s-Indacene (1), a tricyclic, nonalternating [4n]pi system, has been the subject of repeated theoretical and experimental investigations. Formally, 1may be regarded as a [12]annulene perturbed by wto rho bonds, and can therefore be considered as an antiaromatic compound. According to calculations carried out previously for 1, a ground state structure with localized double bonds and the C_(2h)-symmetrical pi-bonding was suggested for isomers 1A and 1B.
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