Interpretation of the proton and nitrogen chemical shifts of pyridine and pyridinium cation

Abstract

A semi-quantitative estimate has been made of the contributions to the proton and nitrogen chemical shifts in pyridine and the pyridinium cation. The difference between the nitrogen chemical shifts in Py and PyHf is due mainly to the large paramagnetic term arising from the n+z* transition of pyridine. The contributions to the proton chemical shifts in Py due to the magnetic anisotropy of the nitrogen atom, and the polarization of the C-H bonds by the nitrogen lone pair, have been calculated. The remaining part of the chemical shifts relative to benzene is found to be well correlated with s.c.f. calculations of the T electron densities.

Synthesis, spectroscopic and electrochemical studies of a series of transition metal complexes with amino- or bis(bromomethyl)-substituted dppz-ligands: Building blocks for fullerene-based donor–bridge–acceptor dyads

“…When the substituted dppz ligands are coordinated to Re in the [Re(CO) 3 (L)Cl] complexes (L = dppz ligand) the phen fragment protons are dominated by the effect of coordination. These protons show a significant down-field shift reflecting the change in nitrogen atom hybridization because of coordination with the subsequent deshielding effect on the protons [21]. The phenazine protons are also displaced to lower field, but the effect of the dppz substituents is retained.…”

Impedance Spectroscopy Study of the AC Conductivity and Dielectric properties of 3, 5-Dimethylpyridine-Iodine Charge Transfer Complex

“…The signals due to protons of pyridine ring at a-& g-positions were upfield which are due to the increase of the shielding of these protons as well as of the change of the magnetic environment on complex formation. The observed shifting of the chemical shifts values of the ring protons of pyridine is governed by two factors; (a) magnetic anisotropy of the N atom and (b) field associated with the nitrogen atomic dipole 19 . The displacement is more pronounced for the a-protons as these carbons are directly connected with the n-donor nitrogen atom.…”

Effect of solvent and anion upon the proton magnetic resonance spectrum of the 1-methylpyridinium ion

“…It has been concluded that proton chemical shifts for the pyridinium ion are partly determined by the effect of the counter ion and by the effect of the electric field of the N-H+ group (9,16). Similarly cation-anion and cationsolvent interactions have a significant effect upon proton chemical shifts in 1,4-diethylpyridinium halides (17).…”

“…The method of estimating n electron densities in aromatic molecules and ions from proton chemical shift measurements has been widely applied (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). It was originally assumed that there was a linear relationship between excess n electron density at a carbon atom and the chemical shift of the adjacent proton relative to internal benzene (1).…”