REINVESTIGATION OF THE THIOUREA CYCLOHEXANE INCLUSION COMPOUND BY DEUTERIUM NMR-SPECTROSCOPY

Deuterium NMR measurements are reported on the thiourea-cyclohexane inclusion compound prepared from perdeuterated cyclohexane over the temperature range -180 to +60-degrees-C. Three regions are identified according to the structure of the host thiourea sublattice is monoclinic and the guest molecules. In the low-temperature region (<-144-degrees-C, phase III), the host sublattice is monoclinic and the guest molecules undergo thermally activated reorientation about their triad axis, but are otherwise fixed in the lattice. Using T1 measurements, it was found that the reorientation in this phase proceeds via 3-fold jumps obeying the rate equation k(J) (kS-1) = 1.83 X 10(13) exp(-2.51 /RT), where R is in kcal mol-1 K-1. In the intermediate-temperature region (-144-degrees-C to -117-degrees-C, phase II), the host lattice is still monoclinic but it gradually transforms into an hexagonal (rhombohedral) form. During this transformation the guest molecules continuously transform between three different species, corresponding to cyclohexane molecules in different dynamic states and apparently also different lattice sites. Finally, in the high-temperature region (> -117-degrees-C, phase I) the host lattice remains hexagonal, while the guest molecules are orientationally high disordered, although the occupy the well-defined crystallographic 32 sites of the lattice channels. Line-shape analysis of cyclohexane-d12 in this phase indicates that the guest molecules undergo fast ring inversion with kinetic equation, k (s-1) = 1.06 X 10(13) exp(-11.1/RT). The results are compared with similar results for the urea-trioxane inclusion compound, and open questions regarding the nature of the guest sublattice in these compounds are discussed.