Neutron scattering studies molecular reorientation

We have been particularly interested in the study of the plastic states of organic compounds,41 which are characterized by high values of AS of formation from the crystalline state, the AS of fusion (plastic-liquid transition) being much smaller. We find that the AH as well as the AS of the crystal-plastic transition generally decrease as the temperature range of stability of the plastic phase increases the AH and the AS of the plastic-liquid transition, on the other hand, increase as the temperature range of stability of the plastic phase increases.42 Neutron scattering, NMR spectroscopy, and several other techniques have been employed to study molecular reorientation in the plastic state.41 We... 

The ordering of the transverse molecular axes, which occurs in certain low-temperature smectic phases, has been studied by NMR and NQR methods [7.40]. These measurements show that the uniaxial reorientation of the molecular cores around their long axes are strongly biased. It is generally assumed that in nematic and smectic A phases, the uniaxial rotation (7-motion) is not biased. However, recent neutron quasielastic scattering experiments [7.41] in the nematic phase of MBBA seem to support the notion that the rigid benzylideneaniline core is restricted to a uniaxial rotational diffusion of finite angular excursion. Restricted libration within 7 =z 00/2 for internal motions in macromolecules has been considered by London and Avitabile [7.42], and Wittebort and Szabo [7.43]. 

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