Resonance pictures

Provided that these structures contnbute equally the resonance picture coincides with the MO treat ment in assigning one third of a positive charge (+ 0 33) to each of the indicated carbons... 

Some of the structural features of this class of compounds which are in accord with the resonance picture are as follows ... 

The EPR spectra of semidione radical anions can provide information on the spin density at the individual atoms. "The semidione derived from butane-2,3-dione, for example, has a spin density of 0.22 at each oxygen and 0.23 at each carbonyl carbon. The small amount of remaining spin density is associated with the methyl groups. This extensive delocalization is consistent with the resonance picture of the semidione radical anion. 

This is an unusually high rotational energy baiiier for a single bond and indicates that the caibon-nitrogen bond has significant double-bond character, as the resonance picture suggests. 

According to the resonance picture, where is the excess negative charge in azide anion Will the center nitrogen or a terminal nitrogen act as the nucleophilic site Examine atomic charges and the electrostatic potential map. Do they substantiate your conclusion Explain. 

Cyclohexatriene to benzene displays a sequence of structures from 1,3,5-cyclohexatriene (withCC single and double bonds initially set to 1.5 and 1.3 A, respectively) to benzene (witb all CC bonds set to 1.4 A) and back to cyclohexatriene. Plot energy (vertical axis) vs. CC bond length (horizontal axis). How many energy minima are there Do the minima look more like 1,3,5-cyclohexatriene or benzene What is the correct interpretation of the resonance picture ... 

In the resonance picture, these structures are considered to contribute ... 

By the resonance picture, the three species may be described as having double bonds in conjugation with, respectively, an unshared pair, an unpaired electron, and an empty orbital (see Chapter 5) ... 

In summary, the covalent/ionic-resonance picture can be used to describe the entire range of neutral and charged H-bonding phenomena. The NRT resonance weights (wcov and / , ) and bond orders (6a—h and 6b...h) are correlated in the expected manner with bond lengths, IR frequencies, intermolecular charge transfer, and other properties. 

As shown in Fig. 5.36, / R and vy become equal near s 0.6, whereas the charge transfer is half-complete near s — 0.3, and the energetic TS is at 5 = 0. Different criteria therefore lead to different estimates of the progress of reaction, but by all criteria the shifts of partial charge and covalency are quite pronounced in the H-bonded complexes. The net charge transfer gH to the evolving hydride Lewis base is seen to parallel bow closely, as the resonance picture (5.68a) suggests. 

Note that we can write a similar resonance picture for esters, and we shall actually need to invoke this when we discuss enolate anions (see Section 10.7). However, electron donation from oxygen is not as effective as from the less electronegative nitrogen. We shall also see that this resonance effect in amides has other consequences, such as increased acidity of the amide hydrogens (see Section 10.7) and stereochemical aspects of peptides and proteins (see Section 13.3). In addition, the amide derivatives have... 

Qualitatively, the resonance picture is often used to describe the structure of molecules, but quantitative valence-bond calculations become much more difficult as the structures become more complicated (e.g., naphthalene, pyridine, etc.). Therefore the molecular-orbital method is used much more often for the solution of wave equations.5 If we look at benzene by this method (qualitatively), we see that each carbon atom, being connected to three other atoms, uses sp1 orbitals to form a bonds, so that all 12 atoms are in one plane. Each carbon has a p orbital (containing one electron) remaining and each of these can overlap equally with the two adjacent p orbitals. This overlap of six orbitals (see Figure 2.1) produces six new orbitals, three of which (shown) are bonding. These three (called it orbitals) all occupy approximately the same space.6 One of the three is of lower energy than... 

Molecular-orbital bond orders are 1.930 for the C-l.C-2 bond, 1.859 for the C-3.C-6 bond and 1.363 for the C-2.C-3 bond.34 Comparing these values with those for butadiene (p. 31), we see that the C-l,C-2 bond contains more and the C-3.C-6 bond less double-bond character than the double bonds in butadiene. The resonance picture supports this conclusion, since each C-l.C-2 bond is double in three of the five canonical forms, while the... 

In alkali atom experiments no explicit resonances have been observed in microwave ionization. However, there are indirect confirmations of the multiphoton resonance picture. First, according to the multiphoton picture the sidebands of the extreme n and n + 1 Stark levels should overlap if E = 1/3n5. In the laser excitation spectrum of Na Rydberg states from the 3p3/2 state in the presence of a 15 GHz microwave field van Linden van den Heuvell et al. observed sidebands spaced by 15.4 GHz, as shown in Fig. 10.15.18 The extent of the sidebands increases linearly with the microwave field, as shown in Fig. 10.15, and the n = 25 and n = 26 sidebands overlap at microwave fields of 150 V/cm or higher, matching the observation that the 25d state has an ionization threshold of 150 V/cm in a 15 GHz field. 

In the allyl cation, with two tt electrons, and in the anion, with four -n electrons, there are two in M(V Note that the nonbonding >Pmo2 is concentrated at the ends of the chain the molecular orbital pictures for these species thus correspond closely to the resonance pictures (see 8, 9, 10, p. 6), which show the charge or unpaired electron to be concentrated at the ends. 

As we have seen, these properties are accounted for in the resonance picture by modifying the model through inclusion of a second structure with double bonds in the alternative locations. 

The positive charge is shared equally by the three carbons indicated. Thus the two carbons ortho to the. s/Z-hybridizcd carbon and the one para to it each bear one third of a positive charge (+0.33). None of the other carbons is charged. The resonance picture and the simple MO treatment agree with respect to the distribution of charge in cyclohexadienyl cation. 

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