evidence for the Formation of a Hydrogen-Bonded Diradical Complex

AuthorsAnglada Rull, Josep M.; Olivella, Santiago CSIC; Solé, Albert
KeywordsAlkylperoxy radicals
Hydroperoxy radical
Key reaction
Gas-phase reaction
Diradical species
Strong negative temperature
Hydrogen-bonded diradical complex
Issue Date18-Apr-2006
PublisherAmerican Chemical Society
CitationJournal of Physical Chemistry A 110(18): 6073-6082 (2006)
AbstractIn an attempt to understand the mechanism of the reaction of alkylperoxy radicals with hydroperoxy radical, a key reaction in both atmospheric and combustion chemistry, the singlet and triplet potential energy surfaces (PESs) for the gas-phase reaction between CH3O2• and HO2• leading to the formation of CH3OOH and O2 have been investigated by means of quantum-mechanical electronic structure methods (CASSCF and CASPT2). In addition, standard transition state theory calculations have been carried out with the main purpose of a qualitative description of the strong negative temperature dependence observed for this reaction. All the pathways on both the singlet and triplet PESs consist of a reversible first step involving the barrierless formation of a hydrogen-bonded prereactive complex, followed by the irreversible formation of products. This complex is a diradical species where the two unpaired electrons are not used for bonding and is lying about 5 kcal/mol below the energy of the reactants at 0 K. The lowest energy reaction pathway occurs on the triplet PES and involves the direct H-atom transfer from HO2 to CH3O2 in the diradical complex through a transition structure lying 3.8 kcal/mol below the energy of the reactants at 0 K. Contradicting the currently accepted interpretation of the reaction mechanism, the observed strong negative temperature dependence of the rate constant is due to the formation of the hydrogen-bonded diradical complex rather than a short-lived tetraoxide intermediate CH3OOOOH.
Description10 pages, 2 tables, 9 figures.
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