Natural resonance theory valency

The model most widely used to explain hypervalence is the three-center, four-electron (3c-4e) model of Rundle and Pimentel [5]. Coulson [31] analyzed the 3c-4e model and suggested a valence bond resonance model that shares some similarities with the MO model. Under this model, the bond is posited to arise primarily from resonance between F-X F and F X -F charge structures (with contributions from other charge configurations). Weinhold and Landis [32] incorporated natural bond orbital analysis and natural resonance theory in what is perhaps the most... 

We have used the concepts of the resonance methods many times in previous chapters to explain the chemical behavior of compounds and to describe the structures of compounds that cannot be represented satisfactorily by a single valence-bond structure (e.g., benzene, Section 6-5). We shall assume, therefore, that you are familiar with the qualitative ideas of resonance theory, and that you are aware that the so-called resonance and valence-bond methods are in fact synonymous. The further treatment given here emphasizes more directly the quantum-mechanical nature of valence-bond theory. The basis of molecular-orbital theory also is described and compared with valence-bond theory. First, however, we shall discuss general characteristics of simple covalent bonds that we would expect either theory to explain. 

This hypothesis has been sometimes put forward, but, for the time being, real proof of individual existence of such valence isomers is still lacking. Although we cannot exclude the possibility that future refinements of experimental techniques may eventually prove the existence of such isomers, it does not seem advisable to accept as a fact the existence of isomers for which no conclusive evidence has ever been produced. We shall then prefer the single bond-no bond resonance theory to the rapid isomerization theory. In so doing we admit that there exists, in trithiapentalenes, an array of atoms which can be represented as in 85, in which the lines between atomic symbols mean only that some sort of bonding exists, without prejudice of the nature of this bonding. 

In practice, the valence bond picture has probably exerted more influence on how chemists actually think than the HMO picture. However most early applications were primarily qualitative in nature. This qualitative VB picture can be summarized under die name of resonance theory [10]. The basic concept is that in general the more ways one has of arranging the spin pairing in the VB wave function, the more stable the molecule is likely to be. Thus, VB theory predicts that phenanthrene with 14 carbon atoms and 5 Kekule structures should be more stable than anthracene with 14 carbon atoms but just 4 Kekule structures, in complete accord with the experimental evidence. It also predicts that benzenoid hydrocarbons with no Kekule structures should be unstable and highly reactive, and in fact no such compounds are knowa Extensions of this qualitative picture appear, for example, in Clar s ideas of resonant sextets [11], which seem to be very powerful in rationalizing much of the chemistry of benzenoid aromatic hydrocarbons. The early ascendancy of HMO theory was thus largely based on the ease with which it could be used for quantitative computations rather than on any inherent superiority of its fundamental assumptions. 

I was inspired too by Linus Pauling (1901-94), another polymath with humanistic concerns. His Nature of the Chemical Bond (1939) brought a new perspective to theories of molecular structure, and refuted the implication of a popular examination question of the time, Is inorganic chemistry a largely closed and finished subject Pauling s resonance theory, formally based on the quantum-mechanical valence-bond (VB) method for... 

Pauling, L. (1960). The Nature of the Chemical Bond, Cornell University Fhess, Ithaca, NY. Another science classic by one of the original heroes of quantum chemistry. Heavily slanted towards Pauling s views on resonance and valence-bond theory. 

Now that we have considered some of the ways in which the idea of resonance has brought clarity and unity into modem structural chemistry, has led to the solution of many problems of valence theory, and has assisted in the correlation of the chemical properties of substances with the information obtained about the structure of their molecules by physical methods, we may well inquire again into the nature of the phenomenon of resonance.1... 

Finally, the use of simple valence bond theory has led recently to a significant discovery concerning the nature of metals. Many years ago one of us noticed, based on an analysis of the experimental values of the saturation ferromagnetic moment per atom of the metals of the iron group and their alloys, that for a substance to have metallic properties, 0.72 orbital per atom, the metallic orbital, must be available to permit the unsynchronized resonance that confers metallic properties on a substance.34 38 Using lithium as an example, unsynchronized resonance refers to such structures as follows. 

In the course of the further investigation of resonating valence bonds in metals the nature and significance of this previously puzzling unstable orbital have been discovered, and it has become possible to formulate a rational theory of metallic valence and of the structure of metals and intermetallic compounds. 

The NRT resonance weights, bond orders, and valencies are generally comparable to those of the older Pauling-Wheland theory (particularly for species of low ionicity) and can be used to rationalize chemical phenomena in a similar fashion. Pauling s classic, The Nature of the Chemical Bond, brilliantly illustrates such reasoning. 

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). 

The contributions of Erich Hiickel to the development of molecular orbital theory have already been mentioned in the subsection on Germany (Section 5.4.1) the development of semi-empirical quantum mechanical treatments in organic chemistry by M. J. S. Dewar has been discussed in Section 5.5. In the early development of the application of quantum mechanics to chemistry, Linus Pauling (1901-1994)359 was pre-eminent. He was associated with CalTech for most of his career. His work before World War II generated two influential books the Introduction to Quantum Mechanics (with E. Bright Wilson, 1935)360 and The Nature of the Chemical Bond (1939).361 He favoured the valence-bond treatment and the theory of resonance. 

The structural formulas used to represent molecules are based on valence bond theory. Double and triple bonds simply represent additional pairs of shared valence electrons. But structural formulas, while useful, don t tell the whole story about the nature of the bonds between atoms in a molecule. Valence bond theory falls flat when it tries to explain delocalized electrons and resonance structures. To get at what is really going on inside molecules, chemists had to dig deeper. 

In valence bond theory, then, we may have to call on the idea of resonance to help us out, as we may only be able to write an approximate wave function for a molecule, in view of its complicated nature compared with a single atom. Extensive use is also required of another concept, hybridisation we will meet this in the next chapter (page 35). 

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