The effects of nitroxide structure upon 1H Overhauser dynamic nuclear polarization efficacy at ultralow-field

The efficacy in H-1 Overhauser dynamic nuclear polarization in liquids at ultralow magnetic field (ULF, B-0 = 92 +/- 0.8 mu T) and polarization field (B-p = 1-10 mT) was studied for a broad variety of 26 different spin probes. Among others, piperidine, pyrrolidine, and pyrroline radicals specifically synthesized for this study, along with some well-established commercially available nitroxides, were investigated. Isotopesubstituted variants, some sterically shielded reduction-resistant nitroxides, and some biradicals were included in the measurements. The maximal achievable enhancement, E-max, and the radio frequency power, P-1/2, needed for reaching E-max/2 were measured. Physico-chemical features such as molecular weight, spectral linewidth, heterocyclic structure, different types of substituents, deuteration, and N-15-labeling as well as the difference between monoradicals and biradicals were investigated. For the unmodified nitroxide radicals, the E-max values correlate with the molecular weight. The P-1/2 values correlate with the spectral linewidth and are additionally influenced by the type of substituents neighboring the nitroxide group. The nitroxide biradicals with high intramolecular spin-spin coupling show low performance. Nitroxides enriched with N-15 and/or H-2 afford significantly higher vertical bar E-max vertical bar and require lower power to do so, compared to their unmodified counterparts containing at natural abundance predominantly N-14 and H-1. The results allow for a correlation of chemical features with physical hyperpolarization-related properties and indicate that small nitroxides with narrow spectral lines have clear advantages for the use in Overhauser dynamic nuclear polarization experiments. Perdeuteration and N-15-labeling can be used to additionally boost the spin probe performance. (C) 2021 Author(s).