Resonance and Conjugation

Qualitative application of VB theory makes use of the concept of resonance to relate structural formulas to the description of molecular stmcture and electron distribution. The case of benzene is a familiar and striking example. Two equivalent Lewis structures can be drawn, but the actual structure is the average of these two resonance structures. The double-headed arrow is used to specify a resonance relationship. 

Resonance is a very useful concept and can be applied to many other molecules. Resonance is associated with delocalization of electrons and is a feature of conjugated systems, which have alternating double bonds that permit overlap between adjacent tt bonds. This permits delocalization of electron density and usually leads to stabilization of the molecule. We will give some additional examples shortly. 

Description of Molecular Structure Using Valence Bond Concepts 

When alternative Lewis structures can be written for a molecule and they differ only in the assignment of electrons among the nuclei, with nuclear positions being constant, then the molecule is not completely represented by a single Lewis structure, but has weighted properties of all the alternative Lewis structures. 

Resonance structures are restricted to the maximum number of valence electrons that is appropriate for each atom two for hydrogen and eight for second-row elements. 

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The huge difference between the hypothetical and observed heats of hydrogenation for benzene cannot be explained solely on the basis of resonance and conjugation. 

A familiar feature of the electronic theory is the classification of substituents, in terms of the inductive and conjugative or resonance effects, which it provides. Examples from substituents discussed in this book are given in table 7.2. The effects upon orientation and reactivity indicated are only the dominant ones, and one of our tasks is to examine in closer detail how descriptions of substituent effects of this kind meet the facts of nitration. In general, such descriptions find wide acceptance, the more so since they are now known to correspond to parallel descriptions in terms of molecular orbital theory ( 7.2.2, 7.2.3). Only in respect of the interpretation to be placed upon the inductive effect is there still serious disagreement. It will be seen that recent results of nitration studies have produced evidence on this point ( 9.1.1). 

Since Stork et al. introduced as a new synthetic method the alkylation and acylation of carbonyl compounds via enamines, this class of compounds has been the subjeet of intensive studies 1-3). The exceptional physical and chemical behavior of the enamine structure can be ascribed to resonance by conjugation of the unshared pair of electrons of the nitrogen atom with the 77 electrons of the double bond ... 

Porphyrin is a multi-detectable molecule, that is, a number of its properties are detectable by many physical methods. Not only the most popular nuclear magnetic resonance and light absorption and emission spectroscopic methods, but also the electron spin resonance method for paramagnetic metallopor-phyrins and Mossbauer spectroscopy for iron and tin porphyrins are frequently used to estimate the electronic structure of porphyrins. By using these multi-detectable properties of the porphyrins of CPOs, a novel physical phenomenon is expected to be found. In particular, the topology of the cyclic shape is an ideal one-dimensional state of the materials used in quantum physics [ 16]. The concept of aromaticity found in fuUerenes, spherical aromaticity, will be revised using TT-conjugated CPOs [17]. 

The discussion in this chapter is limited to cyanine-like NIR conjugated molecules, and further, is limited to discussing their two-photon absorption spectra with little emphasis on their excited state absorption properties. In principle, if the quantum mechanical states are known, the ultrafast nonlinear refraction may also be determined, but that is outside the scope of this chapter. The extent to which the results discussed here can be transferred to describe the nonlinear optical properties of other classes of molecules is debatable, but there are certain results that are clear. Designing molecules with large transition dipole moments that take advantage of intermediate state resonance and double resonance enhancements are definitely important approaches to obtain large two-photon absorption cross sections. 

It is quite brittle and is insoluble, because of the right nature of the Polymer chain that is caused by the presence of aromatic rings linked together through parapositions. It can withstand temperatures even up to 560°C. This property is attributed to the presence of resonance-stabilised conjugated double bonds in the aromatic rings. 

In general, the greater the resonance and hyper-conjugation the greater is the stability of carbocation. The stability also depends on the field strengths. The following examples illustrate this point. 

Now let us go a step further, and conjugate the carbonyl group with a double bond. If we polarize the carbonyl as before, then conjugation allows another resonance form to be written, in which the P-carbon now carries a positive charge. Thus, as well as the carbonyl carbon being electrophilic, the P-carbon is also an electrophilic centre. 

Vitamin C, also known as L-ascorbic acid, clearly appears to be of carbohydrate nature. Its most obvious functional group is the lactone ring system, and, although termed ascorbic acid, it is certainly not a carboxylic acid. Nevertheless, it shows acidic properties, since it is an enol, in fact an enediol. It is easy to predict which enol hydroxyl group is going to ionize more readily. It must be the one P to the carbonyl, ionization of which produces a conjugate base that is nicely resonance stabilized (see Section 4.3.5). Indeed, note that these resonance forms correspond to those of an enolate anion derived from a 1,3-dicarbonyl compound (see Section 10.1). Ionization of the a-hydroxyl provides less favourable resonance, and the remaining hydroxyls are typical non-acidic alcohols (see Section 4.3.3). Thus, the of vitamin C is 4.0, and is comparable to that of a carboxylic acid. 

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