Observation of Both Jahn−Teller Distorted Forms (b1g and b2g) of the Cyclooctatetraene Anion Radical in a 1,2-Disubstituted System

The anion radical of cyclooctatetraene (COT) must undergo a Jahn−Teller distortion from its ideal D 8 h symmetry. Both B3LYP/6-31+G* calculations and EPR observation have shown that the COT moiety in the anion radical of 1,2-dicarbo- tert -butoxycyclooctatetraene exists in a pair of degenerate D 4 h alternating bond angle conformations (a b 1g distortion). The rapid ( k = 3 × 10 7 s - 1 ) interconversion between the two identical conformations leads to specifically predicted line width alternation effects in the EPR spectrum. The COT ring system in the 1,2-dicarbomethoxycyclooctatetraene anion radical [(CH 3 OOC) 2 COT - • ], on the other hand, has a ground state with alternating bond lengths. Since this b 2g distorted (COT - • ) system has a plane of symmetry that can be drawn through the molecule and between the two substituents, no line width alternation effects can take place. However, in this system, the B3LYP/6-31+G* calculations predict an energy difference between the b 1g and b 2g distorted COT - • ring systems of less than 1 kcal/mol. In support of this, the EPR data are best simulated in terms of the presence of both Jahn−Teller distorted forms of the anion radical.