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

Abstract

The ¹⁷O 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 ¹⁷O spectra at 0.037% natural abundance could be obtained in 48 hours, thanks to DNP enhancement of the ¹H polarization by factors ε(¹H) = S_with/S_without = 28, followed by ¹H → ¹⁷O cross-polarization, allowing a saving in experimental time by a factor of ca. 800. The distinct ¹⁷O 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 ¹⁷O sites seems to be leading to large characteristic shifts. Hence, natural abundance ¹⁷O NMR spectra of diamagnetic MOFs can thus be used to probe and characterize the local environment of different ¹⁷O sites on an atomic scale.