A hydrogen-2 nuclear magnetic resonance study of conformational and dynamic characteristics of cyclohexane while trapped within thiourea inclusion-compound channels

We report on ²H NMR spectra from a polycrystalline powder of the thiourea-cyclohexane-d₁₂ inclusion compound within the temperature range 134-345 K. Drastic changes in these spectra as a function of temperature are interpreted in terms of cyclohexane becoming engaged gradually in various uncoupled dynamic modes and undergoing several conformational alterations. At the lowest temperatures, chair-form conformers with a geometry consistent with minimal energy calculations (mec) prevail; the cyclohexane rings are in an upright position within the channels, with the triad axis z′ lying parallel to the channel axis d, about which they spin rapidly. At 137 K the guest molecules tip over suddenly so that z′ becomes tilted at an average angle a relative to d with a concomitant onset of nonuniform reorientation about d. Over the next 125 K the average intrachannel orientation of cyclohexane changes gradually with a determined to an accuracy of ±1.5°. We also find that at 159 K the motion about d becomes uniform and detect an increase of about 2° in the angle between the axial and equatorial C-D bonds. At approximately 240 K, onset of rapid ring inversion is observed.