Resonance structures charge-delocalized

Some fundamental structure-stability relationships can be employed to illustrate the use of resonance concepts. The allyl cation is known to be a particularly stable carbocation. This stability can be understood by recognizing that the positive charge is delocalized between two carbon atoms, as represented by the two equivalent resonance structures. The delocalization imposes a structural requirement. The p orbitals on the three contiguous carbon atoms must all be aligned in the same direction to permit electron delocalization. As a result, there is an energy barrier to rotation about the carbon-carbon... 

According to Branchini et al. (2004), luciferase modulates the emission color by controlling the resonance-based charge delocalization of the anionic keto-form of oxyluciferin in the excited state. They proposed the structure C5 as the yellow-green light emitter, and the structure C6 as the red light emitter. 

Please note that while nitro groups are so electron withdrawing that delocalization of their associated positive charge plays a minimal role in any family of resonance structures, this delocalization is technically possible. Try to identify additional resonance structures where the positive charge is delocalized. 

Remember that neither resonance form adequately represents acetate. The true structure is a hybrid of both structures. In the hybrid, the electron pairs drawn in different locations in individual resonance structures are delocalized. With acetate, a dashed line is used to show that each C-0 bond has a partial double bond character. The symbol 8 (partial negative) indicates that the charge is delocalized on both O atoms in the hybrid. 

Branchini et al3 proposed that L0=0 is the only emitter, and luciferase modulates bioluminescence spectra by controlling the resonance-based charge delocalization of the anionic keto form of the oxyluciferin excited state. A mechanism4 involving twisted conformation to explain color changes in the firefly bioluminescence is not feasible as was shown by experimental5 and theoretical studies.6 Besides, independently of the specific molecular structure of red and green emitters it is the properties of the emitter microenvironment that are responsible for the observed differences in bioluminescence spectra and their pH sensitivity. 

Factor 1 does not answer the question because the negative charge is on oxygen in both cases. Factor 2 also does not answer the question because there are resonance structures that delocalize the charge over two oxygen atoms in both cases. The difference between these compounds is clearly the chlorine atoms. Recall that each chlorine atom withdraws electron density via induction ... 

Write resonance structures for cyclopentadienyl anion suffi cient to show the delocalization of the negative charge over all five carbons J... 

Molecular orbitals are useful tools for identifying reactive sites in a molecule. For example, the positive charge in allyl cation is delocalized over the two terminal carbon atoms, and both atoms can act as electron acceptors. This is normally shown using two resonance structures, but a more compact way to see this is to look at the shape of the ion s LUMO (the LUMO is a molecule s electron-acceptor orbital). Allyl cation s LUMO appear s as four surfaces. Two surfaces are positioned near- each of the terminal car bon atoms, and they identify allyl cation s electron-acceptor sites. 

Another way to assess thiophene s reactivity is to compare the intermediate ions formed by addition of N02. Examine the structures, charge distributions and electrostatic potential maps of thiophene+nitronium at C2 and thiophene+nitronium at C3. Draw all of the resonance contributors needed to describe these structures. Which, if either, better delocalizes the positive charge Compare the energies of the two intermediates. Which product should form preferentially if the reaction is under kinetic control Are these results consistent with FMO theory ... 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

The negative charge of the cyclohexadienyl anion 5 is delocalized over several carbon centers, as is illustrated by the following resonance structures ... 

Thus, in the present approach, the major focus is on the question of how we can influence the external parameters like solvent and counterion and the intrinsic structural parameters within the systems A-l-A to force the electron-hopping process into the timescale of the experiment, or at least to establish clearly the borderline cases. That we are still looking at an electron-hopping process in the case of effective charge delocalization over the entire molecule and not at a pure resonance phenomenon may be reassured by VIS/NIR spectroscopy of the neutral and charged species the absorption of a single chromophore should be detected unless a very fast process > 1012 Hz is taking place. 

Ab initio calculations (MP2/6-31G ) of the parent compound of 8 revealed that the most stable arrangement of the dimer adopts Dih symmetry (Fig. 5). Interestingly, the four Li ions and the two phosphorus centers constitute an octahedral skeleton with relatively short Li-Li and Li-P distances of 2.645 and 2.458 A, respectively. Charge analysis (22) undoubtedly supports the electrostatic bonding model for this system because of the high net charges of the natural atomic orbitals (NBO) at Li (+0.768) and P (-1.583), while NBO-Lewis resonance structures support stabilization through delocalization (Fig. 5). 

The benzyl carbanion has been found to be most stable because of the extensive delocalization of the negative charge over the various resonating structures. 

It is commonly supposed that the stabilization of carbanions by +R groups is due to delocalization of negative charge from the carbanionic carbon. In the case of NO2, resonance structures as in 24 are written184 ... 

While 4bp/-type structures can be considered as resonance structures, 4cp/is also an alternative. The importance of structure 4cp/is supported by the fact that the negative charge at the carbon atoms in the 2 and 5 positions is indeed larger than at the 3 and 4 positions. ° The common feature in all 4ap/-, 4bp7-, and 4cp/-type structures is that the lone pair of the planar tricoordinate phosphorus is fully delocalized, resulting in an enhanced bond order about the tricoordinate planar phosphorus. The d-orbital participation at phosphorus is insignificant in 4, either in the planar or nonplanar form. °... 

Delocalization of charge in the conjugate base anion through resonance is a stabilizing factor and will be reflected by an increase in acidity. Drawing resonance structures allows us to rationalize that the negative charge is not permanently localized on a particular atom, but may be dispersed to other areas of the structure. We should appreciate that a better interpretation is that the electrons are contained in a molecular orbital that spans several atoms. 

A good illustration of this concept is seen in a series of nitrophenols. The nitro group itself has to be drawn with charge separation to accommodate the electrons and our rules of bonding. However, resonance structures suggest that there is electron delocalization within the nitro group. 

Note that m-nitrophenol has pATa 8.4, and is a lot less acidic than o-nitrophenol or p-nitrophenol. We can draw no additional resonance structures here, and the nitro group cannot participate in further electron delocalization. The increased acidity compared with phenol can be ascribed to stabilization of resonance structures with the charge on a ring carbon through the nitro group s inductive effect. 

Now let us look at guanidines, which are even stronger bases. Guanidine itself has pATa 13.6. It can be seen that there is delocalization of charge in the conjugate acid, such that in each resonance structure the charge is favourably associated with... 

The pyridine nucleotides NAD and NADP always function in unbound form. The oxidized forms contain an aromatic nicotinamide ring in which the positive charge is delocalized. The right-hand example of the two resonance structures shown contains an electron-poor, positively charged C atom at the para position to nitrogen. If a hydride ion is added at this point (see above), the reduced forms NADH or NADPH arise. No radical intermediate steps occur. Because a proton is released at the same time, the reduced pyridine nucleotide coenzymes are correctly expressed as NAD(P)H+HT... 

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