Halogen bonding: an electrostatically-driven highly directional noncovalent interaction

A halogen bond is a highly directional, electrostatically-driven noncovalent interaction between a region of positive electrostatic potential on the outer side of the halogen X in a molecule R-X and a negative site B, such as a lone pair of a Lewis base or the pi-electrons of an unsaturated system. The positive region on X corresponds to the electronically-depleted outer lobe of the half-filled p-type orbital of X that is involved in forming the covalent bond to R. This depletion is labeled a sigma-hole. The resulting positive electrostatic potential is along the extension of the R-X bond, which accounts for the directionality of halogen bonding. Positive sigma-holes can also be found on covalently-bonded…