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.

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.

Molecular orbital structures of sulfones

1982 ◽  
Vol 60 (6) ◽  
pp. 730-734 ◽  
Author(s):  
Russell J. Boyd ◽  
Jeffrey P. Szabo
Keyword(s):  
Crystal Structure ◽  
Gas Phase ◽  
Molecular Orbital ◽  
Geometry Optimization ◽  
Membered Ring ◽  
Molecular Orbital Calculations ◽  
Bond Lengths ◽  
Comparison With Experiment ◽  
The Difference

Abinitio molecular orbital calculations are reported for several cyclic and acyclic sulfones. The geometries of XSO2Y, where X, Y = H, F, or CH3 are optimized at the STO-3G* level. Similar calculations are reported for the smallest cyclic sulfone, thiirane-1,1 -dioxide, as well as the corresponding sulfoxide, thiirane-1-oxide, and the parent sulfide, thiirane. Where comparison with experiment is possible, the agreement is satisfactory. In order to consider the possibility of substantial differences between axial and equatorial S—O bonds in the gas phase, as observed in the crystal structure of 5H,8H-dibenzo[d,f][1,2]-dithiocin-1,1-dioxide, STO-3G* calculations are reported for a six-membered ring, thiane-1,1-dioxide, and a model eight-membered ring. Limited geometry optimization of the axial and equatorial S—O bonds in the chair conformations of the six- and eight-membered rings leads to bond lengths of 1.46 Å with the difference being less than 0.01 Å.

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.

Octahedral point-charge model and its application to fragment molecular orbital calculations of chemical shifts

2014 ◽  
Vol 593 ◽  
pp. 165-173 ◽  
Author(s):  
Qi Gao ◽  
Satoshi Yokojima ◽  
Dmitri G. Fedorov ◽  
Kazuo Kitaura ◽  
Minoru Sakurai ◽  
...  
Keyword(s):  
Molecular Orbital ◽  
Point Charge ◽  
Chemical Shifts ◽  
Molecular Orbital Calculations ◽  
Point Charge Model ◽  
Fragment Molecular Orbital ◽  
Charge Model

Substitutent Effects on the Geometrical Properties of 1-Phenylallyl Alcohol

2005 ◽  
Vol 60 (4) ◽  
pp. 265-270
Author(s):  
Salim Y. Hanna ◽  
Salim M. Khalil ◽  
Moafaq Y. Shandala
Keyword(s):  
Electron Density ◽  
Molecular Orbital ◽  
Substituent Effects ◽  
Heats Of Formation ◽  
Geometrical Parameters ◽  
Molecular Orbital Calculations ◽  
Electron Densities ◽  
Geometrical Properties ◽  
Stabilization Energies

Abstract Optimized geometrical parameters, electron densities, heats of formation and stabilization energies have been obtained on X-substituted phenylallyl alcohols, where X is H, OCH3, NH2, CN, F and CH3 at ortho, meta, and para positions, using MINDO-Forces SCF-molecular orbital calculations. The substituent effects on the geometrical parameters and the electron density are discussed.

An investigation into the origins of polar substituent effects upon 19F chemical shifts, using 4-substituted β,β-difluorostyrenes

1980 ◽  
Vol 58 (8) ◽  
pp. 839-845 ◽  
Author(s):  
William F. Reynolds ◽  
Victoria G. Gibb ◽  
Nick Plavac
Keyword(s):  
Chemical Shift ◽  
Electron Density ◽  
Molecular Orbital ◽  
Excellent Agreement ◽  
Field Effect ◽  
Chemical Shifts ◽  
Substituent Effects ◽  
Molecular Orbital Calculations ◽  
Chemical Shift Difference ◽  
Polar Substituent

19F, 13C, and 1H chemical shifts have been determined for β,β-difluorostyrene and eight 4-substituted derivatives. The β-fluorine chemical shift difference, ΔδF, is used to evaluate the constant in the Buckingham equation. A = 3.0 × 10−11 esu for C—F bonds which is in excellent agreement with the value derived by Adcock and Khor. This allows accurate estimates of direct field effect contributions to 19F chemical shifts in aryl fluorides. Substituent parameter correlations demonstrate that the primary polar effect on 19F chemical shifts is field-induced π polarization. Abinitio molecular orbital calculations confirm that the substituent-induced 19F chemical shifts reflect changes in fluorine π electron density.

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