Substituent Effect on the σ- and π‑Electron Structure of the Nitro Group and the Ring in <i>Meta</i>- and <i>Para</i>-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 <i>meta</i>- and <i>para</i>-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 <i>para</i> position. The observed substituent effect on the σ-electron structure of the nitro group, sEDA­(NO₂), is significantly greater (∼1.6 times) for <i>meta</i> derivatives than for the <i>para</i> ones

Dependence of the Substituent Effect on Solvent Properties

The influence of a solvent on the substituent effect (SE) in 1,4-disubstituted derivatives of benzene (BEN), cyclohexa-1,3-diene (CHD), and bicyclo[2.2.2]­octane (BCO) is studied by the use of polarizable continuum model method. In all X–R–Y systems for the functional group Y (NO₂, COOH, OH, and NH₂), the following substituents X have been chosen: NO₂, CHO, H, OH, and NH₂. The substituent effect is characterized by the charge of the substituent active region (cSAR­(X)), substituent effect stabilization energy (SESE), and substituent constants σ or <i>F</i> descriptors, the functional groups by cSAR­(Y), whereas π-electron delocalization of transmitting moieties (BEN and CHD) is characterized by a geometry-based index, harmonic oscillator model of aromaticity. All computations were carried out by means of B3LYP/6-311++G­(d,p) method. An application of quantum chemistry SE models (cSAR and SESE) allows to compare the SE in water solutions and in the gas phase. Results of performed analyses indicate an enhancement of the SE by water. The obtained Hammett-type relationships document different nature of interactions between Y and X in aromatic and olefinic systems (a coexistence of resonance and inductive effects) than in saturated ones (only the inductive effect). An increase of electric permittivity clearly enhances communications between X and Y for BEN and CHD systems