Dynamic nuclear polarization of protons, 19F and 13C nuclei at 21 kilogauss

We have investigated the dyn amic nuclear polarization (DNP ) of protons, 19F and 13C nuclei in solutions of radicals at 21 kG. The corresponding V-band microwaves (59 GHz) were radiated quasi-optically into the sample placed in a high resolution NMR spectrometer. Several partially methyl- and fluorine-substituted benzene derivatives were investigated. The polarization of the protons was found to be negative and considerably smaller than that observed in lower fields. On the other hand, the 19F nuclei gave positive DNP values of around 10, corresponding to a predominantly scalar coupling with the unpaired electrons. The different positive and negative polarizations observed with 13C nuclei can be explained in terms of the three-spin effect and of mixed scalar and dipolar coupling. This could be further worked out by triple resonance experiments, i.e., observing the DNP of the 13C nuclei when irradiating the resonance frequencies of the electrons and the protons or 19F nuclei, respectively