Ring current definition

Turning to the ring current definition of aromaticity, the chemical shift calculations for different structures for [18]annulene can now be carried out in a straightforward manner by quantum mechanics. These calculations predict quite a large ring current, and the chemical shifts are substantially larger for the inner protons if there is bond alternation than if the bonds were to be equivalent. (By equivalent here is meant similar to those in benzene and to each other, and the quantum mechanical stationary... 

The conclusion appears to be that furan, if regarded as an aromatic compound, probably has less than about 20% of the aromaticity of benzene, and perhaps has none. Only a definition of aromaticity that identifies it with NMR shielding by ring currents, and little else, clearly suggests an aromaticity of 60% or more. 

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]. 

Aromaticity The property of having (4 +2)7t electrons in a closed circuit of molecular orbitals. The commonest example is benzene, where n equals one, around a six-carbon ring. A more precise definition is the ability to sustain an induced diamagnetic ring current, i.e. one that is diatropic. 

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