Field-cycled dynamic nuclear polarization (FC-DNP) of 14N and 15N nitroxide free radicals at low magnetic field

Theoretical Overhauser-detected EPR spectra of N-14 and N-15 nitroxide systems in low magnetic field by field-cycled dynamic nuclear polarization ( FC-DNP) were described by a combination of DNP theory and a model of FC-DNP. Spectra were simulated at magnetic fields between 0 and 8 mT. The simulations were able to predict both the EPR peak positions and their amplitudes, corresponding to those from FC-DNP experiments with 14N and 15N TEMPOL solutions. EPR irradiation was in the 45-133MHz range while NMR signal detection occurred at a field of 59 mT. At this frequency range, four p transitions of a 14N system and three p transitions of a 15N system were observed. The simulation programmes were also used to predict the spectral amplitudes of the FC-DNP with EPR irradiation power in the 1-15W range. Theoretical FC-DNP systems were in good agreement with experimental results; however, at low magnetic fields the inhomogeneity of our magnet system resulted in the EPR peaks being left-shifted and somewhat broader than those from the theoretical prediction.