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.

The Variable Electronegativity Method. V. Pyridine, the Pyridinium Cation, and the Evaluation of Core-Attraction Integrals

1959 ◽  
Vol 12 (4) ◽  
pp. 554 ◽  
Author(s):  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Dipole Moment ◽  
Electron Distribution ◽  
Chemical Properties ◽  
Theoretical Treatment ◽  
Matrix Elements ◽  
Molecular Dipole ◽  
Theoretical Methods ◽  
Pyridinium Cation ◽  
Electron Repulsion Integrals ◽  
Better Than

A study of pyridine and the pyridinium cation by the VESCF method is reported. Several different methods of evaluating core attractions and electron repulsion integrals have been tried to determine their effect on the resultant energies and orbitals. Satisfactory results for the π-electron distribution and molecular dipole moment of pyridine are obtained. Values of ionization potentials are little better than those given by other theoretical methods and the variable electronegativity technique does not appear to improve the theoretical treatment of ultraviolet spectra. H�ckel MO parameters, evaluated from the VESCF matrix elements, accord with values found in similar studies of other heterocycles and also with values of parameters found necessary to account for the chemical properties of quinoline.

The ' Variable Electronegativity ' Method. III. The Pyrrole Anion and Electronegativity Reversal

1959 ◽  
Vol 12 (3) ◽  
pp. 330 ◽  
Author(s):  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Electron Density ◽  
Electron Distribution ◽  
Ionization Potentials ◽  
Alternative Methods ◽  
Electron Densities ◽  
Tertiary Nitrogen

The π-electron distribution in the pyrrole anion has been evaluated by the VESCF method using two alternative methods of estimating core attraction terms. The results indicate that the π-electron distribution around the conjugated system is very nearly uniform, supporting previous speculations that the relative attracting powers of tertiary nitrogen and carbon reverse when their π-electron densities exceed a certain value, suspected to be not too much greater than unity. The present calculations indicate that the critical π-electron density is around 1.2. The VESCF estimates of ionization potentials of the pyrrole anion are also reported.

scholarly journals Synthesis and properties of seven- to nine-membered ring nitrogen heterocycles. Cyclic amidines and cyclic amidinium salts

ARKIVOC ◽  
2018 ◽  
Vol 2018 (1) ◽  
pp. 288-318 ◽  
Author(s):  
Isabel A. Perillo ◽  
María C. Caterina ◽  
Alejandra Salerno
Keyword(s):  
Nitrogen Heterocycles ◽  
Membered Ring ◽  
Ring Nitrogen

Xanthate Transfer Cyclisation of Glycine Radicals; Synthesis of 5- and 6-Membered Ring Nitrogen Heterocycles

2010 ◽  
Vol 103 (7-8) ◽  
pp. 329-342 ◽  
Author(s):  
Jan H. Udding ◽  
J. P. M. Giesselink ◽  
Henk Hiemstra ◽  
W. Nico Speckamp
Keyword(s):  
Nitrogen Heterocycles ◽  
Membered Ring ◽  
Ring Nitrogen

The Variable Electronegativity Method. IV. Glyoxaline, Its Cation and Anion

1959 ◽  
Vol 12 (4) ◽  
pp. 543 ◽  
Author(s):  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Dipole Moment ◽  
Electrophilic Substitution ◽  
Electron Distribution ◽  
Chemical Properties ◽  
Ionization Potentials ◽  
Substitution Reactions ◽  
Resonance Parameters ◽  
Electron Distributions ◽  
Electrophilic Substitution Reactions ◽  
Set Up

Results of VESCF calculations of the π-electron distribution and ionization potentials in glyoxaline and its cation and anion are reported. The electron distributions in these systems are in accordance with the electrophilic substitution reactions observed for glyoxaline and bear out the concept of electronegativity reversal in the anion previously suggested to account for chemical properties of glyoxaline. The dipole moment of glyoxaline, evaluated from the calculated π-electron distribution, agrees with the observed moment, providing further vindication for the VESCF method and support for the suggestion that σ-bond polarizations in heterocycles are slight. Values of the Huckel coulomb and resonance parameters required to make the H�ckel method reproduce the VESCF results for these heterocycles are considered in some detail and it seems feasible to set up empirical rules for selecting appropriate values for the coulomb parameters.

