THE OUT-OF-PLANE RING DEFORMATIONS OF 1,3- AND 1,4-DIOXENE AND 3,6-DIHYDRO-2H-PYRAN: COMPARISON OF A SERIES OF CYCLOHEXENES

Abstract The far-IR spectra (100–600 cm − ) of 1,3-dioxene and 3,6-dihydro-2H-pyran in the vapour phase are analyzed for the out-of-plane ring deformations. The fundamentals and a number of combination sequences are assigned. The spectrum of 1,4-dioxene is reexamined. The positions of these deformations in the three compounds are compared with those of cyclohexene and 2,3-dihydropyran. The twisting transitions are analyzed with a one-dimensional quadratic-quartic potential function. The barrier heights so derived, thought to represent approximate values for the barrier to planarity, decrease in the order 1,4-dioxene > 2,3-dihydropyran > 3,6-dihydro-2H-pyran > 1,3-dioxene.