Properties, aromaticity, and substituents effects in poly nitro- and amino-substituted benzenes

Geometrical parameters, aromaticity, and conformational flexibility of the set of polysubstituted benzenes with different number and position of nitro and amino groups were calculated at the MP2/cc-pvdz level of theory. The key factor for structural and energetic changes has been identified. This is related to the presence of nitro and amino groups in vicinal positions that forms strong intramolecular resonance-assisted hydrogen bonds with a binding energy of 7-14 kcal/mol. Increasing number of such bonds facilitates a cooperative effect, inducing notable changes in molecular geometry (particularly increasing bond alternation within H2N-C-C-NO2 fragment and planarization of amino group), drastic increasing of conformational flexibility and decreasing of aromaticity. In spite of well-known pi-electron effects of nitro and amino substituents, influence of their push-pull interaction through aromatic moiety is negligible compared to the effect of the hydrogen bonding. That results in great difference of the ortho-isomers as compared to meta- and para-isomers.