Dynamic Nuclear Polarization (DNP) Using Nitroxide Radicals

The theory of the Overhauser Effect in liquids is well established, but measured DNP enhancements from nitroxide radicals depart significantly from prediction. To achieve large signal enhancements, milli-molar concentrations of radicals are needed, a regime where Heisenberg exchange of the unpaired electron is significant. Therefore, the three electron transitions resulting from hyperfine interactions with the $^{14}$N nucleus cannot be treated as independent in a DNP experiment. Furthermore, the relaxation rate of $^{14}$N can be easily on the same order of magnitude or even greater than the relaxation rate of the unpaired electron, contributing to the mixing of the hyperfine states, even in the absence of Heisenberg exchange. We present a quantitative study and a new model of $^{1}$H DNP enhancement of water by varying radical concentrations and solvent viscosities of natural abundance $^{14}$N versus $^{15}$N isotope enriched 4-Oxo-TEMPO free radical at 0.35 Tesla.