Photoelectron‐Spectroscopic Assignment of Symmetry Species to the Lower Electronic States of Unsaturated Hydrocarbon Radical Cations

The competitive through‐space and through‐bond interaction of two symmetry‐equivalent, semi‐localized orbitals φa, φb, (= π or lone‐pair orbitals) in a molecule M of Cs or higher symmetry can lead either to the natural order of the corresponding delocalized orbitals, i.e., ψ− ≈︁ (φa ‐ φb)/√2 above ψ+ ≈︁ (φa ‐ φb)/√2 or to the inverted order, i.e., ψ+ above ψ−. It is shown that one can discriminate between these two situations through an analysis of the PE spectrum of a molecule M′ which differs from M only by an additional semi‐localized orbital φc of known symmetry behaviour. The method is exemplified by an investigation of the dependence of the n‐orbital sequence in 3,6‐bridged 1,4‐cyclohexadienes on the dihedral angle between the two CCHCHC moieties.