7i-electron systems

The most important classes of functionalized [60]fullerene derivatives, e.g. methanofullerenes [341, pyrrolidinofullerenes [35], Diels-Alder adducts [34i] and aziridinofullerene [36], all give rise to a cancellation of the fivefold degeneration of their HOMO and tlireefold degeneration of their LUMO levels (figure Cl.2.5). This stems in a first order approximation from a perturbation of the fullerene s 7i-electron system in combination with a partial loss of the delocalization. 

For 7i-electron systems Hiickel made the following approximations ... 

Dendrimer chemistry has taught us that these molecules create a nano-sized closed space that, presumably, is the origin of the specific physical properties of this class of materials. As the next stage of dendrimer chemistry, a macromolecule capable of creating such a space inside its molecule is proposed. To create the nano-sized space, porphyrin is considered to be the best candidate for the component molecules, because it has versatile properties associated with its expanded 7i-electron system and the incorporated metal. The resultant multi-detectable properties of porphyrin, that is, a number of its properties are detectable by many physical methods, may reveal the function of the nanometer-sized space. 

The absorption bands for both quinoid and dipolar structures have been calculated by the PPP method.2,12 The calculations for a more simplified model of the colored form of some spirobenzopyrans using the normal parameters are shown in Table 2.12 In this case, the spiro carbon in the indoline moiety is ignored in the 7i-electron system, and the quinoid structure is assumed. 

The scientific interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 7i-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25-27]. 

In five-membered heterocycles, formally derived from benzene by the replacement of a CH=CH unit by a heteroatom, aromaticity is achieved by sharing four p-electrons, one from each ring carbon, with two electrons from the heteroatom. Thus in pyrrole, where the heteroatom is N, all the ring atoms are sp hybridized, and one sp orbital on each is bonded to hydrogen. To complete the six 7i-electron system the non-hybridized p-orbital of N contributes two electrons (Box 1.9). It follows that the nitrogen atom of pyrrole no longer possesses a lone pair of electrons, and the compound cannot function as a base without losing its aromatic character. 

Investigations of unsaturated group 14 element-nitrogen ring systems are primarily focused on cyclic oligomers of the type ( MNR) in which M is in the divalent state. The six-membered rings ( = 3) are potentially Hiickel six 7i-electron systems in which each of the nitrogens contribute two 71-electrons. 

Polyphenols can act as antioxidants by a number of potential pathways. The most important is likely to be by free radical scavenging, in which the polyphenol can break the radical chain reaction. Polyphenols are effective antioxidants in a wide range of chemical oxidation systems, being capable of scavenging peroxyl radicals, alkyl peroxyl radicals, superoxide, hydroxyl radicals, nitric oxide and peroxynitrate in aqueous and organic environments [121]. This activity is due to the ability of donating an H atom from an aromatic hydroxyl group to a free radical, and the major ability of an aromatic structure to support an unpaired electron by delocalization around the 7i-electron system. Phenolic acids... 

The synthesis of diazaazulenes was investigated as mentioned earlier in Section 13.04.2, during the study of the nature of aromaticity of numerous nonbenzenoid carbocyclic and heterocyclic conjugated 7i-electron systems. The synthetic approaches started with reactions of semicyclic 1,5-diketones with various hydrazines to give partially saturated cyclopenta[c]-l,2-diazepines. Then several methods were used to dehydrogenate, as described earlier, the cyclopenta-l,2-diazepines to the respective diazaazulenes without much success <2001TH1>. 

The precise position of the carbonyl band, which is at the lower end of the frequency range, may now be rationalised. It implies that the carbonyl function is in conjugation with the aromatic 7i-electron system leading to reduced double bond character, a weaker carbon-oxygen bond and hence a lower absorption frequency. This conjugation is confirmed by observing the increased intensity of the 1580 cm-1 band associated with the aromatic ring vibration. 

UV data for some parent furopyridines and methyl or chloro derivatives are listed in Table 28. UV data have been collected for a series of furodipyridines. The data indicate that, as expected in comparison with the parent furopyridines, the entire electronic spectrum shifts toward longer wavelengths owing to the extension of the 7i-electron system (90JHC637). 

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