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

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Abstract
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 26°.