Natural Abundance Oxygen-17 Solid-State NMR of Metal Organic Frameworks Enhanced by Dynamic Nuclear Polarization

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 e(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.