Heilbronner proposed an extremely unusual form of this property which may be present in the cyclotetraenyl anion. Molecules with this property are depicted in the Frost circle, which describes the energy levels of the highest occupied and lowest occupied molecular orbitals. Fluorescent spectroscopy of proteins yielding a strong absorptivity at 280 nanometers corresponds to an amino acid with this property, which is tryptophan. While the cyclopentadienyl ion has this property, cyclopentadiene does not. Molecules that contain a multiple of four pi electrons have the "anti" form of this property, while molecules that contain two plus a multiple 
Heilbronner proposed an extremely unusual form of this property which may be present in the cyclotetraenyl anion. Molecules with this property are depicted in the Frost circle, which describes the energy levels of the highest occupied and lowest occupied molecular orbitals. Fluorescent spectroscopy of proteins yielding a strong absorptivity at 280 nanometers corresponds to an amino acid with this property, which is tryptophan. While the cyclopentadienyl ion has this property, cyclopentadiene does not. Molecules that contain a multiple of four pi electrons have the "anti" form of this property, while molecules that contain two plus a multiple of four pi electrons have this property according to Hückel's rule. For 10 points, identify this property which grants unusual stability due to conjugation around a ring, typified by benzene.
Heilbronner proposed an extremely unusual form of this property which may be present in the cyclotetraenyl anion.