Polar structures in the theory of conjugated molecules I. Identification of the ethylene π-electron states

This and two of three parts to be published subsequently are concerned mainly with the so-called valence-bond theory of conjugated and aromatic molecules. An improvement to the method is described, which consists in adding to the usual set of structures some extra ones which are ‘polar’ in the sense that they show two of the π-electrons on one centre, and none on another centre, making these two centres carry respectively negative and positive charges. This adds a certain flexibility to the description of molecular states which is lacking when the electrons are supposed to be distributed one to each centre throughout. In this part a preliminary question is treated which bears on getting the new empirical parameters needed for including polar structures in the theory. This question is the assignment of the long wave-length bands in the spectrum of ethylene. The assignment is made with the help of a theoretical study of the ethylene energy levels in an approximation using antisymmetric molecular orbitals. Using this calculation as a guide, two transitions are assigned. A weak band, appearing at about 2000 Å, is taken to be 1 A g - 1 A g9 and a strong one, having its maximum at about 1630 Å, is taken to be 1 A g – 1 B lu .

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Polar structures in the theory of conjugated molecules. II. Symmetry properties and matrix elements for polar structures

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1950.0024 ◽

1950 ◽

Vol 200 (1062) ◽

pp. 390-401 ◽

Cited By ~ 18

Keyword(s):

Valence Bond ◽

Matrix Elements ◽

Conjugated Molecules ◽

Energy Calculations ◽

Valence Bond Theory ◽

Energy Parameters ◽

Symmetry Properties ◽

The Matrix ◽

Bond Theory ◽

Usual Formalism

The inclusion of polar structures in the valence-bond theory of π-electrons entails some additions to the usual formalism, and these are given in this part. The symmetry properties of sets of structures, both non-polar and polar, and the matrix elements that come into energy calculations, are dealt with. Using the work of part I, and the conclusions of this part, the energy parameters for work with polar structures are evaluated.

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Theoretical Study on Pyramidal C7N6–H3R3 Molecules

Australian Journal of Chemistry ◽

10.1071/ch19015 ◽

2019 ◽

Vol 72 (7) ◽

pp. 501 ◽

Cited By ~ 1

Author(s):

Bing He ◽

Bingke Li ◽

Hongwei Zhou

Keyword(s):

Organic Chemistry ◽

Theoretical Study ◽

Natural Bond Orbital ◽

Valence Bond ◽

Reactive Site ◽

Valence Bond Theory ◽

The Core ◽

Structures And Properties ◽

Bonding Properties ◽

Bond Theory

The pyramidal molecule C7N6H6 and its nine symmetric tri-substituted derivatives C7N6–H3R3 (R=OH, F, CN, N3, NH2, NO2, N=NH, N2H3, and C≡CH) were investigated computationally using the GAUSSIAN 09 program package. Natural bond orbital and atoms in molecules analyses, as well as valence bond theory were applied to investigate the bonding properties. In comparison to their well known analogues C6N7–R3, i.e. generic heptazines, it is found that these 10 molecules are all reactive. Further studies on the topological structures and ionization energy values indicate that the reactive site of the molecules is located at the carbon atom of the core frame. Even though C7N6–H3R3 are neutral molecules, the structures and properties of some are consistent with those of a carbanion, and indeed, they act like carbanions, or so-called carbanionoids. These carbanionoids may have an extensive impact in organic chemistry and organometallic chemistry.

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Electronic levels in simple conjugated systems, I. Configuration interaction in cyclobutadiene

Proceedings of the Royal Society of London Series A - Mathematical and Physical Sciences ◽

10.1098/rspa.1950.0115 ◽

1950 ◽

Vol 202 (1071) ◽

pp. 498-506 ◽

Cited By ~ 28

Keyword(s):

Molecular Orbital ◽

Configuration Interaction ◽

Energy Levels ◽

Valence Bond ◽

Wave Functions ◽

The Other ◽

Orbital Theory ◽

Valence Bond Theory ◽

Symmetry Properties ◽

Bond Theory

In the simplest cyclic system of π-electrons, cyclobutadiene, a non-empirical calculation has been made of the effects of configuration interaction within a complete basis of antisymmetric molecular orbital configurations. The molecular orbitals are made up from atomic wave functions and all the interelectron repulsion integrals which arise are included, although those of them which are three- and four-centre integrals are only known approximately. In this system configuration interaction is a large effect with a strongly differential action between states of different symmetry properties. Thus the 1 A 1g state is several electron-volts lower than the lowest configuration of that symmetry, whereas for 1 B 1g the comparable figure is about one-tenth of an electron-volt. The other two states examined, 1 B 2g and 3 A 2g are affected by intermediate amounts. The result is a drastic change in the energy-level scheme compared with that based on configuration wave functions. Neither the valence-bond theory nor the molecular orbital theory (in which the four states have the same energy) gives a satisfactory account of the energy levels according to these results. One conclusion from the valence-bond theory which is, however, confirmed, is the somewhat unexpected one that the non-totally symmetrical 1 B 2g state is more stable than the totally symmetrical 1 A 1g . On the other hand, it is clear that the valence-bond theory, with the usual value for its exchange integral, grossly exaggerates the resonance splitting of the states, giving separations between them several times too great. Thus the valence-bond theory leads to large values of the resonance energy (larger, per π-electron, than in benzene) and so associates with the molecule a considerable π-electron stabilization. This expectation has no support in the present more detailed and non-empirical calculations.

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