Knowledge Bank

The far-infrared absorption spectra of gaseous cyclopentene oxide, 2-methyl-4,5-dihydrofuran and cyclopentanone have been observed in the range $250-25cm-1$. The oxygen atom in cyclo-pentene oxide introduces asymmetry into the potential function for inversion of the five-membered ring. The eigenvalues of the inversion vibration can be inferred from the spectra and fitted to a potential function of the form $V(x)=ax4-bx2+cx3$, in which x is the ring-inversion coordinate. 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 $98cm-1$. Seven sharp Q branches were observed for cyclopentanone which were fitted to a hindered pseudorotational potential function of the form $V=V2(1-\cos 2\varphi )/2+V4(1-\cos 4\varphi )/2$. The values of $V2$ and $V4$ are respectively 1303 and $29cm-1$.