FAR-INFRARED AND RAMAN-STUDY OF MONOHYDROGENATED CYCLOPENTENE-3-H AND CYCLOPENTENE-4-H RING PUCKERING AND RING TWISTING IN THE GAS-PHASE

Monohydrogenated cyclopentenes C5D(7)-3-h and C5D(7)-4-h have been synthesized and their vapor-phase infrared and Raman spectra have been recorded. For each molecule approximately 10 infrared bands in the 60-130-cm-1 region and a similar number of Raman bands in the 70-180-cm-1 region are assigned to ring-puckering transitions. Some side bands due to ring-puckering transitions issuing from the ring-twisting excited state are present in the infrared spectra. A series of ring-twisting infrared Q branches is also observed for each molecule in the 310-360-cm-1 region. The analysis of the spectra shows that a unidimensional asymmetrical ring-puckering potential energy function V(x) = V2x2 + V3x3 + V4x4 must be used to reproduce the data correctly. The potential energy barrier, dihedral angle, and the energy difference between the two potential wells are determined. They are in good agreement with the results of the previous study of monodeuterated cyclopentenes. This confirms that the isotopic monosubstitution in the allylic positions induces an asymmetry not only in the kinetic energy function but also in the potential energy function. Two distinct cyclopentene conformers with different energies are thus produced, the more stable being that with a CH bond in the axial position. The simultaneous excitation of both ring-puckering and ring-twisting modes is shown to result in a higher potential energy, indicating that the two motions are anticooperative.