Characterization of alkoxycarbonyl radicals by step-scan time-resolved infrared spectroscopy

A series of alkoxycarbonyl radicals has been generated by laser flash photo lysis (355 nm) of fluorenone oxime alkyl oxalates in carbon tetrachloride a nd characterized by time-resolved infrared spectroscopy using the step-scan technique. The alkoxycarbonyl radicals ((v) over tilde (C=O) = 1802 cm(-l) for R = ethyl) generally have a lifetime of several microseconds, decaying by reaction with the solvent to yield esters of chloroformic acid. In some cases, decarboxylation yielding alkyl radicals has also been observed. Thu s, photolysis of fluorenone oxime tert-butyl oxalate results in the formati on of tert-butoxycarbonyl radicals, which subsequently decay, mainly yieldi ng CO, and tert-butyl radicals. The benzyloxycarbonyl radical and the aceto neiminoxycarbonyl radical both decarboxylate too rapidly to be detected wit h our spectrometer (25 ns risetime). Upon purging the solution with oxygen, the alkoxycarbonyl radicals were efficiently quenched, to yield alkoxycarb onylperoxy radicals ((v) over tilde (C=O) = 1845 cm(-l) for R = ethyl), whi ch again had a lifetime of the order of several microseconds. A short-lived transient ((v) over tilde = 1768 cm(-1), tau similar to 200 ns) is assigne d as the carbonyloxy radical 4a on the basis of comparison with time-resolv ed UV/Vis data. A further product of the photolysis of fluorenone oxime oxa lates can be tentatively assigned as the 9-fluorenylideneiminoxy radical 3 ((v) over tilde = 1670 cm(-1)),, which according to our DFT calculations sh ould show a very intense (v) over tilde (C=N-O,as) = 1665 cm(-1). Fluorenon e oxime oxalates are compounds well suited as precursors for alkoxycarbonyl radicals, since they are easily synthesized as crystalline solids, show a convenient absorption at lambda = 355 nm, and exhibit a high degree of ther mal stability.