A high saturation factor in Overhauser DNP with nitroxide derivatives: the role of N-14 nuclear spin relaxation

Overhauser DNP enhancements of toluene were measured at a magnetic field of 0.35 Tesla in a series of chemically functionalized nitroxide radicals. We observe that the enhancements increase systematically with polarizer size and rotational correlation time. Examination of the saturation factor of N-14 nitroxides by pulsed ELDOR spectroscopy led to a quantitative interpretation of the enhancements, for which the saturation factor increases up to almost unity due to enhanced nuclear (N-14) relaxation in the nitroxide radical. The observation has a direct impact on the choice of optimum DNP polarizers in liquids.