Properties of halogen atoms for representing intermolecular electrostatic interactions related to halogen bonding and...

The electrostatic properties of halogen atoms are studied theoretically in relation to their ability of halogen bonding, which is an attractive intermolecular interaction of a covalently bonded halogen atom with a negatively charged atom of a neighboring molecule. The electric quadrupole (of electronic origin) with a positive zz component Theta(zz) of a covalently bonded halogen atom, where the z axis is taken along the covalent bond involving the halogen atom, is mainly responsible for the attractive electrostatic interaction with a negatively charged atom. This positive Theta(zz) is an intrinsic property of halogen atoms with the p(x)(2)p(y)(2)p(z) configuration of the valence electronic shell, as shown by ab initio molecular orbital calculations for isolated halogen atoms with this electronic configuration, and increases in the order of F < Cl < Br < I, in parallel with the known general sequence of the strength of halogen bonding. For halogen-containing aromatic compounds, the substituent effects on the electrostatic properties are also studied. It is shown that the magnitude of Theta(zz) and the electric field originating from it are rather insensitive to the substituent effect, whereas the electric field originating from atomic partial charges has a large substituent effect. The latter electric field tends to partially cancel the former. The extent of this partial cancellation is reduced in the order of Cl < Br < I and is also reducible by proper substitution on or within the six-membered ring of halobenzene. Perspectives on the development of potential function parameters applicable to halogen-bonding systems are also briefly discussed.