CALCULATIONS OF THE EQUILIBRIUM ISOTOPE EFFECTS ON THE REDUCTIONS OF BENZENE-D(6) AND CYCLOOCTATETRAENE-D(8)

B3LYP/6-31+G density functional calculations have been performed in o rder to understand why large, equilibrium isotope effects have been ob served in the reduction of benzene to the corresponding radical anion but not in the reduction of cyclooctatetraene (GOT) to COT.-. The calc ulations reproduce these experimental trends. However, the calculation s also find that reduction of planar GOT, like that of benzene, has a substantial, equilibrium isotope effect. Therefore, the small isotope effect, both calculated and observed, for reduction of COT is due to a n inverse isotope effect on the planarization of the tub-shaped ring i n GOT. The isotope effect computed for forming planar (D-8h) triplet C OT/COT-d(8) at 173 K is 0.38. This is the predicted isotope effect on the adiabatic singlet-triplet splitting in GOT. The isotope effect com puted for forming planar (D-4h) Singlet COT/COT-d(8) at the same tempe rature is 0.41. This is the predicted isotope effect on ring inversion . The reason why planarization of COT has an inverse isotope effect is discussed, and an experimental test of the prediction of an inverse i sotope effect on ring inversion of COT is proposed.