The conformation of 1-vinylcyclohexene

The dipole moment, molar Kerr constant and electronic spectra of 1-vinylcyclohexene were measured at 25°C. Using the molecular mechanics method, modified for diene systems, the equilibrium geometries of the possible conformers have been determined. It is found that the half-chair form is always more stable than the corresponding boat form. Ab initio SCF calculations using the STO-3G basis set and the force field geometries have been performed on the rotamers of the half-chair vinylcyclohexene to estimate the relative energies. The potential energy curve obtained for vinyl group rotation reveals s- trans (absolute minimum) and nearly s- cis minima, 0.53 kcal mol −1 apart, with a barrier of 2.5 kcal mol −1 from s- trans to s- cis . The model system cyclohexene is used to test the applicability of the previously proposed “variable bond polarizabilities” approach to the calculation of the molar Kerr constant: the estimated value is in excellent agreement with experiment. The preferred solution-state conformation of 1-vinylcyclohexene indicated by the analysis of experimental and calculated dipole moment, molar Kerr constant and electronic spectra is shown to be consistent with the theoretical predictions: the s- trans form seems to be preferred but the presence of a 20–30% population of s- cis or nearly s- cis rotamers cannot be ruled out. An estimate of the first ionization potential is also given.