The low frequency Raman spectrum and barrier to planarity of 2‐cyclohexene‐1‐one.

The low frequency Raman spectrum of gaseous 2‐cyclohexene‐1‐one has been obtained at the full vapor pressure at ∼110°C using the 488 nm line of an argon ion laser at ∼2W power. 2‐cyclohexene‐1‐one has been found to exist with all of the heavy atoms except C5 essentially in a plane. The series of Q‐branch transitions in the Raman spectrum between 140–250 cm−1 have been assigned as Δv=2 transitions of a one‐dimensional double‐minimum oscillator. The vibarational mode is the motion of the CH2 group at the 5 position through the plane of the heavy atoms. The potential function in a reduced coordinate Z [V (cm−1) =27.49(Z4−11.67 Z2)] yields a barrier to planarity of 935±70 cm−1 (2.67±0.2 kcal/mole, 11.2±0.8 kJ/mole).