Polarization plays the key role in halogen bonding: a point-of-charge-based quantum mechanical study

The nature of the halogen bond has been under debate over the last decade. Herein, the nature of the halogen bond was reinvestigated using point-of-charge (PoC) approach in which a point of negative or positive charge was used to mimic a Lewis base or acid, respectively. Halogen bond strength was estimated in terms of stabilization energy of the halo molecule in the presence of PoC. Open-ended questions regarding halogen interaction via σ-hole were discussed. A number of fundamental physical terms including σ-node, −σ-hole and +σ-hole interactions were introduced to describe the unconventional behavior of the halogen’s interactions. Several conflicts in the published results and explanations on the halogen bonding were highlighted and clarified. Based on PoC results, it may be claimed that: (i) halogen bond is mainly an electrostatic interaction and (ii) the polarization of the halogen is the key for understanding the reason behind the formation of halogen···Lewis acid/base interaction at halogen···Lewis acid/base angle of 180°. A–X···PoC angle and solvent effects on the molecular stabilization energy were estimated. Furthermore, electron correlation contribution to molecular stabilization energy was evaluated. Natural bonding orbital calculations were performed on the studied halo molecules. Finally, halogen bond test (σn-hole test) was proposed as a theoretical calculation to examine the ability of a halo molecule to form a halogen bond.