# A Wide-Ranging NMR Probe for Multiscale Structures
## 5 Applications
### Organic Porous Materials  
Organic molecules can be used instead of silica to obtain aerogels. For example, it is possible to prepare resorcinol-formaldehyde aerogels via polymerization and consequent drying with supercritical CO\({}_{2}\)[126]. In these materials, the [Resorcinol]/[Catalyst] (R/C) ratio of the starting solution is the main parameter affecting the final gel properties, such as its porous structure. Xenon NMR was used to study aerogels prepared with different (R/C) ratios [10; 127]. Room temperature spectra revealed the presence of both mesoporous spaces and a broad distribution of micropores for all R/C ratios. The obstruction of the micropores was also observed by inserting gaseous CHCl\({}_{3}\) together with Xe into one of the samples and recording NMR spectra at different times. The spectra immediately after CHCl\({}_{3}\) adsorption did not show the signal relative to the micropores. After about 25-30 min following chloroform desorption, the signal reappeared and, after 3 h, the spectra returned to the original state. This confirmed the assignment of the signal to the broad micropore distribution: the small CHCl\({}_{3}\) molecules were able to fill the tiny micropores and block xenon atoms. In addition to that, the reappearance of the signal and the return of the spectra to the original state demonstrated that chloroform adsorption did not disrupt the porous structure. Finally, variable temperature data fit to Equation (10) gave the \(V_{\rm g}/S\) ratios for the various materials, which was observed to increase linearly with increasing R/C ratios. HP \({}^{129}\)Xe NMR has also been used to demonstrate single-file diffusion of xenon in organic porous materials, such as tris(\(o\)-phenylenedioxy)cyclotriphosphate (TPP) [128]. This study was conducted employing the saturation recovery pulse sequence. Hyperpolarized xenon gas was allowed inside the sample for a sufficient time to completely fill its porous structure. Then, a train of 90\({}^{\circ}\) pulses was used to destroy the polarization of the nuclei inside the detection cell. Since the sample was under continuous gas flow, hyperpolarized xenon immediately began refilling the detection cell and diffusing inside the porous structure, replacing saturated xenon. After a buildup time, \(\tau\), a final 90\({}^{\circ}\) excitation pulse was applied, followed by the actual acquisition, in which only the newly diffused gas was detected. Experiments were performed by varying the buildup time, and the ratio \(I_{\text{channel}}(\tau)/I_{\text{gas}}\) was observed to follow the typical trend for single-file diffusion. Moreover, peaks in single pulse spectra showed a positive anisotropy-isotropy-negative anisotropy transition with decreasing temperature and/or increasing xenon concentration (Figure 13). At high temperature/low concentration, xenon mostly interacted with the \(\pi\) electrons of the channel walls, giving rise to a positive and axially symmetric anisotropy with a prevailing \(\sigma_{\parallel}\) component. As xenon loading increased or temperature decreased, Xe-Xe van der Waals interactions became progressively more important, transforming the positive anisotropy to a negative anisotropy. Similar results were obtained by [PERSON] and [PERSON] on L-alanyl-L-valine nanotubes [129], who were also able to observe xenon atoms entering and exiting these nanotubes via a series of 2D EXSY experiments performed at different exchange times and xenon pressures [130]. Assuming Langmuir adsorption for xenon in nanotubes, the authors fit the dependence of the diagonal and the cross-peaks on the mixing time to appropriate analytical expressions, obtaining the mean desorption rate of xenon from the channels. Even though several approximations were made in the proposed kinetic model, the authors demonstrated that the desorption rate decreased with increasing xenon pressure, in accordance with the decreased mobility of Xe in the channels in a diffusion-limited exchange regime.