Vibrational spectra of methylenecyclopentane, methylenecyclopentane-2,2,5,5-d4 and methylene-d2-cyclopentane-2,2,5,5-d4

The 42 fundamental frequencies of methylenecyclopentane, methylenecyclopentane-2,2,5,5-d4 and methylene-d2-cyclopentane-2,2,5,5-d4 are assigned from the vapor and liquid phase infrared spectra and the Raman spectra of liquid samples. Assignments are made on the basis of group frequencies, intensities, isotopic shifts, gas phase band contours, Raman depolarization ratios and comparison with similar molecules. The lowest frequency ring mode is observed as a combination band progression in the mid-infrared spectrum of methylenecyclopentane yielding a progression of bands beginning at 158.0 cm−1 and decreasing in frequency. These bands are interpreted in terms of a one-dimensional harmonic vibration with a small negative quartic anharmonicity. This interpretation is favored over that of pseudorotation in the methylenecyclopentane molecule.