Crystal Structure of the High Temperature Polymorph of Ethyl p-Azoxybenzoate

The high temperature crystal (solid II) of smectogenic ethyl p-azoxybenzoate has a monoclinic unit cell of space group C2/c containing eight molecules. Its parameters are: a = 23.36 A, b = 4.58 A, c = 32.24 A and β = 100.0°. Intensities of 718 independent reflections were estimated visually from Weissenberg photographs, and the structure was solved by direct methods using Sayre's relation. Least-squares refinement using anisotropic temperature factors produced a final R value of 0.14. The molecule is nearly planar in both polymorphs, but solid II is partially disordered due to the presence of both trans-cis and cis-trans rotational conformers, whereas solid I is predominately in the form of the cis-cis conformer. The component of the dipole moment perpendicular to the molecular axis, and in the plane of the molecule, is only 3.1 D. for solid II, as contrasted with 6.73 D. for solid I. The reduced importance of dipolar interactions in the high temperature crystal is evident upon comparing the molecular packing in solids I and II. This observation makes it less likely that dipolar interactions make a significant contribution to the lateral attractions between molecules of ethyl p-azoxybenzoate in the smectic state.