Homonuclear monocyclic compounds

The first question that needed answering was whether the unsaturated NHC are indeed aromatic. The Hiickel rule states that aromatic systems are monocyclic, homonuclear, planar and possess a delocalised 4n-n2 jr-electron system [96]. Unsaturated NHC fulfil all the criteria except homonuclearity, which would make them heteroaromatic compounds. However, there was doubt as to the delocalisation of the 6 7t-electron system. Althongh there are 6 n-electrons, their distribution is extremely unequal, the backbone carbon atoms C and C contribute one rr-electron each, the nitrogen atoms provide a lone pair each, but the carbene carbon atom C gives none. Can there be a true delocalisation in such circumstances ... 

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See also in sourсe #XX -- [ , ]

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