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.

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.

Purine analogues. I. The status of Hückel molecular orbital calculations as predictors of proton shifts, basic strengths, and reactivity

1969 ◽  
Vol 47 (7) ◽  
pp. 1129-1138 ◽  
Author(s):  
Brian M. Lynch ◽  
Allan J. Robertson ◽  
John G. K. Webb
Keyword(s):  
Nucleophilic Substitution ◽  
Molecular Orbital ◽  
Chemical Shifts ◽  
Ionic Species ◽  
Membered Ring ◽  
Molecular Orbital Calculations ◽  
Electron Densities ◽  
Purine Analogues ◽  
The Status ◽  
Proton Chemical Shifts

A detailed series of molecular orbital calculations based on the HMO method has been made for the various possible ionic species of purine, pyrazolo(3,4-d)pyrimidine, v-triazolo(4,5-d)pyrimidine, and pyrazolo(3,4-b)pyridine. π-Electron densities and localization and delocalization energies for nucleophilic substitution have been derived.The results are compared with the observed proton chemical shifts in the conjugate acids of these molecules, with the relative rates of nucleophilic piperidinodehalogenations in the neutral molecules, and with the ionization constants.It is concluded that it is possible to reconcile the calculations with experimental results for the various positions within a six-membered ring, but that positions in six- and five-membered rings cannot be directly compared. The electron densities seem to be of little value in correlating the observed ionization patterns of purines and their analogues.

Correlation of the chemical shifts with ?-electron densities in mono-derivatives of benzene, pyridine, and pyrazine

1968 ◽  
Vol 17 (5) ◽  
pp. 951-958
Author(s):  
G. G. Dvoryantseva ◽  
V. P. Lezina ◽  
V. F. Bystrov ◽  
T. N. Ul'yanova ◽  
G. P. Syrova ◽  
...  
Keyword(s):  
Chemical Shifts ◽  
Electron Densities ◽  
Derivatives Of

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.

The Variable Electronegativity Method. VII. Pyrazole, Its Anion and Cation

1960 ◽  
Vol 13 (1) ◽  
pp. 49 ◽  
Author(s):  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Dipole Moment ◽  
Electron Distribution ◽  
Nitrogen Heterocycles ◽  
Chemical Properties ◽  
Membered Ring ◽  
Matrix Elements ◽  
Electron Densities ◽  
Ring Nitrogen ◽  
Made In ◽  
Tertiary Nitrogen

Results of a VESCF treatment of pyrazole, its anion, and cation are reported. A comparison is made of the chemical properties of pyrazole and those predicted from the calculated π-electron densities. An ambiguity in the comparison owing to the rapid tautomerization of pyrazole is emphasized. The calculated π-electron distribution in the anion supports the suggestion made in previous papers that the relative electronegativities of carbon and tertiary nitrogen reverse when their π-electron densities exceed 1.2. The dipole moment is predicted to be about 2.5 D for pyrazole, in agreement with observations in solution. Theoretical values of ionization potentials are also presented. An analysis is made of VESCF matrix elements and some empirical guides as to suitable values of coulomb and resonance parameters for five-membered ring nitrogen heterocycles are suggested.

Excited-State Proton Transfer: Molecules in a Hurry to Get Rid of Antiaromaticity

2019 ◽  
Author(s):  
Chia-Hua Wu ◽  
Lucas Karas ◽  
Henrik Ottosson ◽  
Judy Wu
Keyword(s):  
Excited State ◽  
Proton Transfer ◽  
Chemical Shifts ◽  
Proton Relay ◽  
Proton Transfers ◽  
Bifunctional Compounds ◽  
Excited State Proton Transfer ◽  
Derivatives Of ◽  
Related Compounds

<p>Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4<i>n</i>+2] π-aromatic in the ground state, become [4<i>n</i>+2] π-antiaromatic in the first <sup>1</sup>ππ* states, and proton transfer (either<i>inter-</i>or <i>intra-</i>molecularly) helps relieve excited-state antiaromaticity. Computed nucleus independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. <i>o-</i>Salicylic acid undergoes ESPT only in the “antiaromatic” S<sub>1</sub>(<sup>1</sup>ππ*) state, but not in the “aromatic” S<sub>2</sub>(<sup>1</sup>ππ*) state. Stokes’ shifts of structurally-related compounds (<i>e.g.</i>, derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with pro tic substrates) vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation.</p>

Proton chemical shifts and electron densities

1962 ◽  
Vol 34 ◽  
pp. 18 ◽  
Author(s):  
B. P. Dailey ◽  
Albert Gawer ◽  
W. C. Neikam
Keyword(s):  
Chemical Shifts ◽  
Electron Densities ◽  
Proton Chemical Shifts

Proton chemical shifts and ?-electron densities

1968 ◽  
Vol 9 (4) ◽  
pp. 540-544
Author(s):  
R. M. Aminova ◽  
I. D. Morozova
Keyword(s):  
Chemical Shifts ◽  
Electron Densities ◽  
Proton Chemical Shifts
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