scholarly journals Regioselectivity and Tautomerism of Novel Five-Membered Ring Nitrogen Heterocycles Formed via Cyclocondensation of Acylthiosemicarbazides

Molecules ◽  
2008 ◽  
Vol 13 (3) ◽  
pp. 501-518 ◽  
Author(s):  
Jana Tomaščiková ◽  
Ján Imrich ◽  
Ivan Danihel ◽  
Stanislav Böhm ◽  
Pavol Kristian ◽  
...  
Keyword(s):  
Nitrogen Heterocycles ◽  
Membered Ring ◽  
Ring Nitrogen

Benzene isomers. II. 3,4-Dimethylenecyclobutene

1968 ◽  
Vol 21 (7) ◽  
pp. 1807 ◽  
Author(s):  
BAW Coller ◽  
ML Heffernan ◽  
AJ Jones
Keyword(s):  
Dipole Moment ◽  
Chemical Shifts ◽  
Current Model ◽  
Chemical Properties ◽  
Membered Ring ◽  
Thermal Rearrangement ◽  
Local Anisotropy ◽  
Satellite Spectrum ◽  
Anisotropy Model ◽  
Physical And Chemical

Theoretical considerations indicate that the electron distribution in 3,4,-dimethylenecyclobutene is non-uniform and a dipole moment of 0.71 D has been predicted by inclusion of non-neighbour core resonance integrals. Preliminary studies of the microwave spectrum of this compound provide a dipole moment of 0.618 � 0.011 D. The term "pseudo-alternant" is proposed to describe this phenomenon. In the present paper additional physical and chemical properties of 3,4,-dimethylene- cyclobutene are described. The observed low-field proton chemical shifts for 3,4-dimethylenecyclobutene are not accounted for using a ring current model and a small calculated paramagnetic current indicates properties usually associated with H�ckel 4n-hydrocarbons. A local anisotropy model provides a better account of the observed shift. The carbon-13- proton satellite spectrum of 3,4-dimethylenecyclobutene indicates similarities in the geometry of the four-membered ring to that in cyclobutene. In addition, infrared and ultraviolet spectra support the conclusion that this hydrocarbon be regarded as a cross-conjugated diene. A detailed discussion of the products arising from the bromination of 3,4-dimethylenecyclobutene under a variety of conditions indicates consecutive 1,4-addition of bromine across the four-membered ring, a property which has been associated with cross-conjugation. Hydrogenolysis, chlorination, iodination, and attempted Diels-Alder addition reactions are also described. 3,4-Dimethylenecyclobutene was prepared by the thermal rearrangement of hexa-1,5-diyne. The mechanisms of thermal and photolytic rearrangement of hexa-1,5-diyne are discussed using Woodward-Hoffmann postulates. 1,2-Dideutero-,1-ethyl-, and 1-n-propyl-3,4-dimethylenecyclobutene were also prepared by the thermal rearrangement of the corresponding diyne. In addition, the thermal rearrangement of hexa-1,5-diyne over the temperature range 290-700� to give variable quantities of 3,4-dimethylenecyclobutene, fulvene, and benzene is described. 3,4- Dimethylenecyclobutene is thermally rearranged to benzene at 680�.

ChemInform Abstract: Xanthate Transfer Cyclization of Glycine Radicals; Synthesis of 5- and 6-Membered Ring Nitrogen Heterocycles.

ChemInform ◽  
2010 ◽  
Vol 26 (14) ◽  
pp. no-no
Author(s):  
J. H. UDDING ◽  
J. P. M. GIESSELINK ◽  
H. HIEMSTRA ◽  
W. N. SPECKAMP
Keyword(s):  
Nitrogen Heterocycles ◽  
Membered Ring ◽  
Ring Nitrogen
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