Examination of the exchange interaction through micelle size. 2. Isotope separation efficiency as an experimental probe

The geminate reaction probabilities (for recombination and disproportionation) of benzoyl/sec-phenethyl radical pairs, generated by the photolysis of [alpha]-methyldeoxybenzoin, for both unlabeled ([sup 13]C in natural abundance at the carbonyl position) and labeled ketones ([sup 13]C in the carbonyl position) were measured in different sized alkyl sulfate micelles (sodium octyl sulfate (C[sub g]) through sodium dodecyl sulfate (C[sub 12])) in zero and high magnetic fields (B = 2400 G). Although the probability of geminate recombination (P[sub t]) diminishes for the unlabeled pair, from 0.549 to 0.436 and for the labeled pair from 0.585 to 0.504 at zero magnetic field with decreasing micelle size (C[sub 12] to C[sub 8]), the efficiency of isotope separation ([alpha]) is found to increase at zero magnetic field from 1.144 to 1.236 with decreasing micelle size. Theoretical considerations of these experimental results show that the rate of geminate reaction of the unlabeled radical pairs in small micelles is sensitive to the electron spin exchange interaction; intersystem crossing is influenced by fast forced reencounters. These effects are not as important for the labeled radical pairs (which possess a strong [sup 13]C hyperfine interaction). 40 refs., 6 figs., 3 tabs.