Properties and interactions – melting point of tribromobenzene isomers

Abstract

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.

Response to comment on Properties and interactions – melting point of tribromobenzene isomers

“…Equation 4 in Wei (1999), and its inversed version above, both involve the enthalpy loss ÁH, depending on the cohesion forces in the crystal. The calorimetric parameters of bromobenzene isomers C 6 H n Br 6-n (Bujak et al, 2021) are not as well and completely determined as those for chlorobenzenes C 6 H n Cl 6-n . Hence in our research we focused on the determination of their crystal structures, and in particular on comparing halogen bonds BrÁ Á ÁBr.…”

“…Our recent paper (Bujak et al, 2021) reported the crystal structures of isomeric 1,2,3tribromobenzene, 1,2,4-tribromobenzene and 1,3,5-tribromobenzene determined by single-crystal X-ray diffraction as a function of temperature (at T = 100, 200 and 270 K). We analysed the intermolecular interactions as well as the crystal and molecular symmetry in relation to the melting point (T m ) of these compounds.…”

Mixed halobenzenes also prefer face⋯face approaches of aromatic rings, revealing a new inversion symthon

“…12 Several crystal structures of halobenzenes have been reported and discussed in a series of papers by Bujak and co-workers. [13][14][15][16] In all of these studies, discussion of the crystal structures has focussed on approaches containing one, two or three favourable connections, sometimes denoted "synthons", including halogen⋯halogen and C-H⋯halogen connections.…”

Comment on the article Properties and interactions – melting point of tribromobenzene isomers

“…In a recent paper, Bujak et al (2021) try to rationalize the melting points of three isomers of tribromobenzene using 'Carnelley's rule': in a set of isomers, the higher the molecular symmetry, the higher the melting point (Carnelley, 1882a,b). Indeed, 1,2,4-tribromobenzene melts at 317.7 K, whereas 1,3,5-tribromobenzene melts at 396.0 K. The authors try to use Carnelley's rule to argue that the higher the molecular symmetry, the more optimal the molecular interactions in the crystal structure and therefore the higher the melting point.…”