Relation between the Substituent Effect and Aromaticity

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Abstract
Molecular geometries of benzene and its 18 monosubstituted derivatives were optimized at B3LYP/6-311+G** level of theory. The changes of π-electron delocalization of the benzene fragment were estimated by use of aromatic stabilization energies (ASE) based on different homodesmotic reaction schemes, geometry-based HOMA model, magnetism-based NICS, NICS(1), NICS(1)zz, and an electronic delocalization index, PDI, derived from the AIM theory. Apart from aromatic stabilization energies the other descriptors of aromaticity vary to a very small extent, indicating high resistance of the π-electron structure to the substituent effect. This is somewhat analogous to a tendency of benzene systems to retain their initial π-electron structure during the reaction course that leads to aromatic substitution.

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