Double bonds resonance structures

Figure 5.36. Schematic representation of the fullerene C60 molecule. Notice its highly symmetric structure (truncated icosahedron) in which all carbon atoms are identical and are located at the connection between two hexagons and one pentagon. The bond lengths are 138.6 pm for the bonds common to two hexagons (having a double-bond resonant structure) and 143.4 pm for the hexagon-pentagon common bonds. The bonding therefore seems to be not completely delocalized as in graphite.

As mentioned in the introduction, the wave function of CO may be approximated as a superposition of three resonance structures (see Fig. la). In the example of Cr(CO)6, we discussed the bonding on the basis of structure I. The only orbitals which seem to incorporate structures II and III are the delocalized recent study of the PtCO molecule we have found that besides the Pt-CO dative bond VBO structure, the Pt=CO structures shown schematically in Fig. 5a contribute quite significantly. The SOPP orbitals for one of the double bond resonance structures are shown in Fig. 5. The bonds between the d,s,p-hybrids on Pt and the hybrids on the carbon atom are seen to be bent. 

The determination of values of interatomic distances in molecules has been found to provide much information regarding electronic structure, especially in the case of substances which resonate among two or more valence-bond structures. The interpretation of interatomic distances in terms of the types of bonds involved is made with use of an empirical function formulated originally for single bond-double bond resonance of the carbon-carbon bond.1 There are given in this... 

Equation 7-3, for single-bond double-bond resonance, was derived by consideration of the two potential functions corresponding to the two structures A—B and A=B. In the same way we may derive an%... 

The resonance of BFj (page 145) is still a matter of some dispute becaise one chemist will point to (he double bond in structure II (favorably) another will point to F+ (unfavorably). Suggest a molecule for which charges completely rule out resonance. 

These characteristics of the benzene molecule can be explained by using the concept of resonance. We note that there are in fact two Kekule structures with exactly the same energy they differ only in the positions of the double bonds (18). As a result of resonance between these two structures, the electron density of the C C double bonds is spread over the whole molecule, thereby giving each bond a length intermediate between that of a single and a double bond. Resonance makes all six... 

The phosphorus ylide has two resonance forms one with a double bond between carbon and phosphorus, and another with charges on carbon and phosphorus. The double-bonded resonance form requires ten electrons in the valence shell of phosphorus, using a d orbital. The pi bond between carbon and phosphorus is weak, and the charged structure is the major contributor. The carbon atom actually bears a partial negative charge, balanced by a corresponding positive charge on phosphorus. 

Some molecules can t be adequately represented by a single Lewis structure. For example, two valid Lewis structures can be drawn for the anion (HCONH). One stmcture has a negatively charged N atom and a C - O double bond the other has a negatively charged O atom and a C - N double bond. These structures are called resonance structures or resonance forms. A doubleheaded arrow is used to separate two resonance structures. 

In addition, resonance can occur in the pairs of double bonds and structures XV and XVI will contribute to the structure of the molecule. 

Figure 9-11 Representations of the bonding in the benzene molecule, CgHg. (a) Lewis formulas of the two valence bond resonance structures, (b) The six p orbitals of the benzene ring, shown overlapping to form the (hypothetical) double bonds of the two resonance forms of valence bond theory, (c) In the MO description the six electrons in the pi-bonded region are dehcalized, meaning they occupy an extended pi-bonding region above and below the plane of the six C atoms.

In the drawing of the molecular structure, one oxygen atom is indicated to be bonded to the adjacent carbon atom by a double bond, and the other by a single bond. In fact, studies of the molecular structure have shown that the two carbon-oxygen distances are equal, with the value 1.25 A. This fact is accounted for by saying that the double bond resonates between the two oxygen atoms. 

The chemistry and physics of nitroxides derived from hindered piperidines attracted enormous attention for years [64,108,176,177]. Information on their behaviour were available prior to the commercial boom of HAS. EPR spectral characteristics of NO have been well defined. The structure was determined by mass spectroscopy. HAS derived nitroxides react in two canonical forms [178]. The contribution of the dipolar double bonded canonical structure 123b (perhaps nearly 50%) makes the nitroxide 123a a resonance hybrid and a species having a substantial dipole moment. 

Figure 25. (a) Interaction between a p orbital of Y and the antibonding double bond-no bond resonance structures. 

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