Structural Study of the Crystal and Mesomorphic States of trans, trans-4?- Butyl-bicyclohexyl-4-Carboxylic Acid (ZLI 1756)

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Abstract
The crystal structure at room temperature and the phase transitions of the title compound (ZLI 1756, Merck, Darmstadt), whose smectic B phase displays catalytic effects on the monomolecular rearrangements of terpene derivatives, have been investigated. The compound changes on heating from a crystalline phase stable at room temperature to another solid phase, then to a smectic B phase and later to a nematic phase. The crystal structure has been determined at room temperature by direct methods and refined by least-squares methods to R = 0.072 for 2339 observed reflections. The compound crystallizes in space group P21/c with a = 21.934(5), b = 5.384(2), c = 27.681(7) Å, β = 97.35(3) ° and Z = 8. The molecules are arranged as hydrogenbonded dimers. The four cyclohexyl rings have the chair conformation and the butyl chains have the trans-planar conformation. The packing is layer-like with the molecules inclined to the layers, which are the (100) planes. The chain-chain interactions occur at the aliphatic surface parallel to the layers. The inclination of the molecules relative to the layers disappears at the change from the solid to the smectic B phase, but the dimeric arrangement remains in this phase too.

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