σ-hole bonding: molecules containing group VI atoms

AbstractIt 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 “σ-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 σ-hole concept and the outer regions of positive electrostatic potential account for the existence, directionalities and strengths of the observed noncovalent interactions. FigureCalculated B3PW91/6-31G** electrostatic potential of F2S, computed on the 0.001 electrons/bohr3 contour of the electronic density. The sulfur atom is toward the reader; the red areas indicate the most positive potentials, reaching +34.4 kcal/mole, along the extensions of the F-S bonds. The purple region (negative) on the left and the one (not totally visible) on the right side of the sulfur are due to its nonbonded s and p electrons. The fluorines (top left and bottom left) also have negative regions of potential (purple areas)