Ionization potentials and vibrational structure in photoelectron spectra by a Green's function method: trans-HNNH, cis-HNNH, and 1,1-dihydrodiazine (H ₂NN)

The ionization potentials of trans-diazene, cis-diazene and 1,1-dihydrodiazine (in its singlet state) have been computed by a many-body Green's function method. For trans-diazene all calculated ionization potentials are within 0.3 eV of the measured values. Koopmans' theorem supplies the correct ordering only for trans-diazene, but fails for the other two molecules. The electronic structures of the three molecules differ considerably. The ordering of the ionization potentials is for trans-diazene: n₊, π, n _-, σ₊(NH), σ_-(NH), and σ(NN); for cis-diazene: n_-, n₊, π σ₊(NH), σ_-(NH), and σ(NN); and for H₂NN: n _-, n₊, π, σ_-(NH), σ ₊(NH), and σ(NN). The n₊/n_- splitting is calculated to be 5.3, 3.6, and 5.7 eV, respectively. The vibrational structure in the photoelectron spectrum of trans-diazene and trans-diazene- d₂ is computed and compared to the experimental one. Similarly looking bands in the spectra of N₂H₂ and N₂D₂ may arise from quite different couplings.