Electron delocalization resonance description

One way to assess the relative stabilities of these various intermediates is to exam me electron delocalization m them using a resonance description The cyclohexadienyl cations leading to o and p mtrotoluene have tertiary carbocation character Each has a resonance form m which the positive charge resides on the carbon that bears the methyl group... 

Resonance description of electron delocalization in carboxylate anion... 

The corresponding resonance description shows the delocalization of the nitrogen lone pair electrons m terms of contributions from dipolar structures... 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

Like pyrrole, its resonance description requires the delocalization of one of the lone pair electrons of the oxygen atom with the cyclopentadiene unit (Scheme 6.21). 

In the valence bond or hybridization model for CO2, we have two resonance (or canonical) structures, as shown in Fig. 3.4.7. In both structures, the two a bonds are formed by the sp hybrids on carbon with the 2pz orbitals on the oxygens. In the left resonance structure, the jv bonds are formed by the 2px orbitals on C and Oa and the 2p-y orbitals on C and O. In the other structure, the jv bonds are formed by the 2px orbitals on C and Ob and the 2py orbitals on C and Oa. The real structure is a resonance hybrid of these two extremes. In effect, once again, we get two a bonds, two jv bonds, and four lone pairs on the two oxygens. This description is in total agreement with the molecular orbital picture. The only difference is that electron delocalization in CO2 is... 

Although benzene is still drawn as a six-membered ring with three alternating Jt bonds, in reality there is no equilibrium between two different kinds of benzene molecules. Instead, current descriptions of benzene are based on resonance and electron delocalization due to orbital overlap, as detailed in Section 17.2. 

We now outline two approaches to a description of valence in the boron hydrides. The first employs three-center bonds. This particular kind of localized molecular orbital seems most suitable for the smaller, more open hydrides. Its use in the more complex hydrides will require delocalization of the bonding electrons either by a molecular orbital modification or a resonance description. The second approach is simply that of molecular orbitals, which is particularly effective in the more condensed and symmetrical hydrides. These approaches merge as the discussion becomes more complete. It is an important result that filled orbital descriptions are obtainable for the known boron hydrides. Also some remarks about charge distribution in the boron hydrides are possible. But the incompleteness of this valence theory in this nontopological form is indicated by the lack of a large number of unknown hydrides, whose existence would be consistent with these assumptions. 

Resonance attempts to correct a fundamental defect in Lewis formulas. Lewis formulas show electrons as being localized they either are shared between two atoms in a covalent bond or are unshared electrons belonging to a single atom. In reality, electrons distribute themselves in the way that leads to their most stable arrangement. This sometimes means that a pair of electrons is delocalized, or shared by several nuclei. What we try to show by the resonance description of ozone is the delocalization of the lone-pair electrons of one oxygen and the electrons in the double bond over the three atoms of the molecule. Organic chemists often use curved arrows to show this electron... 

As might be anticipated, there is an intimate connection between the description of electron delocalization in terms of NBO interactions and resonance structmes. Consider the vic-... 

In Section 7-6 we described resonance formulas for molecules and polyatomic ions. Resonance is said to exist when two or more equivalent Lewis formulas can be written for the same species and a single such formula does not account for the properties of a substance. In molecular orbital terminology, a more appropriate description involves delocalization of electrons. The shapes of molecular orbitals for species in which electron delocalization occurs can be predicted by combining all the contributing atomic orbitals. 

A single electron-dot formula cannot properly describe delocalized bonding. Instead, a resonance description is often used. According to the resonance description, you describe the electron structure of a molecule having delocalized bonding by writing all possible electron-dot formulas. These are called the resonance formulas of the molecule. The actual electron distribution of the molecule is a composite of these resonance formulas. 

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