Theoretical study of intermolecular interactions in crystalline arene-perhaloarene adducts in terms of the electron density

The effect of halogen substitution on the intermolecular interactions and crystal packing of areneperhaloarene adducts was studied by means of theoretical methods. Solid state density functional theory geometry optimizations with the LAPW methodology were carried out for the hexafluorobenzenepyrene, hexafluorobenzene-triphenylene, hexachlorobenzene-pyrene and hexachlorobenzenetriphenylene complexes using as starting points the X-ray crystal geometries measured in our laboratory. The structures of hexachlorobenzene- pyrene and hexachlorobenzene- triphenylene are reported for the first time. Using the tools of the quantum theory of atoms in molecules, the following six types of intermolecular interactions were identified: pi...pi, pi...X, pi...H, X...H, H...H and X..X where X = F or Cl. The electron density and related properties at the bond critical points and the NCI index allow to classify them as closed-shell weak interactions. The alternating regions of positive and negative electrostatic potential of chlorine and a larger deformation density than the one observed in fluorine, allow to understand the stronger pairwise interactions involving Cl. The cohesion energies were also computed, being more negative those for the molecular crystals involving hexachlorobenzene. This observation was rationalized in terms of the properties of the electron density at the intermolecular contacts. It was also found that dispersion is the most stabilizing long-range contribution to the dimerization energies of several related model systems. These results suggest that the properties of the electron density and the energetic stabilizing contributions provide complementary viewpoints for the understanding of the intermolecular interactions in these crystals.