A high resolution quasielastic neutron scattering study of the guest molecules dynamics in the cyclohexane/thiourea inclusion compound

Abstract In cyclohexane/thiourea inclusion compounds, the host substructure contains uni-directional and non-intersecting channels within which the guest (cyclohexane) molecules are located. There are three thermotropic structural phases in cyclohexane/thiourea: phase I (down to 149 K) is rhombohedral, phase II (149–129 K) is monoclinic and phase III (below 129 K) is also monoclinic . In this paper, we report high resolution (1 μeV) incoherent quasielastic neutron scattering experiments performed in the three phases of the inclusion compound. We show that a quasielastic broadening is observed only in phase III, whereas the dynamics of the guest molecules in phases II and I is too fast to be analysed within the instrumental energy window. In phase III, the cyclohexane molecule undergoes 120° jumps about its three-fold symmetry axis with an activation energy E a =9.6(±0.5) kJ mol −1 , the three-fold symmetry axis of cyclohexane being tilted by an angle λ =75° with respect to the channel axis. These experiments confirm the orientational ordering of the guest molecules in phase III of the inclusion compound.