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

Abstract

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.