Resonance stabilization energies

Resonance stabilization energies are generally assessed from thermodynamic data. If we define to be the resonance stabilization energy of species i, then the heat of formation of that species will be less by an amount ej than for an otherwise equivalent molecule without resonance. Likewise, the AH for a reaction which is influenced by resonance effects is less by an amount Ae (A is the usual difference products minus reactants) than the AH for a reaction which is otherwise identical except for resonance effects ... 

In writing the second version of this, the proportionality constant has been set equal to unity as a simplification. Note that the resonance stabilization energy of the reference radical Ri- also cancels out of this expression. 

Each exponential involves the difference between the resonance stabilization energy of the radical and monomer of a particular species. 

We might be hard pressed to estimate the individual resonance stabilization energies in Eqs. (7.23) and (7.24), but the qualitative apphcation of these ideas is not difficult. Consider once again the styrene-vinyl acetate system ... 

Define styrene to be monomer 1 and vinyl acetate to be monomer 2. The difference in resonance stabilization energy ep. - > 1, since... 

Um, Ua, Up Resonance stabilization energies for monomer, attacking radical and product radical, respectively (Chap. V). 

As a general trend, six-membered mononuclear N-heteroaromatics such as pyridine and derivatives are much less prone to undergo hydrogenation than bi-and trinuclear N-ring compounds (e.g., quinolines, benzoquinolines, acridines) due to their higher resonance stabilization energy. 

Because of the opposite nature of the n interactions between the ring and either the nitro or the amino substituent, let us assess the stabilization energies of nitrobenzene and aniline. In equation 13, the resonance stabilizing energy of aniline was defined as the exothermicity of a reaction involving arbitrary reference states. By analogy to equation 13, we may write equation 57 for nitrobenzene and the same arbitrary reference states, R = /-Pr or t-Bu. 

Fig. 30 Correlation of resonance stabilization energy for diaryl telluride radical cations with Pa. Data from Ref. 113.

Resonance Stabilization Energies of Select Molecules (Pauling, 1960)... 

The energy released by a molecular entity as a consequence of resonance, the magnitude of which equals the difference between the actual molecule s energy and the energy of the least energetic canonical form. Because the isolated canonical form does not exist factually, the resonance stabilization energy can only be estimated. 

DEWAR STRUCTURES KEKULE STRUCTURES RESONANCE STABILIZATION ENERGY RESPIRATORY BURST OXIDASE NADPH OXIDASE RESPIRATORY EXCHANGE RATIO Restricted rotation,... 

Problem 11-17. What is the resonance stabilization energy of cyclobutadiene, according to Hiickel theory Express your answer in terms of / . 

Since a calculation of the resonance energy of benzene by the valence bond method shows that the greater part of it is a result of the resonance between the two Kekule structures shown, we might suppose that its homologs would also have significant resonance stabilization energies. Such conclusions are at variance with experimental fact, however, since cyclobutadiene appears to be too unstable to have any permanent existence, and cyclooctatetraene exists as a nonplanar tetraolefin, having no resonance stabilization of the sort considered. 

The stability of phosphabenzene and of arsabenzene in the absence of air and the isolation of the silicon-carbon and silicon-silicon double bonds might suggest that silabenzene, appropriately substituted, could be stable enough to be isolable. Indeed, calculations suggest that it would have a it-resonance stabilization energy about two-thirds that of benzene (78JA6499). 

These resonance stabilization energies of free radicals can be quite large, e.g. 50 kj mol 1 for benzyl-, and 70kjmol 1 for methyl-Np. These must be included in the overall energy balance of the reaction, and can make all the difference between a fast, highly exergonic process, and an endergonic process which in practice does no take place at all. 

In the case of the triphenylmethyl radical shown in Figure 4.86, it is possible to write many different resonance structures but in a small free radical such as the methyl radical there is only one possible structure. The reactivity of the radical decreases as the unpaired spin density at each site decreases, and the radical also becomes more stable because of the resonance energy. This resonance stabilization is zero for the phenyl radical, since the unpaired electron resides in an orbital which is orthogonal to the it system. By contrast, the methylphenyl radical has a resonance stabilization energy of some 10 kcalmol-1, and the larger methylnaphthyl radical is stabilized by about 15 kcalmol-1. These resonance stabilizations can have important consequences for the energy balance of photochemical reactions (see e.g. sections 4.4.2 and 4.4.4). 

Resonance Stabilization Energies of Isomeric Pyrrolizines in fl Units 14... 

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