Delocalization, resonance

This phenomenon is not possible in p-nitrobenzoic acid hence, p-nitrophenol is a stronger acid with respect to p-nitrobenzoic acid than is expected on the basis of a comparison of substituents in which this resonance delocalization is not an important factor. It was, therefore, recommended that Op = 1.27 be used for p-nitro derivatives of phenols and anilines, rather than the Op = 0.78 given in Table 7-10. These enhanced sigma constants, symbolized a, apply primarily to electron-withdrawing groups in reactions aided by low electron density at the reaction site. 

IR VTMCD band in each of the [Fe3S4] spectra, therefore, corresponds to the uniaxial a- a transition of Fe-Fe interaction and the resonance delocalization energy, (3, is determined to be 4290 25 cm for Type 1 clusters and 4350 25 cm for Type 2 clusters. Hence, the Fe-Fe interactions within the valence-delocalized pair are very similar in both locations. 

The range of structural alternatives explored by valency-deficient carbon species and the subtle interplay of substituents is remarkable. Scheme 7.6 (ORTEP adapted from reference 31) illustrates an example of an X-ray structure clearly describing a localized [C-H C+] carbenium ion (A) where a symmetric bridging structure [C-H-C] + (B) could have been assumed. In this case it is proposed that a charge-transfer interaction between the resonance delocalized cation and the adjacent electron-rich carbazol moiety may be responsible for the stabilization of the localized form over the three-center, two-electron (3c-2e) bridging structure. 

As the discussion of Chapter 2 and the numerical charges in (3.190) suggest, the extreme ionic picture such as (3.189a) must be modified by donor-acceptor interactions that create partial covalency by delocalizing significant charge ( 0.5e) from bare fluoride ions into acceptor orbitals of the central cation. Such partial-covalency effects can be represented by resonance delocalization of the form... 

Another type of hypervalency is encountered in textbook descriptions of the oxyanions of common laboratory acids. Generations of chemistry students have been taught that the correct representations of these species are in terms of resonance-delocalized hypervalent Lewis-structure diagrams, such as sulfate (S042-),... 

Thus, neither hypervalency nor strong resonance delocalization is implied by Lewis s original electron-dot picture, which rests on a distinctly ionic (not electroneutral ) picture of the bonding. 

The specific realization (3.259) of the favored 0.10.3.0 styx pattern is only one of many equivalent localized representations. The central 1—4—5 triangle of Fig. 3.110 could equivalently be chosen as any of the 20 possible triangular facets in Fig. 3.111, leading to 20 equivalent three-center Lewis structures that contribute to the overall resonance delocalization of Bi2Hi22. The non-Lewis density of any such structure is found to be... 

In this case, the principal resonance delocalization accompanying H-bond formation involves only the two alternative forms of the terminal carboxylate moiety,... 

In contrast, resonance delocalization and bond alternation in the C—C=C—C backbone are only slightly affected by H-bond formation (namely, the C4—C5 bond order varies by only 0.008 between H-bonded and open conformers), because such resonance shifts do not intrinsically alter the charge distribution in the H-bonded O- H—O triad. This example illustrates the principle that H-bonding is not generally coupled to resonance per se, but only to such resonance as leads to effective CAHB enhancement (Section 5.2.2). 

The most common and also most effective mechanism of radical stabilization involves the resonant delocalization of the unpaired spin into an adjacent 7r system, the allyl radical being the prototype case. A minimal orbital interaction diagram describing this type of stabilization mechanism involves the unpaired electron located in a 7r-type orbital at the formal radical center and the 7r- and tt -orbitals of the n system (Scheme 1). 

X = C6H4Y) have lifetimes of 0.6 ms (Y = 4-OMe) to 26 ns (Y = 4-CF3). They are quenched by azide ion at the diffusion limit. The Yukawa-Tsuno r+ value for these is 2.8, also consistent with a large amount of resonance delocalization. Photolysis of (52) results in the A -methyl-iV-phenylnitrenium ion (53), which presumably gives the observed products (54), aniline and iV-methylaniline as shown. ... 

We have also shown some representations of acetate and methanesulfonate anions that have been devised to emphasize resonance delocalization these include partial bonds rather than double/single bonds. Although these representations are valuable, they can lead to some confusion in interpretation. It is important to remember that there is a double bond in these systems. Therefore, we prefer to draw out the contributing resonance structures. 

We were even less keen on the second example, where, in the resonance delocalization step, an arrow is shown taking electrons away from a positive charge and creating a new positive centre. 

Stabilized by resonance delocalization indeed, they are even more stable than tertiary radicals. In the presence of a suitable initiator, bromine dissociates to bromine atoms that will selectively abstract an allylic or a benzylic hydrogen from a suitable substrate, generating the corresponding allyl and benzyl radicals. 

N-9 proton is lost giving an anion with substantial resonance delocalization of charge. 

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