Proton chemical shifts and electron densities in aromatic and heteroaromatic molecules. I. Procedure and chemical shift corrections; Applications to azines

1967 ◽  
Vol 20 (7) ◽  
pp. 1305 ◽  
Author(s):  
PJ Black ◽  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Chemical Shift ◽  
Electric Field ◽  
Electron Distribution ◽  
Theoretical Calculations ◽  
Chemical Shifts ◽  
Lone Pair ◽  
Excess Charge ◽  
Heteroaromatic Compounds ◽  
Electric Field Dependence ◽  
Proton Chemical Shifts

In this series, the observed proton chemical shifts of heteroaromatic compounds are used to obtain estimates of the electron distribution in these systems. These estimates are deduced from the measured shifts after applying corrections for the shielding contributions arising from ring currents, magnetic anisotropy of neighbouring atoms, and the electric field from the lone-pair dipoles on heteroatoms, in a manner closely related to that of Gil and Murrell. The corrected chemical shifts are then used to deduce an ?experimental? π-electron distribution via the electric field dependence of the proton shifts on the excess charge at all ring positions. However, in most cases, there are not enough experimental shifts to determine uniquely the excess charge at each ring atom, and it is necessary to assume that some inaccessible charges are close to values obtained from theoretical calculations. With these limitations, a comparison is made between the excess charges deduced in the above fashion from chemical shift data on mono- and poly-cyclic azines and diazines and those obtained by theoretical calculations using the BJ VESCF method. It is emphasized that this interpretation is tentative and that the extent of σ-electron polarization is not well understood at present.

Evidence for the existence of an electric field effect from the vinyl proton magnetic resonance spectra of 4-substituted styrenes

1968 ◽  
Vol 46 (24) ◽  
pp. 3813-3820 ◽  
Author(s):  
G. K. Hamer ◽  
W. F. Reynolds
Keyword(s):  
Chemical Shift ◽  
Electric Field ◽  
Field Effect ◽  
Chemical Shifts ◽  
Electric Field Effect ◽  
Vinyl Proton ◽  
Concentration Effects ◽  
Electric Field Effects ◽  
Field Effects ◽  
Proton Chemical Shifts

Vinyl proton chemical shifts of styrene and six 4-substituted styrenes have been determined at infinite dilution in cyclohexane. It is shown that changes in the chemical shift difference of the β protons, Δ(δC − δB) can be accounted for by electric field effects. Reasonable values of the constant in the Buckingham equation of(−3.11 ± 0.50) × 10−12 and (−4.77 ± 0.83) × 10−12 are obtained from two different types of field effect calculations. Residual chemical shift changes for β protons after correction for electric field effects can be explained in terms of mesomeric and possibly inductive mechanisms. α-Proton chemical shift values cannot be satisfactorily rationalized. Small concentration effects are noted, usually resulting in high field shifts with increasing concentration. Previous results are reexamined in order to resolve a conflict in the literature.

Proton chemical shifts and electron densities in aromatic and heteroaromatic molecules. II. Derivatives of pyrrole and furan

1967 ◽  
Vol 20 (7) ◽  
pp. 1325 ◽  
Author(s):  
PJ Black ◽  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Electron Distribution ◽  
Chemical Shifts ◽  
Membered Ring ◽  
Electron Densities ◽  
Heteroaromatic Compounds ◽  
Derivatives Of ◽  
Proton Chemical Shifts

In this paper, the methods outlined in Part I are used to estimate the electron distribution from the proton chemical shifts in a series of five-membered ring heteroaromatic compounds. The systems studied are pyrrole and furan, their benzo and dibenzo derivatives, indazole, indolizine, the azaindolizines, and purine. It is found that the estimated excess charges are not as closely represented by the BJ VESCF method as was the case for the azine derivatives and that the role of the σ-electrons is in urgent need of closer investigation.

Substituent-induced 1H chemical shifts for 4-substituted (2, 2-dichlorocyclopropyl)benzenes: further evidence for intramolecular electric field effects on 1H chemical shifts

1977 ◽  
Vol 55 (3) ◽  
pp. 530-535 ◽  
Author(s):  
Robert H. Kohler ◽  
William F. Reynolds
Keyword(s):  
Electric Field ◽  
Field Effect ◽  
Ring Current ◽  
Chemical Shifts ◽  
Basic Pattern ◽  
Electric Field Effects ◽  
Field Effects ◽  
The Individual ◽  
Induced Changes ◽  
Proton Chemical Shifts

Correlations of cyclopropyl proton chemical shifts for 4-substituted (2,2-dichlorocyclopropyl)-benzenes with σI and σR0 provide evidence that these chemical shifts reflect direct field effects and weak phenyl–cyclopropyl conjugative interactions. Corrections for variable ring current effects due to substituent-induced changes in conformation improve the individual correlations but do not alter the basic pattern of results. Correlation of the β cyclopropyl 1H chemical shift difference with σI with calculated hydrogen electron densities, and with calculated electric field components provides further strong evidence for a field effect on 1H chemical shifts.

Elektronendichte und chemische Verschiebung der Styryldiazin- und Diazaphenanthrenprotonen / Electrondensity and Proton Chemical Shift of Styryldiazines and Diazaphenanthrenes

1972 ◽  
Vol 27 (2) ◽  
pp. 310-319
Author(s):  
H.-H. Perkampus ◽  
Th. Bluhm ◽  
J. Knop
Keyword(s):  
Ring Current ◽  
Chemical Shifts ◽  
Lone Pair ◽  
Current Effect ◽  
Electron Densities ◽  
Paramagnetic Effect ◽  
Nitrogen Lone Pair ◽  
Chemische Verschiebung ◽  
Lone Pair Electrons ◽  
Proton Chemical Shifts

AbstractProton chemical shifts in styryldiazines and diazaphenanthrenes linearly correlate with SCF-π-electron densities of the attached carbon atom and with the electron densities of the hydrogen atom (calculated by the CNDO/2 method). The observed deviations from linearity are discussed in terms of ring current effect, steric effects and the paramagnetic effect of the nitrogen lone pair electrons. An appreciable weakening of ring current is found for diazaphenanthrenes with two adjacent N-atoms. Under the same condition the paramagnetic effect on ortho-hydrogens is increased.

Chemical Shift. VI. The Long-range Shielding Effects of Ethylene–Ketal and −Thioketal Groups

1971 ◽  
Vol 49 (9) ◽  
pp. 1335-1338 ◽  
Author(s):  
J. W. ApSimon ◽  
H. Beierbeck ◽  
D. K. Todd ◽  
P. V. Demarco ◽  
W. G. Craig
Keyword(s):  
Chemical Shift ◽  
Electric Field ◽  
Long Range ◽  
Field Effect ◽  
Chemical Shifts ◽  
Electric Field Effect ◽  
Electric Dipoles ◽  
Derivatives Of ◽  
Shielding Effects

The calculation of chemical shift values by the method used in parts I–V (1–5) has been extended to a derivation of the shielding effects of the ethylene–ketal and −thioketal groups. For these studies ketal and thioketal derivatives of monoketoandrostanes were prepared. The chemical shifts of the C-18 and -19 methyl protons in these compounds are reported for the solvents CDCl3, CCl4 and benzene.Representing both groups by point dipoles, values for the anisotropies and for K, a parameter descriptive of the electric field effect, were derived for various, coincidental, locations of the magnetic and electric dipoles along the symmetry axes of the two groups.

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