Raman Spectra of Gases. I. Cyclopentene and 2,5-Dihydrofuran

The Raman spectra of cyclopentene and 2,5‐dihydrofuran, both in the gaseous state, were recorded from 70 to 3500 cm⁻¹. Even the very weak ring puckering vibrations were observable. The selection rule for this normal mode is Δv=2 for the molecules studied even though the 2,5‐dihydrofuran molecule is a planar molecule. The observation of these double quantum jumps can be a powerful check on the sometimes ambiguous assignment of the far‐infrared transitions. It is also shown that the Raman combination and difference bands in the $\mathrm{C}–\mathrm{H}$ stretching region may be useful for obtaining the ring puckering transitions.