Crystalline solids composed of one-dimensional channels with cross-sectional dimensions below 1 nm represent an intriguing class of materials with important potential applications. A key characteristic for certain applications is the average open channel persistence length, i.e., the ensemble average distance from a channel opening to the first obstruction. This paper introduces an NMR-based methodology to measure this quantity. The protocol is applied to polycrystalline specimens of two different dipeptide nanotubes: l-Ala-l-Val and its retro-analog l-Val-l-Ala. Persistence lengths derived from the NMR measurements are found to be comparable to the typical crystallite dimensions seen in scanning electron microscopy (SEM) images, indicating that the crystals of these AV and VA specimens are essentially hollow with practically no blockages. Applications of the method to an AV sample that has been pulverized in a mortar and pestle showed that the open channel persistence length was reduced from 50 to 6.6 μm, consistent with the crystallite sizes observed in SEM images.

This paper reviews results of our recent studies of diffusion of gaseous sorbates in nanotube systems. 13C pulsed field gradient (PFG) NMR at 17.6 T was used to probe diffusion of tetrafluoromethane in aluminosilicate nanotubes at several sorbate loadings. The results of PFG NMR measurements were compared with the corresponding data obtained by molecular dynamics simulations. The diffusion in this system was found to follow the laws of normal (i.e. Fickian) transport. At the same time, experimental data pointing at single-file diffusion were recorded for xenon gas confined inside dipeptide l-alanyl l-valine (AV) nanotubes. These data were obtained by means of high field 129Xe PFG NMR. The PFG NMR data are compared with recently reported hyperpolarized tracer exchange 129Xe NMR in the same material.Graphical abstractHighlights► Pulsed field gradient NMR was used to study diffusion of gaseous sorbates in nanotubes. ► Experimental data are compared with the results of MD simulations. ► Normal diffusion was observed for tetrafluoromethane in aluminosilicate nanotubes. ► Signatures of single-file diffusion were observed for xenon in AV nanotubes.

Deuterium quadrupolar echo NMR was applied to precision CD3 branched polyethylene at temperatures ranging from below the glass transition up to the melting point. The CD3 branches were placed on every 15th or 21st carbon with zero variation in the branch spacing by acyclic diene metathesis polymerization chemistry. The deuterium lineshapes were simulated and fit to the experimental spectrum assuming appropriate models that approximate the motions in the amorphous and crystalline phases. Spectral contributions of each phase were isolated by T1 fitting. The fitting results comprise the isotropic reorientation correlation time distribution and axial jump angle distribution in these two phases, respectively. The mean jump angle was found to increase monotonically with temperature, approaching 35° near the melting point, consistent with previous carbon-13 NMR results on this same polymer.

The study of crystals of molecular wheels as nanoporous materials is reported. Hyperpolarized 129Xe NMR spectroscopy has been used to characterize the mode of molecular diffusion and Xe interactions within the supramolecular nanochannels formed upon crystallization of the molecular wheels [Ga10(OMe)20(O2CMe)10] and [Ga18(pd)12(pdH)12(O2CMe)6(NO3)6](NO3)6. In agreement with expectations based on the collision diameter of the Xe atom relative to the differing internal diameters of the two types of gallium wheels, single-file diffusion occurs in the Ga10 channels, whereas in the Ga18 system the data are consistent with normal, Fickian diffusion. Information about the electronic environment inside the channels was probed by the Xe chemical shift. The interaction of the gas with the channel walls is found to be substantially stronger than the interaction in organic nanotubes and zeolites. The results establish the ability of crystals of molecular wheel compounds to function as a new class of porous nanotubular materials, and ones of a known and variable diameter, for studying the channel diameter dependence of molecular exchange and unidirectional diffusion on the micrometer length scale.