Aromaticity magnetic susceptibility exaltation

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. 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

The magnetic susceptibility exaltations (A, defined as a difference between the magnetic susceptibility of a given system and a reference one, without cyclic delocalization) are also based on Equation (1). Systems with strongly negative values of A are qualified as aromatic. 

Several aspects of aromaticity have been studied <2002JOC1333> using statistical analyses of quantitative definitions of aromaticity. ASEs, REs, magnetic susceptibility exaltation (A), nucleus-independent chemical shift (NIGS), the harmonic oscillator model of aromaticity (HOMA), (/j) and (Aj), evaluated for a set of 75 five-membered 7t-electron systems and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations between the various aromaticity criteria, provided the whole set of compounds is used. The data in Table 9 have been found for arsole (AsH) 1 (E = As, R = H), its anion (As ), and protonated species (AsH2 ). 

The electronic structure and the aromaticity of 1,2-azaphosphole, 1,2-oxaphosphole, 1,2-thiaphosphole, and 1,2-diphosphole have been theoretically investigated in terms of aromatic stabilization energies (ASEs), resonance energies (REs), magnetic susceptibility exaltations, and nucleus-independent chemical shift (NICS) indexes <2002JOC1333>, as well as based on isodesmic reactions <2003T1657>. 

The structure analysis of the dilithium pentalenediide 45 reveals a C2-sym-metric ion triplet with the two lithium cations located on opposite sides of the two different rings. The structural parameters were extremely well reproduced by ab initio calculations [35] (see Fig. 3). Both the experimental structural parameters and calculated magnetic susceptibility exaltation classify the 107r-elec-tron species 45 as an aromatic compound. Apparently, the lithium counterions in 45 do not exert any significant effect on the bond lenghts of the dianion 22. On the other hand, the antiaromatic pentalene 2 and its aromatic dication 22+ show the characteristic bond length alternation (Fig. 3) [35]. 

Magnetic Susceptibility Exaltation for Some Aromatic Hydrocarbons... 

An obvious limitation of the model is the fact that it can be applied only to carbocyclic systems. Sometimes this approach has been used on species with heteroatoms, and then only CC bonds were taken into account. This limitation results in exclusion of the contributions which are directly due to the interactions between heteroatoms and carbon—heteroatoms leading to a potential misinterpretation of aromaticity of these systems. Julg s index was applied to numerically describe the aromaticity of some jr-elec-tron systems and served also to test the correctness of other aromaticity indices such as HOMA as well as supporting other already accepted measures of aromaticity such as aromatic stabilization energy, ASE, and magnetic susceptibility exaltation. A. ... 

Figure 6 Dependence between A, NICS, NICS(1), and HOMA vs ASE for all 105 structures (a) exaltation of magnetic susceptibility vs ASE (b) NICS vs ASE (c) NICS(1) vs ASE (d) HOMA vs ASE. ASE, aromatic stabilization energy HOMA, harmonic oscillator model of aromaticity NICS, nucleus-independent chemical shift A, magnetic susceptibility exaltation. Reprinted with permission from Cyranski et al. (2002JOC1333). Copyright 2002 American Chemical Society.

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