Cross-polarization experiment relaxation mechanisms

Properties of the selective pulses are used therefore twofold in such experiments. Firstly, a selective pulse selectively perturbs the selected spin and the perturbation is distributed in the course of the experiment among the coupled spins, depending on the type of coupling (scalar, dipolar) and depending on the type of exchange mechanism (polarization transfer, cross polarization or cross relaxation). Secondly, the phase (selective 90° pulse) or the frequency (selective 180° pulse) of the selective pulse serve to label the response of both the selected and the residual coupled spins as positive or negative. 

Anomalous (in the sense of radical pair theory) polarization phases and magnetic field dependence were reported for substrates containing 19F nuclei, and were explained by cross-relaxation [67], Azumi and co-workers [68] investigated the photolysis of benzaldehyde. The polarizations could be accounted for by S-T0 mixing at high fields and by S-T mixing at low fields. At a field of 325 mT, however, the authors could not reconcile the CIDNP phase with the predictions of the radical pair mechanism from additional DNP experiments, they concluded that cross-relaxation with... 

---