Origin of Stacked-Ring Aromaticity

Although strong diatropic currents run in the pi-stacked dimers of 4n pi annulenes, they are still antiaromatic with negative topological resonance energies (TREs). The TRE difference between two annulene molecules and the pi-stacked annulene dimer call be used as a practical measure of stacked-ring aromaticity. As predicted by Corminboeuf et aL, all pi-stacked 4n pi annulene dimers exhibit marked stacked-ring aromaticity. In contrast, the TRE for the entire conjugated system of a pi-stacked benzene dimer is much smaller than twice the TRE for benzene. The analysis of circuit contributions to energetic and magnetic properties shows that such stacked-ring aromaticity and diatropicity arise from many (4n + 2)-site circuits created by the stacking of two antiaromatic rings. Tetragonal faces formed in the dimers are responsible for the creation of these (4n + 2)-site circuits.