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

Theoretical Multipolar Atom Model Transfer in Nitro-Derivatives of <i>N</i>‑Methylaniline

The nitroanilines are an example of compounds in which the coexistence of electron-rich and electron-deficient substituents, connected through a conjugated π-electronic system, makes their molecular second-order hyperpolarizability and second-harmonic generation efficiency particularly high. This property makes them extremely interesting from the point of view of charge density distribution analysis. The electron density of three isomeric molecules, i.e., <i>N</i>-methyl-2-nitroaniline, <i>N</i>-methyl-3-nitroaniline, and <i>N</i>-methyl-4-nitroaniline, was calculated theoretically through the multipolar atom model transfer. Two types of refinement models, i.e., multipolar atom model (MAM) and independent atom model (IAM), have been applied for analysis of model improvement concerning the electron-density parameters transfer. It results in a more precise molecular structure in terms of geometry and thermal displacement parameters along with a reduction of statistical refinement factors and residual electron densities. The proposed approach enables the extraction of relevant electron density-derived information, where the intrinsic quality of X-ray data does not allow a “true” multipolar refinement. The effect of ortho-, meta-, and para-substitution on π-electron distribution and aromaticity of the nitroaniline ring was compared using harmonic oscillator model of aromaticity (HOMA) and nucleus independent chemical shift (NICS) indexes. In the paper, the electronic effects from the charge density parameters have been examined along with the study of intermolecular interactions using two different approaches: one, based on the Hirschfeld surfaces analysis, and the second, based on the dissociation energy estimation from topological analysis

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