Micellar CIDNP

The photolyses of dlbenzyl ketones in aqueous micellar solution have been shown to greatly enhance both geminate radical pair recombination and the enrichment of in recovered ketone compared to homogeneous solution. These observations have been attributed to the combined effects of the reduced dimensionality imposed by mlcelllzatlon and hyperflne 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-dipheny1-2-methyl-1-propa-none and its 2h and derivatives in micellar solution are now described and further demonstrate the enhanced cage and magnetic isotope effects of mlcelllzatlon. 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. 

Figure 5. Steady state CIDNP spectra during the photolysis of di-t-butyl ketone in a, homogeneous CtD, solution and b, HDTCl micellar solution.

The F-palrs, consisting of two t-butyl radicals, may also result In products which exhibit CIDNP effects. Since there Is no g-factor difference we expect to observe multlplet effects In these products. These expectations are confirmed(iA) In the spectrum shown In Figure 5a. Likewise, when this photolysis Is conducted In HDTCL micellar solutions In D20(ii ., an essentially Identical CIDNP spectrum Is observed, as shown In Figure 5b. 

A minor product observed in the photolysis of DBK in aqueous HDTCl (Fig. 5) is the isomeric ketone 4-methylphenylacetophenone (PMAP, 3) 15 18). The question arises as to whether the formation of 3 is due to some special orienting feature of the micelle which operates on the ratio of coupling products 1 and 3. Although 3 has not been detected by the conventional analytical methods for product analyses, (e.g., vpc), some 3 is detectable via chemically induced dynamic nuclear polarization (CIDNP) measurements 28). Thus, it may be that the ratio of formation of 1 to 3 is similar in both micellar and homogeneous solutions, but that the absolute yield of both combination products is so much smaller in homogeneous solution that 3 is not detectable by conventional analyses. 

Because the CIDNP effect is caused by the interplay of spin dynamics, diffusional dynamics, and chemical reactivity, information about each of these fields can be obtained from CIDNP experiments. CIDNP spectroscopy has proven extremely useful for studies of the former two aspects in systems with restricted diffusion (biradicals, and, to a lesser degree, micellar systems see Sections IV.C and IV.D). Concerning the rates and mechanisms of chemical reactions, CIDNP spectroscopy possesses several unique features, which make it a very powerful method. 

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