Far-Infrared Spectra of Ring Compounds. IV. Spectra of Compounds with an Unsymmetrical Potential Function for Ring Inversion

The infrared spectra of gaseous trimethylenimine, trimethylenimine‐N‐d, 2,5‐dihydropyrrole, 2,5‐dihydropyrrole‐N‐d, and cyclopentene oxide have been observed in the range 300–25 cm⁻¹. The N–H bonds of trimethylenimine and 2,5‐dihydropyrrole and the oxygen atom of cyclopentene oxide introduce asymmetry into the potential function for inversion of the ring in these molecules. In consequence the eigenvalues of the inversion vibrations are quite irregular but can be inferred from the spectra and fitted to a potential function of the form ${\mathrm{V(x)\hspace{0.17em}=\hspace{0.17em}ax}}^4{\mathrm{\hspace{0.17em}-\hspace{0.17em}bx}}^2{\mathrm{\hspace{0.17em}+\hspace{0.17em}cx}}^3$ , where $x$ is the ring‐inversion coordinate. Interpretation of the far‐infrared spectrum also enables assignment of the satellite fine structure of certain of the mid‐infrared fundamentals as difference bands involving the inversion vibration.