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Zusammenfassung:
The lower excited singlet and triplet states and the zero-field splitting parameters D of [2.2]paracyclophane are studied within a semiempirical π theory which takes into account overlap effects between the two benzene rings, transanular and through-bond interaction via the methylene bridges. Whereas the singlet energies depend strongly on the through-bond interaction and the mutual polarization of σ core and π system this is not the case for the energies and zero-field splitting parameters D of the two lowest triplet states. The deformations of the benzene rings in [2.2]paracyclophane lead only to a small decrease of the excitation energies of about 0·2 eV. The D parameter can be written as a sum D = DA + DB + DAB with the intrasubunit contributions DA and DB of the conjugated subunits A and B of the phane and an intersubunit term DAB . We demonstrate that the deformations reduce the intrasubunit terms DA and DB and that they are crucial for the decrease of the D values of [2.2]paracyclophane with respect to p-xylene. The difference between the D values of the first and second triplet states is governed by the intersubunit term DAB which has a different sign in the two states. However, this difference does not depend markedly on the transanular interaction. A further reduction of DA and DB in the first triplet state only is caused by transanular interaction by means of symmetrical charge-transfer terms in the wavefunction.
