Abstract
The Hückel approximation in MO LCAO is used to show that the anion system of cyclopent-4-ene-1,3-dione should have two π→π* transitions: a weak one at long wavelengths, N→V1, and a strong one at short wavelengths, C=C(1,3)→V1. An ethylene, phenyl, or acyl group at position 2 gives rise to a new strong band, N→C=C(2), N→benz, or N→C=O (2). A p-nitrophenyl group at position 2 gives rise to a strong N→NO2 band, which overlaps the weak N→V1 band, while the N→benz band becomes weak and is virtually lost from the spectrum.
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Gren, E.Y., Vanag, G.Y. MO LCAO calculations and the electronic spectra of the anions of cyclopent-4-ene-1,3-diones and indane-1,3-diones. Theor Exp Chem 2, 115–120 (1967). https://doi.org/10.1007/BF00523767
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DOI: https://doi.org/10.1007/BF00523767