# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Organic Porous Materials
Single-file diffusion has also been observed in the channels formed by phenylether _bis_-urea macrocycles [131]. In this work, very high chemical shifts, up to 340 ppm, higher than solid xenon (around 300 ppm), were observed due to the very strong distortion experienced by the xenon electron cloud inside the microporous channels. It must be pointed out that single-file diffusion can only be observed in systems with channels long enough, in which xenon resides for the whole duration of the NMR experiment. In this regard, even in these microporous systems, xenon acts as a probe of both the nano-sized diameters of the cavities and the much bigger lengths of the channels. Figure 13: CF HP \({}^{129}\)Xe NMR line shapes for Xe inside tris-(\(o\)-phenylenedioxy)cyclotriphosphazene channels at different temperatures and xenon concentrations. All spectra were acquired at ambient pressure. Adapted with permission from reference [128]. Copyright 2000 American Chemical Society. In 2017, [PERSON] et al. studied xenon adsorption and dynamics in the CC3-R molecular cage [132], which showed very good gas separation performances. The CC3-R molecular cage is constituted by four mimo-aromatic moieties connected with flexible cyclohexane linkers. Two different sites for xenon adsorption are possible: a cage cavity formed by a single molecule, and a window cavity which is the tunnel between two different, adjacent molecules. Thus, the chemical shift of xenon was assumed to be the result of a weighted average of the shifts relative to these two adsorption sites. A combination of variable pressure (xenon loading) and temperature experiments with computational modeling allowed determination of the \(\delta\) values for the cage and the window cavities, the relative xenon populations, and the rates of exchange. In addition to that, diffusion coefficients were determined using pulsed gradient stimulated echo (PGSTE) experiments. Two different diffusion components were observed, attributed to inter and intraparticle exchange. The diffusion coefficient relative to intraparticle diffusion decreased rapidly with increasing xenon loading, reflecting the increasing degree of saturation of the porous structure. Covalent organic frameworks (COFs) have been recently discovered as a new class of crystalline organic materials with a permanent porosity [133]. Like with metal-organic frameworks, it is possible to synthesize stimuli-responsive COFs in which structural changes can be induced by guest molecules, temperature, pressure, or other external factors. [PERSON] et al. studied the dynamic behavior of a covalent organic framework using variable temperature \({}^{129}\)Xe NMR spectroscopy [134]. At high temperatures, only a single broad peak was observed around 220 ppm. A second signal appeared at a higher chemical shift at 248 K, and its relative intensity progressively increased with decreasing temperature. At 193 K, the first signal disappeared and the spectrum showed a single peak around 280 ppm. The same behavior was observed when heating the sample back to 273 K and with increasing xenon pressure. Thus, the two signals were attributed to two different forms of the COF: expanded and contracted. These two forms were also observed with PXRD: spectra demonstrated that solvation with THF induced the transition from the contracted to the expanded form. Unit cell parameters indicated that the latter was able to accommodate more than one xenon atom as opposed to the former. Thus, the signal at high ppm values was attributed to xenon located in the expanded form of the COF, where Xe-Xe interactions were assumed to be significant, while the other signal was assigned to xenon in the contracted form. In a recent work, hyperpolarized xenon NMR spectroscopy has been used to characterize the morphology of porous starch, a type of synthetically modified starch [135].