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

Theoretical Overhauser-detected EPR spectra of ¹⁴N and ¹⁵N 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 45–133 MHz range while NMR signal detection occurred at a field of 59 mT. At this frequency range, four π transitions of a ¹⁴N system and three π 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 1–15 W 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.