First-order resonance structures

These important or first-order resonance structures are defined as those with the minimum number... 

Explain why the three second-order resonance structures shown in Model 8 are NOT considered important resonance structures. That is, why are they "not as important" as a regular, first-order resonance structure such as the one at the far left of Model 8 (Hint Count formal charges.)... 

Explain why the second-order resonance contributors are less important than first-order resonance structures, and contribute only a small amount to our overall understanding of phenol. 

For m = 1, first-order DFB operation is obtained. The laser radiation is observed from the sample edge in this case. For a second-order DFB resonator (m = 2), laser radiation is also emitted perpendicular to the sample surface. This part of the emission does not participate in the feedback of the laser process and acts as radiation loss. Therefore lower coupling coefficients k and higher threshold energy values are expected for second- and higher-order DFB lasers compared to first-order DFB structures. 

Composite Drawing of Aniline (showing a combination of all first- and second-order resonance structures)... 

A resonance effect is the donation or withdrawal of electron density through n bonds as demonstrated by first- or second-order resonance structures (as shown below, right). 

Use curved arrows to show a mechanism for Synthetic Transformation 26.3. Hint the first step is a tautomerization to generate an enol (it may help to draw a second-order resonance structure of this enol). 

Hydrolysis of an enamine yields a carbonyl compound and a secondary amine. Only a few rate constants are mentioned in the literature. The rate of hydrolysis of l-(jS-styryl)piperidine and l-(l-hexenyl)piperidine have been determined in 95% ethanol at 20°C 13). The values for the first-order rate constants are 4 x 10 sec and approximately 10 sec , respectively. Apart from steric effects the difference in rate may be interpreted in terms of resonance stabilization by the phenyl group on the vinyl amine structure, thus lowering the nucleophilic reactivity of the /3-carbon atom of that enamine. 

The next-nearest-neighbor-orbital resonance integrals, /JI3, also remain unaffected by the pure twisting motion. We conclude that a pure twisting motion can therefore represent at best only a relatively small perturbation of the electronic structure of the polysilane chain, suitable for treatment by first-order perturbation theory. The perturbation is represented by changes in the resonance integrals between more distant hybrid orbitals, among which / 14 clearly is the most important. 

So each bond has a bond order of 1.5, which is consistent with the observed bond length. These two resonance structures are often called Kekule structures because they were first proposed in 1865 by Kekule, who imagined that the molecule converted very rapidly from one form to the other. This, however, is not the case the molecule never has either of the Kekule structures but only a single structure, which is intermediate between these two hypothetical structures and is approximately represented as follows ... 

Ester-type dimers illustrate the four-step approach for identification of carbohydrate structural elements in the oligosaccharide cores by NMR spectroscopy. Eor these type of compounds, edited NMR sub-spectra have permitted the assignment of all of the resonances in both monomeric units (22, 78). Spectroscopic simulation of the coupling constants can be deduced for proton resonances with a non-first-order resolution. Figure 2 illustrates this approach for batatin 1 (230) the... 

At variance with our view that coordination number considerations are of first-order importance (which favors III-NIO), others (33, 86) favor both structures for BioHis" (III-NIO, based on smoother charge distribution, and the IV-NlO structure, based on 3-center resonance considerations). Probably charge and resonance considerations are of importance, and possibly all three considerations are subtly interrelated. 

Figure 9-32 High-resolulion nmr spectrum of ethyl iodide, CH3CH2I, at 60 MHz relative to TMS, 0.00 ppm. The first-order splitting pattern is seen in the well-separated three-four line pattern for the CH3—CH2 resonances. The second-order splitting is the additional fine structure superimposed on the three-four pattern.

Various reactivity indices have been derived for benzenoid hydrocarbons from the following purely topological approaches the Huckel model (HMO), first-order perturbation theory (PMO), the free electron MO model (FEMO), and valence-bond structure resonance theory (VBSRT). Since many of the indices that have been known for a long time (index of free valence Fr, self-atom polarizability ir , superdelocalizability Sr, Brown s index Z, cation localization energy Lr+, Dewar reactivity number Nt, Brown s para-localization energy Lp) have been described in detail by Streitwieser in his well-known volume [23] we will refer here only to some more recent developments. 

Carbon-13 nuclear magnetic resonance was used to determine the molecular structure of four copolymers of vinyl chloride and vinylidene chloride. The spectra were used to determine both monomer composition and sequence distribution. Good agreement was found between the chlorine analysis determined from wet analysis and the chlorine analysis determined by the C nmr method. The number average sequence length for vinylidene chloride measured from the spectra fit first order Markovian statistics rather than Bernoullian. The chemical shifts in these copolymers as well as their changes in areas as a function of monomer composition enable these copolymers to serve as model... 

---