Institut für Organische Chemie, Freie Universität Berlin, D-1000 Berlin 33
Organic oligoradicals or multispin systems are characterized by scalar exchange interaction and dipolar coupling of the unpaired electrons in addition to Zeeman splitting and hyperfine interaction. Because of an increasing number of hyperfine components, the resolution of fluid-solution EPR spectra of oligoradicals deteriorates progressively for higher electron spin states (bi-, tri- and tetraradicals). Moreover, relaxation caused by the electron-electron dipolar interaction gives rise to line broadening. The higher resolution of ENDOR spectroscopy allows the determination of hyperfine couplings even in quartet-state triradicals  and quintet-state tetraradicals , but the experiments are more difficult to perform because of the unfavourable relaxation properties of these systems.
Use of 13C labelled radicals offers two advantages . First, the (presumably) large 13C hyperfine coupling may give rise to a resolved hyperfine structure in the EPR spectra even of tetraradicals, allowing a selective field setting in ENDOR experiments. Second, observation of 13C ENDOR signals supplies additional evidence for the identity of the spin state under study. EPR and ENDOR spectra of a series of 13C-labelled oligogalvinoxyls up to a tetraradical are presented, and the influence of the exchange interaction is discussed.
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