Union Monocyclic Nonalternant Hydrocarbons

The procedure developed above can be used quite generally to study the stability of cyclic even AHs, and also of nonalternant even hydrocarbons in which there are two adjacent odd-numbered rings. It cannot, however, be extended to other odd systems since an odd conjugated system cannot be derived by union of two odd AHs. Here we need the third type of perturbation considered in Section 2.4, i.e., intramolecular union between two positions in an AH. 

Intramolecular union between positions r and s in an AH corresponds to a change in the corresponding resonance integral from zero to some final value prs- The corresponding change in n energy [equation (2.13)] is 2p Xs where p s is the rs bond order in the original AH. We can represent the inter-molecular union in AH as follows  

Even systems containing an odd-numbered ring example fulvene  

nonalternant, monocyclic radicals example the cyclopen-tadienate radical  

nonalternant ions formed by union through unstarred positions example the 1,5-bismethylenecyclopentadienate anion  

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