Resonance structures nonequivalent

For nonequivalent resonance structures, the more stable a structure is the greater is its contribution to the hybrid. 

The even greater basicity of guanidine (p/iLa = 13-6) as compared with amidines is due to an even greater resonance stabilization of the cation, since three equivalent resonance structures enter into resonance [6]. The base itself resonates between three nonequivalent structures [37], and has according to Pauling (1939,... 

Whereas in the nitro group the two resonating structures are equivalent, they are made nonequivalent in the carboxyl group and its esters, becoming equivalent again in the corresponding ions ... 

The cyclopentadiene anion is stabilized by five equivalent resonance structures. The anion is an aromatic anion by virtue of it being a six-jr-electron system. The indenyl anion is stabilized by a total of seven resonance contributors. However, they are nonequivalent and all but one require that the aromatic cloud of the benzene ring is disrupted. Thus, while the negative charge is well delocalized, the resonance stabilization is less than that of the cyclopentadiene system. Thus the proton is not as easily removed, making indene a weaker acid. 

Note that the method of counting the number of resonance structures containing a double bond between a given pair of carbon atoms is very crude and cannot distinguish among the last three of the listed bond types, each of which shows a double bond in only one resonance structure. Even within the framework of the limited resonance theory, it would be necessary to know the relative weighting of each of the two equivalent structures (a) and (b) with the nonequivalent structure (c). 

Analogously, NMR spectra exhibit either singlets (Fig. 3a,c) or doublets (Fig. 3b,d,e) for nitrogen sites in the >NC(S)S- groups. However, there is no full correlation between and NMR spectra of thiuram disulfides. For instance, the NMR spectrum of compound 7, R2=(CH2)6 (Table 1) shows a single resonance line for carbon sites in the dithiocarbamate groups (191.7 ppm), while the N NMR spectrum reveals their structural nonequivalence (127.3 and 121.3 ppm. Fig. 3d). Therefore, spectral equivalence of carbon sites can be combined with nonequivalence of peripheral N atoms in the >NC(S)S- groups of thiuram disulfides. 

Both C and NMR spectra of other adducts, 29-31 and 33, show double resonance lines with relative intensities 1 1 (see Figs. 15-18 and Table 4) assigned to C and N sites, in the >NC(S)S- groups. The latter indicates the structural nonequivalence of the dithiocarbamate ligands in these compounds. The... 

In the examples of resonance hybrids that we have examined so far, the contributing structures have been equivalent (or equally valid) Lewis structures. In these cases, the true structure is an equally weighted average of the resonance structures. In some cases, however, we can write resonance structures that are not equivalent. For reasons we cover in the material that follows—such as formal charge—one possible resonance structure may be somewhat better than another. In such cases, the true structure is still an average of the resonance structures, but the better resonance structure contributes more to the true structure. In other words, multiple nonequivalent resonance structures may be weighted differently in their contributions to the true overall structure of a molecule (see Example 9.8). 

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. 

Describe the concept of resonance and the difference between equivalent and nonequivalent resonance structures. 

NMR spectrum of the same ethyl alcohol molecules is given in Figure 8.9b measured by a high-resolution NMR spectrometer. It can be seen that the proton resonance of the different functional groups CH3, CH2 and OH takes place at different values of the magnetic field (on account of the difference in g). There follows, in particular, an important conclusion that the number of NMR signals on the spectrum is equal to the number of structure-nonequivalent protons in the molecule. (For instance, in benzene there will be one signal, in mono-substituted benzene there will be three, in ortho-di-fluorine-benzene—one, in para-di-fluorine-benzine—one, in meta-di-fluorine-benzine—two, etc). 

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. 

When the crystallography of compounds related by polymorphism is such that nuclei in the two structures are magnetically nonequivalent, it will follow that the resonances of these nuclei will not be equivalent. Since it is normally not difficult to assign organic functional groups to observed resonances, solid state NMR spectra can be used to deduce the nature of polymorphic variations, especially when the polymorphism is conformational in nature. Such information is extremely valuable at the early states of drug development when solved single crystal structures for each polymorph or solvate species may not yet be available. 

Superimposed on each of the SF4 resonances is a triplet fine structure of 1 2 1 intensity ratios (Fig. 4) which reflects spin-spin coupling between nonequivalent sets of fluorine atoms. This fine structure establishes the number of nuclei per environment as two by the following line of reasoning. 

All nuclear multiplet structures due to coupling of nonequivalent nuclei are, as noted earlier, subject to effects on line shapes by chemical or positional exchange. For those multiplet structures arising from coupling of nuclei, one of which has a nonzero nuclear quadrupole moment, effects of quadrupole relaxation must be considered. For example, if a proton or fluorine atom is bonded to a nitrogen nucleus (I = 1), a triplet resonance will be expected in the proton or fluorine spectrum. For observation of this fine structure it is necessary that the lifetimes of the nuclear spin states of nitrogen (m = 1, 0, —1) be greater than the inverse frequency separation between multiplet components, i.e., t > l/ANx (106). The lifetimes of N14 spin states can become comparable to or less than 1 /A as a result of quadrupole relaxation. When the N14 spin-state lifetimes are comparable... 

---