Aromaticity magnetic properties

Aromaticity remains a concept of central importance in chemistry. It is very useful to rationalize important aspects of many chemical compounds such as the structure, stability, spectroscopy, magnetic properties, and last but not the least, their chemical reactivity. In this chapter, we have discussed just a few examples in which the presence of chemical structures (reactants, intermediates, and products) and TSs with aromatic or antiaromatic properties along the reaction coordinate have a profound effect on the reaction. It is clear that many more exciting insights in this area, especially from the newly developed aromatic inorganic clusters, can be expected in the near future from both experimental and theoretical investigations. 

Aromaticity, defined as a structural feature, was used as a predictive tool for compounds that had not been prepared previously, whereas reactivity, bond length, or magnetic criteria had to await the isolation of a compound and its experimental investigation. Only recently has the development of quantum-chemical methods reached the point where one can predict with sufficient accuracy the magnetic properties, the bond lengths, and the reactivity patterns of aromatics. The multidimensional character and the definition and measurement of aromaticity generated confusion and conflicts.43 A recent review discussed the multidimensional character of aromaticity and theoretical and experimental approaches to aromatic structures and their predictions, and references are indicated extensively.66... 

Building on and extending earlier studies, Schleyer, Najafian, et al. [57] employed computed geometric, energetic, and magnetic properties to quantify the aromaticity of the closo boranes B H 2 (6 < n < 12), and their isoelectronic counterparts, the CB iH and C2B 2H carboranes, and the NB H azaboranes [34]. All possible heteroatom placements were considered. The most stable iso-... 

It was discovered, however, that the spherical aromaticity of the icosahedral fullerenes C20, Cjq and CgQ depends on the filling of the Jt-sheUs with electrons [107]. As pointed out in Section 14.3.1 no distortion of the cage structure is expected in these fullerenes if their shells are fully filled. Closed-shell situations are realized if the fullerene contains 2(N -1-1) Jt electrons. This is closely related to the stable noble-gas configuration of atoms or atomic ions [108]. In this case the electron distribution is spherical and all angular momenta are symmetrically distributed. Correlation of the aromatic character determined by the magnetic properties is shown in Table 14.3. 

For a review of nmr and other magnetic properties with respect to aromaticity, see Haddon Haddon Jackman Fortschr. Chem. Forsch. 1971, 16, 103-220. For an example of a magentic method other than nmr, see Dauben Wilson Laity, in Snyder, Ref. 55, vol. 2, pp. 167-206. 

The synthesis and characterization of V02+ complexes of hydrazones (94) and (95) derived from benzoylhydrazine and o-hydroxy aromatic aldehydes and ketones were also reported772 and molecular weight determinations, IR, electronic spectra and magnetic properties were explained assuming a dimeric structure (96) with each unit having a five-coordinate square pyramidal geometry. 

The magnetic properties of crystals composed of aromatic polynuclear molecules may give information on the relative orientations of the benzene rings to each other. Thus, Clews and Lonsdale (1937) concluded from the magnetic anisotropy of crystals of o-diphenylbenzene that the planes of the o-phenyl groups are inclined at 50° to the plane of the main ring. 

TABLE 7. Experimental structural and magnetic properties of silole anions and dianions and conclusions regarding their degree of aromaticity... 

The known silole dianions are coordinated to the metallic counter ions either by rj5,rj5-bonding10a 82 or by r/1, r/ -bondinglob (Figures 8c and 8d). Pyramidality at Si is, of course, not a problem for the silole dianions where the Si is dicoordinated, and the known dianions are all planar (Figures 8c and 8d). The structural and magnetic properties of the silole dianions are consistent with charge delocalization and aromatic character of these species, as shown by the data collected in Table 7. 

TABLE 8. Calculated structural and magnetic properties of c-C4H4SiH (48a and 67a) and 0C4fLSi2 (49a) and their lithium complexes and conclusions regarding the degree of their aromaticity... 

Compound ASE or RE" Bond lengths (A) Magnetic properties Degree of aromaticity Ref. 

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