The nature of intermolecular interactions between halogen atoms, X...X (X = Cl, Br, I), continues to be of topical interest because these interactions may be used as design elements in crystal engineering. Hexahalogenated benzenes (C6Cl(6-n)Br(n), C6Cl(6-n)I(n), C6Br(6-n)I(n)) crystallise in two main packing modes, which take the monoclinic space group P2(1)/n and the triclinic space group P1. The former, which is isostructural to C6Cl6, is more common. For molecules that lack inversion symmetry, adoption of this monoclinic structure would necessarily lead to crystallographic disorder. In C6Cl6, the planar molecules form Cl...Cl contacts and also pi...pi stacking interactions. When crystals of C6Cl6 are compressed mechanically along their needle length, that is, [010], a bending deformation takes place, because of the stronger interactions in the stacking direction. Further compression propagates consecutively in a snakelike motion through the crystal, similar to what has been suggested for the motion of dislocations. The bending of C6Cl6 crystals is related to the weakness of the Cl...Cl interactions compared with the stronger pi...pi stacking interactions. The triclinic packing is less common and is restricted to molecules that have a symmetrical (1,3,5- and 2,4,6-) halogen substitution pattern. This packing type is characterised by specific, polarisation-induced X...X interactions that result in threefold-symmetrical X3 synthons, especially when X = I; this leads to a layered pseudohexagonal structure in which successive planar layers are inversion related and stacked so that bumps in one layer fit into the hollows of the next in a space-filling manner. The triclinic crystals shear on application of a mechanical stress only along the plane of deformation. This shearing arises from the sliding of layers against one another. Nonspecificity of the weak interlayer interactions here is demonstrated by the structure of twinned crystals of these compounds. One of the compounds studied (1,3,5-tribromo-2,4,6-triiodobenzene) is dimorphic, adopting both the monoclinic and triclinic structures, and the reasons for polymorphism are suggested. To summarise, both chemical and geometrical models need to be considered for X...X interactions in hexahalogenated benzenes. The X...X interactions in the monoclinic group are nonspecific, whereas in the triclinic group some X...X interactions are anisotropic, chemically specific and crystal-structure directing.

Bending of crystals of molecular solids occurs when the strength of intermolecular interactions in orthogonal directions is significantly different. We report here a survey of 60 molecular crystals and establish a causative correlation between bending and crystal packing. This group contains crystals with 4 and 8 Å crystal axes and includes 1D, 2D, 3D, isostructural, polymorphic, stacked, interlocked, single, and multicomponent crystals and solvates. We found that 17 of these 60 crystals may be bent, whereas the rest are brittle and cannot be bent plastically. The bending crystals could be deformed into many shapes; sometimes, they could even be flattened upon themselves without breakage. A model for bending is proposed using the information obtained from X-ray diffraction, face indexing, and mechanical property measurements on both bending and non-bending (brittle) crystals. The bending and brittleness of these molecular crystals are discussed in comparison with the deformation behavior of metals. Molecular crystals show practically no change in volume and the lengths of the inner and the outer arcs and the sample thickness are unchanged following plastic bending. This is in contrast with the bending of metallic materials, in which a decrease in thickness is evident. Isotropic crystals with comparable intermolecular interactions in the three orthogonal directions are "cross-linked" and do not bend; they are hard and brittle. Mechanical properties of molecular crystals are important because they vary with the crystal form and have major implications for large-scale processing and handling of materials in industry, especially the pharmaceutical industry.

Bending is observed in organic crystals when the packing is anisotropic in such a way that strong and weak interaction patterns occur in nearly perpendicular directions.

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