Natural abundance oxygen-17 solid-state NMR of metal organic frameworks enhanced by dynamic nuclear polarization

The O-17 resonances of zirconium-oxo clusters that can be found in porous Zr carboxylate metal-organic frameworks (MOFs) have been investigated by magic-angle spinning (MAS) NMR spectroscopy enhanced by dynamic nuclear polarization (DNP). High-resolution O-17 spectra at 0.037% natural abundance could be obtained in 48 hours, thanks to DNP enhancement of the H-1 polarization by factors epsilon(H-1) = S-with/S-without = 28, followed by H-1 -> O-17 cross-polarization, allowing a saving in experimental time by a factor of ca. 800. The distinct O-17 sites from the oxo-clusters can be resolved at 18.8 T. Their assignment is supported by density functional theory (DFT) calculations of chemical shifts and quadrupolar parameters. Protonation of O-17 sites seems to be leading to large characteristic shifts. Hence, natural abundance O-17 NMR spectra of diamagnetic MOFs can thus be used to probe and characterize the local environment of different O-17 sites on an atomic scale.

Automated summary

In this study, the investigation of O-17 resonances in zirconium-oxo clusters within porous Zr carboxylate MOFs was carried out using DNP-enhanced MAS NMR spectroscopy. Through high-resolution spectral analysis, O-17 sites were successfully resolved with the assistance of DFT calculations and cross-polarization techniques.