Bemerkung zur Gleichheit der Aufspaltungen ΔI (zwischen den ersten beiden π‐Ionisationspotentialen) und ΔE (zwischen den entsprechenden π* ← π Übergangsenergien) des Spiro[4,4]nonatetraens. Vorläufige Mitteilung

For a given molecule M, the difference ΔI between the first two vertical ionization potentials I_v,2 and I_v,2 (from MOs ψ₁ and ψ₂) and ΔE between the corresponding singlet‐singlet excitation energies E₁ and E₂ (transitions ψ−₁ ←₁, ψ−₁ ψ₂) are related by ΔE = ΔI‐ (J_2,−1−J_1,−1) −2(K_1,−1 − K_2,−1), using Koopmans approximation. A simple MO model suggests that under certain conditions of symmetry and quasi‐alternancy (e. g. in spiro[4,4]nonatetraene 1) the bracketed differences between the Coulomb‐ and exchange‐integrals should vanish to first order, thus leading to the simple (almost) equality ΔE = ΔI. It is shown that the results from a photoelectron‐ and electron‐spectroscopic investigation of 1 support this conclusion i.e. ΔI = 1.23 eV, ΔE = 1.19 to 1.23 eV.