Valence bond approximation

On the other hand, the relative energy of the A" is greatly reduced upon halogen substitution, especially in the case of fluorine. Based on valence-bond approximations (Fig. 3), it was argued that the A"- A splitting in compounds 16-19 should be closely related to the T S splitting of the correspondingly substi-... 

At the present stage of theoretical development it is hardly possible to go far enough, in terms of either band theory or the Pauling valence bond approximation, with a quantitative treatment of crystals involving free electrons and a relatively large electronegativity difference between the components. Nevertheless, the matter merits more attention. 

Although the use of strokes to represent bonds between atoms in molecules comes from the nineteenth century, the electron pair concept as necessary for the understanding of chemical bonding was introduced by G.N. Lewis (1875-1946) in 1916 (ref. 90) following Bohr s, then recently proposed, model of the atom. Indeed, the Lewis model still lies at the basis of much of present-day chemical thinking, although it was advanced before both the development of quantum mechanics and the introduction of the concept of electron spin. In a more quantitative way, it found a natural theoretical extension in the valence-bond approximation to the molecular wavefunction, as expressed in terms of the overlap of (pure or hybridized) atomic orbitals to describe the pairing of electrons, coupled with the concept of electron spin. 

Soon after the development of the quantum mechanical model of the atom, physicists such as John H. van Vleck (1928) began to investigate a wave-mechanical concept of the chemical bond. The electronic theories of valency, polarity, quantum numbers, and electron distributions in atoms were described, and the valence bond approximation, which depicts covalent bonding in molecules, was built upon these principles. In 1939, Linus Pauling s Nature of the Chemical Bond offered valence bond theory (VBT) as a plausible explanation for bonding in transition metal complexes. His application of VBT to transition metal complexes was supported by Bjerrum s work on stability that suggested electrostatics alone could not account for all bonding characteristics. 

The most aesthetically satisfying and generally applicable semi-empirical valence-bond approximation is the so-called diatomics-in-molecules (DIM)... 

Free valence introduced by Daudel and Pullman 9 following a suggestion of Svartholm65 in the valence bond approximation and then by Coulson19 in the molecular orbital method will be more useful here. 

Multiplying out the right-hand side of Eq. (80) leads to terms in 9 (1)v (2) 9> (1)9> (2) and fl9,(l)9> (2) J 2). The last two terms are ionic, i.e., they refer to that portion of the wave function (p which allows both electrons on the same nucleus. By ignoring these terms we arrive at the valence-bond approximation, so named because it includes only those terms that have a valence electron on each nucleus and thus form a valence bond. 

The approximate bonding energy of the four valence electrons in HgCl2 may be written down by applying Mulliken s general formula 124), based on the valence-bond approximation of perfect pairing. Representing the structure Cl—Hg —Cl in the manner... 

A calculation of spin-spin coupling constants using the valence-bond approximation was carried out by Klose 211) for alkyl derivatives of Si, P, Se, Cd, Sn, Te, Hg, Tl, and Pb. The agreement with observed values was gratifying in most cases, which included opposite signs for CH3—M and -CH2—M (see discussion of observed spectra for ethyl derivatives, below). The effect of substituents on coupling constants for Si —(and —H)... 

Homonuclear Diatomic Molecules in the Valence-Bond Approximation... 

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