Investigation of zeolitic imidazolate frameworks using 13C and 15N solid-state NMR spectroscopy
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Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) with extended three-dimensional networks of transition metal nodes (bridged by rigid imidazolate linkers), with potential applications in gas storage and separation, sensing and controlled delivery of drug molecules. Here, we investigate the use of 13C and 15N solid-state NMR spectroscopy to characterise the local structure and disorder in a variety of single- and dual-linker ZIFs. In most cases, a combination of a basic knowledge of chemical shifts typically observed in solution-state NMR spectroscopy and the use of dipolar dephasing NMR experiments to reveal information about quaternary carbon species are combined to enable spectral assignment. Accurate measurement of the anisotropic components of the chemical shift provided additional information to characterise the local environment and the possibility of trying to understand the relationships between NMR parameters and both local and long-range structure. First-principles calculations on some of the simpler, ordered ZIFs were possible, and provided support for the spectral assignments, while comparison of these model systems to more disordered ZIFs aided interpretation of the more complex spectra obtained. It is shown that 13C and 15N NMR are sufficiently sensitive to detect small changes in the local environment, e.g., functionalisation of the linker, crystallographic inequivalence and changes to the framework topology, while the relative proportion of each linker present can be obtained by comparing relative intensities of resonances corresponding to chemically-similar species in cross polarisation experiments with short contact times. Therefore, multinuclear NMR spectroscopy, and in particular the measurement of both isotropic and anisotropic parameters, offers a useful tool for the structural study of ordered and, in particular, disordered ZIFs.
Sneddon , S , Kahr , J , Orsi , A F , Price , D J , Dawson , D M , Wright , P A & Ashbrook , S E 2017 , ' Investigation of zeolitic imidazolate frameworks using 13 C and 15 N solid-state NMR spectroscopy ' Solid State Nuclear Magnetic Resonance , vol 87 , pp. 54-64 . DOI: 10.1016/j.ssnmr.2017.09.001
Solid State Nuclear Magnetic Resonance
© 2017 the Authors. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
The authors are grateful to EPSRC computational support through the Collaborative Computational Project on NMR Crystallography (CCP-NC), via EP/M022501/1, and for other support through EP/G062129/1 (JK) and EP/M506631/1 (SS). AFO acknowledges funding from the European Community Seventh Framework Program (FP7/2007-2013 [grant agreement number 608490], Project M4CO2). SEA would also like to thank the Royal Society and Wolfson Foundation for a merit award. AFO would also like to acknowledge the SCI for a scholarship for her PhD studies. Some of the calculations were performed on the ARCHER UK National Supercomputing Service, and were supported by CCP-NC. The research data (and/or materials) supporting this publication can be accessed at http://dx.doi.org/10.17630/7959a81e-161d-4ada-9914-08d3d235ce88
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