Characteristics of a σ-Hole and the Nature of a Halogen Bond.

The nature of halogen bonding in 128 complexes was investigated using advanced quantum mechanical calculations. First, isolated halogen donors were studied and their σ-holes were described in terms of size and magnitude. Later, both partners in the complex were considered and their interaction was described in terms of DFT-SAPT decomposition. The whole set of complexes under study was split into two categories on the basis of their stabilisation energy. The first subset with 38 complexes possesses stabilisation energies in the range 7-32 kcal/mol, while the second subset with 90 complexes has stabilisation energies smaller than 7 kcal/mol. The first subset is characterised by small intermolecular distances (less than 2.5 Å) and a significant contraction of van der Waals (vdW) distance (sum of vdW radii). Here the polarisation/electrostatic energy is dominant, mostly followed by induction and dispersion energies. The importance of induction energy reflects the charge-transfer character of the respective halogen bonds. Intermolecular distances in the second subset are large and the respective contraction of vdW distance upon the formation of a halogen bond is much smaller. Here the dispersion energy is mostly dominant, followed by polarisation and induction energies. Considering the whole set of complexes, we conclude that the characteristic features of their halogen bonds arise from the concerted action of polarisation and dispersion energies and neither of these energies can be considered as dominant. Finally, the magnitude of the σ-hole and DFT-SAPT stabilisation energy correlates only weakly within the whole set of complexes.