Aromaticity, aromatic character homoaromatic compounds

In order for the orbitals to overlap most effectively so as to close a loop, the sp atoms are forced to lie almost vertically above the plane of the aromatic atoms. In 107, Hb is directly above the aromatic sextet and so is shifted far upfield in the NMR. All homoaromatic compounds so far discovered are ions, and it is questionable as to whether homoaromatic character can exist in uncharged systems. Homoaromatic ions of 2 and 10 electrons are also known. 

In late 1950s, a novel class of cyclic compounds was discovered, that exhibited aromatic character from a chemical, thermodynamic and spectroscopic viewpoint despite the presence of sp -hybridized atoms within the ring [49]. In his pioneering work, Winstein [49] named such compounds as homoaromatics due to their... 

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