Sigma-holes from iso-molecular electrostatic potential surfaces.

Visualization of the halobenzene σ-hole region of molecules (PhX, X = Cl, Br, I) was conducted to investigate the nature of the σ-hole present between covalently bonded elements of groups IVB-VIIB (known as halogen bonding for group VIIB) and corresponding negative sites, such as Lewis base lone electron pairs, π-electrons, or anions. The σ-hole consists of a region of poor electron density and often relatively positive electrostatic potential surrounding the outermost portion of the halogen atom along the A-X bond axis. In this work, molecular electrostatic potential (MEP) isosurfaces for PhX obtained from ab initio calculations are examined to determine the σ-hole in 3D, showing the surfaces of corresponding positive and negative regions. Surfaces were mapped for isopotentials of PhX molecules as low as 0.003 V and scaled up by factors of 10 up to 3 V. The σ-hole is revealed as a positive region exposed underneath a predominantly negative MEP isosurface. As isopotential values move away from zero, this hole grows in radius; conversely, its presence completely fades as potential approaches zero with increasing distance from the molecule. This technique can also be used to compare behaviors of neutral molecules and their ionic counterparts like the case of neutral PF