An X-Ray Study of the p-n-Alkoxybenzoic Acids. Part VI. Isotypic Crystal Structures of Four Smectogenic Acids Having Seven, Eight, Nine, and Ten Alkyl Chain Carbon Atoms

Abstract

The isotypic crystal form for the smectogenic p-n-alkoxybenzoic acids has been determined by room-temperature single-crystal x-ray diffraction analyses for the acids having 7, 8, 9, and 10 carbon atoms in the alkyl chain. The molecules are present in the crystals as centrosymmetric hydrogen-bonded dimers. The alkyl chains adopt an all-trans extended conformation but a gauche-relation of the ether oxygen and C(γ) about the C(α)-C(β) bond of the chains imparts a markedly non-linear and non-planar conformation to the molecules. With these chain lengths, adoption of this conformation makes possible an imbricated stratified layer structure in which the alkyl chains and the central aromatic nuclei are segregated into separate close-packed strata. Antiparallel interdigitation of chains in adjacent layers gives a 2:1 ratio of chains to nuclei in the strata. Evidence is presented that the molecules in the smectic mesophase probably retain the conformation found in the crystal and it is shown that the change in chain tilt angle in passing from solid to smectic phases is not as great as previously supposed. C₁₄H₂₀O₃ (70BAC). Triclinic, Pī, a = 4.739(5), b = 8.034(7), c = 17.948(18) Å, α = 82.32(8), β = 93.78(7), γ = 102.19(2)°, Z = 2. C₁₅H₂₂O₃ (80BAC). Triclinic, Pī, a = 4.825(1), b = 8.123(2), c = 19.036(4) Å, α = 81.15(2), β = 94.57(2), γ = 102.98(2)°, Z = 2. C₁₆H₂₄O₃ (90BAC). Triclinic, Pī, a = 4.743(1), b = 8.049(2), c = 20.484(4) Å, α = 82.38(2), β = 93.36(2), γ = 102.55(2)°, Z = 2. C₁₇H₂₆O₃ (10OBAC). Triclinic, Pī, a = 4.819(1), b = 8.112(2), c = 21.598(4) Å, α = 80.94(2), β = 94.05(2), γ = 102.87(2)°, Z = 2. The small even-odd effect in the cell dimensions reflects a slight difference in the mode of chain packing in the solids.