Magnetic exaltation

Dauben suggested using the magnetic exaltation as a measure of aromaticity [32-34]. According to this criterion a conjugated molecule is aromatic if A > 0, nonaromatic if A 0, and anti-aromatic if A <0. Practically, is either determined by experiment, or more conveniently calculated by quantum chemical calculations. Increments are available from Haberditzel [25]. The latter increment system also includes local contributions from binding electrons. The following calculation for benzene may serve as an example (susceptibiUty increments in —10 cm mol ) ... 

Schleyer and jiao ( v) extensively using magnetic exaltation criterion to study aromaticity M... 

Many more mean values have been reported than full tensors or anisotropies (see, for example, refs 104— 109). Very early in the development of this field it was found that these mean values were additive, being sums of atomic terms plus some so-called constitutive corrections (see, for example, refs 110 and 111 and Chapter 6 of ref 106). Any deviation from these (Pascal s"°) rules was said to be a measure of magnetic exaltation (see section III.A.4, which follows). 

The magnetic exaltation is defined as the difference between the true mean molar magnetic susceptibility and the one calculated by a hypothetical, additive incremental scheme by use of atom and bond increments according to the... 

The magnetic exaltation E is the difference in the average magnetic susceptibility (averaged over all orientations of the molecule with respect to the applied magnetic field) and empirically estimated susceptibility [3]. The latter is calculated as the sum of the atomic susceptibilities and an increment due to the presence of double bonds [4,5] = EaXa + where n is the number of double bonds). Systems... 

In Tabled theoretical magnetic anisotropies AK for selected molecules, calculated assuming the bond lengths shown in Fig. 5, are listed (in units of d.Kbenzene) d Compared with the experimental exaltations reported by Dauben et a/. . Theoretical values are in good agreement... 

Table 4. Calculated magnetic susceptibilities (AK) and experimental exaltation (A)...

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. 

Ring currents cannot be directly determined by experimental methods. However, comparison of experimental values of magnetic susceptibilities and their exaltations and anisotropies as well as of H-NMR chemical shifts with the respective data calculated from the ring current model points to the adequacy of this model for the interpretation of experimental results. The magnetic susceptibility associated with the ring current / (83BCJ1853), known as the London susceptibility, is given by... 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

Another quantitative characteristic of the magnetic manifestation of aromaticity is represented by the exaltation of the total magnetic susceptibility A (68JA811 75MI2). For conjugated compounds, this parameter is given by the difference between Xm and Xm standing, respectively, for the experimentally measured molar susceptibility and the molar susceptibility... 

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