Photoelectron‐Spectroscopic Characterization of 3,4‐Dimethyl‐2,4‐hexadienes

The He(Ia) photoelectron (PE) spectra of the ( E,E )‐, ( E,Z )‐ and ( Z,Z )‐isomers of the title compound have been recorded to obtain information about their conformation in the gas phase. For a valid correlation with the PE data of other dienes it is necessary to take the potentials V (φ) for internal rotation and the corresponding conformer population densities P (φ) into account, as well as the rather complicated way wy in which the π −1 ionization energy gap Δ I (φ) depends on the direct π‐orbital interaction and the long‐range ‘through‐space’ interaction between the semi‐localized methyl‐group orbitals and the π‐orbitals. These factors being taken into account, the mean twist angles, φ , compatible with the PE‐spectroscopic results are φ ( E,E ) ≈ O° ± 30°, φ ( E,Z ) ≈ 80° or 110° within ± 15°, and φ (Z,Z) ≈ 85° to 105°. These results are in rough agreement with electron diffraction data by Traetteberg [15], other spectroscopic results and, for the ( E, E )‐ and (Z,Z)‐isomers, internal rotation potentials V (φ) previously calculated by Roth [17]. On the other hand the potential V (φ) proposed for the ( E,Z )‐isomer does not seem to be compatible with our findings.