Spectra and structure of small ring molecules: Part XLIV. Vibrational spectra and conformational stability of cyclobutylmethyl ketone

Abstract

The infrared (3500 to 30 cm −1) and Raman (3200 to 30 cm −1 spectra have been recorded for the gaseous and solid states of cyclobutylmethyl ketone, c-C 4H 7C(CH 3)O. Additionally, the Raman spectrum of the liquid has been recorded and qualitative depolarization values have been obtained. Evidence is presented based on the band contours in the infrared spectrum that the molecule exists predominantly in the equatorial- gauche conformation (the C(CH 3)O group is in the equatorial position relative to the four-membered ring with the CO nearly eclipsing one of the CC bonds of the ring) in the gaseous and liquid states and exclusively in this conformation in the solid state. However, there is clear evidence in the Raman spectra of the fluid phases that a second conformer is present at ambient temperature and this conformer is believed to be the equatorial- trans where the CO is eclipsing the α-CH bond of the four-membered ring. The asymmetric torsion was observed as a very broad, structureless band centered at about 51 cm −1. From the relative intensities of the Raman lines of the liquid at 386 (equatorial- gauche) and 442 cm −1 (equatorial- trans) as a function of temperature, the enthalpy difference was found to be 480 ± 62 cm −1 (1.37 kcal mol −1) with the equatorial- gauche being more stable. A complete vibrational assignment is proposed based on infrared band contours, depolarization values, and group frequencies. The methyl torsion was tentatively assigned at 112 cm −1 from which a threefold barrier to internal rotation was calculated to have a value of 0.94 kcal mol −1 (331 cm −1). A few of the normal modes were observed to be split in the infrared spectrum of the solid which indicates that there are at least two molecules per primitive cell. These results are compared to similar quantities in some related molecules.