An X-Ray Study of the p-n-Alkoxybenzoic Acids. Part V. Crystal Structures of the Nematogenic Acids Having Three and Five Alkyl-Chain Carbon Atoms

Abstract

The crystal structures of p-n-propoxy- and p-n-pentoxy-benzoic acids (3OBAC and 5OBAC) have been determined at room temperature by single-crystal x-ray analysis. Both crystals have sheet structures. Within each sheet the molecules are arranged as hydrogen-bonded dimers in head-to-tail fashion in parallel staggered rows. The long axes of the aromatic nuclei are almost normal to crystal b. The alkyl chains are in planar extended form, but the ether oxygen and C(γ) are in a gauche relation about the C(α)-C(β) bond of the chain. The stacking of sheets is slightly different in 3OBAC which undergoes a solid-solid transition before yielding a nematic phase, than in 5OBAC which transforms directly to the nematic state. The tight face-to-face overlap of phenyl rings characteristic of the crystal structures of the two lowest homologs in the series, which are non-mesogenic at atmospheric pressure, is absent in these crystal structures where the ring planes in adjacent sheets are inclined to one another by about 55°. Nematogenic behavior is attributed to parallel alignment of molecules in the solid coupled to a weakening of the solid-state intermolecular forces, arising from difficulty in accomodating the increasing steric bulk of the alkyl chains with respect to the aromatic nuclei.

C₁₀H₁₂O₃ (3OBAC). Monoclinic, P2₁/c, a = 15.303(2), b = 7.491(1), c = 8.177(1) Å, β = 95.70(2)°, Z = 4.

C₁₂H₁₆O₃ (5OBAC). Monoclinic, P2₁, a = 16.099(5), b = 7.357(2), c = 10.018(3), β = 102.89(2)°, Z = 4.