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Symmetry anomaly in disubstituted benzenes



A striking feature in disubstituted benzenes is that the well-known degeneracy scheme of the π levels of the unperturbed benzene ring is conserved when donor and acceptor groups of the same strength are introduced in para positions (antisymmetrical substitution). This degeneracy is lifted when substituting with two identical donor or acceptor groups (symmetrical substitution). In the antisymmetrical

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substitution, even though the perturbation affects only one of the two degenerate π levels, a compensation effect has the result that the degeneracy of the π levels is not only conserved but their energies remain unchanged; thus, the perturbation only affects the energy of the lowest occupied π level and that of the highest unoccupied π level. The same feature is observed for other even annulenes. This symmetry anomaly is analyzed by various methods. The connection with Berry's phase problem is also discussed. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002

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