Resonance forms nonequivalent

The canonical structures of nonequivalent resonance forms, on the other hand, are not related by a symmetry element. Consider the three possible Lewis structures for SCN shown in Figure 6.7. There is no rotational axis about which the canonical forms can be interchanged to form an equivalent configuration. In the case of nonequivalent canonical structures, the weighting is unequal and some of the canonical structures will make a larger contribution to the resonance hybrid than will others. 

Altliough both forms are contributors to the true structure of the ion, we shall see that one contributes more than the other. The question is, which one If we extend our consideration of nonequivalent resonance forms to include those containing atoms without electron octets, the question becomes more general. 

When two resonance forms are nonequivalent, the actual structure of the resonance hybrid is closer to the more stable form than to the less stable form. I bus, we might expect the true structure of the acetone anion to be closer to the resonance form that places the negative charge on an electronegative oxygen atom than to the form that places the charge on a carbon atom. 

If resonance forms are nonequivalent, the structure of the actual molecule resembles the more stable resonance form(s). 

The result is the second-best resonance form—okay for octets, but charges are separated, which makes it less of a contributor than the Lewis structure on the left. The hybrid will more closely resemble the left-hand structure, with two nonequivalent NO bonds. The contribution of the right-hand structure, while small, will tend to make the NO bond at the end the most polar one in the molecule, with the O at the negative end. 

When the canonical forms all contribute equally to the bonding, they are called equivalent canonical forms when they contribute unequally, they are nonequivalent canonical forms. Therefore, the structures shown in Figure 6.5 for ozone represent equivalent canonical forms. They are related to each other by a symmetry element—in this case, a twofold rotational axis that passes through the central oxygen. As a result, each canonical structure contributes exactly 50% to the resonance hybrid. The double-headed arrow is used to indicate the concept of resonance. The carbonate ion, shown in Figure 6.6, is another example of equivalent canonical forms only in this case, each canonical structure contributes 33.3% to the resonance hybrid. 

Three nonequivalent canonical forms for the thiocyanate ion. Canonical structure B makes the largest contribution to the resonance hybrid, whereas canonical form A contributes the least. 

The resonance structures for molecules such as O3 are equivalent and contribute equally to the structure of the molecule. However, many molecules have nonequivalent resonance structures that do not contribute equally to the structure of the molecule. To decide which resonance form is the more important, we can use the following four guidehnes. The rules are applied with priority 1 > 2 > 3 > 4. 

The chlorosulfite derived from the alcohol is an allyl derivative that is prone to react by an mechanism. Two resonance forms can be written for the carbocation, which are equal in energy, but nonequivalent because of the deuterium atom. 

Because they are resonance hybrids of two nonequivalent forms, enolate ions can be looked at either as vinylic alkoxides (C=C-0 ) or as -keto... 

Diastereomeric 1,3-diols 5 could be easily identified by the equivalence and nonequivalence of the geminal methyl carbon resonances in the meso- and <7,/-form, respectively415. 

Because amorphous and crystalline solid-state forms contain nonequivalent spatial relationships at the molecular level, they often display differences in functional group vibrational modes that can be measured by IR spectroscopy. Total attenuated reflectance IR spectroscopy is utilized because it is non-destructive and can be used to directly measure actual tablet and capsule samples. Similarly, solid-state NMR spectroscopy is another non-destructive direct analytical method that can detect and measure differences in nuclear resonance frequencies and relaxations, such as those displayed by amorphous and crystalline material. Cross-polarization... 

However, we must also consider the minor isotopomers containing two Hg atoms, [ Hg- Hg-Hg]2+ and [ Hg-Hg- Hg]2+. The latter is not detected because it contains two equivalent nuclei whose resonances will be coincident with those of [ Hg-Hg-Hg]2+. In contrast, the nonequivalent spins in the remaining isotopomer, [ Hg- Hg-Hg]2+, are expected to couple strongly with each other. The direct Hg- Hg coupling (139 600 1000 Hz ) is large compared to the difference in chemical shifts ( 45 000 Hz), so an AB spin system is formed. The lines at —1400 and -1550 8 are the intense inner transitions of the AB spin system, while the two outer transitions are well outside the spectral width and are not observed. This is believed to be the first observation of direct Hg- Hg coupling. 

The enantiotopic protons of the prochiral methyl groups in the iminium salt 36 exhibited distinct resonances in the presence of Eu(hfc)3 . As already discussed for achiral lanthanide S-drketonates, the system likely forms an ion pair between the organic cation and the species [Ln( S-dik)3X]. The spectrum of racemic 37, which as its bromide salt has been studied as an ionic liquid, exhibits nonequivalence in the presence of Eu(tfc)3 and Eu(hfc)3. No splitting of the resonance occurs in the presence of Eu(fod)3. In addition to the likely ion-pairing interaction of 37 with [Ln(/ -dik)3X] , rather substantial shifts of some of the OCH2 protons implied that the ether oxygen atoms also likely coordinated with the europium ion. A similar ion-paired system explains the enantiomeric discrimination observed in the spectrum of the tris(phenanthroline) complexes of Ru(II) ([Ru(phen)3]Cl2) in the presence of Eu(tfc)3 . 

As resonance hybrids of two nonequivalent forms, enolate ions can be looked at either as a-keto carbanions ("C-C=0) or as vinylic alkoxides (C=C-0 ). Thus, enolate ions can react with electrophiles either on carbon or on oxygen. Reaction on carbon yields an 01-substituted carbonyl compound, while reaction on oxygen yields an enol derivative (Figure 22.6). Both kinds of reactivity are known, but reaction on carbon is more common. 

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