Far-infrared spectra and two-dimensional potential energy surface for the out-of-plane vibrations of 2-cyclopenten-1-one and its deuterated isotopomers

The far-infrared spectra of 2-cyclopenten-1-one and its 5-d1 and 5,5-d2 derivatives have been recorded and analyzed. Each molecule has a ring-puckering series of bands characteristic of mixed quartic–quadratic potential energy functions. The fundamental puckering frequencies were observed at 94.4, 89.0, and 84.9 cm−1, respectively, for the d0, d1, and d2 species. In addition to the main puckering series for each molecule, side bands arising from the ring-twisting excited states were also observed. Bands resulting from ring-twisting transitions from various puckering states were observed for all three species in the 280–290 cm−1 region. The data demonstrate that the molecular skeleton is planar. A two-dimensional potential energy function of the form V=a1x41 +b1x21+a2x42+b2x22 +cx21x22, where x1 and x2 are the ring-puckering and ring-twisting coordinates, was determined. For all three isotopomers, this gives rise to calculated frequencies which are in excellent agreement with the observed values. The interaction term c is larger than in related molecules, reflecting the effects of asymmetry and conjugation.