Deuterium nuclear magnetic resonance study of several cyclophosphazene inclusion compounds: Guest conformation, dynamics, and intrachannel orientation

We report on a ²H NMR study of polycrystalline powders of several cyclophosphazene inclusion compounds between 370 and 150 K. With p- and o-xylene we encounter rapid internal methyl reorientation. The aromatic rings orient, on the average, in the upright position within the inclusion channels and reorient about their C₂ symmetry axis. With p-xylene this mode is rapid above room temperature, slows down at lower temperatures, and consists of threefold jumps about the channel axis d. With o-xylene we find that although the rate of this motion stays high throughout the temperature range investigated, its symmetry decreases from C_3υ (or higher) to C_2υ at roughly 308 K. Cyclohexane executes simultaneously rapid ring inversion and fast reorientation about the molecular symmetry axis z′, whereas dioxane only spins about the latter. Dramatic temperature-induced spectral modifications are interpreted in terms of the extent of z′ ordering along d or in terms of a temperature-dependent tilt of z′ with respect to d. We find that tetrahydrofuran assumes a C₂ envelope conformation, orients in an upright position within the channels, and spins rapidly about the channel axis. The spectral consequences of varying the temperature are interpreted in terms of changes in the molecular geometry. The acetone deuterons experience rapid methyl rotation with the internal diffusion axis fluctuating about the a mean orientation. It is suggested that two types of methyl groups, differing in the extent of this wobbling motion, prevail at higher temperatures and that the maximum amplitude of these fluctuations is temperature dependent.