Isomorphism and pseudosymmetry in 2,6-dichloro- and 2, 6-dibromobenzonitrile

The title compounds are isomorphous, with two half-molecules in the asymmetric unit of a monoclinic unit cell in the space group C2/m. For the chloro compound: a = 18.0525 (8), b = 20.7374 (10), c = 3. 8334 (2) A, beta = 101.143 (1) degrees; for the bromo compound: a = 18.2269 (15), b = 21.349 (2), c = 3.9663 (3) A, beta = 101.446 (2) degrees. The dominant intermolecular interactions are between halogen atoms rather than between halogen and nitrile, as is the case in the corresponding 2,4,6-trihalobenzonitriles. The molecules pack into layers with a fourfold pseudosymmetry. The pseudosymmetry appears to be the consequence of halogen-halogen intermolecular interactions.