Contributing structures

Most of the qualitative relationships between color and structure of methine dyes based on the resonance theory were established independently during the 1940 s by Brooker and coworkers (16, 72-74) and by Kiprianov (75-78), and specific application to thiazolo dyes appeared later with the studies of Knott (79) and Rout (80-84). In this approach, the absorptions of dyes belonging to amidinium ionic system are conveyed by a group of contributing structures resulting from the different ways of localization of the 2n rr electrons on the 2n l atoms of the chromophoric cationic chain, rather than by a single formula ... 

Atomic positions (connectivity) must be the same in all resonance structures only the electron posi tions may vary among the various contributing structures... 

Although such alkoxides have never been isolated it is assumed that with bulky alcohols such as steroidal alcohols, the main contributing structure in such an equilibrium (especially when excess lead tetraacetate is present) is the one in which n = 1. An advantage of this procedure held in common with the hypoiodite reaction is the fact that the alcohol derivative is formed in situ. Intermolecular hydrogen abstraction e.g., reaction with solvent)... 

In 1955 Boyer d al challenged this formulation, and suggested a static, mesomeric system rather than a dynamic, tautomeric one, with Contributing structures of type 9 and 10 to a symmetrical resonance hybrid, proposing the name -o-dinitrosobenzene for the parent System. This notion, however, raised more problems than it solved,... 

Valence bond theory (Chapter 7) explains the fact that the three N—O bonds are identical by invoking the idea of resonance, with three contributing structures. MO theory, on the other hand, considers that the skeleton of the nitrate ion is established by the three sigma bonds while the electron pair in the pi orbital is delocalized, shared by all of the atoms in the molecule. According to MO theory, a similar interpretation applies with all of the resonance hybrids described in Chapter 7, including SO S03, and C032-. 

Symmetrical cyanine dyes, because of the resonance shown in Figure 6.4 (in which the two contributing structures are exactly equivalent), are completely symmetrical molecules. X-ray crystal structure determinations and NMR spectroscopic analysis have demonstrated that the dyes are essentially planar and that the carbon-carbon bond lengths in the polymethine chain are uniform. The colour of cyanine dyes depends mainly on the nature of the terminal groups and on the length of the polymethine chain. The bathochromicity of the dyes is found to increase... 

Hammond and co-workers have suggested that the exciplex should be described in terms of the four contributing structures... 

The energy of the actual molecule is lower than the energy that might be estimated for any contributing structure. 

To indicate resonance forms, we use a doubleheaded arrow between the contributing structures. This arrow is reserved for resonance structures and never used elsewhere. The difference between the two structures is that the electrons in the n bonds have been redistributed, and we can illustrate this by use of another type of arrow, a curly arrow. This arrow is used throughout chemistry to represent the movement of two electrons. In the benzene case, a cyclic movement of electrons accounts for the apparent relocation of double bonds, though there are two ways we might show this process both are equally satisfactory. 

The presence of resonance leads to stabilization, which means that the species is more stable than any of the contributing structures. 

The energy of the hybrid, E, is always less than the calculated energy of any hypothetical contributing structure, E. The difference between these energies is the resonance (delocalization) energy, E/. 

Contributing structures (a) differ only in positions of electrons (atomic nuclei must have the same positions) and (b) must have the same number of paired electrons. Relative energies of contributing structures are assessed by the following rules. 

Problem 2.24 Write contributing structures, showing formal charges when necessary, for (a) ozone, O, (b) CO (c) hydrazoic acid, HN, (d) isocyanic acid, HNCO. Indicate the most and least stable structures and give reasons for your choices. Give the structure of the hybrid. ... 

Problem 2.26 Indicate which one of each of the following pairs of resonance structures is the less stable and is an unlikely contributing structure. Give reasons in each case. 

Problem 2.43 Arrange the contributing structures for (a) vinyl chloride, H,C=CHC1, and (h) formic acid, HCOOH, in order of increasing importance (increasing stability) by assigning numbers starting with 1 for most important and stable. 

Isomers are real compounds that differ in the arrangement of their atoms. Contributing structures have the same arrangement of atoms they differ only in the distribution of their electrons. These imaginary structures are written to give some indieation of the electronic structure of certain species for which a typical Lewis structure cannot be written. 

Structure (i) has 11 bonds and makes a more significant contribution than the other two structures, which have only 10 bonds. Since the contributing structures are not equivalent, the resonance energy is small. 

No. The product consists of an equal number of HjC = CH CH2Br and CHj = CHCHjBr molecules. H-abstraction produces a resonance hybrid of two contributing structures having both and as equally reactive, free-radical sites that attack Br,. 

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