Annulene, perimeter model bridged

SJAnnulene dianion, frontier orbitals. 90 [l4)Annulene perimeter. 87, 174 Annulene, perimeter model, 78. 85 aniiaromatic. 167.205,445 bridged. 70... 

Alternatively, the porphyrin ring can be constructed starting from [16]annulene. In the first step, two electrons are added to form the corresponding [16]annulene dianion, which is transformed into porphyrin Cl") by adding bridges and heteroatoms. Unlike the structure derived from [18]annulene, the dianion-based model has a fourfold symmetry, and was considered suitable for the description of metal complexes [23] (see Sect. 2.3.2). In yet another approach [24], based on the so-called perimeter model, the porphyrin macrocycle is derived from the [20]annulene dication (I "). Both the [16]- and [20]annulene models were employed to describe electronic absorption spectra and magnetic circular dichroism of porphyrinoids [24, 25],... 

Various substituted l,6-methano-[10]annulenes have been prepared and characterized, and in particular the n.m.r. of such compounds reflects their delocalized nature 17,120,124-127) (a 71-complex formed from reaction of l,6-methano-[10]annulene with hexacarbonylchromium has also been characterized 128>). The X-ray crystallographic analysis 129> of the 2-carboxylic acid derivative 24 shows the bridge angle to be 99.6 °, and thus that the Cio perimeter is even flatter than expected from molecular models. Furthermore, the ten perimeter bonds have lengths which vary over the narrow range 1.38 to 1.42 A, the values being typical of benzenoid aromatic bonds. This lack of appreciable... 

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