Kekulé index for valence bond structures of conjugated systems containing cyclobutadiene

1978 ◽  
Vol 43 (5) ◽  
pp. 1375-1392 ◽  
Author(s):  
Ante Graovac ◽  
Ivan Gutman ◽  
Milan Randić ◽  
Nenad Trinajstić
Keyword(s):  
Valence Bond ◽  
Conjugated Systems

Electronic levels in simple conjugated systems II. Butadiene

1951 ◽  
Vol 206 (1086) ◽  
pp. 287-296 ◽  
Keyword(s):  
Oscillator Strength ◽  
Configuration Interaction ◽  
Empirical Work ◽  
Valence Bond ◽  
Ultra Violet ◽  
Molecular Orbitals ◽  
Triplet States ◽  
Absorption Frequency ◽  
Conjugated Systems ◽  
Singlet And Triplet States

The method of molecular orbitals, with configuration interaction, as described in part I, is applied to butadiene. The total energy, ultra-violet absorption frequency, oscillator strength and charge distribution are calculated for several singlet and triplet states in general agreement with similar more empirical work using the valence-bond approximation. It appears that without configuration interaction, no satisfactory calculation of the absolute intensities of the N → V transitions can be made. Best-possible molecular orbitals, compounded linearly of 2 pπ atomic orbitals, are obtained. Their use reduces the magnitude of configuration interaction from 2·8 to 1·7 eV in the ground state. It also gives a satisfactory measure of the energy of the first allowed transition, but not of the oscillator strength.

π-σ Electronic states in molecules II. The singlet spectrum of ethylene and derivatives

1952 ◽  
Vol 210 (1102) ◽  
pp. 343-354 ◽  
Keyword(s):  
Ground State ◽  
Valence Bond ◽  
Resonance Energy ◽  
Theoretical Explanation ◽  
Conjugated Systems ◽  
Alkyl Substitution ◽  
Transition Rules ◽  
The Right ◽  
Double Nature ◽  
Bond Method

It is suggested that the failure of the Heitler-London-Slater-Pauling method to give the right transition rules for the excited levels in conjugated systems is not due chiefly to shortcomings in the valence bond method (like neglect of ionic structures) but rather to the use of the Hückel approximation (see part I). This is confirmed by a detailed treatment of ethylene as a twelveelectron problem by the valence-bond method. The Hückel approximation is dispensed with, so that six canonical structures are considered. Of these, one corresponds to the perfect pairing scheme, one to π-σ resonance in the double bond and four to π- h resonance ( h being the hydrogen 1 s electron) in the C-H bonds. No purely σ-σ resonance is considered. Most of the necessary exchange integrals are computed in this paper. Two of them, however, are estimated. The treatment can be extended to substituted ethylenes by changing a few exchange integrals. It is necessary here to estimate another integral. A π-σ resonance energy of 0.47 eV is obtained for the ground state of ethylene. The actual positions of the peaks of absorption obtained are not very accurate, as could be expected. They are, however, of the same order as those given by the method of the antisymmetrized molecular orbitals. A first band 1 A 1g - A 1g , forbidden, is obtained. This is identified with the observed faint absorption starting at 2000 Å. The second band, allowed, is double, given by the transitions 1 A 1g - 1 B 1M and 1 A 1g 1 B 2u separated by ca. 0.2 eV (experim ental 0.06 eV). These bands are identified with the strong absorption at 1600 Å. The double nature of this band was hitherto without theoretical explanation. The distance between the first forbidden ban d and the centre of the second is 1.4 eV (experimental ca. 1.1 eV). The second band is shifted towards the red by tetra-alkyl substitution by 0.9 eV (experimental 1 eV). There is no need to have recourse to hyperconjugation to explain this last fact.

Valence bond description of spin properties of π-conjugated systems

1996 ◽  
Vol 204 (2-3) ◽  
pp. 447-461 ◽  
Author(s):  
Xiangzhu Li ◽  
Paldus Josef
Keyword(s):  
Valence Bond ◽  
Conjugated Systems ◽  
Spin Properties

Resolving the Quadruple Bonding Conundrum in C2 Using Insights Derived from Excited State Potential Energy Surfaces: A Molecular Orbital Perspective

2019 ◽  
Author(s):  
Ishita Bhattacharjee ◽  
Debashree Ghosh ◽  
Ankan Paul
Keyword(s):  
Excited State ◽  
Potential Energy ◽  
Molecular Orbital ◽  
High Spin ◽  
Potential Energy Surfaces ◽  
Valence Bond ◽  
Deep Minimum ◽  
State Potential ◽  
Energy Surfaces ◽  
Mo Theory

The question of quadruple bonding in C<sub>2</sub> has emerged as a hot button issue, with opinions sharply divided between the practitioners of Valence Bond (VB) and Molecular Orbital (MO) theory. Here, we have systematically studied the Potential Energy Curves (PECs) of low lying high spin sigma states of C<sub>2</sub>, N<sub>2</sub> and Be<sub>2</sub> and HC≡CH using several MO based techniques such as CASSCF, RASSCF and MRCI. The analyses of the PECs for the<sup> 2S+1</sup>Σ<sub>g/u</sub> (with 2S+1=1,3,5,7,9) states of C<sub>2</sub> and comparisons with those of relevant dimers and the respective wavefunctions were conducted. We contend that unlike in the case of N<sub>2</sub> and HC≡CH, the presence of a deep minimum in the <sup>7</sup>Σ state of C<sub>2</sub> and CN<sup>+</sup> suggest a latent quadruple bonding nature in these two dimers. Hence, we have struck a reconciliatory note between the MO and VB approaches. The evidence provided by us can be experimentally verified, thus providing the window so that the narrative can move beyond theoretical conjectures.

A Nanoscale Jigsaw-Puzzle Approach to Large π-Conjugated Systems

2010 ◽  
Vol 49 (38) ◽  
pp. 6764-6767 ◽  
Author(s):  
Xingfa Gao ◽  
Shengbai B. Zhang ◽  
Yuliang Zhao ◽  
Shigeru Nagase
Keyword(s):  
Jigsaw Puzzle ◽  
Conjugated Systems
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