Knowledge Bank

The reduction of symmetry by isotopic substitution for cyclopentene-1-$d1$ and cyclopentene-$1,2,3,3-d4$ allows the observation of the normally infrared inactive ring-twisting mode. The fine structure observed for this vibration in both molecules makes is possible to determine the ring-puckering energy levels in the twisting excited state. Using these levels (and others for cyclopentene-$d0$ and -$d8$), a two-dimensional potential energy for the out-of-phase ring modes has been calculated.