Raman spectra of gases. VII. Barriers to planarity in 1,4- and 1,3-cyclohexadiene

The Raman spectra of gaseous 1,4- and 1,3-cyclohexadiene have been recorded between 80 and 3300 cm−1. A band at 216 cm−1 in the 1,4-cyclohexadiene spectrum has been assigned to the ring-puckering mode which is only consistent with D 2h (planar) configuration. A series of Q branches appearing at ∼195 cm−1 has been assigned to the low frequency twisting mode in 1,3-cyclohexadiene. The assignment of these lines to a double minimum potential function leads to a barrier to planarity of 1099 ± 50 cm−1. The CH2 stretching modes of 1,4-cyclohexadiene have been reassigned in light of the gas phase Raman data and a tentative vibrational assignment is given for the observed Raman lines of 1,3-cyclohexadiene.