Mesomeric effect resonance

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

The term has been deemed obsolescent or even obsolete (see mesomeric effect, resonance effect). Many have used phrases such as enhanced substituent resonance effect that imply the operation of the electromeric effect without using the term, and various modern theoretical treatments parametrize the response of substituents to electronic demand, which amounts to considering the electromeric effect together with the INDUCTOMERIC EFFECT. 

The underlying principle of the PEOE method is that the electronic polarization within the tr-bond skeleton as measured by the inductive effect is attenuated with each intervening o -bond. The electronic polarization within /r-bond systems as measured by the resonance or mesomeric effect, on the other hand, extends across an entire nr-system without any attenuation. The simple model of an electron in a box expresses this fact. Thus, in calculating the charge distribution in conjugated i -systems an approach different from the PEOE method has to be taken. 

Over the years, the Hammett equation has been modified many times, usually by defining an alternative scale of a constants, the better to allow for special features found in some mechanisms, such as resonance stabilization and mesomeric effects. Thus, there are substituent scales known at cr+, er , crj, etc. The reader is referred to specialized treatises for further details.5-811... 

The marked acid-strengthening effect of p-NOy is usually attributed to the influence of the electron-attracting inductive effect (+/), augmented by a small electron-attracting mesomeric or resonance effect (+R). The smaller acid-strengthening effect of m-NOy is explained as the resultant of the inductive effect and a small relayed influence of the resonance effect. If op is regarded simply as a sum of oj and or (Section II.B) and a/ is taken as 0.67 (Section III.A), a value of 0.78 — 0.67 = 0.11 is indicated for or. The relay factor of 0.33 for the resonance effect accounts reasonably well for the value of om as ay + 0.33or = 0.67 + 0.04 = 0.71 cf 0.71 above. 

This electron-donating effect from lone pair electrons is simply a resonance effect, but is often termed a mesomeric effect. A mesomer is another term for a... 

Electron withdrawal arising from the inductive effect in the a and from the resonance (mesomeric) effect in the fi position of nitroalkenes (Table 4.48) induces deshieldings of similiar magnitudes and large two-bond carbon-proton coupling constants (up to 8 Hz). 

In Table 4.77 a small selection of carbanionic species has been compiled [502-510]. The carbanionic carbon shift of methyllithium is — 16.6 ppm (Table 4.71) in comparison to - 23.1 ppm, which is predicted for an sp2 carbon containing two electrons in a p orbital, following the empirical carbon-13 shift to charge density correlation [76, 507]. Carbanion carbon shifts become progressively more positive with increasing delocalization of the negative charge by resonance (mesomeric) effects, as shown for allyl and pentadienyl anions [503-505] in Table 4.77. 

Table 4.5 Examples of the different electronic substitution constants used in QSAR studies. Inductive substituent constants (crO are the contribution the inductive effect makes to Hammett constants and can be used for aliphatic compounds. Taft substitution constants (cr ) refer to aliphatic substituents but use propanoic acid (the 2-methyl derivative of ethanoic acid) as the reference point. The Swain-Lupton constants represent the contributions due to the inductive (.F) and mesomeric or resonance (R) components of Hammett constants. Adapted from An Introduction to the Principles of Drug Design and Action by Smith and Williams 3rd Ed. (1998) Ed. H.J.Smith. Reproduced by permission of Harwood Academic Publishers.

So far, only one detailed discussion of boron-11 nuclear magnetic resonance spectra of aminoborane systems has been reported 31>. It was found that the 1 lB chemical shifts of aminoborane systems can be described fairly well in terms of a set of additive substituent contributions. In consonance with earlier work on trisubstituted boron compounds 35> these contributions depend on the mesomeric effects of substituents rather than their electronegativity. 1,3,2-diazaboracycloalkanes can be considered as aminoborane derivatives and in the case of the known heterocycles the exocyclic boron substituent will govern primarily the boron chemical shifts and will do so by mesomeric effects. However, the available data are rather limited and it may be possible that additional factors must be considered. Steric effects appear to be negligible, however, since the heterocycles with either six or seven annular atoms have almost identical shifts (Table 5). 

With respect to a-substituents bearing p- or 7r-electrons which are directly attached to the C—Cl bond (Table 6, Z = CH2=CH to CH3CH20), these may delocalize their electrons through resonance or mesomeric effects with the positively charged carbon atom in the transition state. Because of this, they were not plotted in the Taft figure for a-substituted ethyl chlorides. Furthermore, the rates for these substituents also could not be correlated with the electrophilic substituent constants a+. The o+ parameters have been defined for substituents on the benzene ring which are far from the reaction site. Even though steric effects may interfere with the coplanarity and hence with delocalization, the effect of these substituents was believed to be polar in nature. 

Despite the success in parameterizing acid/base and many other properties for a range of different compounds, it is obvious that the simple electrostatic model used by Taft and extended by others [27, 28] have fundamental weaknesses - both with regards to the domain of validity and at a more fundamental level. The model is more intuitive than physical, in the sense that the inductive effect, polarisation effect, resonance effect, mesomeric effect and steric effect have no proper quantum mechanical definition, and can therefore not be derived directly from the system s wave function [29,30]. 

Resonance effect is an energy stabilization due to delocalization of electrons in the bond network of the molecule and can be attributed to a mesomeric effect, i.e. the delocalization of Jt electrons on the jr orbital network, a hyperconjugation effect, i.e. a delocalization of a electrons in a ji orbital aligned with the o bond, and secondary mesomeric effects, such as repulsion of the ir electrons by nonbonded electrons on a substituent or solvent, or by time-dependent effects due to polarizabilities (for the last, the term electromeric effect is sometimes used). 

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