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Rees polycyanated hydrocarbons and related compounds are extremely powerful Broensted superacids in the gas-phase and DMSO - a density functional B3LYP study

The spatial structure and acidic properties of demethylated Rees hydrocarbons and related fluoradene, as well as their polycyanated derivatives, are considered by the B3LYP/6-311+G(2d, p)//B3LYP/6-31G(d) method. The molecular frameworks 1c, 2e and 3g involve the pi-networks possessing 10, 14 and 18 pi-electrons on the molecular rims, held in the rigid tricyclic structures by the central tertiary C(sp3) carbon atom. It is found that 1c is the tricyclic [10]annulene with almost uniformly delocalized pi-electrons over the molecular perimeter, whereas other systems with 14 and 18 pi-electrons exhibit pronounced bond alternation. It is conclusively shown that extended pi-electron networks, like e.g. that in 3g include also smaller pi-electron patterns conforming to Huckel's (4n + 2) rule, thus representing an interesting case of molecular "philogenesis". Further, all these molecules undergo prototropic tautomerism. The number of prototropic tautomers is considerably increased by polycyanation inter alia by forming keteneimine moiety. The most important result of the present study is that polycyano derivatives of studied molecules are strong organic Broensted acids both in the gas-phase (GP) and DMSO. More specifically, percyano molecules 1aCN, 2aCN and 3aCN possess the gas-phase delta_H_acid values as low as 261.8, 259.0 and 246.3 kcal mol-1, respectively. If the thresholds of superacidity and hyperacidity are accepted as delta_H_acid(HClO4) = 300.0 kcal mol-1 and the protonation energy of 1, 8-bis(dimethylamino)naphthalene DMAN, PA(DMAN) = 245.3 kcal mol-1, then 1aCN and 2aCN are NH superacids, whereas 3aCN is the C(sp3)H hyperacid. The origin of the dramatic amplification of the acidity in multiply cyanated derivatives is identified as the increase in the anionic resonance of the resulting conjugate bases.

aromaticity ; bridged annulenes ; polycyano annulenes ; superacidity

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