Raman spectra of gases. XII. Twisting mode and barriers to planarity and interconversion in 2,3-dihydropyran, 1,4-dioxene, and cyclohexene

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Abstract
The Raman spectra of gaseous 2,3‐dihydropyran, 1,4‐dioxene, and cyclohexene have been recorded in the low frequency region. Both the low frequency ring‐twisting and ring‐bending modes have been observed. The bending modes had ``type B'' band contours and provided little information on the potential surface. The ring‐twisting overtones were of Alg species and had very sharp Q branches. The numerous hot bands associated with these ring‐twisting overtones confirm the earlier assignment of the twisting mode from the far infrared analysis of the bend and difference tone of the twist and bend for 2,3‐dihydropyran. Our assignment of the twisting mode in 1,4‐dioxene required a reassignment of the bending mode for this molecule. By using these data, potential barriers to planarity and interconversion have been estimated by using perturbation theory. Analysis of the ring twisting mode in cyclohexene was carried out in a similar manner. The barrier of 2630±200 cm−1 to interconversion for cyclohexene was found to be nearly the same as that found for the oxygen containing molecules, but the barrier of 8800±1700 cm−1 to planarity was significantly larger than the corresponding quantity for the other molecules.