Comments on the interpretation of dynamic deuterium NMR spectra from solid inclusion compounds

Guest molecules trapped within the regular voids of crystalline adducts, called inclusion compounds, are likely to experience mobility. Recent solid-state ²H NMR studies indicated these motions to be primarily discrete jumps related to the molecular symmetry, to that of the environment, or to some other geometric feature of the system investigated. In this work we offer a more consistent picture based primarily on the symmetry of the voids, but also including the molecular symmetry. We show that the ²H NMR spectra from a whole class of guest molecules can be interpreted in terms of planar threefold jumps between unequally populated sites, such that the populations follow a simple Boltzmann law. The enthalpy difference between these sites is of the order of 1-2 kcal reflecting a rather weak but nonuniform guest-host interaction.