Resonance structures major contributor

A similar concept exists when comparing resonance structures. One compound might have three resonance structures, but all three resonance structures might not contribute equally to the overall resonance hybrid. One resonance structure might be the major contributor (like the peach), while another resonance structure might be insignificant (like the kiwi). In order to understand the true nature of the compound, we must be able to compare the resonance structures and determine which structures are major contributors and which structures are not significant. 

While the majority of molecules may be adequately represented by a single resonance contributor, there are numerous situations where two or more contributors are needed. The simplest case is where all the contributing resonance structures are equivalent. Here, the proper description is in terms of an unweighted average. 

A molecule or ion for which two or more valid Lewis structures can be drawn, differing only in the placement of the valence electrons. These Lewis structures are called resonance forms or resonance structures. Individual resonance forms do not exist, but we can estimate their relative energies. The more important (lower-energy) structures are called major contributors, and the less important (higher-energy) structures are called minor contributors. When a charge is spread over two or more atoms by resonance, it is said to be delocalized and the molecule is said to be resonance stabilized, (pp. 14-18)... 

The phosphorus ylide has two resonance forms one with a double bond between carbon and phosphorus, and another with charges on carbon and phosphorus. The double-bonded resonance form requires ten electrons in the valence shell of phosphorus, using a d orbital. The pi bond between carbon and phosphorus is weak, and the charged structure is the major contributor. The carbon atom actually bears a partial negative charge, balanced by a corresponding positive charge on phosphorus. 

These structures are not equal in estimated energy. The first structure has the positive charge on nitrogen. The second has the positive charge on carbon, and the carbon atom does not have an octet. The first structure is more stable because it has an additional bond and all the atoms have octets. Many stable ions have a positive charge on a nitrogen atom with four bonds (see the Summary Table, page 13). We call the more stable resonance form the major contributor, and the less stable form is the minor contributor. The structure of the actual compound resembles the major contributor more than it does the minor contributor. 

The first structure, with more bonds and less charge separation, is possible because sulfur is a third-row element with accessible d orbitals, giving it an expandable valence. For example, SFg is a stable compound with 12 electrons around sulfur. Theoretical calculations suggest that the last structure, with octets on all atoms, may be the major resonance contributor, however. We cannot always predict the major contributor of a resonance hybrid. 

When two resonance structures are different, the hybrid looks more like the better resonance structure. The better resonance structure is called the major contributor to the hybrid, and all others are minor contributors. The hybrid is the weighted average of the contributing resonance structures. What makes one resonance structure better than another There are many factors, but for now, we will learn just two. 

Comparing resonance structures X and Y, X is the major contributor because it has mote bonds and fewer charges. Thus, the hybrid looks more like X than Y. 

Problem 1.15 Label the resonance structures in each pair as major, minor, or equal contributors to the hybrid. Then draw the hybrid. 

Although the resonance hybrid is some combination of all of its valid resonance structures, the hybrid more closely resembles the most stable resonance structure. Recall from Section 1.5C that the most stable resonance structure is called the major contributor to the hybrid, and the less stable resonance structures are called the minor contributors. Two identical resonance structures are equal contributors to the hybrid. 

Because A contains a positive charge and a ione pair on adjacent atoms, a second resonance structure B can be drawn. Because B has more bonds and aii second-row atoms have octets, B is more stabie than A, making it the major contributor to the hybrid C. Because the hybrid is more stable than either resonance contributor, the order of stability is ... 

The two resonance structures that contain an intact aromatic ring and place a negative charge on an O atom are major contributors to the hybrid. Resonance stabilizes phenoxide but not as much as... 

The 2 polymorphs of CaCOs, aragonite and calcite, can readily be distinguished on the basis of their " Ca MAS spectra (Figure 8.28B). Calcite shows a characteristic second-order quadrupolar lineshape from which the NMR parameters can he extracted by spectral simulation. The narrower Ca MAS resonance from aragonite shows no discernible structure, but the corresponding static aragonite spectrum is about 20 times broader than under MAS conditions. This suggests that CSA is a major contributor to the static linewidth, which is confirmed by satisfactory simulation of the spectrum... 

Structures with the lowest formal charges usually have the lowest energy (major contributors to the resonance hybrid). 

