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

In cyclohexane/thiourea inclusion compounds, the host substructure contains uni-directional and non-intersecting channels within which tile guest (cyc lohexane) molecules are located. There are three thermotropic structural ph ases in kcyclohexane/thiourea: phase I (down to 149K) is rhombohedral, phas e II (149-129K) is monoclinic and phase III (below 129 K) is also monoclini c. Tn this paper, we report high resolution (1 mu eV) incoherent quasielast ic neutron scattering experiments performed in the three phases of the incl usion 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 i s too fast to be analysed within the instrumental energy window. In phase I II, the cyclohexane molecule undergoes 120 degrees jumps about its three-fo ld symmetry axis with an activation energy E-u = 9.6(+/- 0.5) kJ mol(-1) th e three-fold symmetry axis of cyclohexane being tilted by an angle lambda = 75 degrees with respect to the channel axis. These experiments confirm the orientational ordering of the guest molecules in phase III of the inclusio n compound. (C) 2001 Elsevier Science B.V. All rights reserved.