The Non-bonding Molecular Orbital Method

As a general treatment of the problem of substitution, Wheland s method has the disadvantage of not being very easy to apply. For one class of aromatic substances, namely the alternant hydrocarbons and heterocyclic compounds isoaromatic with them, a comprehensive and rapid treatment has been developed. Alternant hydrocarbons have no odd-numbered rings. They have the property that the constituent carbon atoms fall into two sets distinguished as starred and unstarred, such that no atom of one set is adjacent to another of the same set. Odd alternant hydrocarbons are radicals or ions. As an 

Longuet-Higgins fi has shown how such considerations can be used to illuminate a number of problems in aromatic chemistry. His results will be referred to later at appropriate points, but the method is usefully illustrated here by reference to aromatic substitution, as for example in the case of nucleophilic substitution in benzene and pyridine. The essential point is that not only benzene and pyridine themselves, but also the transition states (12), (13), (14) and (15) form isoaromatic series. Qualitatively, it is obvious that, since in (12) negative charge resides on the starred atoms. 

The method is capable of more exactitude One way of seeing this is by using the equation dEldof = qr (p. 37). From the isoaromatic pair represented by an alternant hydrocarbon and its aza-derivative, we have — Ettj jj = where or is the difference between the coulombic 

This treatment reproduces exactly the predictions of simple resonance theory , but with a degree of semi-quantitative differentiation. However, a full utilization of the more exact equation for given above demands 

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