Transition-state spectroscopy of cyclooctatetraene

The 351-nanometer photoelectron spectrum of the planar cyclooctatetraene radical anion (COT-) shows transitions to two electronic states of cyclooctatetraene (COT). These states correspond to the D-4h (1)A(1g) state, which is the transition slate for COT ring inversion, and the D-8h (3)A(2u) state. The electron binding energy of the (1)A(1g) transition state is 1.099 +/- 0.010 electron volts, which is lower by 12.1 +/- 0.3 kilocalories per mole than that of the (3)A(2u) state. The photoelectron spectrum shows that the singlet lies well below the triplet in D-8h COT and confirms ab initio predictions that the molecule violates Hund's rule. Vibrational structure is observed for both features and is readily assigned by use of a simple potential energy surface.