For a given molecule M, the difference ΔI between the first two vertical ionization potentials Iv,2 and Iv,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‐ (J2,−1−J1,−1) −2(K1,−1 − K2,−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.
All Science Journal Classification (ASJC) codes
- Drug Discovery
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry