Hypershifted spin spectroscopy with dynamic nuclear polarization at 1.4 K.

Dynamic nuclear polarization (DNP) enhances nuclear magnetic resonance (NMR) sensitivity by transferring polarization from unpaired electrons to nuclei, but nearby nuclear spins are difficult to detect or "hidden" due to strong electron-nuclear couplings that hypershift their NMR resonances. Here, we detect these hypershifted spins in a frozen glycerol-water mixture doped with TEMPOL at ~1.4 K using spin diffusion enhanced saturation transfer (SPIDEST), which indirectly reveals their spectrum. Additionally, we directly observe H NMR lines spanning 10 MHz. The spectrum is confirmed by simulations and density functional theory (DFT) calculations, which verify that the signals originate from intramolecular protons on TEMPOL. Using two-dimensional NMR, we demonstrate polarization transfer from hypershifted to bulk nuclei across a spin diffusion barrier. This methodology provides new insights into the structures of radicals and could aid in designing more efficient DNP polarizing agents. It also complements information on hyperfine interaction accessible by electron paramagnetic resonance (EPR).