Hypershifted Spin Spectroscopy with Dynamic Nuclear Polarization at 1.4 K

Dynamic nuclear polarization (DNP) is a nuclear magnetic resonance (NMR) hyperpolarization technique that mediates polarization transfer from unpaired electrons to nearby nuclear spins, which then relay the polarization to more remote nuclear spins in the bulk via spin diffusion. Despite widespread applications of DNP, the role of these nearby nuclear spins has never been properly characterized because they were hitherto believed to be unobservable or ‘hidden’ due to severe shifts or line broadening due to strong electron-nuclear couplings, i.e., their NMR resonances are ‘hypershifted’ beyond detection limit. By using DNP on a frozen glycerol-water mixture (‘DNP juice’) doped with TEMPOL at 1.4 K, we employed a newly introduced technique known as SPIn Diffusion Enhanced Saturation Transfer (SPIDEST), to indirectly reveals the spectrum of these hypershifted spins. Then, we performed direct two-pulse echo experiment, and we report here the results of the first direct observation of these hypershifted 1H nuclear spins under actual DNP conditions. The inhomogeneously broadened 1H NMR lines span a range of 10 MHz and can be directly observed by acquiring a series of frequency-stepped NMR spectra. Experiments repeated with deuterated TEMPOL proves that the hypershifted 1H signals indeed originate from methyl and methylene protons that are covalently attached to TEMPOL. In addition to characterizing the relaxation times (T1 and T2) of these nearby nuclei, we show that their polarization can be transferred to the bulk via spin diffusion using 2D NMR, i.e., the transport is not impeded by a spin diffusion barrier, as has been widely believed so far. This work presents a new form of spectroscopy that directly characterizes the nearby nuclei that we like to refer to as “hypershifted” nuclei. This could lead to the design of more efficient DNP polarizing agents and to a better understanding of the role of the molecular structures of paramagnetic agents.