Crystal and Molecular Structure of the Nematogenic Compound 4-Cyanophenyl-4′-n-Heptylbenzoate (CPHB)

Abstract

It is now well established that for a proper understanding and interpretation of several physical properties of liquid crystalline phases, a knowledge of the molecular arrangement in the crystalline state is very useful. A preliminary survey of the present knowledge of the solid-mesophase relationships has been given by Bryan¹. The classical view is that in typical nematogen crystals the long narrow molecules lie more or less parallel and are interleaved giving what was described by Bernal and Crowfoot² as an imbricated packing and that the transformation from the solid to the nematic phase is characterised by the breakdown of the positional order of the molecules but not of the orientational order. Leadbetter³ has however pointed out that although this is true of at least the majority of the small number of cases so far known, it is a generalisation which must at this stage be treated with caution. Hence the determination of the crystal and molecular structure of the title nematogenic compound was undertaken. Bhattacharjee et al. ⁴ have previously studied the transition temperatures of this compound by X-ray diffraction and optical microscopy and found as follows:

These transition temperatures were found to be identical to those reported by M/s. F. Hoffmann-La-Roche, (the only additional fea-ture being the supercooling). Bhattacharjee et al. also determined the orientational order parameters and distribution functions of mag-netically aligned samples, intermolecular distances and apparent mo-lecular lengths. In this paper we describe the crystal and molecular structure of CPHB and discuss the change in the molecular packing which accompanies the solid to nematic phase transition.

Key Words:

• crystal structure
• nematogen