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 $\text{∼195\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ 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 $\text{1099 ± 50\hspace{0.17em}}{\mathrm{cm}}^{\text{−1}}$ . 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.