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

We have investigated 44 crystal structures, found in the Cambridge Structural Database, containing the X 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 motif is present demonstrate that weak synergy occurs only in the case of the I motif. In the present paper we computationally investigate crystal structures in which the X 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 motif. Only in the case of two other structures containing the I synthon a very weak or weak synergy, i.e. the cooperative effect being stronger than -0.40 kcal mol, 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.