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 model to observed single-crystal diffuse X-ray scattering has been applied to (hk0) scattering data obtained for the guest-host system (inclusion compound) cyclohexane-perhydrotripheny-lene (cyclohexane-PHTP), C₆H₁₂/C₁₈H₃₀, at 295 K. A simple two-dimensional model representing the projection of the structure down c was used, which allowed refinement of short-range order parameters describing the mutual orientation of guest molecules within neighbouring channels, together with a parameter describing the coupling of these orientations to distortions of the host framework. Since the guest site still appears very disordered in projection, only a crude model for the shape of the guest molecule was used. The resulting calculated diffraction patterns showed good qualitative agreement with the observed pattern, but quantitatively the achieved agreement factor (R ≃ 38%) was considerably higher than that achieved in comparable studies of a different system. Calculations of the contribution to the diffraction pattern of the guest alone and the host alone clearly indicate that the host framework distortion is modelled well, but the guest is not so well described. The lattice distortion revealed by the study indicates a tendency for the hexagonal network to distort towards local monoclinicity. This is consistent with the observation that at ∼273 K, the compound undergoes a reversible phase transition to a micro-twinned monoclinic phase.