STRUCTURE OF SMECTIC A PHASES OF COMPOUNDS WITH CYANO END GROUPS.

The smectic layer spacings (d) have been determined for a number of cyano derivatives which have an anomalous d/molecular length ratio of similar 1. 4. It is shown that the quantity (d-molecular length) is directly related to the length of the alkyl or alkoxy tail of the molecule but is not simply related to the length of the aromatic core. This is consistent with a bimolecular layer structure having overlapping molecular cores at the layer centre. Using a combination of X-ray and neutron diffraction measurements of the intensity of the layer reflections on several isotopically substituted forms this model was positively confirmed and quantified for one compound (4-cyano-4 prime -n-octylbiphenyl, 8CB).