Knowledge Bank

The far-infrared spectra in the region of $50-450cm-1$ have been recorded for a large variety of cyclohexane and cyclohexene derivatives in the vapor phase. The spectra exhibit one or more typical sequences of Q branches characteristic of low energy anharmonic skeletal deformations. From the positions of the appropriate sequences, barriers to planarity are estimated, and in some cases where no structural information is available from other sources the likely form of the most stable conformation is suggested. Molecules used to demonstrate the work include 3,6-dihydro-2H-pyran, 3,6-dihydro-2H-thiopyran, 3-methylene and 4-methylene cyclohexene, 3-cyclohexen-1-one,cyclohexanone, tetrahydro-4H-pyran-4-one, tetrahydro-4H-pyran-2-one, and 1,3-dioxan-5-one.