Two-state model of antiaromaticity: The low lying singlet states

Aromatic and antiaromatic compounds are resonance hybrids of two cyclic covalent Kekulé structures. In both, two combinations can be formed, an in-phase and an out-of-phase one. In aromatic compounds having an odd number of conjugated double bonds, the in-phase combination is the ground state and the out-of-phase one is an excited state. In antiaromatic compounds, having an even number of conjugated electron pairs, the situation is reversed; the ground state is formed by the out-of-phase combination. This causes the ground state of these molecules to be a non-totally symmetric one, which in turn means that it has a biradical character. Moreover, the out-of-phase combination is necessarily unstable, being a transition state between the two bond-alternating Kekulé structures. By comparison to noncyclic biradicals such as perpendicular olefins, the antiaromatic cyclic structures are strongly stabilized, reducing the activation barrier from around 50-60 kcal/mol to around 3-5 kcal/mol. Therefore, the bond-alternating structures are easily interconverted at ambient temperatures and in the process acquire biradical character, making them highly reactive and difficult to synthesize. The in-phase combination of the two Kekulé structures is a strongly stabilized totally symmetric excited state which has a similar geometry to that of the ground transition state.