Far-Infrared Spectra of Four-Membered-Ring Compounds. I. Spectra and Structure of Cyclobutanone, Cyclobutanone-d4, Trimethylene Sulfide, and Perfluorocyclobutanone

Two series of very sharp Q branches have been observed in the far-infrared spectra of cyclobutanone and cyclobutanone-d4 vapor. For cyclobutanone, these Q branches have wavenumbers of 35.3, 57.03, 64.99, 72.17, 77.77, 81.85, and 85.33 cm−1, and for cyclobutanone-d4 the wavenumbers are (∼30), 47.1, 54.2, 60.6, 65.6, 69.8, 73.2, and 78.9 cm−1. Relative frequency values were calculated with the energy levels of a pure quartic oscillator which fitted rather accurately all the transitions except the first observed one. This suggests that the four-membered ring of cyclobutanone is planar. The far-infrared spectrum of trimethylene sulfide revealed at least six Q branches of varying intensity and spacing. Their wavenumbers were 62.27, 84.50, 92.14, 100.18, 105.75, and 139.1 cm−1. It is impossible to fit the series to the energy levels of either a pure quartic oscillator or one perturbed by a quadratic term. Comparison of the statistical entropy with the measured calorimetric entropy indicated a double-minimum potential with a barrier above the first energy level. In the far-infrared spectrum of perfluorocyclobutanone no band was observed which could be confidently assigned to the ring-puckering vibration of this molecule.