Diffuse scattering and Monte Carlo simulations of cyclohexane-perhydrotriphenylene (PHTP) inclusion compounds, C6H12/C18H30

A recently developed method for fitting a Monte Carlo computer simulation m odel to observed single-crystal diffuse X-ray scattering has been applied t o (hk0) scattering data obtained for the guest-host system (inclusion compo und) cyclohexane-perhydrotriphenylene (cyclohexane-PHTP), C6H12/C18H30, at 295 K. A simple two-dimensional model representing the projection of the st ructure down c was used, which allowed refinement of short-range order para meters describing the mutual orientation of guest molecules within neighbou ring channels, together with a parameter describing the coupling of these o rientations to distortions of the host framework. Since the guest site stil l appears very disordered in projection, only a crude model for the shape o f the guest molecule was used. The resulting calculated diffraction pattern s showed good qualitative agreement with the observed pattern, but quantita tively the achieved agreement factor (R similar or equal to 38%) was consid erably higher than that achieved in comparable studies of a different syste m. Calculations of the contribution to the diffraction pattern of the guest alone and the host alone clearly indicate that the host framework distorti on is modelled well, but the guest is not so well described. The lattice di stortion revealed by the study indicates a tendency for the hexagonal netwo rk to distort towards local monoclinicity. This is consistent with the obse rvation that at similar to 273 K. the compound undergoes a reversible phase transition to a micro-twinned monoclinic phase.