RING-PUCKERING VIBRATIONAL-SPECTRA OF CYCLOPENTENE-1-D1 AND CYCLOPENTENE-1,2,3,3-D4
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The far-infrared and low-frequency Raman spectra of cyclopentene-1-d1 and -1,2,3,3-d4 have been recorded. For each molecule approximately a dozen infrared bands in the 60-200-cm-1 region and a like number of Raman bands in the 90-250-cm-1 region were assigned to ring-puckering transitions. A series of ring-twisting infrared Q branches was also obtained for each molecule, near 370 cm-1 for the d1 and near 338 cm-1 for the d4. The d4 molecule showed a series of side bands resulting from the ring-twisting excited state. The seventh excited state (v = 7) of the ring puckering was found to be split by an unusual Fermi resonance with the v = 2 puckering state in the twisting excited state. Extensive potential energy calculations were carried out using various models (with and without CH2 rocking) for the puckering vibration for the d0, d1, d4, and d8 molecules. Inclusion of rocking gave slight improvement for the frequency fit, but rocking parameters could not be well determined. The barriers to inversion of the four molecules were found to be 233, 231, 224, and 215 cm-1 in order of increasing deuteration. The dihedral angle of each isotopic form was determined to be 26.
