THE PHOTOOXIDATION OF 2,6-DIMETHYLPHENOL AND MONOPHENYLPHENOLS BY URANYL-ION IN AQUEOUS-SOLUTION

The photo-oxidation of 2,6-dimethylphenol and o-, m- and p-phenylpheno ls by uranyl ion was investigated in aqueous solution. Steady state an d dynamic luminescence quenching studies at pH 0.8 show the rapid dyna mic deactivation of excited uranyl ions by the phenols. At the natural pH of uranyl salt solutions (pH 2.3), differences are observed betwee n the steady state and dynamic quenching behaviour, and it is suggeste d that these differences are due to uranyl hydrolysis. Flash photolysi s studies with uranyl ion in the presence of 2,6-dimethylphenol and m- and p-phenylphenols show that the initial photoreaction leads to phen oxyl radical formation. The photolysis products were identified by hig h performance liquid chromatography (HPLC) and UV absorption spectrosc opy. With 2,6-dimethylphenol in aerated solution, both quinone and dim er formation are observed. Kinetic studies show that these processes o ccur concurrently. In contrast, photolysis of degassed solutions leads to dimer formation only. The quantum yields for these processes are r eported. The photo-oxidation of aerated solutions of o-phenylphenol in the presence of uranyl ions leads to the production of two dimers and the quinone, whereas with degassed solutions only the dimers are obse rved. The photo-oxidation of these substrates by uranyl ion is contras ted with the behaviour of [Co(NH3)(5)N-3](2+) as photooxidant of the s ame substrates. With m-phenylphenol, quinone and dimers are observed i n aerated solution, whereas only the dimer is observed in deoxygenated solution. With p-phenylphenol, only dimer formation is observed. Poss ible mechanistic origins of the differences in the selectivity of oxid ation by the different metal complexes are discussed. (C) 1997 Elsevie r Science S.A.