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 H NMR chemical shifts in benzenes bearing an electron-donating (NH ) or an electron-withdrawing (NO ) 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 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.

Importance of the Amino Moiety of the Hydrazone Group in the Electrochemical and UV‐Vis Absorption Properties of Simple Isomeric Hydrazones

“…[21] In turn, signals 5 and 4a are shifted from 7.99 and 7.89 ppm in 1-E to 7.40 and 7.17 ppm in 1-Z, respectively, given that, in the Z configuration, the corresponding protons are no longer deshielded by the in-plane lone pairs of electrons belonging to the nitrogen of the imine-like group and one of the pyridine rings. [22] In the case of 1-Me, which we synthesized for comparing its electrochemical properties with those of 1-E and 1-Z, the aromatic signals of its 1 H NMR spectrum remind those of 1-E. As expected, the broad singlet assignable to the proton of the NÀ H group is not observed at all, but instead a sharp singlet at 3.70 ppm is observed owing to the methyl group (see Figure 3C).…”

“…In particular, the signal belonging to the N−H group is shifted from 9.10 ppm in 1‐E to 14.06 ppm in 1‐Z because this group is involved in a very strong intramolecular hydrogen bond [21] . In turn, signals 5 and 4a are shifted from 7.99 and 7.89 ppm in 1‐E to 7.40 and 7.17 ppm in 1‐Z , respectively, given that, in the Z configuration, the corresponding protons are no longer deshielded by the in‐plane lone pairs of electrons belonging to the nitrogen of the imine‐like group and one of the pyridine rings [22] . In the case of 1‐Me , which we synthesized for comparing its electrochemical properties with those of 1‐E and 1‐Z , the aromatic signals of its 1 H NMR spectrum remind those of 1‐E .…”

A theoretical and electrochemical impedance spectroscopy study of the adsorption and sensing of selected metal ions by 4-morpholino-7-nitrobenzofuran

“…Due to the presence of the nitro group (NO 2 ) in the ortho-position of this hydrogen atom, the H19 proton had a higher unshielded signal than the H20 proton. Due to the strong electron delocalization in the benzene ring, these protons resonate at a slightly lower field than those cited in the literature [ 63 ]. The aromatic ring protons detected in this range are attributed to a potent ICT that involves the thoroughly examined molecule.…”

N‐Carbon‐Doped Binary Nanophase of Metal Oxide/Metal–Organic Framework for Extremely Sensitive and Selective Gas Response

“…This downfield shift is attributed to the highly electrophilic nature of the NO 2 groups, leading to the deshielding of the carbon atoms. [ 58 ] The 15 N spectra, on the other hand, discernably exhibit only a single peak at 170 ppm denoted with a *, likely due to the high signal‐to‐noise ratio, which complicates the resolution of multiple peaks for nitrogen atoms in the sample (Figure 3d). Nonetheless, the signal position resembles the hydrogen‐terminated N 1″ of the 2‐MeIM linker, potentially signifying the cleavage of Zn─N bonds (Figure 3d).…”