Substituent effects and aromaticity of six-membered heterocycles

Structure, aromaticity, relative stability, and conformational flexibility of nitro and amino substituted monoheterocyclic analogous of benzene were studied by ab initio quantum-chemical method at MP2/aug-cc-pvDZ level of theory. Amino derivatives were found to be slightly less aromatic than nitro derivatives. Strong push–pull interactions were found in α- and γ-aminochalcogenopyrylium cations and, in less extent, in α- and γ-aminopyrydines. These molecules are less aromatic but more stable, as compare to their β-isomers. All heterocycles with 3rd and 4th row heteroatoms reveal C–NO₂ bond elongation accompanied by C–Heteroatom bond shortening in β-nitro isomers, and strong inequality of two endocyclic C–Heteroatom bonds in α-amino isomers.