Halogen bonds in some dihalogenated phenols. Copyright IUCr 2014
Halogen bonds in some dihalogenated phenols. Copyright IUCr 2014

Compounds containing halogen atoms, such as chlorine and bromine have many uses as flame retardants, wood preservatives, insecticides, and antibacterial substances. However, a new study on the nature of the interactions between halogen atoms in such various materials points to the possibility of designing bendy crystals, according to results published in the IUCrJ.

Gautam Desiraju and Arijit Mukherjee at the Indian Institute of Science in Bangalore, hope to unravel the halogen-halogen intermolecular bond and how and why it behaves in the way it does in different halogenated phenolic compounds and related substances. The studies might have implications for understanding the environmental persistence of such chemicals, their effects on the endocrine system in our bodies, the efficacy of powerful halogen-containing pharmaceuticals and the design of building blocks for crystal engineering of solids with useful mechanical properties.

The team has investigated the crystal structure of 3,4-dichlorophenol, an organic compound used in wood treatment, leather tanning and paper production. They explain that the compound crystallizes with a tetragonal symmetry but uniquely there are two distinct types of non-bonding interaction between the chlorine atoms in the crystal labelled simply type I and type II in which the angles between interacting pairs of chlorine atoms relative to their attached carbon atoms are distinct.

In type I the two angles are the same, in type II they are about 90 degrees apart, a point the team has demonstrated by obtaining crystal structures of various other halogenated phenols. Usually, only one type of chlorine-to-chlorine non-bonding interaction occurs in any given compound because all of the chlorine atoms are equivalent, sharing the same surroundings. In 3,4-dichlorophenol the interactions between the oxygen atom of the phenolic "OH" groups and neighboring hydrogen atoms produce two different environments allowing both types of chlorine-chlorine interactions to form.

Additional experiments with analogues of 3,4-dichlorphenol containing either one or two chlorine atoms substituted for bromine show that the distances between the halogen atoms lengthens with increasing temperature. This elongation is substantially greater in the type II contacts than in the type I, which Desiraju and Mukherjee suggests is consistent with the stronger type II interactions being a bit more like conventional chemical bonds able to sustain themselves at higher temperatures over greater separation than the looser interactions in type I.

Moreover, the team has found that the type II interaction between two bromine atoms is stronger than that observed in chlorine-only compounds and that this makes crystals of 4-bromo-3-chlorophenol more elastic than 3,4-dichlorophenol. This discovery could lay the foundations for the development of springy, functional materials based on materials with different mixes of carbon-halogen bonds.