EXAMINATION OF THE EXCHANGE INTERACTION THROUGH MICELLE SIZE .2. ISOTOPE-SEPARATION EFFICIENCY AS AN EXPERIMENTAL PROBE

The geminate reaction probabilities (for recombination and disproporti onation) of benzoyl/sec-phenethyl radical pairs, generated by the phot olysis of alpha-methyldeoxybenzoin, for both unlabeled (C-13 in natura l abundance at the carbonyl position) and labeled ketones (C-13 in the carbonyl position) were measured in different sized alkyl sulfate mic elles (sodium octyl sulfate (C-8) through sodium dodecyl sulfate (C-12 )) in zero and high magnetic fields (B = 2400 G). Although the probabi lity of geminate recombination (P(r)) diminishes for the unlabeled pai r, 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-12 to C-8), the e fficiency of isotope separation (alpha) is found to increase at zero m agnetic field from 1.144 to 1.236 with decreasing micelle size. Theore tical 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 cr ossing is influenced by fast forced reencounters. These effects are no t as important for the labeled radical pairs (which possess a strong C -13 hyperfine interaction). In addition, paramagnetic relaxation due t o anisotropic hyperfine and electron-electron dipole interactions is s hown to be important in the quantitative description of the micelle si ze dependence of the reaction ability of the labeled radical pairs.