EPR Spectroscopy by Dynamic Nuclear Polarization in Low Magnetic Field

Heisenberg spin exchange effects of nitroxide radicals on Overhauser dynamic nuclear polarization in the low field limit at 1.5mT

“…In Eq. (3) the nitrogen Larmor frequency is neglected and while the equations give the same result described by Guiberteau and Grucker [16] the simpler formalism of Fedin et al is used [38]. At high magnetic fields C 2 and C 3 converge to zero.…”

“…We begin by describing the ESR transitions at low magnetic fields then relate these to the DNP factors of the nuclei, for the purpose of showing where the theory breaks down at higher radical concentrations. More detailed accounts of the theory for both 14 N and 15 N nitroxide radicals can be found in the literature [16,18,32,37].…”

“…Previously, Overhauser DNP has been described using the earth's magnetic field (0.05 mT [17]) and fields between 1 and 9 mT [16,18,[31][32][33]). All of these experiments were performed with field-cycling DNP [32,34], where the field is cycled to a polarization field for electron saturation then returned to a lower or higher value for NMR detection.…”

“…Very inhomogeneous polarization fields can be used and either permanently placed around the sample (prepolarization coils) or installed as a separate magnet (sample shuttling). Prepolarization has the disadvantage of potentially complicating magnet design but more importantly limits the type of samples that can be used, because the sample needs to have a longitudinal relaxation time T 1 long enough for the prepolarization to last through the time to switch off the prepolarization coil (40-150 ms [15][16][17][18]) or physically shuttle the sample. The second signal enhancement method is hyperpolarization, where the higher polarization from a second group of spins is transferred to the target nuclei for detection.…”

The life of π and σ—A tutorial review of the ubiquitous use of these symbols in Zeeman and magnetic‐resonance spectroscopy

“…One example in recent EPR spectroscopy of the usefulness of the p and s notation is for the polarization (population) effects on protons in solutions using the two types of EPR transitions generated in nitroxide free radicals at relatively low magnetic fields (46)(47)(48)(49).…”