Aromatic systems cydopentadienyl

Fig. (a) Cydopentadienyl anion (b) cycloheptatrienyl cation. Bicyclic and polycyclic systems can also be aromatic. 

Anti-aromaticity was predicted by the Hiickel approach for conjugated cyclic planar structures with 4n 7i electrons due to the presence of two electrons in antibonding orbitals, such as in the cydopropenyl anion, cydobutadiene, and the cydopentadienyl cation (n = 1), and in the cydoheptatrienyl anion and cydooctatetraene (n = 2). It has been argued that a simple definition of an anti-aromatic molecule is one for which the 1H NMR shifts reveal a paramagnetic ring current, but the subject is controversial. The power of the Hiickel theory indeed resides not only in the aromatic stabilization of cydic 4n + 2 electron systems, but also in the destabilization of those with An electrons [22, 27, 42]. 

NMR studies of He-encapsulating compounds showed a reduced He shielding in He 10h compared to He 9b [18], which implies that the -system of the cation lOh is less aromatic relative to its precursor 9h. This can be explained by reduced delocalization in the six-membered rings (shaded rings in Scheme 9.8) adjacent to the cationic center as a result of the diminished resonance contribution of the antiaromatic cydopentadienyl cation structure lOh. ... 

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