Resonance structures, unequal

Model 7 Resonance Structures—Unequal (but still important)... 

Pure HN3 (atom sequence HNNN) is a very explosive compound. In aqueous solution, it is a weak acid (comparable to acetic acid) that yields the azide ion, N3. Draw resonance structures to explain why the nitrogen-nitrogen bond lengths are equal in N3 but unequal in HN3. 

In contrast to the symmetrical charge distribution in the parent allyl cation, unequal charge distributions can result when substituents are present. Consider the methyl-substituted allylic ion represented by the following two resonance structures. 

It is important to realize that if resonance structures contribute unequally, the actual structure of the hybrid most resembles the structure that contributes most. The electrostatic potential map of acetone shows the negative charge (red) on oxygen and the positive charge (blue) on carbon in agreement with the results we derive from the resonance treatment. 

Describe the bonding using the valence-bond method, assuming the single structure with alternating single and double bonds. The correct structure more nearly resembles one of these resonance structures with bonds of unequal length. The molecule is not aromatic. ... 

Secondary Oxonium Ions [RR OH ]. 1H NMR and IR investigation75 of protonated ether salts (hexachloroantimonates) in dichloromethane solutions showed the formation of both (a) dialkyloxonium ions in which the proton is bound to only one oxygen (21) and (b) bidentate complexes in which the proton is shared between two ether molecules (22). Structural analysis of such a bidentate complex of diethyl ether with a complex anion shows a broad H+ resonance at 81 H 16.3 and unequal O—H bond distances (1.39 and 1.11 A).76... 

The experiment described above is termed selective population transfer (SPT), or more precisely in this case with proton spin inversion, selective population inversion, (SPI). It is important to note, however, that the complete inversion of spin populations is not a requirement for the SPT effect to manifest itself. Any unequal perturbation of the lines within a multiplet will suffice, so, for example, saturation of one proton line would also have altered the intensities of the carbon resonance. In heteronuclear polarisation (population) transfer experiments, it is the heterospin-coupled satellites of the parent proton resonance that must be subject to the perturbation to induce SPT. The effect is not restricted to heteronuclear systems and can appear in proton spectra when homonuclear-coupled multiplets are subject to unsymmetrical saturation. Fig. 4.20 illustrates the effect of selectively but unevenly saturating a double doublet and shows the resulting intensity distortions in the multiplet structure of its coupled partner, which are most apparent in a difference spectrum. Despite these distortions, the integrated intensity of the proton multiplet is unaffected by the presence of the SPT because of the equal positive and negative contributions (see Fig. 4.19d). Distortions of this sort have particular relevance to the NOE difference experiment described in Chapter 8. 

This is the dispersion of the geometrical structure factor in crystallography. The overall effect of resonant scattering is to cause the breakdown of Friedel s law so that the Bijvoet pairs of reflections S(h) and S(—h) are unequal (see e.g. ). The difference... 

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. 

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. 

Not all structures contribute equally to a resonance hybrid. We describe four ways to predict which structure contributes more to the hybrid. But before examining these preferences, let s consider how to think about the fact that contributing structures may contribute unequally to many resonance hybrids. Suppose you combined yellow and... 

When two or more Lewis structures differing only in the positions of the electrons are needed to describe a molecule, they are called resonance forms. None correctly describes the molecule, its true representation being an average (hybrid) of all its Lewis structures. If the resonance forms of a molecule are unequal, those which best satisfy the rules for writing Lewis structures and the electronegativity requirements of the atoms are more important. 

The hydrogen NMR of 2,2,3,3-tetrachlorobutane has a single resonance, a singlet, at 25 °C. Decreasing the temperature to -50 °C yields a spectrum with two singlets of unequal intensity. What structures account for the low temperature spectrum ... 

---