# A Wide-Ranging NMR Probe for Multiscale Structures
## 1 Introduction  
The element xenon was first discovered by [PERSON] and [PERSON] in 1898 through the study of liquefied air, shortly after the discovery of krypton and neon. [PERSON] proposed to call this new element _xenon_ after the Greek word \(\xi\)&vov, meaning foreign, strange. Like all the other noble gases, xenon is odorless, colorless, and dense. For a very long time, it was considered completely unable to form chemical compounds, but, in 1962, [PERSON] managed to produce the first known xenon compound, xenon hexafluoroplatinate, by mixing Xe with PtF\({}_{6}\), a powerful oxidizing agent. Since then, many other xenon compounds were discovered, such as fluorides, oxides, oxyfluorides, and complexes with carbon, nitrogen, and metal ions, making this element interesting for inorganic chemistry. Nowadays, xenon is obtained commercially in the distillation of liquid air. It is used in specialized light sources, in ion propulsion systems in spacecrafts, and as a general anesthetic. Natural xenon consists of seven different stable isotopes. Among them, \({}^{129}\)Xe is particularly interesting thanks to its relatively high natural abundance (26.4%) and its nuclear spin, \(I\), which is equal to 1/2. In addition to that, with 54 electrons, xenon has a very large electron cloud that makes it a highly polarizable atom, extremely sensitive to the surrounding environment. Thanks to these combined features, this nucleus is very suitable for nuclear magnetic resonance (NMR) spectroscopy: any change in its electron cloud is reflected in its chemical shift value which covers a range of approximately 7500 ppm. Another interesting isotope is \({}^{131}\)Xe since it also has a high natural abundance, around 21.2%. However, this nucleus has a nuclear spin of 3/2 and, thus, a nonzero quadrupolar moment which leads to complex spectra even for the simplest materials. Consequently, \({}^{129}\)Xe is by far the most used isotope for xenon NMR spectroscopy. In the 1980s, [PERSON] and [PERSON] found that the chemical shift of xenon adsorbed in zeolites was highly influenced both by the size of the cavities and the presence of cations [1,2]. Since these pioneering works, \({}^{129}\)Xe NMR spectroscopy has become a very important technique for studying porous species such as zeolites [3, 4, 5, 6, 7], mesoporous silica [8, 9], organic porous materials [10, 11], polymers [12, 13, 14, 15, 16], metal organic frameworks [17, 18], and, more recently, ionic liquids [19, 20, 21, 22, 23]. Several different models for determining the size of the pores via the \({}^{129}\)Xe chemical shift have been developed. In addition to that, this technique has proven capable of studying pore connectivity, pore blocking, and hierarchical materials, and to distinguish between different nanosegregated domains and to determine their sizes. Even though many reviews have been published on this technique [24, 25, 26, 27, 28, 29], especially in recent years [30, 31], to the best of our knowledge there are no reviews focused on the characterization of materials between the nano and the micro scale. These materials, often constituted by a hierarchical arrangement of nanostructures, have proven to be very interesting both in practical applications and in providing deep insight into transport phenomena. The characterization of these materials is particularly challenging: microscopy needs to be specifically tuned to explore these dimensions and the use of techniques such as conventional NMR and XRD has proven difficult due to the high sensitivity needed to properly characterize the structure and the organization of these materials. In this context, \({}^{129}\)Xe NMR spectroscopy, exploiting the high sensitivity of the xenon nucleus to the surrounding environment as well as its inherent mobility, has established itself as a very useful technique, capable of giving detailed information regarding porous structure, morphology, hierarchy, presence and size of different domains, and phase separation.