INFRARED AND RAMAN-STUDY OF MONOHYDROGENATED CYCLOPENTENES-3-H1 AND CYCLOPENTENES-4-H1 - CH STRETCHING VIBRATION AND ITS 2 1ST OVERTONES IN GAS-PHASE

The infrared and Raman spectra of gaseous monohydrogenated cyclopentenes-3-h1 (Cy3H) and -4-h1 (Cy4H) have been investigated in order to determine the ring-puckering potential in the first three excited states of the CH stretching vibration. All the spectra present a triplet structure accompanied by shoulders and weaker bands, but the relative intensities of the latter and of the central peak of the triplet decrease from DELTA-upsilon = 1 to DELTA-upsilon = 3. It has been possible to interpret the observed structure and its intensity evolution in terms of transitions from the known ground-state ring-puckering levels to excited-state ring-puckering levels in a potential of increasing asymmetry. The two more intense bands of the triplet are thus assigned to transitions \0,0] --> \upsilon,0] and \0,1] --> \upsilon,1]. They correspond to the excitation of the CH oscillator in its axial and equatorial conformation, respectively. The anharmonicity of the axial and equatorial nu(CH) vibrations are nearly the same, but are higher in the -3-h1 than in the -4-h1 derivative. These results confirm nicely the conclusions previously obtained from a similar study of monodeuterated cyclopentenes: in all the isotopic monosubstituted derivatives, the asymmetry of both the kinetic and potential energy terms is definitively evidenced. Furthermore, the observed asymmetry in the potential of the monohydrogenated derivatives is in perfect agreement with the predictions made from the previous study of the monodeuterated ones and with ab initio calculations of the CH bond length difference between axial and equatorial positions.