Origin of the Surprising Enhancement of Electrostatic Energies by Electron-Donating Substituents in Substituted Sandwich Benzene Dimers

A recent study of substituted face-to-face benzene dimers by Lewis and co-workers [J. Am. Chem. Soc. 2011, 133, 3854-3862] indicated a surprising enhancement of electrostatic interactions for both electron-withdrawing and electron-donating substituents. Here we demonstrate that charge penetration (an attractive electrostatic interaction arising from the overlap of the electron densities on the two monomers) is the cause of this counterintuitive effect. These charge penetration effects are significant at typical pi-pi interaction distances, and they are not easily described by multipole models. A simple measure of a substituent's electron-donating or electron-withdrawing character, such as the Hammett parameter sigma(m), is unlikely to capture subtle charge penetration effects. Indeed, correlation of the relative total energies or relative electrostatic energies with Sigma sigma(m) breaks down for multiply substituted face-to-face benzene dimers.