Far-Infrared Spectra of Ring Compounds. IX. Far-Infrared Spectrum and Hindered Pseudorotation in Cyclopentanone

The high resolution far-infrared spectra of cyclopentanone-α-d4, and d8 have been observed. The energy levels of the low-frequency vibration have been calculated as a problem of a particle in a two-dimensional potential well with a variable reduced mass. The eigenstates of the hindered pseudorotation have been described in terms of a mixture of the eigenstates for the two orthogonal motions, pure bending and pure twisting around the twisted equilibrium conformation. The conformational energy of the molecule is estimated to be V(x, y) in cm−1 = 1302 x2 − 14531y2 + 366x4+ 14765x2y2 + 70352y4 where x and y are dimensionless bending and twisting coordinates. The interconversion between the two twisted conformations occurs through the planar form whose potential energy is about 750 cm−1 higher than the equilibrium form. To account for the observed deuterium substitution effects, some coupling of the α-methylene rocking motions with the hindered pseudorotation has been postulated on an empirical basis. The major features of the far-infrared spectra have been explained and some of the implications of the model calculation have been discussed.