Knowledge Bank

The conformation and electronic structure of cyclooctatetraene (COT) and its anions have been of interest for many years. COT has a nonplanar tub structure ($D2d$) and is nonaromatic. The dianion ($COT2-$) is almost certainly planar, aromatic, and of $D8h$ symmetry. The situation is less clear with respect to the monoanion $(COT-)$. We have measured the magnetic circular dichrosim (MCD) of COT isolated in Ar matrices by codepositing COT and alkali metal atoms. Bands are observed at about 340 and 300 nm. The former shows a remarkably well resolved series of vibronic A terms. The MCD is temperature independent, strongly supporting the view*** that the ground state structure of $COT-$ is consistent with an orbitally degenerate planar configuration subject to a dynamic Jahn-Teller Effect in the $b1g$ in-plane C-C stretching vibration. The lack of MCD temperature dependence allows us to place a lower limit on the strength of the Jahn-Teller coupling.