Unimolecular reaction chemistry of a charge-tagged beta-hydroxyperoxyl radicalElectronic supplementary information (ESI) available: Mass spectrometry of the photodissociation and collision induced dissociation to produce the target radical anion. Master equation simulation with and without pre-reaction complex using M06-2X and G3SX barrier heights at 307 K and 2.5 mTorr He. Optimized geometries, vibrational frequencies and moments of inertia of the wells and transition states at M06-2X level of

β-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 β-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 β-hydroxyalkyl radical anion, &z.rad;CH 2 C(OH)(CH 3 )CH 2 C(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 γ-methylene group and from the β-hydroxyl group, with both reaction channels eventually forming isobaric product ions due to loss of either &z.rad;OH + HCHO or &z.rad;OH + CO 2 . Isotope labeling studies confirm that a 1,5-hydrogen shift from the β-hydroxyl functionality results in a hydroperoxyalkoxyl radical intermediate that can undergo further unimolecular dissociations. Furthermore, this study confirms that the facile decomposition of β-hydroxyperoxyl radicals can yield &z.rad;OH in the gas phase. The study of unimolecular isomerization and decomposition of a charge-tagged β-hydroxyperoxyl radical anion &z.rad;CH 2 C(OH)(CH 3 )CH 2 C(O)O − using mass spectrometry, quantum mechanical calculations and master equation kinetic simulations.