Aromaticity/antiaromaticity, magnetic

Ring Currents Aromatic and Antiaromatic Magnetic Resonance Imaging Spectra by the Thousands Gas Chromatography GC/MS and MS/MS... 

As the collection of recent reviews in the topic shows [12-20], a rather large consensus appears in the computation or experimental tests for the diagnosis of the aromaticity/antiaromaticity (energetic, structural, magnetic, chemical reactivity, and electronic diagnostic tools), whereas the mechanisms themselves still remain open to the debate. The primary controversy in the area involves the questions of whether aromaticity/antiaromaticity can be quantified and, if so, which of the methods commonly used to evaluate aromaticity/antiaromaticity is most appropriate. The literature on aromaticity and its measure is so vast that I must be content here with outlining briefly only the aromaticity indicators which have been extensively used to diagnose aromaticity/antiaromaticity in the domain of all-metal aromatics. 

Nucleus-independent chemical shifts (NICS) provide a useful criterion of aromaticity/antiaromaticity. Absolute magnetic shieldings are computed at ring centers (nonweighted mean of the heavy atom coordinates) and correspond to the NMR chemical shift convention the signs are reversed, so that negative NICS values denote aromaticity and positive NICS values denote antiaromaticity. 

The stepwise transfer of electrons from alkali metals to Ti-conjugated systems enables the study of the relationship between the number of Ti-electrons and the aromatic/antiaromatic properties of the systems vis d vis the Hiickel rule. The magnetic criterion for aromaticity serves as a probe for the aromatic nature of the systems under study. 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

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. 

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. 

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