Ring puckering of cyclooctatetraene and cyclohexane is induced by pseudo-Jahn-Teller coupling

Ring puckering of cyclooctatetraene and cyclohexane (an aromatic and a saturated hydrocarbon) is caused by pseudo-Jahn-Teller coupling between the ground state and excited states of (pi, sigma*) and (sigma, sigma*) type, respectively, as shown by symmetry-restricted CASSCF calculations. The effect can be also understood as due to interaction between the bonding and antibonding orbitals involved in the excitation.