Molecular conformation and electronic structure. Part III. Crystal and molecular structure of the stable form of N-(p-chlorobenzylidene)-p- chloroaniline

The title compound crystallizes in the orthorhombic space group Pccn with a = 24.503(5), b = 6.334(1), c = 7.326(1) Å, and Z = 4. The structure was solved by direct methods from 1 259 reflections measured on a Syntex P1 automated diffractometer with graphite-monochromated Mo-K_α radiation, and refined anisotropically by full-matrix least-squares to R 0.056. The structure is disordered about a cristallographic two-fold axis leading to a novel conformation of the molecule with benzene rings twisted by 24.8° in opposite directions from the plane of the central four atoms, in contrast to the metastable triclinic form, which is planar. The two structures represent a striking example of conformational polymorphism which is discussed in terms of the packing.