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Fourier transform electron paramagnetic resonance (FT-EPR) has been used to identify the transient free radicals formed in the photolysis of 4-chlorophenol (4CP) dissolved in alcohols and to get information on their precursor from CIDEP effects.

Abstraet. The mechanism of 4-chlorophenol (4CP) photolysis was investigated with the aid of Fou-¡ Transform Electron Paramagnetic Resonance (FT-EPR) and pulsed-laser photolysis combined with High Performance Liquid Chromatography (HPLC) deteetion of stable (diamagnetic) products. With FT-EPR transient free radicals produced by pulsed-laser excitation of solutions of 4CP in alcohols could be identified. Time profiles of the FT-EPR spectra provided information on reaction kinetics and Chemically Indueed Dynamic Electron Polarization (CIDEP) effects. Ir was found that 4CP photolysis in alcohols leads to the simultaneous formation of the phenoxyl radical and radieals produced by hydrogen abstraction from the solvent. CIDEP pattems establish that these radicals are formed in a reaction sequence involving a triplet state precursor and radical pair intermediate. Results of earlier transient optical absorption measurements indicate that the t¡ precursor must be the carbene 4-oxocyclohexa-2,5-dienylidene. This assignment is supported by the finding that photolysis of quinone diazide in a hydrogen-donating solvent gives the same free radical products as those obtained from 4CP. The formation of the phenoxyl radical intermediate accounts for the finding that photolysis of deoxygenated solutions of 4CP in alcohols gives phenol as stable diamagnetic product. By contrast, photolysis of aerated and deoxygenated aqueous solutions of 4CP produces benzoquinone and hydroquinone as primary produets, respectively.

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