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The acidity of a number of all-transpolyenes, and some of their phenyl and naphthyl derivatives, was examined by the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d) method. A dramatic increase in their acidity was established upon multiple cyanation. The origin of the acidity amplification was identified as the increased stability of the resulting conjugate bases, which is mirrored in Koopmans' term appearing in the triadic analysis. It is important to emphasize that many of the studied polycyano compounds have already been synthesized. Others are probably synthesizable and their preparation is strongly recommended. Some of the examined systems come close to the acidity threshold of 245 kcal mol−1that characterizes hyperacids. The acidity of different tautomeric forms of the same organic acid is briefly discussed. [ABSTRACT FROM AUTHOR]

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 π-networks possessing 10, 14 and 18 π-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 a tricyclic [10]annulene with almost uniformly delocalized π-electrons over the molecular perimeter, whereas other systems with 14 and 18 π-electrons exhibit pronounced bond alternation. It is conclusively shown that extended π-electron networks, like e.g. that in 3g include also smaller π-electron patterns conforming to Hückel’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 Brønsted acids both in the gas-phase (GP) and DMSO. More specifically, percyano molecules 1aCN, 2aCN and 3aCN possess gas-phase ΔHacid 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 ΔHacid(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 1aCN and 2aCN are NH superacids, whereas 3aCN is a 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. [ABSTRACT FROM AUTHOR]

The gas-phase proton affinities and basicities of a large number of extended π-systems possessing imino nitrogens as the most basic sites, which are parts of the [3]iminoradialene or quinonimine “spearheads”, are examined by the DFT B3LYP/6-311+G(2df,p)//B3LYP/6-31G(d) method. Some of the systems are neutral organic superbases exhibiting the gas phase proton affinities over 300 kcal mol−1 and basicity values in the range of 245.3–294.7 kcal mol−1. The backbone of the polycyclic π-electron networks contains either [3]radialene, methylenecyclopropene or quinoid substructures. Construction of the “tails” of planar organic superbases was at the focus of the research efforts. It is shown that the =P(NR2)3 and =C(NR2)2 fragments as well as the 1,3-diamino-2-methylenecyclopentene ring enable very efficient cationic resonance across the extended linear π-system, thus considerably contributing to the amplified basicity. Conclusive evidence is provided that protonation triggers aromatization of the quinoid six-membered ring (as well as that of the 1,3-diamino-2-methylenecyclopentene) particularly if they are linearly aligned. Triadic analysis has shown: (1) that the extremely large basicities are obtained, if a synergistic effect is obtained between the three contributions corresponding to the initial state, intermediate stage and final state triadic terms, and (2) that the methyl groups enhance basicity predominantly via destabilization of the principal molecular orbitals of the initial bases. It is argued that powerful organic superbases should lead to a spontaneous proton transfer, when interacting with (organic and inorganic) superacids. Formation of their ion pair might provide new interesting molecular materials. [ABSTRACT FROM AUTHOR]