p-Methoxycinnamic acid (I) is nematogenic but cinnamic acid (II), although as a hydrogen-bonded dimer it has the necessary molecular anisotropy, is not. The crystal structures of the two compounds have been determined at room temperature, and this difference can be directly attributed to differences in molecular packing.

Crystals of (II) are monoclinic, space group P2₁/c with a= 5.644(1), b= 18.011(6), c= 9.019(3)Å, β= 121.47(2)°, and Z= 4. The structure was solved by direct methods and refined by least squares to R 0.059 for 899 independent reflections whose intensities were measured by counter diffractometry. Crystals of (I) are triclinic, space group P with a= 8.336(2), b= 12.026(4)c= 4.842(2)Å, α= 81.80(3), β= 99.28(2), γ= 111.90(3)°, and Z= 2. The structure was solved by a Fourier-transform method, and refined by least squares to R 0.048 for 1 675 independent reflections.

The molecular packing in crystals of (II) is zig-zag type with the long axes of the hydrogen-bonded dimers inclined at 45°. Crystals of (I) have all molecules parallel in a three-dimensionally interlocked arrangement characteristic of nematogenic crystals.