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

Abstract

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 ${\mathrm{V(x)\hspace{0.17em}=\hspace{0.17em}ax}}^4{\mathrm{\hspace{0.17em}+\hspace{0.17em}bx}}^2$ . 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 ${\mathrm{V(x)\hspace{0.17em}=\hspace{0.17em}ax}}^4{\mathrm{-bx}}^2$ 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 $\mathrm{V\hspace{0.17em}=\hspace{0.17em}}\frac{1}{2}{V}_2\text{(1−}\cos \text{2φ)\hspace{0.17em}+\hspace{0.17em}}\frac{1}{2}{V}_4\text{(1−cos4φ)}$ . 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.