Halogen⋯halogen interaction is a highly debated and important topic in crystal engineering and supramolecular chemistry. There are controversies about the nature and geometry of these interactions. F, Cl, Br and I are the four halogens involved in these interactions. Very often the lighter and the heavier halogens behave differently. The nature of the interactions also depends upon the nature of the atom which is covalently bonded to the halogens. In this review different homo‐halogen⋯halogen, hetero‐halogen⋯halogen, halogen⋯halide interactions, their natures, and preferred geometries have been discussed. Different motifs of halogen⋯halogen interactions, interchangeability of the halogen⋯halogen interactions with other supramolecular synthons and interchangeability between different halogens and other functional groups also have been discussed. Some of the important applications, in which halogen⋯halogen interactions have been successfully employed, are mentioned.
Statistical analysis and DFT calculations have been performed on NO2···NO2 interactions between nitrobenzene moieties to understand the preferred geometry and energy of this interaction. The favorable interplanar angle, N···O distance, ∠C–N···O, and ∠N–O···N have been determined from these two analyses which agree with each other on all the parameters. While cam-B3LYP reproduces the geometrical parameters well, energetic trends are better reflected at M06-2x level of theory. The most favorable angle is obtained when the two NO2 groups are parallel to each other and are located around an inversion center. The inversion center ensures that there would be two equivalent N···O interactions. The parallel form is computed to be more stable than the perpendicular one by 0.93 kcal/mol at M06-2x/Aug-cc-pvTZ. However, the N···O distance is interestingly slightly shorter in perpendicular geometry compared to the parallel geometry. The interacting O atom sits on the interacting N atom which makes the ∠C–N···O to be around 90°, indicating involvement of the N atom in this interaction. In the perpendicular geometry, the preferred ∠N–O···N is found to be around 155° in the statistical analysis and 148° in the DFT calculation. This indicates the involvement of the lone pair of the O atom in the interaction. Comparison of C–N and N–O bond lengths between the monomer and dimers point to the involvement of the π* LUMO, though NBO analysis shows these interactions to be minimal.
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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.