STRUCTURE AND DYNAMICS OF THE UREA TRIOXANE INCLUSION COMPOUND PHASES, STUDIED BY H-2 NMR-SPECTROSCOPY

Deuterium NMR measurements are reported on the solid phases of the urea-trioxane inclusion compound (UTIC) prepared from isotopically normal urea and perdeuterated trioxane. The results are interpreted in terms of the dynamic state and orientational disorder of the trioxane sublattice in the various solid phases of the UTIC. At the low-temperature region of phase IV (<-140°C) the trioxane molecules are static on the NMR time scale, but above this temperature range 3-fold jumps about the molecule C3 axis sets in with a rate equation kj (s-1) = 2.2 × 1013 exp(-4.8/RT), where R is in kcal/(mol deg). Concomitantly the orientational order of the guest molecules gradually decreases and becomes asymmetric reaching at the transition to phase III the values S = 0.83 and η= 0.1. The transition to phase III is discontinuous, resulting in a complex line shape due to two distinct species, A and B, with intensity ratio of approximately 2:1. There is no discontinuity at the phase III to II transition and the spectrum remains essentially unchanged over the whole range of both phases. The transition to the high-temperature phase I is discontinuous. Only one species is observed in this phase with a relatively small orientational order, changing from 0.22 at the transition temperature to 0.12 at the melting point. Within the phase I region the trioxane molecules undergo ring inversion at a rate very similar to that observed in liquid solution, k (s-1) = 2.7 × 2.7 x× 1013 exp(-11.7/RT). Deuterium NMR spectra of a UTIC sample composed of urea-π4 and protonated trioxane are also reported. The spectra exhibit gradual line-shape change throughout the whole range of solid phases due to π-flips of the urea molecules in the walls of the host channels, but no discontinuities that can be associated with phase transformations are observed. The deuterium NMR results are discussed against earlier proton NMR and X-ray measurements, and for several phases alternative dynamic models are proposed. Remaining open questions related to the structure of the UTIC and the dynamic state of its host and guest sublattices are discussed. © 1990 American Chemical Society.