Molecular crystals and liquid crystals: new results for t -butyl chloride

Abstract

The main theme of this paper is the use of incoherent quasielastic scattering to study molecular reorientational motions in molecular and liquid crystals at temperatures where a classical description is appropriate. The relation to quantum mechanical tunnelling between orientational potential wells of hydrogenous species (e.g. CH ₃ , NH ⁺ ₄ ) is mentioned briefly. The use of this technique is illustrated with a description of the determination of the nature and dynamics of the molecular reorientations in the solid phases of t -butyl chloride. This shows how the measurement of the quasielastic spectrum as a function of scattering vector permits an identification of the type of reorientation and its characteristic time scale, provided this is fast enough to be observed in a neutron experiment (r < 10 ^-9 s). Finally a brief resume is given of the results that have emerged from such studies about the various molecular motions that characterize and distinguish different liquid crystal phases.