Interlayer molecular rotation

The final group of lamellar phases to be introduced here are the so-called crystal smectic phases. These are more ordered than the previous smectic examples and are characterized by the appearance of interlayer correlations and, in some instances, by the removal of freedom of molecular rotation. Consequently the (crystal) B, G and J phases are derived from the Sb, Sf and S] phases, respectively, but with the presence of interlayer correlations. Further the (crystal) E, H and K phases are B, G and J phases which have lost rotational freedom. These phases are still disordered and hence, still intermediate between the solid and liquid states. 

ESR studies of other paramagnetic metal ions (Mn " ", VC)2+) as surface probes have confirmed that cation rotational mobility on fully wetted smectites with interlayer spacings of 1.0 nm or more is not more than 50% reduced relative to aqueous solution (12, 14). Apparently, about four molecular layers of water in the interlayer provide sufficient space for solution-like rotation rates of hydrated metal ions. 

Returning to the smectic A arrangement, then introduction of hexagonal symmetry within the layers produces a more ordered molecular arrangement assigned as the smectic B phase (Sb). Again the phase is fluid and interlayer diffusion occurs, although rotation about the molecular axis is concerted. In a similar manner to the way that the molecules in the smectic C phase are tilted compared to the smectic A,... 

Structural Elucidation of IsoX Phase. Spectral analysis by C NMR and X-ray diffraction measurements suggests that upon the SmC IsoX transition, an interlayer correlation may be broken, whereas dimerization or tetramerization via a stereospecific intermolecular interaction through fluorines at chiral centers within each layer is assumed to be operating. The dimerization or tetramerization reduces the anisotropy of molecular motion to increase rotation around the short axis. This change in molecular dynamics may release entropy of the system to induce the endothermic transition. The IsoX phase is not a result of the competition between helical structure and mesophase ordering but a result of the chirality-dependent stereospecific interaction or chiral molecular recognition (7. 

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