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).

17O NMR spectroscopy of crystalline microporous materials

“…A number of 17 O DNP experiments have been carried out on non-porous solids (often also on enriched samples), 119 but the first example of natural abundance 17 O DNP experiment on a MOF (Zr-containing MIP-206) has recently been shown. 120 A DNP enhancement of 28 was seen for 1 H, with magnetisation then transferred to 17 O using CP, enabling a MAS NMR spectrum of 30 mg of sample to be obtained in 48 hours. While not possible at natural abundance without DNP, this does highlight that there is still a long way to go – much greater enhancements are needed to perform the multi-dimensional and multi-nuclear experiments that will be needed to understand the detailed structure of porous solids.…”

Residue-Specific High-Resolution 17O Solid-State Nuclear Magnetic Resonance of Peptides: Multidimensional Indirect 1H Detection and Magic-Angle Spinning

“…Reports of CP for quadrupolar nuclei typically employ it for selection of sites proximal to other nuclei of interest 35−37 and, more recently, also for enhancement via dynamic nuclear polarization (DNP). 38,39 For CP, continuous rf fields spin-lock the polarization of both spin species. Their rf fields match the modified Hartman−Hahn conditions during MAS and result in flip-flop and flop-flop dipolar Hamiltonians under the ZQ and DQ CP matching conditions, respectively.…”

“…Transfer of polarization to or from quadrupolar nuclei, such as 17 O, is an area of active research due to the complex behavior of multilevel quadrupolar spin systems under radio-frequency (rf) irradiation and MAS. − The use of cross-polarization (CP) for quadrupolar nuclei often yields mixed results, in contrast to the case of spin I = 1/2 nuclei, because the necessary spin-locking tends to be less efficient for quadrupolar nuclei. Reports of CP for quadrupolar nuclei typically employ it for selection of sites proximal to other nuclei of interest − and, more recently, also for enhancement via dynamic nuclear polarization (DNP). , For CP, continuous rf fields spin-lock the polarization of both spin species. Their rf fields match the modified Hartman–Hahn conditions during MAS and result in flip-flop and flop-flop dipolar Hamiltonians under the ZQ and DQ CP matching conditions, respectively .…”

Probing oxygen exchange between UiO‐66(Zr) MOF and water using 17O solid‐state NMR

“…Recently, 17 O NMR spectra of the MIP-206(Zr) MOF built with isophtalate ligands coordinated to two types of inorganic clusters containing either six or twelve Zr atoms have been recorded in natural abundance, thanks to the sensitivity enhancement provided by dynamic nuclear polarization (DNP) under magic-angle spinning (MAS) conditions. [18] However, these experiments require a specific MAS DNP-NMR spectrometer and the introduction of polarizing agents in the MOF, which can affect its structure and change its hydration state. For instance, it was shown that the μ 3 -O sites of MIP-206(Zr) were protonated during the impregnation with an aqueous solution of nitroxide biradicals.…”