Low-frequency vibrational spectra and ring puckering of cyclopentene-d8

More than 25 ring-puckering transitions have been observed in the vapor-phase Raman and far-infrared spectra of cyclopentene-d8. The ring-puckering potential energy function in reduced form was determined to be V=18.20(Z4-6.88 Z2) cm-1, which represents a barrier to inversion of 215 cm-1 (0.61 kcal/mole). The barrier differs from that of the undeuterated cyclopentene by 17 cm-1 owing to mixing of other motions (presumably CH2 rockings). The equilibrium value of the molecule is calculated to have a dihedral angle of 260