On the Correlation between Ionization Potentials and Excitation Energies, part III: Pyrazine

A recently suggested scheme for relating ionisation potentials ( IP ) to excitation energies ( E ) has been applied to pyrazine and 2,6‐dimethylpyrazine. The results indicate that in these systems the energy gap (Δ E ) between the two lowest lying 1 n ,π*‐states is significantly smaller than that (Δ IP ) between the corresponding 2 n ‐states of the parent radical cations. The values estimated for Δ E on the basis of Δ IP , measurable by photoelectron spectroscopy, disagree with those calculated theoretically but seem to be supported by the experimental evidence available. Since Δ E = Δ IP would normally be expected, the relationship Δ E < Δ IP actually present in these systems is discussed and related to the different shape of the n ‐MO's involved in the ionization and excitation processes.