The 'Variable Electronegativity ' Method. II. Pyrrole

1959 ◽  
Vol 12 (3) ◽  
pp. 319 ◽  
Author(s):  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Dipole Moment ◽  
Molecular Orbital ◽  
Electron System ◽  
Orbital Method ◽  
Molecular Orbital Method ◽  
Consistent Field ◽  
Inductive Effects ◽  
Self Consistent Field ◽  
Negligible Contribution ◽  
Good Agreement

The properties of the π-electron system in pyrrole have been studied by a " variable electronegativity " self-consistent field molecular-orbital method and the results compared with those obtained by the conventional SCF procedure. The π-electron distribution calculated by the conventional SCF procedure cannot be satisfactorily reconciled with the observed dipole moment, but the distribution calculated by the VESCF method leads to a predicted dipole moment in good agreement with observation. Polarization of σ-bonds makes a negligible contribution to the dipole moment.Derivation of the coulomb parameters for the simple H�ckel molecular-orbital method from the VESCF results is considered and the factors responsible for auxiliary inductive effects are discussed. The π-electron ionization potential and the positions of the lowest excited states of pyrrole have been calculated by the VE method.

Electronic structure of the first row hydrides BH, CH, NH, OH and FH. I. Ground states

1958 ◽  
Vol 248 (1252) ◽  
pp. 119-135 ◽  
Keyword(s):  
Molecular Orbital ◽  
Electron Correlation ◽  
Dipole Moments ◽  
Valence Bond ◽  
Orbital Method ◽  
Constant Amount ◽  
Consistent Field ◽  
Valence Electrons ◽  
Self Consistent Field ◽  
Experimental Values

Some recent calculations by the self-consistent field molecular orbital method are generalized to allow for electron correlation. Correlations between the motions of the valence electrons are introduced explicitly by means of configuration interaction, whilst the effects of intra-atomic electron correlation are estimated semi-empirically. Both forms of correlation, but especially the latter, are found to have a profound effect on the calculated properties of the hydrides. The total electronic energies obtained in the final calculations fall consistently above the experimental values by an almost constant amount (0.5 to 0.7 eV). The wave functions and dipole moments of the molecules are analyzed in the frameworks of both the valence-bond and molecular orbital theories.

Theoretical study on the diffusion of gases in hexagonal ice by the molecular orbital method

2003 ◽  
Vol 81 (1-2) ◽  
pp. 251-259 ◽  
Author(s):  
A Hori ◽  
T Hondoh
Keyword(s):  
Molecular Orbital ◽  
Lower Energy ◽  
Theoretical Study ◽  
Orbital Method ◽  
Molecular Orbital Method ◽  
Hexagonal Ice ◽  
Diffusion Constants ◽  
Ice Lattice ◽  
Semi Empirical ◽  
Good Agreement

To estimate the diffusion constants for various gases in ice, the barrier energies during interstitial diffusion are calculated for model ice clusters by the molecular orbital method. For He and Ne, the calculated values for diffusion along the c-axis were 0.11 and 0.26 eV, respectively. These are in good agreement with the experimental results. However, the calculated values for the diffusion of He perpendicular to the c-axis are not in close agreement with the experimental data. The barrier energies for O2, N2, and CH4 were calculated by the semi-empirical molecular orbital method and estimated to be 0.35, 0.47, and 0.75 eV, respectively. The lower energy for O2 in comparison with N2 is attributed to the formation of a quasi chemical bond between the O2 molecule and the ice lattice. The diffusion constants for O2, N2, and CH4 were estimated to be 1.8 x 10–11, 2.5 x 10–12, and 2.0 x 10–14 m2s–1, respectively. PACS Nos.: 31.15Ar, 31.15Ne, 66.30Jt, 66.30Ny

Theoretical Studies of some Nonbenzenoid Hydrocarbons. II

1974 ◽  
Vol 52 (1) ◽  
pp. 155-166 ◽  
Author(s):  
Archana DasGupta ◽  
Nadna K. DasGupta
Keyword(s):  
Molecular Orbital ◽  
Configuration Interaction ◽  
Resonance Integral ◽  
Theoretical Studies ◽  
Molecular Orbital Study ◽  
Consistent Field ◽  
Orbital Study ◽  
Self Consistent ◽  
Self Consistent Field ◽  
Good Agreement

A semiempirical self-consistent field molecular orbital study has been made on some nonbenzenoid hydrocarbons using a resonance integral value proposed by Lo and Whitehead (2, 4), Chung and Dewar (3), Dewar and Harget (7), and Yamaguchi et al. (6) without using configuration interaction. It has been observed that spectral transitions calculated using the β-value proposed by Lo and Whitehead and Chung and Dewar are in good agreement with experiment and comparable to those calculated by the β-value proposed by Yamaguchi et al. and Dewar et al.

The chemistry of Pyridine, Pyrimidine, and Pyrazine

1956 ◽  
Vol 9 (1) ◽  
pp. 83 ◽  
Author(s):  
RD Brown ◽  
ML Heffernan
Keyword(s):  
Molecular Orbital ◽  
Nitrogen Heterocycles ◽  
Chemical Properties ◽  
Detailed Comparison ◽  
Orbital Method ◽  
Molecular Orbital Method ◽  
Good Agreement

The results of a study of pyridine, pyrimidine, and pyrazine by the molecular- orbital method are reported, and a detailed comparison is made with the chemical properties of these molecules. Good agreement is found, indicating that the present theoretical technique is satisfactory for interpreting and predicting the chemical properties of nitrogen heterocycles.

A theoretical study of the chemisty of Furan, Phrrole, Benzofuran, Indole, Dibenzofuran, and Carbazole

1959 ◽  
Vol 12 (2) ◽  
pp. 152 ◽  
Author(s):  
RD Brown ◽  
AW Coller
Keyword(s):  
Molecular Orbital ◽  
Orbital Method ◽  
Field Calculation ◽  
The Core ◽  
Oxygen Heterocycles ◽  
Inductive Effects ◽  
Electron Distributions ◽  
Self Consistent Field ◽  
Special Case ◽  
Coulomb Parameter

It is shown that it is possible, within the framework of the simple H�ckel molecular- orbital method, to account fully for all observations on electrophilic substitution in furan, pyrrole, benzofuran, indole, dibenzofuran, and carbazole, in terms of the π-electron distributions in these compounds. The values of coulomb parameters required to obtain this correlation are physically reasonable and in particular are in agreement with values found independently by a variable electronegativity self-consistent field calculation on pyrrole. The previously puzzling difference in orientation of substitution in benzofuran and indole is accounted for in the present study in terms of a difference in the auxiliary inductive effects of the two hetero-atoms. It seems possible to attribute this difference ultimately to differences in the CO and CN bond lengths and to differences in the effective nuclear charges for 2pπ atomic orbitals on the two hetero-atoms. Attention is drawn to an " isoprotonic principle " for estimating the value of the primary coulomb parameter of a hetero-atom. (It is probably a special case of a more general principle-that the primary coulomb parameter is predominantly determined by the charge on the core and scarcely depends upon the nature of the nucleus at the centre of the core.) A spurious H�ckel molecular-orbital model for the oxygen heterocycles is noted. It accounts for the chemistry of the heterocycles but represents a physically unsatisfactory view of the electronic properties of oxygen in these compounds.

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