Electron paramagnetic resonance spin trapping detection of short-lived radical intermediates in the direct photolysis of 4-chlorophenol in aerated aqueous solution

Abstract

The photolysis of 4-chlorophenol (I) in aqueous solution (pH 6.5–10.5) in the presence of the spin trap 5,5-dimethylpyrroline- N-oxide (DMPO) results in the formation of at least five distinct electron paramagnetic resonance (EPR) spectra. We identified three of these as arising from the DMPO spin adducts of the hydrogen atom (or a hydrated electron plus later protonation) (spectrum A), an aryl radical (probably the p-hydroxyphenyl radical) (spectrum B) and the ·OH radical (spectrum C). Spectrum D probably arises from a decomposition product of one or more of the spin adducts, and spectrum E is that of the p-benzosemiquinone anion. By examining EPR spectra after various lengths of UV irradiation, and determining the time course of the growth of each spectrum, we established that spectra A and B appear immediately on initiation of the photolysis. In contrast, the growth of spectrum C is sigmoidal, indicating that it arises from a secondary product. We present a mechanism, based on primary radicals detected, that accounts for the photoconversion of I to p-benzoquinine (II).