Aromaticity and conceptual density functional theory

Pratim Kumar Chattaraj, Ranjita Das, Soma Duley and Santanab Giri  

Aromaticity is one of the most fascinating popular qualitative chemical concepts in chemistry Michael Faraday isolated benzene by distillation in 1825. He noticed that although benzene is an unsaturated compound with H C 1 1 it is much less reactive than the related unsaturated aliphatic compounds. Moreover, it undergoes substitution reactions rather than addition reactions exhibited by alkenes and alkynes. Eilhard Mitscherlich synthesized benzene by heating benzoic acid with lime. 

A characteristic smell or aroma is the hallmark of benzene derivatives which are exceptionally stable in comparison to their unsaturated aliphatic counterparts. In 1855, A. W. Hofmann coined the term aromatic to be used in the chemistry vocabulary. A. Kekule also made use of the word aromatic . Since then the aromaticity and stability of these compounds are used almost synonymously. Robert Robinson tried to explain this stability through the aromatic sextet of electrons which resists any type of change. 

Although aromaticity concept has been talked about by all chemists for almost two centuries there exists no unambiguous quantitative definition of aromaticity mainly because it is not an experimentally measurable quantity. It is also not possible to theoretically calculate it as the expectation value of a linear hermitian operator in quantum mechanics, mainly due to the same reason stated above. 

The empiricism in the concept of aromaticity is inherent. It forces chemists to understand the exact nature of an aromatic system through various indirect criteria An widely accepted definition of aromaticity has been put forward by Paul v. R. Schleyer and his coworkers which may be stated to describe aromaticity as a manifestation of electron delocalization in closed circuits, either in two or three dimensions. This results in energy lowering, often quite substantial, and a variety of unusual chemical and physical properties. These include a tendency toward bond equalization, unusual reactivity and characteristic spectroscopic features. Since aromaticity is related to induced ring currents, magnetic properties are particularly important for its detection and evaluation . 

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