Photoelectron‐Spectroscopic Evidence for the Orbital Sequence in Fulvene and 3, 4‐Dimethylene‐cyclobutene

The photoelectron spectra of fulvene (II) and of 3,4‐dimethylene‐cyclobutene (III) have been recorded. The PE. bands are correlated, in order of increasing ionisation potentials, with the following orbitals: II: 1 a 2 (π), 2 b 1 (π), 7 b 2 (π), 1 b 1 (π); III: 2 b 1 (ω), 1 a 2 (π), 10 a 1 (ω), 8 b 2 (ω), 1 b 1 (π). This assignment is based on a semi‐quantitative perturbation MO‐model and on the SCF‐LCAO‐MO calculations reported by Praud , Millie & Berthier [5] for benzene, II and III.