Sigma-hole bonding: molecules containing group VI atoms.
It has been observed both experimentally and computationally that some divalently-bonded Group VI atoms interact in a noncovalent but highly directional manner with nucleophiles. We show that this can readily be explained in terms of regions of positive electrostatic potential on the outer surfaces of such atoms, these regions being located along the extensions of their existing covalent bonds. These positive regions can interact attractively with the lone pairs of nucleophiles. The existence of such a positive region is attributed to the presence of a "sigma-hole." This term designates the electron-deficient outer lobe of a half-filled p bonding orbital on the Group VI atom. The positive regions become stronger as the electronegativity of the atom decreases and its polarizability increases, and as the groups to which it is covalently bonded become more electron-withdrawing. We demonstrate computationally that the sigma-hole concept and the outer regions of positive electrostatic potential account for the existence, directionalities and strengths of the observed noncovalent interactions.