Pauling valence bond

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. 

The remainder of this book will be devoted to the modem ideas of bonding in several important classes of molecules. The emphasis will be on the molecular-orbital theory, with comparisons made from time to time to the valence-bond theory. Of the many scientists involved in the development of these theories, the names of R. S. Mulliken (molecular-orbital theory) and Linus Pauling (valence-bond theory) are particularly outstanding. 

This increased-valence description of electron conduction combines features of both the delocalized molecular orbital and the Pauling valence-bond and theories. The simplest increased-valence theory need use only the 2s orbitals for bonding (cf the simplest form of delocalized molecular orbital theory), and it uses localized bonds as does the valence-bond theory. 

The increased-valence theory represents a natural extension of the more familiar Lewis-Pauling valence-bond theory. Therefore an understanding of it may be useful for all chemists who have an interest in qualitative valence-bond descriptions of electronic structure. It will be shown that all Lewis-type valence-bond structures with lone-pairs of electrons can be stabilized easily via one-electron delocalizations from doubly-occupied atomic orbitals into diatomic bonding molecular orbitals when the relevant atomic orbitals overlap, as is shown here for the two sets of oxygen n electrons of N2O. 

Very recently, Singleton has suggested a new way of calculating isotope effects illustrated by the bromonium ion [61]. This method could possibly also be used for tautomeric systems. Another new approach is the multicomponent molecular orbital method for direct treatment of nuclear quantum effects [62]. The basic idea is to incorporate the nuclear wave function and in particular the proton wave function directly into the electronic structure calculation. This approach has great potential but has so far been tested only for secondary isotope effects on chemical shifts [63]. The geometric isotope effect has also been looked into based on Pauling valence-bond orders [20]. 

The concepts of directed valence and orbital hybridization were developed by Linus Pauling soon after the description of the hydrogen molecule by the valence bond theory. These concepts were applied to an issue of specific concern to organic chemistry, the tetrahedral orientation of the bonds to tetracoordinate carbon. Pauling reasoned that because covalent bonds require mutual overlap of orbitals, stronger bonds would result from better overlap. Orbitals that possess directional properties, such as p orbitals, should therefore be more effective than spherically symmetric 5 orbitals. 

In his valence bond theory (VB), L. Pauling extended the idea of electron-pair donation by considering the orbitals of the metal which would be needed to accommodate them, and the stereochemical consequences of their hybridization (1931-3). He was thereby able to account for much that was known in the 1930s about the stereochemistry and kinetic behaviour of complexes, and demonstrated the diagnostic value of measuring their magnetic properties. Unfortunately the theory offers no satisfactory explanation of spectroscopic properties and so was... 

In the 1930s a theoretical treatment of the covalent bond was developed by, among others, Linus Pauling (1901-1994), then at the California Institute of Technology. The atomic orbital or valence bond model won him the Nobel Prize in chemistry in 1954. Eight years later, Pauling won the Nobel Peace Prize for his efforts to stop nuclear testing. 

Linus Pauling, A Resonating-Valence-Bond Theory of Metals and Intermetallic Com-... 

Pauling, L. (1975) Valence-bond theory of compounds of transition metals, Proc. Natl. Acad. Sci. USA 72,4200-4202. 

Pauling, L. Herman, Z.S. Valence-Bond Concepts in Coordination Chemistry and the Nature of Metal-Metal Bonds J. Chem. Educ. 1984, 61, 582-587. 

The classic HLSP-PP-VB (Heitler-London-Slater-Pauling perfect-pairing valence-bond) formalism and its chemical applications are described by L. Pauling, The Nature of the Chemical Bond. 3rd edn. (Ithaca, NY, Cornell University Press, 1960 G. W. Wheland, The Theory of Resonance (New York, John Wiley, 1944) and H. Eyring, J. Walter, and G. E. Kimball, Quantum Chemistry (New York, John Wiley, 1944). 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

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