Molecular rotations in the plastic phases of C6F9H3 and C6F12

Abstract

Incoherent quasi elastic neutron scattering experiments at relatively high momentum transfer have been made on the plastic crystalline phase of C₆F₉H₃ at ∼295 K. These data are consistent with a rotational diffusion motion with a rotational diffusion constant D_r= 3 × 10¹⁰ s^–1. However, analysis of the intensities of the quasi elastic scattering together with results obtained earlier at lower momentum transfer and a lower temperature indicate the existence of an appreciable librational component of the total mean square vibrational displacements. This indicates that the molecules spend a significant fraction of time librating about a quasi-fixed orientation. It is concluded that the overall rotational motion is that of an itinerant librator.

Preliminary coherent inelastic scattering measurements have also been made on a single crystal of C₆F₁₂ in its plastic phase at ∼295 K. A very low energy excitation has been observed and it is suggested that this may be associated with cooperative reorientational motions of the molecules.