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Anomalous cyclic conjugation in benzenoid molecules with a small number of Kekule structures

The currently used Kekule-structure-based models for assessing the extent of cyclic conjugation in benzenoid hydrocarbons predict that there is no or very little cyclic conjugation in the rings possessing "fixed" doubleand single carbon-carbon bonds. With the example of a suitably chosen class of benzenoid systems with a small number of Kekule structures, we show that the cyclic conjugation in the rings with "fixed" bonds (measured by its energy-effect) may be unexpectedly high, contradicting the results of standard Kekule-structure-based considerations. Consequently, our study reveals that the analysis of the conjugation modes of polycyclic aromatic compounds, based solely on Kekule structures, may sometimes be insufficient and may lead to erroneous conclusions.