Direct observation of the gas phase reaction of the cyclohexyl radical with dioxygen using a distonic radical ion approach
Alkylperoxyl radicals are intermediates in the oxidation of hydrocarbons. The reactive nature of these intermediates, however, has made them elusive to direct observation and isolation. We have employed ion trap mass spectrometry to synthesize and characterize 4-carboxylatocyclohexyl radical anions (•C6H10-CO2 -) and observe their reactivity in the presence of dioxygen. The resulting reaction is facile (k ) 1.8 × 10-10 cm3 molecule-1 s-1 or 30% of calculated collision rate) and results in (i) the addition of O2 to form stabilized 4-carboxylatocyclohexylperoxyl radical anions (•OO-C6H10-CO2 -), providing the first direct observation of a cyclohexylperoxyl radical, and (ii) elimination of HO2 • and HO• radicals consistent with recent laser-induced fluorescence studies of the reaction of neutral cyclohexyl radicals with O2. Electronic structure calculations at the B3LYP/6-31+G(d) level of theory reveal viable pathways for the observed reactions showing that formation of the peroxyl radical is exothermic by 37 kcal mol-1 with subsequent transition states as low as -6.6 kcal mol-1 (formation of HO2 •) and -9.1 kcal mol-1 (formation of HO•) with respect to the entrance channel. The combined computational and experimental data suggest that the structures of the reaction products correspond to cyclohexenes and epoxides from HO2 • and HO• loss, respectively, while alternative pathways leading to cyclohexanone or ring-opened isomers are not observed. Activation of the charged peroxyl radical •OO-C6H10-CO2 - by collision induced dissociation also results in the loss of HO2 • and HO• radicals confirming that these products are directly connected to the peroxyl radical intermediate.