A high saturation factor in Overhauser DNP with nitroxide derivatives: the role of (14)N 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 (14)N 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 ((14)N) relaxation in the nitroxide radical. The observation has a direct impact on the choice of optimum DNP polarizers in liquids.