Radical Pair Reactions in Micellar Solution in the Presence and Absence of Magnetic Fields

Abstract

The photolyses of dibenzyl ketones in aqueous micellar solution have been shown to greatly enhance both geminate radical pair recombination and the enrichment of 13C in recovered ketone compared to homogeneous solution. These observations have been attributed to the combined effects of the reduced dimensionality imposed by micellization and hyperfine induced intersystem crossing in the geminate radical pairs. This latter effect is the basis of Chemically Induced Dynamic Nuclear Polarization (CIDNP), a phenomenon which is well known in homogeneous solution. The photolyses of 1,2-diphenyl-2-methyl-1-propanone and its 2H and 13C derivatives in micellar solution are now described and further demonstrate the enhanced cage and magnetic isotope effects of micellization. We report also the observation of CIDP during the photolyses of micellar solutions of several ketones, and demonstrate the validity of the radical pair model to these systems. Analyses of the CIDNP spectra in the presence and absence of aqueous free radical scavengers (e.g., Cu2+) allow us to differentiate between radical pairs which react exclusively within the micelle and those that are formed after diffusion into the bulk aqueous phase. In some cases this allows us to estimate a lifetime associated with the exit of free radicals from the micelles.