Anisotropy due to Aromatic Systems Aromaticity, Antiaromaticity, Ring-current Phenomena, etc

Anisotropy due to aromatic systems aromaticity, antiaromaticity, ring-current phenomena, etc. 

Dehydroannulenes reported by Sondheimer and co-workers include l,3,7,9,13,15-hexadehydro-[18]-annulene (14), in which the protons (all outside ) resonate at low field, suggesting the presence of a diamagnetic aromatic ring current. In comparison, the protons of 

Other studies on systems claimed to be aromatic include calculations on di- and triphenylcyclopropenium cations, in which modifications of the conventional Johnson-Bovey-Waugh-Fessenden model 

The spectrum of l,4,5,8-tetraphenyl-9,10-dimethylanthracene (20) shows methyl resonances shifted 1 -3 p.p.m. upheld from the position in 9,10-dimethylanthracene (21). The calculated shift (Johnson— Bovey model) is much larger (47 p.p.m.) presumably because the ring 

Chemical shift non-equivalence of methylene and methyl protons, etc. 

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