Structural Properties of Low-Temperature Phase Transitions in the Prototypical Thiourea Inclusion Compound: Cyclohexane/Thiourea

The prototypical thiourea inclusion compound, cyclohexane/thiourea, in which cyclohexane guest molecules
are located within tunnels in a hydrogen-bonded thiourea host structure, is known to exist in three phases
(denoted I, II, and III). The stable phase at ambient temperature (phase I) undergoes a second-order transition
to phase II at ca. 148 K, and phase II undergoes a first-order transition to phase III at ca. 127 K. The structural
properties of phases I−III have been determined in the present work from synchrotron X-ray powder diffraction
data, employing the Rietveld refinement technique. The structural properties determined for phase I are in
agreement with those reported previously from single-crystal X-ray diffraction data. The structures of the
two low-temperature phases (phases II and III) have not been reported previously, and we emphasize the
advantages of using X-ray powder diffraction techniques in these cases due to the occurrence of crystal twinning
on entering these phases. In phase II, the observed distortion of the thiourea tunnel is consistent with an
increase of orientational ordering of the guest molecules in comparison with phase I, although a unique
determination of the disordered guest substructure could not be established. On entering phase III, however,
the Rietveld refinement indicates a further distortion of the thiourea tunnel structure, which is associated
with ordering of each guest molecule in a single discrete orientation. Importantly, structural aspects of the
guest substructure in this phase (particularly concerning the tilt angle of the C3 axis of the guest molecule
relative to the tunnel axis) are in excellent agreement with results on the orientational characteristics of the
guest molecules in phase III deduced from a previous single-crystal 2H NMR study.