Far-Infrared Spectra of Ring Compounds. V. Ring-Puckering Potential Functions of Some Oxygen-Containing Molecules

The infrared absorption spectra of gaseous 2,5-dihydrofuran, 3-oxetanone, 2-methyl-4,5-dihydrofuran, diketene, cyclopentanone, and cyclohexanone have been observed in the range 250–25 cm−1. Under high resolution the spectrum of 2,5-dihydrofuran shows a satellite series of Q branches in addition to those observed by Shimanouchi and Ueda. The satellites arise from transitions between ring-puckering energy levels in an excited ring-twisting state with altered spacing due to higher-order cross terms in the potential energy. The spectrum of 3-oxetanone is that of a planar molecule, and the energy levels of the ring-puckering mode fit a potential function of the form V(x) = ax4 + bx2. The absorption of 2-methyl-4,5-dihydrofuran is complicated by several low-frequency modes, but the spectrum can be fairly well interpreted with a double-minimum potential function of the form V(x) = ax4−bx2 with a barrier height of 98 cm−1. Seven sharp Q branches were observed for cyclopentanone, which were fitted to a hindered pseudorotational potential function of the form V = 12V2(1−cos2φ) + 12V4(1−cos4φ). Although one might also expect diketene and cyclohexanone to have anharmonic out-of-plane ring modes, both compounds gave far-infrared spectra which appear to be essentially harmonic.