Properties and interactions – melting point of tri­bromo­benzene isomers

Single crystals of isomeric 1,2,3-tri­bromo­benzene (123TBB), 1,2,4-tri­bromo­benzene (124TBB) and 1,3,5-tri­bromo­benzene (135TBB) have been grown from different solvents and their structures determined by X-ray diffraction at 100, 200 and 270 K. The melting-point differences of ca 40 K between 135TBB, 123TBB and 124TBB have been correlated with the molecular symmetry and packing preferences in the crystal, as well as with the main types of intermolecular halogen interactions, i.e. Br⋯Br, Br⋯C (Br⋯π) and Br⋯H. The relationship between symmetry and melting point in Carnelley’s rule has been extended to the accessibility of terminal atoms for the formation of intermolecular interactions, their occurrences and distribution, and the close packing. The electrostatic potential mapped on molecular surfaces demonstrates that in more symmetric molecules the more evenly distributed substituents are more accessible and form more optimum intermolecular interactions.