Intermolecular nuclear-electron interaction and scalar spectral intensities in solutions of 1,3,5-trifluorobenzene and substituted fluorobenzenes containing free radicals

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Abstract
From 19F dynamic nuclear polarization experiments at five widely separated magnetic fields between 16 and 13 200 gauss and at various temperatures, the relative strengths and the spectral densities of intermolecular hyperfine couplings between solvent nuclei and radical free electron orbitals have been determined. The N.M.R. enhancements depend upon competition between the dipolar and the scalar contact part of the coupling. Separation of the two contributions and evaluation of the scalar intensity functions have been achieved by using proton dynamic nuclear polarization as a probe for the dipolar interaction. The importance of the scalar interaction and the particular form of the spectral intensity function depend upon the chemical and electronic properties of both interacting molecules. Detailed data are given for solutions of bisdiphenylene phenyl allyl in 1,3,5-trifluorobenzene, fluorobenzene, 1-fluoro-4-iodobenzene, and 1-fluoro-4-nitrobenzene, and for solutions of tri-t-butyl phenoxyl in fluorobenzene. The hyperfine coupling and its random modulation by molecular motion is explained in terms of a generalized diffusion model with pulse-shaped fluctuations of the scalar interaction. Conclusions as to the nature and the mechanism of the coupling are discussed.