Planar supported lipid membranes were first prepared and studied as simplified structural models of cell membranes [4,6, 32], and more recently as biocompatible coatings for sensor transducers and other synthetic materials [33-37], A major advantage of the planar geometry relative to vesicles, and a major contributor to the expansion of this field, is the availability of powerful surface-sensitive analyti-cal/physical techniques. Confining a lipid membrane to the near-surface region of a solid substrate makes it possible to study its structural and functional properties in detail using a variety of techniques such as surface plasmon resonance, AFM, TIRF, attenuated total reflection, and sum frequency vibrational spectroscopy [38 -2]. 

Resonance forms may be drawn for (b) and hjfi of Problem 28—the structures containing double bonds. (It is always possible to draw a resonance form for a structure with a multiple bond, although the resonance form you get is not necessarily a major contributor.)... 

The proton NMR spectra of 50b and 50c revealed the C-3 proton at 6.05 and 5.95 ppm, respectively. These shifts led the authors to conclude that these munchnones are not aromatic and that resonance structure 50 is the major contributor to the hybrid. Compound 50a was too labile to be studied by NMR. The UV and IR spectra of 50a-c were also recorded, although with difficulty, since decomposition was occurring. The authors believe that the (CH2CI2) at 360 and 353 nm for 50a and 50b, respectively, are 71 71 transitions. The IR spectra of 50a-c aU show carbonyl bands at 1700-1708 cm and 1726-1747 cm , in addition to the acetoxy absorption at 1770 cm for 50c. 

A variety of compounds such as hydrocarbons, alcohols, furans, aldehydes, ketones, and acid compounds are formed as secondary oxidation products and are responsible for the undesirable flavors and odors associated with rancid fat. The off-flavor properties of these compounds depend on the structure, concentration, threshold values, and the tested system. Aliphatic aldehydes are the most important volatile breakdown products because they are major contributors to unpleasant odors and flavors in food products. The peroxidation pathway from linoleic acid to various volatiles is determined in several researchs, - by using various techniques (Gas chromatography mass spectrometry, GC-MS, and electron spin resonance spectroscopy, ESR), identified the volatile aldehydes that are produced during the oxidation of sunflower oil. In both cases, hexanal was the major aldehyde product of hydroperoxide decomposition, whereas pentanal, 2-heptenal, 2-octenal, 2-nonenal, 2,4-nonadienal, and 2,4-decadienal were also identified. 

In most cases, the various resonance structures of a molecule are not equivalent and do not contribute equally to the resonance hybrid. The electron distribution in the molecule resembles that of its major contributor more closely than any of its alternative resonance stmctures. Therefore, it is important that we develop some generalizations concerning the factors that make one resonance form more important (more stable) than another. Table 1.6 outlines the structural features that alert us to situations when resonance needs to be considered and lists criteria for evaluating the relative importance of the contributing structures. 

We encountered enols earlier as intermediates in the hydration of alkynes (see Mechanism 9.2). Enolates, represented as a hybrid of the resonance structures shown, are the conjugate bases of enols. The major enolate contributor is the structure with the negative charge on oxygen. It is, however, the carbanionic character of the a carbon that is responsible for the importance of enolates in organic synthesis, and we will sometimes write the enolate in the form that has the negative charge on carbon to emphasize this. 

One further, particularly informative experiment, involves the detection and characterization of the proton sites within the zeolite structure by NMR as pioneered by Freude and Pfeiffer (20). Even when they are not major contributors to the overall lattice structure, they may be central to the catalytic reactions. Since they are relatively dilute in the lattice, a simple MAS experiment yields spectra of sufficient resolution to identify the different functionalities as shown in Figures lA and 15. Spectra of this type will be critical in the probing of the catalytic activities of these systems and optimization of techniques for their activation. The protons detected in these experiments may also be used as magnetization sources for crossipolar-ization to Si and Al. These will yield spectra where the relative intensities of the resonances are weighted by their proximity to the proton source. Experiments of this t3 e have been reported (21). 

This determinant size is a strict minimum. Even for benzene, determinants can be quite large. For example, Norbeck and Gallup used 175 resonance structures in an ab initio valence bond calculation for benzene and found that the ionic structures are major contributors to the calculated structure Norbeck, J. M. Gallup, G. A. /. Am. Chem. Soc. 1973, 95, 4460. 

Judgment must be exercised in cases where two of the general resonance criteria are in conflict. For example, the structure B is an important contributor to the resonance hybrid for the intermediate in the nitration of methoxybenzene (anisole), even though the structure has a positive charge on oxygen. This unfavorable feature is compensated for by the additional covalent bond present in structure B compared with the other major contributor, structure A, in which the positive charge is on carbon. 

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