Contributing forms resonance

The Dewar-type structure for the central ring of the anthracene system in 19 is possible only because the 9,10 substituents prevent the system from being planar. From 19 is the actual structure of the molecule and is not in resonance with forms like 20, although in anthracene itself, Dewar structures and structures like 20 both contribute. This is a consequence of rule 2 (p. 41). In order for a 20-like stracture to contribute to resonance in 19, the nuclei would have to be in the same positions in both forms. 

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

If two planar unsaturated groups are linked by a single bond (conjugated), the planar conformation is stabilized. In terms of valence bond theory, the planar structure is stabilized due to the contribution of resonance forms (Scheme 4). 

C- and N-NMR spectroscopy has been used to gain information on the contributions of resonance forms of type 3 to the hybrid structure of the ground state of diazoazoles. The C chemical shifts of the ipso carbon of the diazo group in some diazoazoles are reported in Table I where those of diazomethane and of two diazocyclopentadienes are compared. 

A few words should be said about the difference between resonance and molecular vibrations. Although vibrations take place, they are oscillations about an equilibrium position determined by the structure of the resonance hybrid, and they should not be confused with the resonance among the contributing forms. The molecule does not resonate or vibrate" from one canonical structure to another. In this sense the term resonance is unfortunate because it has caused unnecessary confusion by invoking a picture of vibration. The term arises from a mathematical analogy between the molecule and the classical phenomenon of resonance between coupled pendulums, or other mechanical systems. 

The synthesis and metal complexes of the pyrrole dithiocarbamate ligand were reported by Kellner et al. (386). At a later date two independent studies dealt with the coordination chemistry of this ligand (26, 211). An outstanding feature of this molecule is the pyrrole ring, which, by preserving aromaticity makes the contribution of resonance form B (Fig. 4) insignificant (26) and creates a dominant -accepting character at the sulfur atoms (26). 

Clearly, it is inconvenient and tedious to write the structures of the contributing forms to show the structure of a resonance hybrid. A shorthand notation is therefore desirable. Frequently, dashed rather than full lines are used where the bonding electrons are expected to be delocalized over several atoms. For benzene, 16a or 16b is quite appropriate ... 

Resonance has a significant influence on the electron distributions and energies of molecules. The delocalization of electrons is described by the contributions of resonance forms, which are themselves localized structures with discrete bonds. Such structures are known as valence bond (VB) structures, and this approach... 

Conjugation lowers the absorption frequency of each conjugated group due to a lowering of the bond order due to the contributions of resonance forms with lower bond orders. 

In the case of sensitization of halouracil-containing DNA with ionizing radiation, attention has been drawn to potential contributions of resonant electron capture of low-energy electrons (Abdoul-Carine et al. 2001). The reaction of eaq with 8-Br-dGuo (Chap. 10.7) incorporated into DNA leads to a conversion of 8-Br-G into G in high yields with Z-form DNA being more effective than B-form DNA (Kimura et al. 2004). 

The ketenimine complex is unusual in that angle y for the complex is 11.9(10)° and the average S is 174°. These angles are indicative of a coordinated triple bond, rather than the expected double bond. This seemingly anomalous result can be rationalized on the basis of a large contribution of resonance form B to the structure of the ketenimine. This... 

The pi bond between the metal and carbon atom arises as a result of a backbonding interaction between a filled metal rf-orbital of the appropriate symmetry and the empty pi star orbital of the carbonyl. Any factor that increases the electron density at the metal will increase the strength of the M C pi bond, and thus the contribution from resonance form b. The result is a... 

For the complex [ (bpy)2Ru 2(p.-adc)]3+ (38), researchers invoked a contribution of resonance form II to the ground state in order to explain ESR and electronic absorption spectroscopy results. Alternatively, if Ru orbitals closely match the energy of the LUMO of the bridging ligand, the following resonance forms may result ... 

In reaonatkce language, there are two contributing forms. Since tlof amide resonance stabilisation would be lost if the nitrogen atom urm pro tonated, pimtonation is disfavored. 

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