ELECTRONIC-STRUCTURE OF BENZO[3,4]CYCLOBUTA[1,2]CYCLOHEPTENYL IONS - THE AROMATICITY TROPICITY DILEMMA

An H-1 and C-13 NMR study of the benzo[3,4]cyclobuta[1,2]cycloheptenyl cation and anion has been undertaken in order to unravel the electronic structure of these species. The formal addition of two electrons to the cation can be seen as a conversion of a slightly diatropic and aromatic cation into an unstable, highly diatropic but antiaromatic anion. Self-consistent iterative Huckel, MNDO, and AM1 as well as graph-theoretical ring current calculations were carried out in order to clarify this confusion. Comparisons to the isoelectronic biphenylene and biphenylene dianion are especially revealing.