Knowledge Bank

The low frequency Raman spectra of 2,3-dihydrofuran, 2,3-dihydrothiophene, 1,4-cyclohexadiene, and 1,3-cyclohexadiene in the gas phase have been obtained. A band at $216cm-1$ in the 1,4-cyclohexadiene spectrum has been assigned to the ring puckering mode which is consistent only with a $D2h$ (planar) configuration. For each of the other three molecules, a series of Q branches was found in the range 100 - $200cm-1$ and has been assigned as arising from low frequency ring bending motions which are confirmed by double minimum potential functions. The barriers to planarity were found to be 85, 325, and $1099cm-1$ for the 2,3-dihydrofuran, 2,3-dihydrothiophene and 1,3-cyclohexadiene molecules, respectively.