Unraveling the Origin of Substituent Effects in the Benzene Dimer

Steven Wheeler, UCLA and Center for Computational Chemistry, (UGA)

The prevailing view of substituent effects in the benzene dimer is flawed.  In the Hunter-Sanders model (J. Am. Chem. Soc. 1990, 112, 5525), electron-withdrawing substituents enhance binding in the benzene dimer by withdrawing electron density from the pi-cloud of the substituted ring, reducing the repulsive electrostatic interaction with the non-substituted benzene.  Conversely, electron-donating substituents donate excess electrons into the pi-system and diminish the pi-stacking interaction.  We present computed interaction energies for the sandwich configuration of the benzene dimer and 24 substituted dimers, as well as sandwich complexes of substituted benzenes with perfluorobenzene.  While the computed interaction energies correlate well with sigma meta values for the substituents, in accord with the recent experimental results of Hunter and co-workers (Org. Biomol. Chem. 2007, 5, 1062), interaction energies for related model systems demonstrate that this trend is independent of the substituted ring.  Instead, the observed trends can be explained by direct interactions of the substituents with the unsubstituted ring.

Oral