Following the unusual breathing behaviour of¹⁷O-enriched mixed-metal (Al,Ga)-MIL-53 using NMR crystallography

Abstract

<p>Mixed-metal (Al,Ga)-MIL-53 materials were synthesised and enriched in ¹⁷O. An NMR crystallographic approach reveals the cation distribution on the atomic level, and the effect of this on the breathing behaviour of the framework.</p>

17O NMR spectroscopy of crystalline microporous materials

“…51 For Al-MIL-53, there is a significant shift difference between the carboxyl signal at ~203 ppm, and that for the bridging hydroxyl oxygens (~20 ppm), and also a difference in quadrupolar couplings (~7 MHz and ~ 5.5 MHz). 25,66 The presence of the quadrupolar interaction can also pose challenges for the acquisition of quantitative spectra, which are necessary to determine the uniformity of enrichment and to extract accurate information on the proportion of different species present). 7,17 In addition to accounting for relaxation (as is required in all quantitative NMR experiments), it is also necessary to take into account the different nutation rates (i.e., the different responses to the application of a radiofrequency pulse) of species with different C Q .…”

“…For microporous materials, it is important to accurately account for dispersion forces in the calculations, and semi-empirical dispersion correction schemes are often added to the commonly-used PBE functional. 10,91 In the absence of these, particularly flexible materials can adopt different and often much more open conformations than expected (as 25,51,54,56,66,92 There is, however, considerably more scope for closer interaction between 17 O NMR experiments and computation with recent improvements in hardware and software, and with the growing expertise within the scientific community. relationships (if present) can then be used to determine detailed structural information directly from an NMR spectrum.…”

“…54 The 17 O C Q is also diagnostic of the bonding environment, often being large for both carboxyls (7)(8) and aryl-O -(9-10 MHz), smaller for hydroxyls (4-6 MHz) and much smaller (1-2 MHz) for oxo species. 25,54,106,66 Although the significant variation in 17 could be resolved and were assigned using DFT calculations. 107,108 17 O NMR spectra of a Cu(II)-based MOF, HKUST-1 (prepared using DGC) have also been acquired.…”

“…This can be addressed using a number of approaches, including selective enrichment of different components of the MOF, 106 comparison between chemically or structurally similar sets of materials, 25,54 use of CP to selectively excite O bonded to H 25,54 and prediction of NMR parameters using first-principles calculations. 25,54,66 Although the latter is extremely powerful, care must be taken to account for any molecules adsorbed within the pores and, for flexible MOFs, any structural changes associated with these or with temperature, if accurate parameters are to be obtained. The sensitivity of the 17 O NMR spectra to some of these changes is shown in Figure 5a, materials such as MIL-53.…”

Synthesis of Magnetic Metal-Organic Frame Material and Its Application in Food Sample Preparation

“…It's worth noting that the uniformity of the MMOFs prepared by this method needs to be improved [122,123]. method, the reactants were first separated from the solvent and steamed at a high temperature to induce the MOFs material to grow around MNPs [119,120]. The preparation of MMOFs by dry gel conversion method was first proposed by Tan et al, in 2017 [121].…”