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

摘要

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(sp(3)) carbon atom. It is found that 1c is a 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 Bronsted acids both in the gas-phase (GP) and DMSO. More specifically, percyano molecules 1a(CN), 2a(CN) and 3a(CN) possess 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), respectively, then 1a(CN) and 2a(CN) are NH superacids, whereas 3a(CN) is a C(sp(3))H hyperacid. The origin of the dramatic amplification of the acidity in multiply cyanated derivatives is identiified as the increase in the anionic resonance of the resulting conjugate bases.