Segregation of charge in ions of dibenzo[a,c]naphthacene : relation of topology and electronic structure. An NMR, ESR, and ENDOR study

1H- and 13C-chemical shifts of the dianion of dibenzo[a,c]naphthacene (1) have unambiguously been assigned by 2D-NMR spectroscopy. They indicate a remarkable charge distribution, as most of the negative charge is localized on the ‘anthracenic’ moiety, while the ‘phenanthrenic’ moiety is almost neutral. Association of 12− with alkali-metal counterions has only a minor effect on the chemical shifts. The charge partitioning in 12−, which is reproduced by ωβ calculations in the frame of the Huckel model, must, thus, be considered as an intrinsic property of the 4nπ-electron system of 12−. It is rationalized in terms of differing energy contents of the constituent anthracenic and phenanthrenic moieties. ESR and ENDOR studies of the radical anion 1 and the radical cation 1 show that the π-charge distribution in 12− is reflected by the π-spin distributions in the two radical ions of the alternant hydrocarbon 1.