Types of Resonance Interactions

AN UNSHARED PAIR OF ELECTRONS NEXT TO A PI BOND 0 ACETATE ANION, ACETIC ACID, 0 ACETONE ANION. 

This is the acetate anion. The curved arrows are used to help keep track of how electrons are moved to get from the first resonance structure to the second. An unshared pair of electrons on the lower oxygen is moved in to become the pi electrons in the second structure. The pi electrons are moved to become an unshared pair on the upper oxygen. Resonance structures must always have the same total charge — in this case — I. These structures happen to be equivalent in other respects also, so they contribute equally to the resonance hybrid. With two important resonance structures, the acetate anion has a large resonance stabilization. It is significantly more stable than would be predicted on the basis of examination of only one of the structures. 

This is acetic acid, a neutral molecule. Similar resonance structures can be written for acetic acid as are shown in part 0 for the acetate anion. In this case the two structures are not the same. The second structure is still neutral overall, but it has two formal charges. Therefore, the first structure is more stable and contributes much more to the resonance hybrid than the second does. Acetic acid has a smaller resonance stabilization than that of acetate anion — it is only a little more stable than the first structure would indicate. 

The allyl radical has an odd number of electrons. The odd electron is in a p orbital, so the species is conjugated. It has two equally important resonance structures. The octet rule is not satisfied, so this radical is an unstable, reactive species. However, because of its large resonance stabilization, it is not as unstable as would be predicted on the basis of examination of a single structure without delocalization. Single-headed arrows are used to show movement of one electron, rather than electron pairs. Radicals are discussed in more detail in Chapter 21. 

These diagrams show the overlap of a series of five conjugated p orbitals in this cation. The hydrogens have been omitted for clarity in the drawing at the left. 

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Examine the structure and determine the conjugated series of orbitals that is involved in resonance. Next, determine the types of resonance interactions that are present (see Figures 3.16-3.20). Draw the resonance structures and evaluate their relative stabilities... 

Types of Resonance Interactions Mastery Goal Quiz Molecular Model Problems... 

Despite the complication which resonances introduce into the analysis of a spectrum and the theoretical treatment of the hamiltonian, when they can be analysed they often give valuable information on the force field which cannot be obtained directly in the absence of a resonance. We consider briefly the two commonest types of resonance interaction, Fermi (or anharmonic) resonance and Coriolis resonance, to illustrate this point. 

In general, the dissection of substituertt effects need not be limited to resonance and polar components, vdiich are of special prominence in reactions of aromatic compounds.. ny type of substituent interaction with a reaction center could be characterized by a substituent constant characteristic of the particular type of interaction and a reaction parameter indicating the sensitivity of the reaction series to that particular type of interactioa For example, it has been suggested that electronegativity and polarizability can be treated as substituent effects separate from polar and resonance effects. This gives rise to the equation... 

There are two common features of these two types of hyperfine interactions. First, both 8C and 8pc are inversely dependent on temperature. Second, they both depend on the total electronic spin 5 of the iron atom as shown in Eqs. (1) and (2). The magnitude of the hfs resonances depends on the 5 value of each Hb derivative. For example, met-Hb is a high-spin ferric complex with five unpaired electrons per heme, deoxy-Hb is a high-spin ferrous complex with four unpaired electrons per heme, and both cyanomet- and azidomet-Hb are low-spin ferric complexes, each with one unpaired electron per heme. The hyperfine interactions can shift resonances either upfield or downfield from their diamagnetic counterparts. It should be noted that both HbC>2 and HbCO are low-spin ferrous complexes that are diamagnetic systems (S = 0) and they will not give rise to hyperfine interactions. 

An additional requirement for high-temperature superconductivity is that such hypo-electronic atoms as La, Y, Ba, or Sr can interact with the hyperelectronic Cu atoms. This results in electron transfer from the Cu atoms to the hypoelectronic atoms, which leads to the formation of covalent bonds that resonate among the Y-Y and Y-Cu positions, conferring electronic conductivity on the substance. These two types of resonance caused by the combination of crest and trough metals couple with the phonons to yield superconductivity at relatively high temperatures. 

The type of orbitals involved in sulfur-bond formation varies considerably by structure and can be represented as involving orbitals which have p, sp hybrid or d character. The role of the sulfur d orbitals has been a matter of some debate but at least in compounds containing unsaturated bonds and/or electron-withdrawing groups the d orbitals are credited with conferring enhanced stability on the ground state of the molecules as a result of resonance interaction. In photochemical rearrangements of thiophene derivatives for example (vide infra) the transition states proposed all involve d-orbitals participation by sulfur. 

Also the analysis of relative intensities (as a function of primary energy and geometric conditions) is important to assess the type of mechanism acting on the excitation. In polystyrene and for electronic excitations, that analysis showed that the behavior of relative intensities as a function of primary energy is typical of resonant interactions. This is compatible with the fact that triplet excitation (optically forbidden for spin reasons) intensity is favored relative to singlet excitation. 

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