The low-temperature form (solid I) of the smectogenic compound ethyl p-azoxybenzoate (C18H18N2Os) is stable below 90 °C. The triclinic unit cell of space group P] and dimensions a = 7.56 + 0.02, b--18"35 _+ 0.07, c= 6.28 + 0.01 A, ¢¢= 94.38 ° + 0.30 °, ,8= 97.21 _+ 0.31 °, and y = 83.97 ° + 0.25 ° contains two molecules and has a calculated density of 1.325 g.cm-3. The observed density is 1.315 + 0.013 g.cm-3. Integrated intensities were measured at room temperature by a 20--~ scan on a

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... cle diffractometer equipped with a post-diffraction monochromator and scintillation detector. The crystal structure was refined by full-matrix least-squares methods with 1460 independent reflections to a final R index of 0"093. The molecules are nearly planar (the normals to the planes of the two benzene rings diverge by only 0-5 °) and are packed in a parallel array, as contrasted with the imbricated structure which has been observed for several nematogenic crystalline solids. The carbonyl and nitroso oxygen atoms are all in the cis conformation, resulting in a large dipole component in the plane of the molecule and perpendicular to its long axis. It is speculated that this feature of the molecular structure may be characteristic of smectogenic compounds.