Solid state NMR studies of the bromocyclohexane/thiourea inclusion compound

Solid state C-13-MAS and H-2-NMR investigations have been performed on the bromocyclohexane/thiourea inclusion compound. The main objective of this study was the evaluation of the molecular features of the guest molecules and their changes over a large temperature range (100 K < T < 350 K). Particular emphasis was placed on the molecular behaviour in the vicinity of a solid-solid phase transition at T = 237 K and in the low temperature phase, which was hitherto unknown. The H-2-NMR lineshape and relaxation studies reveal that, in the low temperature phase, restricted but fast overall motions (MHz to GHz region) of the guest molecules are dominant and reflect the distorted symmetry of the thiourea channels. On heating above the solid-solid phase transition almost unrestricted overall motions appear, together with a large degree of orientational disorder at higher temperature. Furthermore, the ring interconversion process presents a major relaxation process in the MHz region. The conformational order is unusual in the sense that the axial conformational state of the guest molecules is stabilized in the thiourea channels. It turns out that this unique property is also preserved in the low temperature phase.