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The Zr-based Metal Organic Framework (MOF) UiO-66(Zr) is widely employed owing to its good thermal and chemical stabilities. Although the long-range structure of this MOF is preserved in the presence of water during several days, little is known about the formation of defects, which cannot be detected using diffraction techniques. We apply here 17 O solid-state NMR spectroscopy at 18.8 T to investigate the reactivity of UiO-66, through the exchange of oxygen atoms between the different sites of the MOF and water. For that purpose, we have selectively enriched in 17 O isotope the carboxylate groups of UiO-66(Zr) by using it with 17 O-labeled terephthalic acid prepared using mechanochemistry. In the presence of water at 50 °C and a following dehydration at 150 °C, we observe an overall exchange of O atoms between COO - and μ 3 -O 2- sites. Furthermore, we demonstrate that the three distinct oxygen sites, μ 3 -OH, μ 3 -O 2- and COO - , of UiO-66(Zr) MOF can be enriched in 17 O isotope by post-synthetic hydrothermal treatment in the presence of 17 O-enriched water. These results demonstrate the lability of Zr-O bonds and the reactivity of UiO-66(Zr) with water., (© 2024 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

Estuaries are key ecosystems with unique biodiversity and are of high economic importance. Along the estuaries, variations in environmental parameters, such as salinity and light penetration, can modify the characteristics of dissolved organic matter (DOM). Nevertheless, there is still limited information about the atomic-level transformations of DOM in this ecosystem. Solid-state NMR spectroscopy provides unique insights into the nature of functional groups in DOM. A major limitation of this technique is its lack of sensivity, which results in experimental time of tens of hours for the acquisition of 13 C NMR spectra and generally precludes the observation of 15 N nuclei for DOM. We show here how the sensitivity of solid-state NMR experiments on DOM of Seine estuary can be enhanced using dynamic nuclear polarization (DNP) under magic-angle spinning. This technique allows the acquisition of 13 C NMR spectra of these samples in few minutes, instead of hours for conventional solid-state NMR. Both conventional and DNP-enhanced 13 C NMR spectra indicate that the 13 C local environments in DOM are not strongly modified along the Seine estuary. Furthermore, the sensitivity gain provided by the DNP allows the detection of 15 N NMR signal of DOM, in spite of the low nitrogen content. These spectra reveal that the majority of nitrogen is in the amide form in these DOM samples and show an increased disorder around these amide groups near the mouth of the Seine., (© 2021 Wiley-VCH GmbH.)

The measurement of dipolar and J- couplings between 29 Si and 17 O isotopes is challenging owing to (i) the low abundance of both isotopes and (ii) their close Larmor frequencies, which only differ by 19%. These issues are circumvented here by the use of isotopic enrichment and dedicated triple-resonance magic-angle spinning NMR probe. The surface of 29 Si-enriched silica was labelled with 17 O isotope and heated at 80 and 200 °C. 29 Si- 17 O connectivities and proximities were probed using two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J- and D-HMQC) experiments between 17 O and 29 Si nuclei. The simulation of the build-up of the J- and D-HMQC signals allowed the first experimental measurement of J- and dipolar coupling constants between 17 O and 29 Si nuclei. These HMQC experiments allow distinguishing two distinct siloxane (SiOSi) oxygen sites: (i) those covalently bonded to Q 3 and Q 4 groups, having a hydroxyl group as a second neighbour and (ii) those covalently bonded to two Q 4 groups. The measured J- and dipolar coupling constants of siloxane 17 O nucleus with Q 4 29 Si nuclei differ from those with Q 3 29 Si nuclei. These results indicate that the 29 Si- 17 O one-bond J-coupling and Si-O bond length depend on the second neighbours of the Si atoms., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier Inc. All rights reserved.)