NMR Study of the Orientational Phase Transition in the Monohalogen-substituted Cyclohexane/Thiourea Inclusion Compounds C6H11X/SC(NH2)2 (X=Cl, Br)

Thiourea host molecules form inclusion compounds with monohalogen-substituted cyclohexane guest molecules. C6H11X / SC(NH2)(2) (X=Cl, Br) undergoes a phase transition from a high-temperature phase (HT phase), where the guest molecules reorient rapidly, to a low-temperature phase (LT phase). To study the guest motion in a thiourea host lattice, we measured the spin lattice relaxation time T-1 of carbon nuclei. In the HT phase, log(T-1) of both nuclei in guest molecules was linearly related to the inverse temperature. The relaxation was affected by the sufficiently isotropic motion of the guest molecule. In the LT phase, the temperature dependences showed a single-mode behavior. The mode can either be a three-fold reorientation (C-3 reorientation) in the channel direction or two-fold reorientations (C-2 reorientations) in the directions perpendicular to the channel. The observed mode was found to be a C-3 reorientation for both samples. Thus, C-3 reorientation shows a T-1 minimum and freezes at a, higher temperature than the C-2 reorientations.