Resonance theory/principles

See Lead zirconate titanate ceramics Quality number 219 Quantum transmission 59 Reciprocal space 123, 353 Reciprocity principle 88 Reconstruction 14, 327 Au(lll) 327 DAS model 16 Si(lll)-2X1 14 Recursion relations 352 Repulsive atomic force 185, 192 Resonance frequency 234, 241 piezoelectric scanners 234 vibration isolation system 241 Resonance interactions 171, 177 and tunneling 177 Resonance theory of the chemical bond 172... 

It is possible and probably very likely that both types of electronic effects are occurring in the acetal function. In other words, 2 could be more stable than 2 because 2 is stabilized relative to 2 by a partial electron transfer and 2 is destabilized relative to 2 by electronic repulsions. There is presently no experimental technique to differentiate between the two effects. At the present time, many chemists, including myself, prefer to consider the anomeric effect as a stabilizing rather than a destabilizing effect. The main reason is that the concept of stabilization of a system through electronic delocalization is a well established principle in organic chemistry. The resonance theory is indeed based on this principle. I believe that this concept rather than the dipole - dipole or electron pair - electron pair repulsions allows the organic chemist to rationalize his results better. 

Resonance was introduced when it was found that there are many molecules whose properties cannot be accounted for by means of a single electronic structure of the VB type, but rather by a combination of several structures [1], Although there is an element of arbitrariness in the resonance theory, in the sense of choosing VB structures, Wheland [50] systemized the basic principles to select the important resonance structures as well as to estimate their relative contribution to the ground state of a molecule. In fact, the qualitative resonance theory enjoyed such a great success due to its convenience and usefulness that resonance has become one of the most fundamental concepts in chemical theory. 

Keep in mind the following basic principles of resonance theory. 

If benzene were 1,3,5-cyclohexatriene, the carbon-carbon bonds would be alternately long and short as indicated in the following structures. However, to consider the structures here as resonance contributors (or to connect them by a double-headed arrow) violates a basic principle of resonance theory. Explain. 

Nuclear magnetic resonance theory and principles of interpretation ... 

We have come to associate electron delocalization with increased stability. On that basis alone, benzene ought to be stabilized. It differs from other conjugated systems that we have seen, however, in that its it electrons are delocalized over a cyclic conjugated system. Both Kekule stmctures of benzene are of equal energy, and one of the principles of resonance theory is that stabilization is greatest when the contributing structures are of... 

In 1952, Fukui published his Frontier MO theorywhich went initially unnoticed. In 1965, Woodward and Hoffmann published their principle of conservation of orbital symmetry, and applied it to all pericyclic chemical reactions. The immense success of these rules" renewed interest in Fukui s approach and together formed a new MO-based framework of thought for chemical reactivity (called, e.g., giant steps forward in chemical theory in Morrison and Boyd, pp. 934, 939, 1201, and 1203). This success of MO theory dealt a severe blow to VB theory. In this area too, despite the early calculations of the Diels-Alder and 2-1-2 cycloaddition reactions by Evans,VB theory missed making an impact, in part at least because of its blind adherence to simple resonance theory. All the subsequent VB derivations of the rules (e.g., by Oosterhoff in Ref. 90) were after the fact and failed to reestablish the status of VB theory. 

To clarify the discussion further concerning conducting polyaromatics, it is worthwhile to elaborate on some principles described by the valence-bond (VB) theory. The ideas of the (VB) (resonance) theory are found to be of aid to organic chemists for rationalizing in a simple manner the facts of organic structure and reactions. However, quantitative application is much harder for the VB method than for the molecular orbital (MO) method. Indeed, for quantitative calculations, the MO method has overshadowed the VB method. The main advantage of the VB method is that it is based on the familiar chemical ideas of bonding and structure [1050]. 

Each resonance structure implies that electron pairs are localized in bonds or on atoms. In actuality, resonance allows certain electron pairs to be delocalized over two or more atoms, and this delocalization of electron density adds stability. A molecule with two or more resonance structures is said to be resonance stabilized. We will return to the resonance hybrid in Section 1.5C. First, however, we examine the general principles of resonance theory and leam how to interconvert two or more resonance structures. 

Use the principles of resonance theory to explain why carbocation A is more stable than carbocation B. 

Classical structures in general, and double bonds in particular, are extremely well supported by a wide variety of chemical and physical evidence. Resonance theory is merely an extension of this, and is independently supported by its explanatory successes as a new semi-empirical structural principle which is compatible with quantum mechanics and supported by agreement with the facts obtained by experiment [Pauling, 1970, 1001]. Note that Pauling requires only consistency of the applications with the fundamental theory, and he appeals to both physics and classical chemistry (and experiment, of course) for justification. This style and outlook is characteristic of both his research papers [Park, 2000] and his textbooks [Gavroglu and Simoes, 2000]. 

Classical structures and bonds are well supported by a wide variety of chemical and physical evidence. Resonance theory is independently supported by its explanatory successes as a new semi-empirical structural principle which is compatible with quantum mechanics and supported by agreement with the facts obtained by experiment. [Pauling 1970, 1001]... 

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