Lack of Cooperativity in the Triangular X(3) Halogen-Bonded Synthon?

[Image: see text] We have investigated 44 crystal structures, found in the Cambridge Structural Database, containing the X(3) 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 X(3) motif is present demonstrate that weak synergy occurs only in the case of the I(3) motif. In the present paper we computationally investigate crystal structures in which the X(3) 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 X(3) motif. Only in the case of two other structures containing the I(3) 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.