Resonance acceptor effect

Resonance acceptor effect of R3M substituents towards Rff (d- r conjugation) and X (d-n conjugation) decreases as M changes in the order Si > Ge > Sn > Pb. The question whether the R = Pb group still has resonance acceptor properties is under debate. 

Sn > Pb. This trend agrees with a maximal resonance acceptor effect (d-jr conjugation) for M = Si mentioned above repeatedly, which weakens as the atomic number of M increases. 

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... 

A complex mechanism of the resonance acceptor effect (the d-jr conjugation in R3MRjr and the d-n conjugation in R3MX, for M = Ge, Sn, Pb). This effect is absent when M = C. The acceptor effect includes the participation of unoccupied nd-orbitals of M and of antibonding a -orbitals of the M—R bonds of the R3M fragments (R = alkyl). 

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. 

According to Figure 13.1, carbon-bound H atoms are acidic if they are bound to carbon atoms that are in the a-position with respect to an electron acceptor that can stabilize a negative charge via resonance (-M effect). Carbon-bound H atoms are even more acidic if they are located in the opposition of two such electron acceptors, which is the case in the so-called active-methylene compounds. Enolates derived from active-methylene compounds require three resonance forms for their description, and resonance forms A and B (Figure 13.6) are the more important ones. Compounds that contain an FI atom in the Opposition with respect to three electron acceptors are even more acidic than active-methylene compounds. However, such compounds do not play a significant role in organic chemistry. 

A study of substituent effects on the N chemical shift (5 N) (Table 10) for 4-substituted anilines in DMSO was interpreted in terms of substituent solvation-assisted resonance (SSAR) effects. Solvation of certain conjugated r-electron-acceptor (-I-R) substituents has been found to give signihcant enhancements in the acidities of anilines, phenols and other acids , and the magnitudes of these enhancements increase with... 

The substituent is an electron acceptor and the active site a strong electron donor by resonance. Delocalized effect substituent constants for use in these two cases are designated... 

Table 7.4 lists the Q and e values for an assortment of common monomers. The extremes in the column of e values in Table 7.4—which are listed in order-quantify the range of donor-acceptor properties which is used as the basis for ranking in Fig. 7.2. The Q values perform a similar ranking with respect to resonance effects. The eight different Q-e combinations in Table 7.4 allow the estimation of ri and values for 28 different copolymers. Of course, in these systems Q and e values were assigned to give the best fit to r values which had already been measured. As an illustration of the predictive values of the Q-e scheme, consider the following example ... 

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

Notice that the MO picture gives the same qualitative picture of the substituent effects as described by resonance structures. The amino group is pictured by resonance as an electron donor which causes a buildup of electron density at the /3 carbon, whereas the formyl group is an electron acceptor which diminishes electron density at the /3 carbon. 

Those sets for which the resonance effect is predominant are the sets which are most likely to give rise to the free carbonium ion 5, as the substituents in these sets (sets 15-14 and 15-17 and possibly 15-18) are all donors by resonance, as is shown by their Or values. Those sets for which the localized effect is predominant may be accounted for in terms of intermediates 3 or 4. Sets 15-5, 15-7B2, and 15-12 gave significant values of jS. It is difficult to account for this fact in terms of intermediate 4. The results can be accounted for in terms of intermediate 3, however, if this species resembles other three-membered rings, such as cyclopropane, in its behavior. Sets 15-6, 15-8, 15-9, 15-12, and 15-15 include both donor and acceptor substituents. The successful correlation of... 

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