Use of nitroxide spin labels in studies of solvent–solute interactions

Complexities imposed on the studies of dynamic polarization of solvent nuclei by the three-line ESR spectrum of solute nitroxide radicals are examined. These include (1) the field dependence of proton polarizations, (2) the unequal magnitude of the polarizations at frequencies corresponding to the nitroxide radical peaks, and (3) the radical concentration dependence of the extrapolated enhancements. These features are analyzed with respect to existing models and with respect to a new mathematical model based on an exchange interaction that couples radicals in different 14N spin states. This model successfully accounts for the nonlinear extrapolation of enhancements as a function of reciprocal radical concentration. The model is tested experimentally using samples of 4-hydroxy-2,2,6,6-tetramethyl piperid-1-yloxy free radical in benzene, with both DNP and relaxation data used in the analysis.