Aromaticity reversals and their effect on bonding in the low-lying electronic states of cyclooctatetraene

Aromaticity reversals and their effect on chemical bonding in the low-lying electronic states of cyclooctatetraene (COT) are investigated through a visual approach which examines the variations in isotropic magnetic shielding in the space surrounding the molecule. The ground state (S₀) of COT is shown to be strongly antiaromatic at the π-bond-shifting transition state (TS), a regular octagon of D_8h symmetry; S₀ antiaromaticity decreases at the D_4h planar bond-alternating tub-to-tub ring-inversion TS but traces of it are shown to persist even at the tub-shaped D_2d local minimum geometry. The lowest triplet (T₁) and first singlet excited (S₁) states of COT are found to have very similar D_8h geometries and visually indistinguishable shielding distributions closely resembling that in benzene and indicating similarly high levels of aromaticity. Unexpectedly, COT diverges from its antiaromatic predecessor, cyclobutadiene, in the properties of the second singlet excited state (S₂): In cyclobutadiene S₂ is antiaromatic but in COT this state turns out to be strongly aromatic, with a shielding distribution closely following that around S₂ benzene.