A Comparative Study of the Two Isomers of Bicalicene

Two isomers of bicalicene are possible. The trans-isomer is a stable aromatic compound, known since the 1980s. The cis-isomer has never been experimentally prepared and has so far attracted little attention of theoretical chemists. The properties of cis-bicalicene are now studied theoretically, using the B3LYP/6-311G(d,p) quantum-chemical method. All data obtained (geometric, energetic, as well as those based on harmonic oscillator model of aromaticity and nucleus-independent chemical shift indices of local aromaticity) unequivocally indicate that the “missing” isomer of bicalicene is an unstable antiaromatic species, and that the lack of its existence is no surprise whatsoever.