Shedding light on nuclei near the spin diffusion barrier via electron decoupling
Dynamic nuclear polarization (DNP) has evolved as the method of choice to enhance NMR signal intensities and to address a variety of otherwise inaccessible chemical, biological and physical information. Despite its success, there is no detailed understanding of how the large electron polarization is transferred to the surrounding nuclei or where these nuclei are located relative to the polarizing agent. We have previously shown using three-spin solid effect that the size of the spin diffusion barrier surrounding the trityl radical in a glassy glycerol-water matrix is < 6 Å. In this contribution, we demonstrate that a combined DNP and electron decoupling approach enables direct NMR detection of intramolecular 1H’s on the trityl radicals. Although the 1H’s reside outside of the spin diffusion barrier, they are still broadened by the electron-nuclei hyperfine interaction, which can be quenched using electron decoupling. We achieved an ~80% improvement in the NMR peak intensity using a ~2 MHz microwave Rabi frequency at 0.35 T and 80 K. The direct detection of 1H’s closest to the unpaired electrons allows us to extract the relaxation parameters of the 1H’s and shed light on the spin diffusion pathways from the unpaired electron to the bulk nuclei. This information could provide insights important in designing more robust DNP polarizing agents. Finally, we showed that the electron decoupling method can be applied to study magnesium hexahydroxytriphenylene metal-organic framework (MgHHTP MOF).