Resonance structures definition

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

Never break a single bond when drawing resonance structures. By definition, resonance stractures must have aU the same atoms connected in the same order. 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

Although the theory behind BLW is more general, a typical application of the method is the energy calculation of a specific resonance structure in the context of resonance theory. As a resonance structure is, by definition, composed of local bonds plus core and lone pairs, a bond between atoms A and B will be represented as a bonding MO strictly localized on the A and B centers, a lone pair will be an AO localized on a single center, and so on. With these restrictions on orbital extension, the SCF solution can be... 

Closely related to the above merit of VB methods, the unique definition of diabatic states also allows us to derive the energy profiles for diabatic states. Since for many reactions the whole process can be described with very few resonance structures, the comparison between the diabatic and adiabatic state energy profiles can yield insight into the nature governing the reactions [22-24]. In fact, even for complicated enzymatic reactions, simple VB ideas have shown unparalleled value [25, 26]. However, the further utilization of the VB ideas at the empirical and semi-empirical levels should be carefully verified by benchmark ab initio VB... 

Zhang, X., Boyar, W., Toth, M. J., Wennogle, L., and GonneUa, N. C. (1997). Structural definition of the C5a G terminus by two-dimensional nuclear magnetic resonance spectroscopy. Proteins 28, 261-267. 

A number of resonance energy definitions are based on theoretical quantum-chemistry approaches. It was also recognized that the main difference between the proposed approaches lies in the definition of the nonconjugated reference structure, not in the use of different MO theories. 

Figure 3.7. Definition of regions from A to E of bond length alternation with the percentage contribution of neutral (structure I) and charge separated (structure II) resonance structures to the ground state. This pattern demonstrates the evolution of the charge separated structure from the neutral from passing through polar structure with almost equal bond lengths (region C). (From Ref. [319] with permission of the American Association for the Advancement of Science.)...

Figure 12 shows preliminary results that have been obtained for the microcanonical rate k(E) (Eq. (8)) as a function of energy E, compared to the experimental values. Though individual features do not match up one-for-one—which is quite beyond the accuracy of the present potential energy surface—the density and widths of the resonance features are in quite good agreement with each other. The calculations definitely lend credence to the validity of the experimental results and their interpretation as metastable resonance structure. 

By definition, all resonance structures are descriptions of the same molecule. We use the notation in which the different resonance structures are connected by double tipped arrows with all the formulas placed within parentheses. 

Acids and bases can also be considered from another point of view. In this view acids are electron acceptors while bases are electron donors. Lewis has used this definition to extend the concept of acidity and basicity to the compounds which don t have hydrogen. The molecule of amine is a stronger base if the electron lone pair on nitrogen is more localized. Localization or delocalization can be estimated by writing the appropriate resonance structures. Electrons are localized if only one resonance formula can be drawn. In principle, if the electron pair is delocalized over a large part of the molecule, the electron energy is lower, the system is more stable and does not require the addition of a proton. 

There are phenomena such as mesomerism or resonance that cannot be described in terms of our graph theoretical model of a molecule. The reason is that it is no longer possible to associate a unique and integral multiplicity to a covalent bond in aromatic structures. Instead, we introduce several resonance structures in order to represent aromatic compounds. Such structures are not generally isomorphic in the sense of Definition 1.17. 

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