The far-infrared spectra and the ring inversion vibrations of a series of methylenecyclohexanes

The far-infrared spectra in the region of 50 to 400 cm−1 are reported for the vapours of methylenecyclohexane and the related compounds 4-methylenetetrahydro-4H-pyran, 4-methylenetetrahydro-4H-thiopyran and 3-methylenetetrahydro-4H-pyran. The absorptions are assigned to the three out-of-plane deformations of the six-membered rings. The observed series of Q branches are ascribed to combination sequences involving the three modes. The lowest wavenumber sequence in each compound is believed to arise from a vibration which if completely executed would take the chair conformation to its equivalent via an inversion through the planar form. With the use of a one-dimensional Hamiltonian incorporating a quadratic-quartic double minimum potential function, barriers to planarity for the rings are estimated. The similarities between these compounds and the corresponding ketones are discussed.