ISOTOPE EFFECTS ON THE RING INVERSION OF CYCLOOCTATETRAENE

H-1 NMR studies (400 MHz) on isopropoxycyclooctatetraene (C8H7-O-C3H7) and its ring-d(7) analogue (C8D7-O-C3H7) show that there is an invers e isotope effect on the rate of ring inversion of k(H)/k(D) = 0.68-0.5 7 in the temperature range from -10 to -50 degrees C. These experiment s confirm the validity of previously published DFT calculations on cyc looctatetraene/cyclooctatetraene-d(8), which predict an inverse isotop e effect of about this size on the rate of ring inversion in this temp erature range. The large inverse isotope effect upon ring inversion ex plains why the similarly large normal equilibrium isotope effect, foun d in the reduction of benzene to its corresponding anion radical, is n ot observed in the analogous reduction of cyclooctatetraene.