Nucleus Independent Chemical Shift dianion

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

NMR shifts, nucleus-independent chemical shifts, NICS, and magnetic susceptibility exaltation, A.9 It has been found that counterions play a more important role in the behavior of dianions than dications, as shown by the improvement in the agreement of experimental and calculated shifts for dianions with the inclusion of counterions. 

The antiaromaticity of a series of dianions of p-substituted benzylidene dibenzo-[fl,d]cycloheptenes (1) was examined through calculated measures of antiaromaticity. The nucleus-independent chemical shifts and magnetic susceptibility exaltation both showed substantial antiaromatic character in the benzannulated tropylium anion. When the antiaromaticity was normalized for the area of the ring, the tropylium anions (1) were shown to be among the most antiaromatic anions in the chemical literature. 

The synthesis of the cyclooctatetraene dianion fused at the [e]-position of trans-10b,10c-dimethyl-10b,10c-dihydropyrene (1) has been described, and by comparison of H NMR properties and nucleus-independent chemical shifts to the analogous benzene-fused dihydropyrene (2), the relative aromaticity of the dianion has been found to be at least as great as that of benzene and substantially larger than that of the cyclopentadienide anion. ... 

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