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Core excitations of symmetrical aromatic molecules. Specific correlations in the valence shell and localization in the core shells

1987; Elsevier BV; Volume: 117; Issue: 1 Linguagem: Inglês

10.1016/0301-0104(87)80098-8

1873-4421

W. Schwarz, Tse‐Chiang Chang, Ute Seeger, Kuang-Hung Hwang,

Spectroscopy and Quantum Chemical Studies

Highly symmetrical molecules exhibit interesting core to valence (v*) excitation phenomena. Due to symmetry, orbital degeneracies occur in the core and valence shells. For instance, He3 or benzene possess three empty π* MOs, two of which are degenerate. However, neardegeneracy correlation results in a specific splitting pattern of the corresponding excited states. The splittings of MC calculations are qualitatively reproduced by broken symmetry SCF and by fully localized equivalent core hole MO calculations, while symmetry adapted RHF results are completely misleading. Because of symmetry, excitations to only the lower group of states are dipole allowed. Substituted benzenes, e.g., fluorobenzene, behave similarly, because the perturbation of the high symmetry of the π shell of benzene by substitution is small as compared to the perturbation by a localized core hole. Concerning the highest π* MO in aromatic compounds, no C1s → π* single particle excitation exists. The corresponding intensity is distributed over a series of double excitations, the first one lying rather low. Parametrized single particle approaches (g - Hartree, Xα, HAM) may at most approximate this lowest state with respect to its energy. Concerning the equivalent core shells in benzene and fluorobenzene, the question of core hole localization is discussed. The core holes created under normal experimental conditions are delocalized in benzene, and are partially or fully localized in fluorobenzene. Finally some comments on the gin - Hartree approach are made. A long-range Coulomb potential seems not to be the optimal model for the short-range correlation potential, especially if the self-interaction is not explicitly eliminated.