Aromaticity/antiaromaticity magnetic indicators

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. 

One of the well-accepted magnetic criteria of aromaticity is the nucleus independent chemical shift (NICS) which is defined as the negative value of the absolute magnetic shielding calculated at some selective points. The NICS(O) and NICS(l) values are computed at the centre and lA above the aromatic ring respectively. Aromatic systems possess negative NICS values as that indicates the presence of an induced diatropic ring current. Positive NICS values refer to paratropicity and are present in antiaromatic... 

It can thus be seen from this selection of the most important and most often used aromaticity criteria that the classification of a molecule as aromatic, nonaromatic, or antiaromatic as well as the quantification of the degree of aromaticity can only be made if accurate values of a series of energetic, geometric, or magnetic properties are available, both from theory and experiment. This is especially true for the indicators which are calculated from the differences of the properties of the parent system and a reference system. 

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