On the correlation between photoelectron and electronic spectra in trans- azomethane

A possibility of correlating electronic and photoelectron spectra is discussed, using trans-azomethane as an example. The Coulomb and exchange integrals required were obtained by three semi-empirical SCF-methods: MINDO/2, CNDO/2, and a modified CNDO method. The orbital energies were taken as minus the corresponding experimental ionization potentials. The sequence of the transition energies ΔE (n_s → π*) Δ E (n_a → π*) < ΔE (π → π*) is found to be different from the ionization potential sequence IP (n_s) < IP (π) < IP (n_a), in agreement with previous spectroscopic studies; the results support the latest view that the π → π* transition of the azo group occurs at around 12 eV.