An application of a charge of the substituent active region concept to 1-Y,4-X-disubstituted derivatives of bicyclo[2.2.2]octane (BCO) [where Y = NO 2 , COOH, OH, and NH 2 and X = NMe 2 , NH 2 , OH, OMe, Me, H, F, Cl, CF 3 , CN, CHO, COMe, CONH 2 , COOH, NO 2 , and NO] provides a quantitative information on the inductive component of the substituent effect (SE). It is shown that the effect is highly additive but dependent on the kind of substituents. An application of the SE stabilization energy characteristics to 1,4-disubstituted derivatives of BCO and benzene allows the definition of inductive and resonance contributions to the overall SE. Good agreements with empirical approaches are found. All calculations have been carried out by means of the B3LYP/6-311++G(d,p) method.

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 = NMe, NHMe, NH, OH, OMe, Me, H, F, Cl, CF, CN, CHO, COMe, CONH, COOH, NO, 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 NO 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(NO) 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(NO), is significantly greater (∼1.6 times) for meta derivatives than for the para ones.

Numerous studies on nitro group properties are associated with its high electron-withdrawing ability, by means of both resonance and inductive effect. The substituent effect of the nitro group may be well described using either traditional substituent constants or characteristics based on quantum chemistry, i.e., cSAR, SESE, and pEDA/sEDA models. Interestingly, the cSAR descriptor allows to describe the electron-attracting properties of the nitro group regardless of the position and the type of system. Analysis of classical and reverse substituent effects of the nitro group in various systems indicates strong pi-electron interactions with electron-donating substituents due to the resonance effect. This significantly affects the pi-electron delocalization of the aromatic ring decreasing the aromatic character, evidenced clearly by HOMA values. Use of the pEDA/sEDA model allows to measure the population of electrons transferred from the ring to the nitro group. Keywords Nitro group. Substituent effects. Molecular modeling. Sigma and pi electron structure. Substituent effect stabilization energy. Charge of the substituent active region This review is dedicated to our friend and mentor Professor Tadeusz Marek Krygowski of the Department of Chemistry of the Warsaw University in gratitude for his guidance, support, and engagement in our research.

Electron-accepting properties of the nitro group were studied in a series of meta-and para-X-substituted nitrobenzene derivatives (X = NMe 2 , NH 2 , OH, OMe, CH 3 , H, F, Cl, CF 3 , CN, CHO, COMe, CONH 2 , COOH, COCl, NO 2 , NO). For this purpose Hammett-like approaches were applied based on quantum chemistry modeling; the B3LYP/6-311++ G(d,p) method was used. The substituent effect (SE) was characterized by the mutually interrelated descriptors: the charge of the substituent active region, cSAR(X), and substituent effect stabilization energy, SESE, as well as substituent constants, σ. Classical SE is realized by dependences of the structural parameters of the nitro group (ONO angle and NO bond lengths) and cSAR(NO 2 ) on the above mentioned SE descriptors. The reverse substituent effect was clearly documented by a comparison of cSAR(X) values for monosubstituted benzenes, meta-and para-substituted nitrobenzenes as well as, additionally, for meta-and para-X-substituted anilines. For para-substituted systems the electron-accepting ability of the nitro group increases from cSAR(NO 2 ) = −0.170 up to −0.284 in dinitrobenzene and nitroaniline, respectively.