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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 17O 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 17O isotope the carboxylate groups of UiO‐66(Zr) by using it with 17O‐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 μ3‐O2− sites. Furthermore, we demonstrate that the three distinct oxygen sites, μ3‐OH, μ3‐O2− and COO−, of UiO‐66(Zr) MOF can be enriched in 17O isotope by post‐synthetic hydrothermal treatment in the presence of 17O‐enriched water. These results demonstrate the lability of Zr−O bonds and the reactivity of UiO‐66(Zr) with water. [ABSTRACT FROM AUTHOR]

[Display omitted] • First 2D 17O-29Si through-bond correlation in solids. • First measurement of 17O-29Si J- and dipolar coupling constants. • 17O-29Si one-bond J- and dipolar coupling constants affected by Si second neighbours. • Distinction between siloxane (SiOSi) moieties bonded or not to OH groups near silica surface. • Double-resonance NMR experiments on isotopes with close Larmor frequencies. The measurement of dipolar and J - couplings between 29Si and 17O 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 29Si-enriched silica was labelled with 17O isotope and heated at 80 and 200 °C. 29Si-17O connectivities and proximities were probed using two-dimensional (2D) through-bond and through-space heteronuclear multiple-quantum coherences (J - and D -HMQC) experiments between 17O and 29Si 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 17O and 29Si nuclei. These HMQC experiments allow distinguishing two distinct siloxane (SiOSi) oxygen sites: (i) those covalently bonded to Q3 and Q4 groups, having a hydroxyl group as a second neighbour and (ii) those covalently bonded to two Q4 groups. The measured J - and dipolar coupling constants of siloxane 17O nucleus with Q4 29Si nuclei differ from those with Q3 29Si nuclei. These results indicate that the 29Si-17O one-bond J -coupling and Si-O bond length depend on the second neighbours of the Si atoms. [ABSTRACT FROM AUTHOR]