Quasielastic neutron scattering and proton N.M.R. study of molecular motions in the ‘rotator’ and liquid phases of octaphenylcyclotetrasiloxane (OPCTS)

Abstract

Rotational and translational motions of octaphenylcyclotetrasiloxane (OPCTS) are analysed in its solid and liquid phases, on the three different time scales accessible from N.M.R. and quasielastic neutron scattering. From time-of-flight neutron spectroscopy (time scale 10^-11 s) an elastic incoherent structure factor is extracted which corresponds to a model based upon a uniaxial rotation of the whole molecule about an axis of the siloxane ring together with large amplitude oscillations of the phenyl groups, which are made easier by a deformation of the siloxane ring. On the time scale of 10^-9 s (neutron back-scattering experiments), the motion appears more isotropic. The axis of the molecule can reach all directions in space and the motion of the whole molecule is more likely described by a rotational diffusion of a symmetric top with two diffusion constants for motions about and perpendicular to the axis. The relevant correlation times are obtained for both motions. Moreover, in the liquid state, just above the transition, values of the translational self-diffusion coefficient could also be obtained. This translational motion was also investigated over all the isotropic solid phase, using proton N.M.R. (time scale 10^-6 s). Finally a short comparison is made with some results obtained for cyclohexane.