Ground and excited state aromaticity and antiaromaticity: magnetic criteria and the valence-bond picture

The idea that the concepts of aromaticity and antiaromaticity apply to molecules in other than their electronic ground states can be traced back to Baird who argued that the well-known 4n+2 and 4n rules for ground-state aromaticity in cyclic conjugated hydrocarbons are reversed in their lowest triplet states: rings with 4n π electrons become aromatic while 4n+2 systems should be regarded as antiaromatic. We show that Baird’s ‘triplet’ aromaticity and antiaromaticity represent particular cases of broader concepts of excited state aromaticity and antiaromaticity. Magnetic criteria including nucleus-independent chemical shifts (NICS), proton shieldings and magnetic susceptibilities calculated using complete-active-space selfconsistent field (CASSCF) wavefunctions constructed from gauge-including atomic orbitals (GIAOs) strongly suggest that, in contrast to the well-known aromaticity of the S0 state of benzene, the T1 and S1 states of this molecule are antiaromatic, whereas in the two most popular examples of antiaromatic cycles, square cyclobutadiene and D8h cyclooctatetraene, the T1 and S1 states are aromatic.These findings can be rationalized using valence-bond models in which aromaticity and antiaromaticity are associated with resonance and antiresonanc