Abstract
Excited and ionized states of p-benzoquinone (p-BQ) and the ground and excited states of its anion radical were studied by the SAC (symmetry adapted cluster)/SAC−CI (configuration interaction) method. Calculated ionization energies were in good agreement with the experimental ionization spectra, and the ionization peaks up to ca. 20 eV were assigned. The lowest four ionized states were calculated to be ng-, nu+, πu+, and πg+ in increasing energy order differently from the Koopmans' energy order, πg+, πu+, ng-, and nu+. The adiabatic electron affinity of p-BQ was calculated to be 1.96 eV in comparison with the experimental values, 1.86, 1.89, and 1.99 eV. The lowest allowed excitation of p-BQ anion radical was assigned to 2Au (πg-*SOMO−πu-* shape resonance) for both the anion and neutral-state geometries. The assignment to 2B3u as the lowest allowed state by Tripathi et al. seems to be due to the exchange of the 2Au and 2B3u states by the hydrogen bonds with water solvent, which was supported by the SAC−CI calculations for p-benzosemiquinone (p-BQH) radical. With support from the proposal in the literature, two forbidden n−π*SOMO transitions were calculated below the lowest peak (2Au) for the anion geometry, whereas for the neutral geometry, these transitions were calculated to be less stable than the 2Au state. The two states observed above the 2Au state in various spectra were both assigned to 2B3u (πu+−πg-*SOMO and πg-*SOMO−πu+*): the order of these states depends on the geometries, i.e., πg-*SOMO−πu+* Feshbach resonance was lower in the anion geometry, whereas πu+−πg-*SOMO shape resonance was lower in the neutral geometry. The ordering is explained by the effect of geometry on the orbitals.
Published Version
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