Aromaticity magnetic susceptibility

Magnetic susceptibilities of 2-aminothiazole have been measured (139) in order to evaluate the aromatic character of the compound. 

It is difficult to draw general conclusions from such a small number of values. Nevertheless, it can be noted that, like other five- or six-membered unsaturated rings (thiophene, pyridine) thiazole exhibits a certain aromatic behavior in its magnetic susceptibility. 

The aromatic character of thiazole has been deduced from the magnetic susceptibility anisotropy of the molecule (319). 

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 aromaticity of five-membered rings with two or more heteroatoms was discussed in detail in earlier reviews.52 100 111 In a comprehensive survey on the quantitative measurements of aromaticity,112 it has been shown that basicity-based quantification of aromaticity gave more reproducible resonance energies than other methods, such as heats of formation, ring currents, magnetic susceptibilities, and theoretical indices. 

Density functional theory and MC-SCF calculations have been applied to a number of pericyclic reactions including cycloadditions and electrocyclizations. It has been established that the transition states of thermally allowed electrocyclic reactions are aromatic. Apparently they not only have highly delocalized structures and large resonance stabilizations, but also strongly enhanced magnetic susceptibilities and show appreciable nucleus-independent chemical-shift values. 

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). 

Since the magnetic susceptibility anisotropy Ax is a characteristic attribute of aromatic molecules (66MI1 75MI2), its value could play the role of an aromaticity index... 

Direct application of Ax for the quantitative evaluation of aromaticity is, however, not practicable since its magnitude is not determined by ring currents only. Quite substantial effects may be played by a local contribution by the 7r-bond anisotropy and the anisotropy of CC and CH (r-bond magnetic susceptibilities as well as by the anisotropy due to local paramagnetic currents (for more detail, see, e.g., 66MI1). 

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... 

More precisely, Eq. (33) represents the difference between the magnetic susceptibility of a cyclic conjugated system and that of a hypothetical cyclic system with localized double bonds in which the ring current vanishes. A molecule is aromatic when A>0 and antiaromatic when A<0, and at A 0 it is nonaromatic [61JCP1996 68JA811 75MI2 83JMS( 102)377]. 

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