Observing Nearby Nuclei on Paramagnetic Trityls and MOFs via DNP and Electron Decoupling

Dynamic nuclear polarization (DNP) is an NMR sensitivity enhancement technique that mediates polarization transfer from unpaired electrons to NMR-active nuclei. Despite its success in elucidating important structural information on biological and inorganic materials, the detailed polarization-transfer pathway from the electrons to the nearby and then the bulk solvent nuclei, and finally to the molecules of interest-remains unclear. In particular, the nuclei in the paramagnetic polarizing agent play significant roles in relaying the enhanced NMR polarizations to more remote nuclei. Despite their importance, the direct NMR observation of these nuclei is challenging because of poor sensitivity. Here, we show that a combined DNP and electron decoupling approach can facilitate direct NMR detection of these nuclei. We achieved an similar to 80 % improvement in NMR intensity via electron decoupling at 0.35 T and 80 K on trityl radicals. Moreover, we recorded a DNP enhancement factor of epsilon similar to 90 and similar to 11 % higher NMR intensity using electron decoupling on paramagnetic metal-organic framework, magnesium hexaoxytriphenylene (MgHOTP MOF).

Automated summary

Dynamic nuclear polarization is a technique that enhances the sensitivity of nuclear magnetic resonance by transferring polarization from unpaired electrons to nuclei. However, the pathway of polarization transfer from electrons to nearby nuclei, and then to bulk solvent nuclei and molecules of interest, remains unclear. Nuclei in paramagnetic polarizing agents play a significant role, but direct observation is challenging due to poor sensitivity. In this study, a combined DNP and electron decoupling approach enabled direct NMR detection of these nuclei in trityl radicals and paramagnetic metal-organic frameworks.