Can Variations of 1H NMR Chemical Shifts in Benzene Substituted with an Electron-Accepting (NO2)/Donating (NH2) Group be Explained in Terms of Resonance Effects of Substituents?

Abstract

The classical textbook explanation of variations of 1H NMR chemical shifts in benzenes bearing an electron-donating (NH2) or an electron-withdrawing (NO2) group in terms of substituent resonance effects was examined by analyzing molecular orbital contributions to the total shielding. It was found that the π-electronic system showed a more pronounced shielding effect on all ring hydrogen atoms, relative to benzene, irrespective of substituent +R/−R effects. For the latter, this was in contrast to the traditional explanations of downfield shift of nitrobenzene proton resonances, which were found to be determined by the σ-electronic system and oxygen in-plane lone pairs. In aniline, the +R effect of NH2 group can be used to fully explain the upfield position of meta-H signals and partly the upfield position of para-H signals, the latter also being influenced by the σ-system. The position of the lowest frequency signal of ortho-Hs was fully determined by σ-electrons.