Violation of Hund's multiplicity rule in the lowest excited singlet–triplet pairs of cyclic bicalicene and its higher homologues

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Abstract
The relative energy ordering of the lowest excited singlet–triplet pair in the title molecules has been examined by using the Pariser–Parr–Pople–type SCF MO method and by taking into account configuration interactions. In cyclic bicalicene, the excited singlet state is predicted to lie below the triplet state by about 4 kcal mol–1, at least within this MO approach. This violation of Hund's multiplicity rule is ascribed essentially to the correlation effects brought about by spin polarization terms. Such a violation of Hund's rule is expected to occur in cyclic bisesquifulvalene and cyclic bitriaheptafulvalene of D2h symmetry.

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