# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Other Materials
Smaller particles displayed a more significant degree of exchange between Xe in the porous structure and the outer surface, reducing the corresponding chemical shift values compared to the larger particles. From a series  
Figure 14: CF HP \({}^{129}\)Xe NMR spectra of different building stone materials. Reprinted from reference [138]. of variable temperature experiments and using Equation (10), the authors determined the physical parameters of the adsorption of xenon. The estimated pore sizes, calculated assuming a cylindrical pore model, were much smaller than those derived by SEM, TEM, and nitrogen adsorption. Building on this important information and on the analysis of temperature dependent line shapes and linewidths, it was proposed that the porous structure of the MSS was constituted by mesoporous channels containing small microporous \"pockets\". The authors performed a similar study on mesoporous silicon sponges grafted with octadecylsilane (H\({}_{3}\)Si-C\({}_{8}\)) in 2018 [144]. It was demonstrated that the grafting process created a thin surface layer (about 2 nm) which covered the micropores embedded in the mesoporous channels, resulting in very different porous structures and interconnectivity compared to those of non-grafted materials. Metal chalcogenide aerogels and xerogels can be viewed as assemblies of quantum dots that have both tunable semiconducting properties and intrinsic porosities [145]. Even without varying the chemical composition, the properties of these two types of gel can be drastically different due to the diverse drying processes employed for their preparation, which create completely different structures. The differences in morphology between CdS aerogels and xerogels was explored in 2010 by employing variable temperature hyperpolarized xenon NMR [146]. Aerogels showed peaks attributable to different populations of mesopores, in agreement with nitrogen adsorption data. On the other hand, xerogels could not be analyzed with N\({}_{2}\) adsorption, and only a single resonance at high chemical shift values was observed in a wide range of temperatures, compatible with xenon dissolved in an organic compound. Indeed, thermogravimetric analysis and \({}^{13}\)C CP MAS NMR spectra revealed the presence of residual organic compounds from the synthetic process in both gels, with the xerogel containing a much higher amount of them. Thus, the authors concluded that, while the supercritical drying used to obtain the aerogel allowed to maintain a well-defined porous structure, the xerogel collapsed during the drying process. This collapse was only arrested by the organic compounds, which, in turn, occluded the pores of the resulting material. Figure 15: HP \({}^{129}\)Xe NMR spectra of various mesoporous silicon sponges acquired at 300 K. The labels indicate pore sizes determined by SEM images (20 nm and 50 nm, respectively) and the particle sizes (5 \(\upmu\)m and \(>\)50 \(\upmu\)m, respectively). Reprinted with permission from reference [143]. Copyright 2017 _Journal of Materials Research_. Multiple-quantum (MQ) NMR has been established as one of the most powerful techniques for the study of materials characterized by anisotropic motions, such as crosslinked polymers and liquid crystals [147]. This technique generally employs complex pulse sequences composed of an excitation block, in which multiple-quantum coherence, dominated by double-quantum (DQ) coherence, is generated, and a reconversion block, with the same pulses but with different phases, that converts multiple-quantum into single-quantum coherences, detectable after a final 90\({}^{\circ}\) readout pulse. [PERSON] and co-workers reported the use of a DQ filter in the \({}^{129}\)Xe CP MAS NMR analysis of the xenon clathrate of [PERSON]'s compound [148]. This approach allowed the signals of singly and doubly-occupied cavities to be distinguished between, in which \({}^{129}\)Xe-\({}^{129}\)Xe dipolar coupling was measurable since the two nuclei maintained sufficient spatial proximity for the whole duration of the experiment.