Aromaticity and Antiaromaticity in the Low-Lying Electronic States of Cyclooctatetraene

The levels of aromaticity of the most important geometries on the ground-state (S-0), lowest triplet-state (T-1), and first singlet excited-state (S-1) potential energy surfaces (PESs) for cycloocta-1,3,5,7-tetraene (COT) are assessed using a wide range of magnetic criteria including nucleus-independent chemical shifts (NICSs), proton shieldings, and magnetic susceptibilities calculated using complete-active-space self-consistent-field (CASSCF) wave functions constructed from gauge-including atomic orbitals (GIAOs). It is shown that the ground state of D-8h COT (transition state for the pi-bond-shift process on the S-0 PES) is markedly antiaromaic, even more so than the classical example of an antiaromatic system, the ground state of square cyclobutadiene. The CASSCF-GIAO magnetic properties of the ground state of D-4h COT (transition state for the ring-inversion process on the S-0 PES) strongly suggest that it is much less antiaromatic than the ground state of D-8h COT, whereas those of the ground state of D-2d COT (local minimum on the S-0 PES) indicate that it is decidedly nonaromatic. The lowest triplet state and the first singlet excited state of D-8h COT (local minima on the T, PES and the S-1 PES, respectively) exhibit surprisingly similar magnetic properties. These, in turn, are very close to the magnetic properties of benzene, which is a strong indication of a high degree of aromaticity.