THE STRUCTURE OF A NUMBER OF NEMATOGENS

X-ray diffraction measurements have been made on five nematogens in the nematic and isotropic liquid phases. The data have been interpreted to give information about the molecular packing. Two Schiff's bases R1 [MATH] CH = N [MATH] R2 : MBBA (R1 = CH3O, R2 = nC4H9) and EBBA (R1 = C2H5O, R2 = nC4H9) have very similar structures, the higher homologue being much the more ordered. A third Schiff's base MBCA (R1 = CH3O, R2 = CN) is thought to have a more crystal-like packing resulting in a repeat distance ("d" spacing) along the texture axis 3 Å shorter than the molecular length. Two cyanobiphenyls R [MATH] CN : 5 CB (R = C5H11) and 7 CB (R = C7H15) have very similar structures, with pronounced local ordering. This is revealed most strikingly in a repeat distance along the texture axis of ~ 1.4 molecular lengths, which is interpreted as arising from an overlapping head to tail arrangement of molecules resulting in a quasi-layer (smectic A) structure on a local scale. The order extends over 100-150 molecules and is more pronounced for the higher homologue. In all cases, local order very similar to that in the nematic phase persists in the isotropic liquids.