Testing the Clar theory: Cyclic conjugation in some benzo- and naphthaleno-annelated ovalenes

Ivan Gutman,Jelena Djurdjevic,Sabina Gojak-Salimovic

doi:10.5937/kgjsci1436087g

Abstract

Cyclic conjugation (assessed by its energy effect) is studied in benzo- and naphthaleno-annelated ovalenes, in which annelation is in positions d and o. These benzenoid species possess a large number of Clar aromatic sextet formulas, making the application of Clar theory difficult and ambiguous. Some predictions based on Clar formulas are not in agreement with the calculated energy effects. This indicates that Clar theory is not universally applicable, even in the case of fully conjugated benzenoid molecules.

Highlights

Benzenoid hydrocarbons form a class of conjugated π-electron systems whose theory is elaborated in due detail [1,2,3,4,5,6,7]
Erich Clar’s “aromatic sextet theory” [1,3] provides the simplest and most direct way by which one can get an insight into the dominant modes of cyclic conjugation of a benzenoid molecule
In our earlier studies [13,14,24,25,26,27], examples of benzenoid hydrocarbons were found in which the predictions of Clar

Summary

Introduction

Benzenoid hydrocarbons form a class of conjugated π-electron systems whose theory is elaborated in due detail [1,2,3,4,5,6,7]. We have detected a class of fully conjugated benzenoid molecules (i.e., molecules without fixed single and double carbon-carbons bonds), in which the modes of cyclic conjugation are not in full agreement with the Clar model These are the benzo- and naphthaleno-derivatives of ovalene, in which annelation is in position d and o, see Fig. 1. A quantitative measure of cyclic conjugation in π-electron systems, especially in benzenoid hydrocarbons, is achieved by the energy-effect of the respective cycles. Details of this method have been earlier described several times, e.g., in the articles [8,9,10] and in the review [11]. The energy-effects of the rings 5, 6, and 7 are omitted because, by symmetry, these are always equal to the ef-values of the rings 3, 2, and 1, respectively

Agreements with Clar theory

Discussion and concluding remarks