IS SUPERAROMATICITY A FACT OR AN ARTIFACT - THE KEKULENE PROBLEM

If a cyclic array of benzene rings in kekulene brings about extra thermodynamic stabilization, this molecule may be classified as a superaromatic species. A simple but decisive method for determining whether kekulene is superaromatic or not was devised. This method is based on the graph-theoretical analysis of the total pi-electron energies of true kekulene and hypothetical superantiaromatic kekulene. Kekulene was found to be essentially non-superaromatic with a vanishingly small superaromatic stabilization energy (ca. 0.0035\beta\), and that aromaticity stems primarily from relatively small (4n + 2)-membered conjugated circuits. Other graph-theoretical approaches support this interpretation. Superaromatic ring currents, i.e., additional ring currents caused by a cyclic array of the benzene rings, were evaluated for the first time.