Dynamic Polarization of Fluorine Nuclei in Solutions of Selected Free Radicals

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Abstract
Dynamic polarization of fluorine nuclei has been examined with 14 free radicals dissolved in hexafluorobenzene (C6F6) and 1,1,1-trifluoro-2,2,2-trichloroethane (CF3CCl3). The radicals may be broadly grouped by their tendency to produce either positive or negative fluorine NMR enhancement. A large positive enhancement indicates that Fermi-contact coupling between the nucleus and the radical electron is the dominant nuclear relaxation mechanism; a large negative enhancement indicates that dipolar coupling is dominant. The correlation times for dipolar and contact relaxation components are comparable, and approximately the values expected from molecular diffusion. The intermolecular contact coupling may be transmitted by exchange polarization of bonding electrons on the fluorocarbon, or by charge transfer between radical and fluorocarbon, or by a combination of both mechanisms. Steric shielding appears to weaken the contact interaction in certain cases; but it is not the sole physical influence governing the contact coupling, nor even necessarily the most important one. The highest fluorine NMR enhancements observed at 74 G were −220 for CF3CCl3 with 2,4,6-tritertiary butyl phenoxyl, and +260 for C6F6 with 2,4,6-triphenyl pyrylyl in acetone.