Koopmans-based analysis of the optical spectra of p-phenylene-bridged intervalence radical ions

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The optical spectra of 10 p-phenylene-bridged delocalized intervalence compounds MC6H4M.- or .+ are analyzed using the Koopmans-based method, which considers only transitions from filled orbitals to the singly occupied orbital (SOMO), called Hoijtink type A transitions, and from the SOMO to unoccupied orbitals, Hoijtink type B transitions, and ignores configuration interaction. The radical ions with quinonoid structures, those that form ring-M double bonds with M = C(CN)2, NMe2, 3-oxo-9-azabicyclo[3.3.1], NPPh3, and O when the odd electron of the intervalence oxidation level is removed, are calculated to have the lowest-allowed type B transition lying mostly above the lowest-allowed A transition, with Bi - Aj decreasing in the order shown from +14 370 to -1390 cm-1, and the more intense second-lowest- allowed type B transition Bi - Aj from +14 940 to +7070 cm-1. The five radical anions with benzenoid structures, which form ring-M single bonds with X = CN, CO2Me, CHO, C3HMeBF 2O2, and NO2 when the odd electron of the intervalence oxidation level is removed, have a Bi - Aj value of the opposite sign that increases in magnitude from -2880 to -17 050 cm-1 in the order shown. Configuration interaction is of course present in the observed spectra, and the predictions ignoring it mostly overestimate transition energies by 1900-2600 cm-1 for the quinonoid compounds (but by 450 cm-1 for the M = C(CN)2 radical anion), and by 1000-1400 cm-1 for the benzenoid compounds (2500 cm-1 for the M = CN radical anion). The very simple Koopmans-based model is useful for considering the optical spectra of these radical ions. © 2005 American Chemical Society.

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Journal of Organic Chemistry