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The far-infrared and Raman spectra of four isotopic species of cyclopentene have been assigned in order to obtain a detailed energy map for the low-frequency ring modes. The data for each molecule, consisting of a main series of ring-puckering bands, a series of side bands, and a series of ring-twisting absorption peaks (for the asymmetrically deuterated d1 and d4 molecules), have been analyzed by means of a two-dimensional potential energy calculation. The calculation yields a potential function of V = 7.880 X 10%4 -0.2706 X 10%2 + 0.5158 X 10%2 + 1.904 X 10%%2, where x1 and x2 represent the ring-puckering and ring-twisting coordinates, respectively. The barrier to inversion is calculated to be 232 cm'1 and the sign of the cross term reflects the fact that twisting of the ring tends to hinder the puckering process.

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