On the Stability of Large [4n]Annulenes

The stabilization energies (B3LYP/6-31G*) of planar [4n]annulenes, evaluated by a new indene-isoindene isomerization method (see Abstract graphic), reveal that all 4n π-electron rings larger than the energetically unfavorable cyclobutadiene are only slightly destabilized by the π-electron interactions. Cyclooctatetraene prefers the “tub” conformation because of strain effects. Generally, the antiaromatic character of the larger systems with 4n π-electrons is revealed best by their magnetic properties rather than by their energies.