Knowledge Bank

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 $60-200cm-1$ region and a like number of Raman bands in the $90-250cm-1$ were assigned to ring-puckering transitions. A series of ring-twisting Infrared Q branches was also obtained for each molecule, near $370cm-1$ for the $d1$ and near $338cm-1$ for the $d4$. The $d4$ molecule showed a series of side bands resulting from the ring-twisting excited state. The seventh excited state $(\nu =7)$ of the ring-puckering was found to be split by an unusual Fermi resonance with the $\nu =2$ 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 $215cm-1$ in order of increasing deuteration. The dihedral angle of each isotopic form was determined to be $26\circ$.