Substituent Effect on the σ- and π‑Electron Structure of the Nitro Group and the Ring in Meta- and Para-Substituted Nitrobenzenes

An application of quantum chemical modeling allowed us to investigate a substituent effect on a σ and π electron structure of a ring and the nitro group in a series of meta- and para-X-substituted nitrobenzene derivatives (X = NMe₂, NHMe, NH₂, OH, OMe, Me, H, F, Cl, CF₃, CN, CHO, COMe, CONH₂, COOH, NO₂, and NO). The obtained pEDA and sEDA parameters (the π- and σ-electron structure characteristics of a given planar fragment of the system obtained by the summation of π- and σ-orbital occupancies, respectively) of the NO₂ group and the benzene ring allowed us to reveal the impact of the substituents on their mutual relations as well as to analyze them from the viewpoint of substituent characteristics. The decisive factor for dependence of pEDA on sEDA of the ring is electronegativity of the atom linking the substituent with the ring; in subgroups an increase of sEDA is associated with a decrease of pEDA. The obtained mutual relation between pEDA­(NO₂) and pEDA­(ring) characteristics documents strong resonance interactions for electron-donating substituents in the para position. The observed substituent effect on the σ-electron structure of the nitro group, sEDA­(NO₂), is significantly greater (∼1.6 times) for meta derivatives than for the para ones.