Abstract
We have investigated 44 crystal structures, found in the Cambridge Structural Database, containing the X3 synthon (where X = Cl, Br, I) in order to verify whether three type II halogen–halogen contacts forming the synthon exhibit cooperativity. A hypothesis that this triangular halogen–bonded motif is stabilized by cooperative effects is postulated on the basis of structural data. However, theoretical investigations of simplified model systems in which the X3 motif is present demonstrate that weak synergy occurs only in the case of the I3 motif. In the present paper we computationally investigate crystal structures in which the X3 synthon is present, including halomesitylene structures, that are usually described as being additionally stabilized by a synergic interaction. Our computations find no cooperativity for halomesitylene trimers containing the X3 motif. Only in the case of two other structures containing the I3 synthon a very weak or weak synergy, i.e. the cooperative effect being stronger than −0.40 kcal mol–1, is found. The crystal structure of iodoform has the most pronounced cooperativity of all investigated systems, amounting to about 10% of the total interaction energy.
Highlights
There are two types of halogen−halogen (X···X) contacts occurring in molecular crystals
BOWRUC02,72 and IODOFO0444) were measured at high pressure. These results suggest that a short distance between halogen atoms is not a simple indicator of a significantly increased interaction strength in the X3 synthon
The hypothesis, based on the structural data, that cooperativity of interactions occurs in the halogen−halogen bonded X3 synthon was proposed in papers devoted to crystal structures pubs.acs.org/crystal
Summary
There are two types of halogen−halogen (X···X) contacts occurring in molecular crystals. In the case of the third group of compounds, these containing the I3 motif, the cooperativity is negligible (ΔEsyn > −0.40 kcal mol−1) for all but two investigated structures; among the systems containing the I3 motif, there are two systems exhibiting weak cooperativity These findings are in line with the results obtained previously for 16 simple model systems.[27] At this point it is important to stress that neither tribromomesitylene (PUZHIB0543 refcode) nor triiodomesitylene (SAQZOY018 refcode) trimers, which have been described as “made up of three cooperative halogen− halogen interactions”,8 do not display any cooperativity according to our computations. We decided to calculate the interaction energies of the 44 studied model systems but with the X···X distance constrained to the sum of van der Waals radii (values estimated by Bondi[78]) It should be noted that the differences between interaction energies in most of the trimers do not exceed 0.50 kcal mol−1
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