Bending properties in the 4-halobenzo­nitrile crystals and C–halogen...N[triple bond]C halogen bonds

The single crystal of 4-iodo­benzo­nitrile (C₇H₄IN) is brittle, whereas those of 4-bromo­benzo­nitrile (C₇H₄BrN) and one of the two forms of 4-chloro­benzo­nitrile (C₇H₄ClN) are com­pliant in nature. The chloro crystal exhibits elastic bending, but in spite of having stronger halogen bonds, the bromo crystal exhibits plastic bending. Crystal structures have been analyzed to understand the different bending properties of these three crystals. In all three cases, the mol­ecules form C—X...N[triple bond]C (X = halogen) halogen-bonded chains in their respective crystal structures. Statistical analyses and DFT calculations on the C—X...N[triple bond]C halogen bonds reveal that the optimum geometry of all three halogen bonds is linear and the C—I...N[triple bond]C bond is strongest among the three. However, when the geometry deviates from linearity, the energy loss is very high in the case of the C—I...N[triple bond]C bond com­pared to the other two systems. This explains why 4-iodo­benzo­nitrile is brittle, whereas the other two are flexible. The interactions in 4-bromo­benzo­nitrile are more isotropic than those in 4-chloro­benzo­nitrile. The iodo and chloro com­pounds crystallize in centrosymmetric space groups, whereas the crystal of the bromo com­pound lacks inversion symmetry. In spite of this difference in their space groups, the chloro and bromo crystals have very similar crystal packing. In the case of the bromo crystal, the halogen-bonded chains are parallel to the bending axis (long axis) of the crystal. However, these chains are significantly tilted in the case of the chloro crystal. The isotropic/anisotropic interactions, presence/absence of an inversion centre and the different alignment of the halogen-bonded chains with respect to the bending axis could explain the different bending properties of the chloro and bromo crystals.