Contributing structures contributors

The situation is more complicated when the set of reasonable contributing structures are not all equivalent. Examine the geometry and atomic charges forphenoxide anion. Do these data fit any one of the possible resonance structures (draw all reasonable possibilities), or is a combination of two or more resonance contributors necessary ... 

X-Ray analysis confirms the structure shown for the polymer, and the bond distances found for the chain indicate that B is the major contributing structure - . Intense bands for C=C and C=C in the Raman spectra also indicate that B is the major contributor, but the relatively low frequencies for these vibrations as well as the linear correlation found between the two frequencies for various polymers suggest that A makes a significant contribution . Both frequencies are found to increase with decreasing phase perfection. 

Treating naphthalene comparably reveals three resonance contributors, 3, 4 and 5. The valence-bond treatment predicts quite well the non-equivalence of the bond lengths in naphthalene in two of the three contributing structures, C-l-C-2 is double and in one it is single, whereas C-2-C-3 is single in two and double in one. Statistically, then, the former may be looked on as 0.67 of a double bond and the latter as... 

There are a substantial number of heterocyclic substances for which no plausible, unpolarised canonical structure can be written such systems are termed mesoionic . Despite the presence of a nominal positive and negative charge in all resonance contributors to such compounds, they are not salt-like, are of course overall neutral, and behave like organic substances, dissolving in the usual solvents. Examples of mesoionic structures occur throughout the text. Amongst the earliest mesoionic substances to be studied w ere the sydnones, for which several contributing structures can be drawn. 

Two or more contributing structures comprise a hybrid structure, which approximates the actual molecule. These contributors are connected by a characteristic double-headed arrow,... 

The relative positions of atomic nuclei must not change from one contributing structure to another. Only the electrons change location in the contributors that make up the hybrid. 

In most cases, the various resonance structures of a molecule are not equivalent and do not contribute equally to the resonance hybrid. The electron distribution in the molecule resembles that of its major contributor more closely than any of its alternative resonance stmctures. Therefore, it is important that we develop some generalizations concerning the factors that make one resonance form more important (more stable) than another. Table 1.6 outlines the structural features that alert us to situations when resonance needs to be considered and lists criteria for evaluating the relative importance of the contributing structures. 

The contributing structure with the negative charge on the O atom is the major contributor to the hybrid because O is more electronegative than C, so O is better able to accommodate the negative charge. 

This delocalization, however, produces a contributing structure that has one less bond than the major contributor. Consequently, electron release to double bonds by alkyl substituents should be, and is observed to be, less pronounced than comparable stabilization of carboca-tions and free radicals—species in which the major and minor contributors have the same number of bonds. 

Electron delocalization in the acyl cation derived from propanoyl chloride is represented by the following two resonance contributors. Note that electron release from oxygen generates a contributing structure that satishes the octet rule and disperses the posihve charge. 

The hydroxyl oxygen atom can donate an electron pair to carbon to give resonance structure 3 in which every atom has a Lewis octet. This stabilizes the C=0 group, and the carbonyl carbon atom is less electrophilic than that of aldehydes or ketones. However, since this contributing structure has a positively charged oxygen atom, it is only a minor contributor to the resonance hybrid. 

A similar concept exists when comparing resonance structures. One compound might have three resonance structures, but all three resonance structures might not contribute equally to the overall resonance hybrid. One resonance structure might be the major contributor (like the peach), while another resonance structure might be insignificant (like the kiwi). In order to understand the true nature of the compound, we must be able to compare the resonance structures and determine which structures are major contributors and which structures are not significant. 

The selection of essential topics and expert authors was not an easy task. We tried to include the most representative applications of CL and BL in analytical chemistry. The contributors were invited to elaborate on the subjects according to their knowledge and experience in the field, and we think we have succeeded in unifying the contents of the overall volume. We heartily thank the contributing authors for agreeing to collaborate on this project their efforts led to the comprehensive structure of this book. 

Wegner, in his first publication (190), favored the butatriene-type structure for the backbone. In fact, subsequent work showed that this is a reasonable description of some structures (194,195), but the majority of solved structures are closer to the ene-yne description (196-199). It is customary to consider these two forms as limiting contributors to a mesomeric hybrid. It has been argued, on the basis of Raman studies, that changes in the relative contributions of the... 

Protein chains are not the sprawling, ill-defined structures that might be expected from a single polypeptide chain. Most proteins are compact molecules, and the relative positions of atoms in the molecule contribute significantly to its biological role. A particularly important contributor to the shape of proteins is provided by the peptide bond itself. Drawn in its simplest form, one might expect free rotation about single bonds, with a variety of conformations possible (see Section 3.3.1). However,... 

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