FT-EPR AND HPLC STUDY OF THE MECHANISM OF 4-CHLOROPHENOL PHOTOLYSIS

The mechanism of 4-chlorophenol (4CP) photolysis was investigated with the aid of Fourier Transform Electron Paramagnetic Resonance (FT-EPR) and pulsed-laser photolysis combined with High Performance Liquid Chr omatography (HPLC) detection of stable (diamagnetic) products. With FT -EPR transient free radicals produced by pulsed-laser excitation of so lutions of 4CP in alcohols could be identified. Time profiles of the F T-EPR spectra provided information on reaction kinetics and Chemically Induced Dynamic Electron Polarization (CIDEP) effects. It was found t hat 4CP photolysis in alcohols leads to the simultaneous formation of the phenoxyl radical and radicals produced by hydrogen abstraction fro m the solvent. CIDEP patterns establish that these radicals are formed in a reaction sequence involving a triplet state precursor and radica l pair intermediate. Results of earlier transient optical absorption m easurements indicate that the triplet precursor must be the carbene 4- oxocyclohexa-2,5-dienylidene. This assignment is supported by the find ing that photolysis of quinone diazide in a hydrogen-donating solvent gives the same free radical products as those obtained from 4CP. The f ormation of the phenoxyl radical intermediate accounts for the finding that photolysis of deoxygenated solutions of 4CP in alcohols gives ph enol as stable diamagnetic product. By contrast, photolysis of aerated and deoxygenated aqueous solutions of 4CP produces benzoquinone and h ydroquinone as primary products, respectively.