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Further measurements of the nuclear electron Overhauser effect in solutions are reported for fluorine and protons. The proton-electron interaction is predominantly dipolar and the results are satisfactorily interpreted in terms of a model of randomly diffusing spheres. Fluorine-electron interactions, however, involve a scalar contribution, and current theories for the time dependence of this interaction give qualitative but not quantitative agreement with experiment. The effect of stimulated emission on the observed enhancements is discussed and concluded to be small in these experiments.

The frequency dependence of the nuclear relaxation times in solutions containing free radicals, previously determined by Gutowsky and Tai, has been re-interpreted in terms of the relaxation theory due to Hubbard. The good agreement between theory and experiment confirms the conclusions of other workers that, for protonic systems, the major contribution to the relaxation mechanism is from dipolar electron-nuclear coupling modulated by translational diffusion. In aromatic fluorine compounds, however, asscalar coupling is also evident. Although Hubbard's diffusion model gives an adequate description of the relaxation in these systems, it may not be entirely satisfactory over a larger range of magnetic field strengths.

The line-widths of an AB spectrum are calculated on the assumption that the relaxation is caused by a paramagnetic ion or radical interacting with the nuclei by both scalar and dipolar perturbations. The bases of the interaction mechanism are the 'sticking' and 'diffusion' models of Hubbard [3]: these models enable the line-width variation of the spectrum to be calculated explicitly and compared with results obtained for two AB systems containing tri-tert-butyl phenoxy radical.

Fluorine-electron double resonance experiments have been carried out at 3300 and 12 500 gauss on C6F6 and CF3CCl3 solutions of the free radical tri-tert-butylphenoxyl in order to determine the dependence of the Overhauser effect upon the diffusional correlation time, τc, of the species in solution. τc was varied by changing the temperature of the sample or by the addition of a chemically inert, miscible solvent which had a much higher (or lower) viscosity than the compounds under study. A scalar contribution to the interaction between the electron of the radical and the fluorine nuclei is evident in these systems and the experimental results have been used to test two current theories of the time-dependence of the interaction. Good qualitative agreement was generally found but neither theory provided a completely satisfactory quantitative description.

The positive Overhauser enhancement observed for the tert-butyl groups of tri-tert-butyl phenol in solutions of tri-tert-butyl phenoxyl radical has been shown to be associated with the proton exchange between the radical and phenol molecules. It is postulated that in the radical molecule the scalar coupling between the electron and the tert-butyl protons occurs at least in part by a hyperconjugative mechanism and therefore the coupling is modulated by random rotation of the tert-butyl groups. This leads to a positive polarization of the tert-butyl protons, which persists when the radical accepts a proton and becomes a tri-tert-butyl phenol molecule.