Molecular wheels as nanoporous materials: differing modes of gas diffusion through Ga10 and Ga18 wheels probed by hyperpolarized 129Xe NMR spectroscopy.

The study of crystals of molecular wheels as nanoporous materials is reported. Hyperpolarized (129)Xe 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 [Ga(10)(OMe)(20)(O(2)CMe)(10)] and [Ga(18)(pd)(12)(pdH)(12)(O(2)CMe)(6)(NO(3))(6)](NO(3))(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 Ga(10) channels, whereas in the Ga(18) 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.