Lack of Cooperativity in the Triangular X₃Halogen-Bonded Synthon?

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.