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

The classical textbook explanation of variations of (HNMR)-H-1 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 -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 sigma-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 sigma-system. The position of the lowest frequency signal of ortho-Hs was fully determined by sigma-electrons.This is peer-reviewed version of the following article: Baranac-Stojanović, M. 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? Chemistry - An Asian Journal 2018, 13 (7), 877–881. [https://doi.org/10.1002/asia.201800137]Supplementary material: [http://cherry.chem.bg.ac.rs/handle/123456789/3181