Unimolecular reaction chemistry of a charge-tagged beta-hydroxyperoxyl radical

beta-Hydroxyperoxyl radicals are formed during atmospheric oxidation of unsaturated volatile organic compounds such as isoprene. They are intermediates in the combustion of alcohols. In these environments the unimolecular isomerization and decomposition of beta-hydroxyperoxyl radicals may be of importance, either through chemical or thermal activation. We have used ion-trap mass spectrometry to generate the distonic charge-tagged beta-hydroxyalkyl radical anion, (CH2C)-C-center dot(OH)(CH3)CH2C(O)O-, and investigated its subsequent reaction with O-2 in the gas phase under conditions that are devoid of complicating radical-radical reactions. Quantum chemical calculations and master equation/RRKM theory modeling are used to rationalize the results and discern a reaction mechanism. Reaction is found to proceed via initial hydrogen abstraction from the gamma-methylene group and from the beta-hydroxyl group, with both reaction channels eventually forming isobaric product ions due to loss of either (OH)-O-center dot + HCHO or (OH)-O-center dot + CO2. Isotope labeling studies confirm that a 1,5-hydrogen shift from the beta-hydroxyl functionality results in a hydroperoxyalkoxyl radical intermediate that can undergo further unimolecular dissociations. Furthermore, this study confirms that the facile decomposition of beta-hydroxyperoxyl radicals can yield (OH)-O-center dot in the gas phase.