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Can Variations of 1 H NMR Chemical Shifts in Benzene Substituted with an Electron‐Accepting (NO 2 )/Donating (NH 2 ) Group be Explained in Terms of Resonance Effects of Substituents?
Author(s) -
BaranacStojanović Marija
Publication year - 2018
Publication title -
chemistry – an asian journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.18
H-Index - 106
eISSN - 1861-471X
pISSN - 1861-4728
DOI - 10.1002/asia.201800137
Subject(s) - substituent , chemistry , aniline , benzene , chemical shift , lone pair , proton , ring (chemistry) , resonance (particle physics) , nitrobenzene , electron , group (periodic table) , crystallography , stereochemistry , atomic physics , molecule , physics , organic chemistry , quantum mechanics , catalysis
The classical textbook explanation of variations of 1 H NMR chemical shifts in benzenes bearing an electron‐donating (NH 2 ) or an electron‐withdrawing (NO 2 ) 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 NH 2 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.