Resonance donor effect

The resonance donor effect of R3M substituents towards Rff (a-n conjugation), shown schematically in 1, increases as M changes in the order C < Si < Ge < Sn < Pb. One can observe a weak a-n conjugation effect even in organic compounds. 

The low PRF at the mef -position of anisole is noteworthy. At the raeta-position, there is no resonance effect, so we see the inductive withdrawal effect of the OMe group. At the ortho- and /2 ra-positions, this is masked by the much greater resonance donor effect. 

The resonance-mediated coupling mechanisms described above involve subtle quantal intramolecular/intermolecular donor-acceptor effects that tend to be inadequately described by current-generation empirical potentials. Simulations based on these potentials are therefore likely to be inherently defective for describing realistic folding processes in proteins. However, approximations such as those illustrated in Example 5.8 may ultimately make it feasible to incorporate additional resonance-mediated effects into empirical force fields of tractable form. 

One-electron oxidation of toluene results in the formation of a cation radical in which the donor effect of the methyl group stabilizes the unit positive charge. Furthermore, the proton abstraction from this stabilized cation radical leads to the conjugate base, namely, the benzyl radical. This radical also belongs to the it type. Hence, there is resonance stabilization in the benzyl radical. This stabilization is greater in the benzyl radical than in the tt cation radical of toluene. As a result, the proton expulsion appears to be a favorable reaction, and the acid-base equilibrium is shifted to the right. This is the main cause of the acidylation effects that the one-electron oxidation brings. 

For all benzene substituents which are resonance donors, ap > ap 1. It follows from equation 23 that the differences ap 1 — ap characterizing the strengthening of the a-n conjugation increase with the enhancement of the polarizability of all the bonds within the substituent bonded to the aromatic ring. The quantitative characteristic of the overall substituent polarizability is the sum of the refractions of its bonds, E R > (see Section I). The values of ap+ and ap are approximately equal for organic substituents which are resonance acceptors. If the R3- X M substituents had only a resonance acceptor effect (the d-7T conjugation), the correlation in equation 23 would fail for compounds R3-nX MPh. In fact, equation 24... 

The presence of a strong resonance donor 0-71 conjugation effect in R3MCH2Rjt molecules. 

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