Theoretical study of electron-attracting ability of the nitro group: classical and reverse substituent effects

Abstract

This study presents a quantum chemistry modeling of the substituent effect. For this purpose, a uniform approach—the model termed as “charge of substituent active region” (cSAR, defined as a sum of atomic charges at the substituent and the ipso carbon atom)—has been used. Its reliability has been confirmed by a quantitative description of the electron-attracting ability of the nitro group, and finally, the reverse substituent effect, showing the influence of reaction site (Y) on the properties of substituent (X), has been introduced and documented. The cSAR model has been applied to para-substituted nitrobenzene derivatives in which the nitro group acts either as a reaction site (classical substituent effect, with X = NO2, CN, CHO, COOMe, COMe, CF3, Cl, H, Me, OMe, NH2 and NHMe) or as a substituent (reverse substituent effect, with Y = NH−, NH2, H and NH3 +). Calculation was performed using BLYP functional with DFT-D3 correction and TZ2P basis set. Application of different assessments of atomic charges (Voronoi, Hirshfeld, Bader, Weinhold and Mulliken) leads to good correlation between calculated cSAR values. Electron-attracting ability of the nitro group depends dramatically on the electron-donating properties of the reaction site