Improved Resonance Energies of Nonalternant Hydrocarbons. An ω-Technique Approach

The ω-technique of Wheland and Mann was introduced into the graph theory of aromaticity in order to improve the resonance energies of nonalternant hydrocarbons. For these compounds, the ω-technique resonance energy was more or less smaller than the resonance energy calculated on the basis of the simplest Hückel MO model. Especially, several compounds thus far considered to be aromatic were predicted to be substantially olefinic with much reduced resonance energies. They are calicene, sesquifulvalene, azulene, dibenzo[cd,gh]pentalene, and cyclohepta[def]fluorene. Many aspects of their chemistry were evidently consonant with the improved resonance energies. Possible limitation of the use of electrophilic substitution as a criterion of aromaticity was discussed. Resonance energies of alternant hydrocarbons are not modified by application of the ω-technique.