Far-infrared spectra, two-dimensional vibrational potential energy surface, and conformation of cyclohexene and its isotopomers

Article abstract of DOI:10.1021/ja00026a004

The far-infrared spectra of cyclohexene and its 1-d₁, 1,3,3-d₃, 1,2,3,3-d₄, 3,3,6,6-d₄, and d₁₀ isotopomers have been recorded and analyzed as a function of its ring-bending and ring-twisting vibrations. More than one-hundred transition frequencies corresponding to single and double quantum jumps of the bending, to the twisting, and to several twist-bend combinations were observed for the six isotopomers. From these data a two-dimensional vibrational potential energy surface was determined which defines the conformational energy of the molecule in terms of the two vibrational coordinates. Appropriate kinetic energy (reciprocal reduced mass) expansions were determined for these computations. The resulting potential energy surface is V (cm^-1) = (3.25 × 10⁴)x⁴ - (1.19 × 10⁴)x² + (2.18 × 10⁴)r⁴ - (2.02 × 10⁴)r² + (1.53 × 10⁵)x²r², where x and r represent the bending (A?) and twisting (radians) coordinates, respectively. Cyclohexene has an energy minimum at a twist angle of 39.0 ± 1.8°. This lies 4700 ± 500 cm^-1 (13.4 ± 1.4 kcal/mol) below the planar conformation and approximately 3600 cm^-1 (10.3 kcal/mol) below the bent form, which corresponds to a saddle point on the two-dimensional surface. The barrier to planarity for cyclohexene is 400-1200 cm^-1 higher than in several analogues of oxygen and sulfur.