Single valence bond

The concept of connection tablc.s, a.s shown. so far, cannot represent adequately quite a number of molecular structures. Basically, a connection table represents only a single valence bond structure. Thus, any chemical species that cannot he described adequately by a single valence bond (VB) structure with single or multiple bonds between two atom.s is not handled accurately. 

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

The idea that the properties of many organic compounds, especially the aromatic compounds, cannot be simply correlated with a single valence-bond structure, but require the assignment of a somewhat more complex electronic structure, was developed during the period 1923 to 1926 by a number of chemists, including Lowry, Lapworth, Robinson, and Ingold in England, Lucas... 

It is sometimes found, however, that an unambiguous assignment of a single valence-bond structure to a molecule cannot be made two or more structures that are about equally stable may suggest themselves, no one of which accounts in a completely satisfactory way for the properties of the substance. Under these circumstances some new structural concepts and symbols might be introduced we might, for... 

The discussion in Section 1-3 about the element of arbitrariness in the concept of resonance may be recalled at this point with reference to the nitrous oxide molecule and the other molecules that are described in this chapter as resonating among several valence-bond structures. It is not necessary that the structures A, B, and C be used as the basis of discussion of the nitrous oxide molecule. We might say instead that the molecule cannot be satisfactorily represented by any single valence-bond structure, and abandon the effort to correlate its structure and properties with those of other molecules. By using valence-bond structures as the basis for discussion, howrever, with the aid of the concept of resonance, we are able to account for the properties of the molecule in terms of those of other molecules in a straightforward and simple way. It is for this practical reason that we find it convenient to speak of the resonance of molecules among several electronic structures. 

Empirical Values of Resonance Energies.—The tables of bond energies permit the calculation of values of the heats of formation of molecules to which a single valence-bond structure can be assigned that agree with the experimental values to within a few kcal/mole. On carrying out a similar calculation for a resonating molecule on the... 

The difference between the observed heat of formation and that calculated for a single valence-bond structure for a molecule with use of the table of bond energies is an empirical value of the resonance energy of the molecule relative to the assumed valence-bond structure. 

The theory of resonance has been applied to many problems in chemistry. In addition to its use in the discussion of the normal covalent bond (involving the interchange of two electrons, with opposed spins, between two atoms) and to the structure of molecules for which a single valence-bond structure does not provide a satisfactory description, it has rendered service to chemistry by leading to the discovery of several... 

Now let us consider benzene. There is no single valence-bond structure that accounts satisfactorily for the properties of benzene. The simple description of benzene that is given by the theory of resonance involves two valence-bond structures, the two Kekul6 structures... 

I feel that the greatest advantage of the theory of resonance, as compared with other ways (such as the molecular-orbital method) of discussing the structure of molecules for which a single valence-bond structure is not enough, is that it makes use of structural elements with which the chemist is familiar. The theory should not be assessed as inadequate because of its occasional unskillful application. 11 becomes more and more powerful, just as does classical structure theory, as the chemist develops a better and better chemical intuition about it. 

In Chapter 3 ana other chapters of this book much use is made of bond-energy values. These values are chosen in such a way that their sum over all of the bonds of a molecule which can be satisfactorily represented by a single valence-bond structure is equal to the enthalpy of formation of the molecule from its constituent atoms in their normal states. For example, the value of the O—H bond energy, 110 kcal /mole, is one-half the enthalpy of formation of U20(g) from 2H(g) and 0(g). 

It must not be forgotten that a purely chemical view of surface action has its adherents. If we regard molecules of hydrogen, adsorbed on the surface of an active nickel catalyst, as held by single valency bonds to the atoms of nickel we are already taking a very definitely chemical view of the matter. The system we are considering differs from a hydride of nickel only in that the nickel remains attached to the surface instead of being carried away in the gas phase. Under these circumstances we can only expect catalytic phenomena to be of an extremely specific nature. 

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. 

Figure 1-11. The resonance forms of some ligands that cannot be represented by a single valence bond structure (acetate anion, acetylacetonate anion and pyridine).

Using Standard Molecular Orbital Software to Compute Single Valence Bond Structures or Determinants... 

The first condition is satisfied automatically with all reactions containing closed shell molecules only. A systematic examination for this type of reactions was performed by Snyder and Basch The theoretical (SCF) heats of reactions were claimed to be more accurate than those obtained using seraiempirical relations of bond energies for reactions of strained molecules, or those not well represented by a single valence-bond structure. However, Snyder and Basch concluded... 

The energy of a single valence bond structure Xi is given by the... 

The essential requirement for a substance to be rubbery is that it consist of long flexible chainlike molecules. The molecules themselves must therefore have a backbone of many noncollinear single valence bonds, about which rapid rotation is possible as a result of thermal agitation. Some representative molecular subunits of rubbery polymers are shown in Figure 1.1 thousands of these units linked together into a chain constitute a typical molecule of the elastomers listed in Figure 1.1. Such molecules change their shape readily and continuously at normal temperatures by Brownian motion. They take up random conformations in a stress-free state but assume somewhat oriented... 

Resonance hybrid n. A structure which cannot be represented by a single valence-bond Lewis structure but rather is shown as a combination, or average, of two or more structures. 

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