Reinvestigation of the Thiourea-Cyclohexane Inclusion Compound by Deuterium NMR Spectroscopy

Article abstract of DOI:10.1021/j100154a071

Deuterium NMR measurements are reported on the thiourea-cyclohexane inclusion compound prepared from perdeuterated cyclohexane over the temperature range -80 to +60 deg C.Three regions are identified according to the structure of the host thiourea sublattice and the behavior of the guest cyclohexane molecules.In the low-temperature region (<-144 deg 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 (s^-1)=1.83 x 10¹³ exp(-2.51/RT), where R is in kcal mol^-1 K^-1.In the intermediate-temperature region (-144 to -117 deg 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 deg C, phase I) the host lattice remains hexagonal, while the guest molecules are orientationally highly disordered, although they 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 a kinetic equation, k (s^-1) = 1.06 x 10¹³ exp(-111.1/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.