Systematic Evaluation of Polarizing Agents for Dynamic Nuclear Polarization Enhanced NMR

Dynamic nuclear polarization (DNP) can significantly enhance the sensitivity of nuclear magnetic resonance (NMR) spectroscopy, via the transfer of polarization from unpaired electrons to nuclei. As a result, there is considerable interest in improving DNP efficiency, particularly through the development of new polarizing agents (PAs). Here, a series of 32 PAs, including 26 dinitroxide and 6 hetero biradicals, of which 11 are introduced for the first time, were evaluated for their performance in DNP NMR experiments at magnetic field strengths of 9.4 T, 14.1 T, and 21.1 T. The PAs that are soluble in aqueous media were compared for different proton concentrations. A detailed comparison of the enhancement factors, polarization build-up times, contribution factors, and the resulting overall sensitivity factors is provided. We find that the class of narrow-line-broad-line biradicals not only yield the best performance at high field (21.1 T and 14.1 T) but remain among the best at intermediate fields (9.4 T). Specifically, SNAPol-1 and HyTEK2 are consistently among the best performing radicals at any field. Among the dinitroxides, notably at 9.4 T, the AsymPol-POK and M-TinyPol families stand out in proton-dense aqueous matrices, and a newly developed dinitroxide, NaphPolCbo, is found to provide the best overall sensitivity factor in organic solvent.