Date of Award


Degree Type


Degree Name

Doctor of Philosophy


The behaviour of photochemically generated radical pairs on a silica gel surface was compared with results obtained in other restricted environments and in solution. Particular emphasis was placed upon the adsorbed radicals' surface mobility as measured by the amount of nongeminate recombination of the original radical pair.;Benzyl radical pairs were generated by photolysis of 4-methylbenzyl-4-methoxyphenyl acetate, 1-(4-methylphenyl)-3-(4-methoxyphenyl)-2-propanone and 4,4'-methoxy-methyldibenzyl sulfone adsorbed on silica gel. The amounts of geminate recombination determined from the dibenzyl product ratios indicated that substantial movement occurs for absorbed benzyl radicals. The surface mobility was influenced by both the photolysis temperature and the radical pair multiplicity but was relatively insensitive to changes in surface coverage, partial dehydroxylation of the silica gel and the presence of coadsorbates on the surface. The observation of rearranged starting materials among the products produced by photolysis of adsorbed dibenzyl ketone and dibenzyl sulfone indicated the increased restrictions to radical motion on the shorter timescale of the phenylacetyl-benzyl and benzyl sulfonyl-benzyl radical pairs. The substantial ('13)C enrichment of dibenzyl ketone recovered after partial photolysis of silica gel provided further evidence for restrictions to the motion of adsorbed phenylacetyl-benzyl radical pairs. The relative rates of the Type I cleavage of a number of monosubstituted dibenzyl ketones were measured by CIDNP.;Cyanopropyl radicals generated photochemically from adsorbed azobisisobutyronitrile yielded both carbon-nitrogen and carbon-carbon coupling products, indicating that the surface did not restrict the rotational motion of these radicals, as previously claimed. Deuterium labelling experiments indicated that there was predominantly geminate recombination of singlet cyanopropyl radical pairs, although some radicals were sufficiently translationally mobile to escape from their original partners. The longer-lived radical pairs generated by triplet sensitization yielded mainly nongeminate recombination.



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