Classical and reverse substituent effects in meta- and para-substituted nitrobenzene derivatives

Electron-accepting properties of the nitro group were studied in a series of meta- and para-X-substituted nitrobenzene derivatives (X = NMe2, NH2, OH, OMe, CH3, H, F, Cl, CF3, CN, CHO, COMe, CONH2, COOH, COCl, NO2, 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(NO2) 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(NO2) = −0.170 up to −0.284 in dinitrobenzene and nitroaniline, respectively.