Resonance Lewis structures

Two Lewis structures (resonance forms) of formamide the atoms are connected in the same order but the arrangment of the electrons is different... 

The blended structure is a resonance hybrid of the contributing Lewis structures. Resonance should be thought of as a blend of the individual Lewis structures rather than as the flickering of a molecule between different structures, just as a mule is a blend of a horse and a donkey, not a creature that flickers between the two. 

We can elaborate this VB formulation for the cycloaddition by replacing the nearest-neighbour active-space AOs in VB structures 50 and 51 with Coulson-Fischer orbitals [34(b)]. Thus if a and b are now the singly-occupied carbon and oxygen AOs of HCNO, and c and d are the singly-occupied carbon AOs of HCCH, the c and d AOs in structure 50 can be replaced by the Coulson-Fischer MOs c + k d and d + k"c. In structure 51, a + Ad, b + Ac, c + K b and d + K"a can replace the a, b c and d AOs. Use of these orbitals permits additional canonical Lewis VB structures to be included in the equivalent Lewis structure resonance scheme. The mechanism can then accommodate some charge transfer between the HCNO and HCCH reactants. The more-flexible wavefunction of Eq.(13),... 

Because of the inclusion of Dewar-type as well as the Kekul6-type structures in the Lewis structure resonance scheme, the increased-valence structures are more stable than are the familiar Kekule-type Lewis structures from which they are derived, provided that the one-electron bond polarity parameters, are chosen variationally. Therefore as discussed already in Section 8, a better (i.e. lower energy) VB description of the bonding may be obtained when increased-valence structures rather than only the component Kekul6-type structures, are used to provide VB representions of electronic structure. 

The first chapter of the text covers the basic features associated with the bonding together of atoms to make molecules. Much of the material (at least through Section 1 -8) is really a review of topics with which you may have some familiarity from freshman chemistry. In other words, it describes just those topics from freshman chemistry that are the most important to know in order to get off to a good start in organic chemistry bonds, Lewis structures, resonance, atomic and molecular orbitals, and hybrid orbitals. Read the chapter, try the problems, read the comments below, and, if necessary, look to other supplementary sources for additional problems and examples. 

This brings us to a new term, resonance. A molecule shows resonance when more than one Lewis structure can be drawn for the molecule. In such a case we call the various Lewis structures resonance structures. 

In Lewis structures, resonance occurs when there are two or more equivalent ways to draw a double bond. The actual structure is an average of all of the equivalent structures. 

Covalent Lewis Structures, Resonance, and Formal Charge... 

Molecular orbital theory is a tool that is used more frequently in advanced organic courses than in this course. This activity is designed as a brief introduction to the topic. Though you may be asked to generate a simple MO diagram of the pi system of a small symmetrical molecule, generating an MO diagram of a complex or asymmetrical molecule is a task beyond the scope of this course. Most predictions we will make in this course will use the older, simpler tools we have become familiar with like Lewis structures, resonance structures, and hybridization. 

As indicated already in Chapter 2, unless stated otherwise (see for example Chapter 23), the equivalent Lewis structure resonance theory assumes that electron-pair bond wavefunctions are of the Heitler-London atomic orbital type - for example y(l)a(2) + a(l)y(2) a.nAy l)b(2) + b )a 2) for structures (6) and (7). Atomic formal charges are not indicated in the generalized valence bond structures that involve the Y, A, B, C and D atoms. 

Each of the increased-valence structures of Fig. 13-4 involves an increased-valence bonding unit of type (17) for the six rc-electrons. It summarizes resonance between the standard Lewis structure (18) and the two long-bond Lewis stractures (19) and (20). (Because structure (21) involves three electrons located in the A-atom atomic orbital, this structure cannot be included in the Lewis structure resonance scheme.)... 

Example 11.3 Writing Lewis Structures Resonance Structures... 

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