Bonds in resonance structures

From the NRT resonance weights Wr and number of A-B bonds in resonance structure r, one can determine the natural bond order Pab between atoms A and B as the resonance-weighted average,... 

Including structures in which there are multiple bonds results in resonance structures like... 

A structure midway between the two resonance structures represents the ozone structure best. The bonds in this structure are stronger than a single bond but weaker than a double one. 

A good illustration of this concept is seen in a series of nitrophenols. The nitro group itself has to be drawn with charge separation to accommodate the electrons and our rules of bonding. However, resonance structures suggest that there is electron delocalization within the nitro group. 

Many molecules that have several double bonds are much less reactive than might be expected. The reason for this is that the double bonds in these structures cannot be localized unequivocally. Their n orbitals are not confined to the space between the double-bonded atoms, but form a shared, extended Tu-molecular orbital. Structures with this property are referred to as resonance hybrids, because it is impossible to describe their actual bonding structure using standard formulas. One can either use what are known as resonance structures—i. e., idealized configurations in which n electrons are assigned to specific atoms (cf pp. 32 and 66, for example)—or one can use dashed lines as in Fig. B to suggest the extent of the delocalized orbitals. (Details are discussed in chemistry textbooks.)... 

The optimized geometry of 4, obtained at the MP2/6-31G level and shown in Figure 22.5, offered more clues about the type of delocalization that is not responsible for the stabilization of 4 than about the type of delocalization that is. If delocalization of the strained bonds between the carbons, a and p to the cationic center, were involved and occurred in the manner depicted in resonance structure C, the bonds between the ipso and a carbons should have considerable double-bond... 

The contribution of structure III would be expected to amount to a few percent for BrNO and C1NO, and much more for FNO, because of the greater stability of the fluorine-nitrogen double covalent bond. A resonating structure with 50 percent contribution of II, 25 percent of I, and 25 percent of III is compatible with both the observed F—N distance in FNO and the observed value18 1.81 D of the electric dipole moment. 

Curved-arrow notation is also a very useful device with which to generate resonance structures. In this application it is truly a bookkeeping system. Since individual canonical forms do not exist but are only thought of as resonance contributors to the description of a real molecule, the use of curved-arrow notation to convert one canonical form to another is without physical significance. Nevertheless it provides a useful tool to keep track of electrons and bonds in canonical structures. For example, the structures of carboxylate resonance contributors can be interconverted as follows ... 

The organic chemist made an important step in the understanding of chemical reactivity when he realized the importance of electronic stabilization caused by the delocalization of electron pairs (bonded and non-bonded) in organic molecules. Indeed, this concept led to the development of the resonance theory for conjugated molecules and has provided a rational for the understanding of chemical reactivity (1, 2, 3). The use of "curved arrows" developed 50 years ago is still a very convenient way to express either the electronic delocalization in resonance structures or the electronic "displacement" occurring in a particular reaction mechanism. This is shown by the following examples. 

Figure 3.22 shows resonance structures for a compound that has a CO double bond in conjugation with a CC double bond. The resonance structures are a combination of the types in Figures 3.18 and 3.19. Structure (a) has the octet rule satisfied at all atoms and has no formal charges, so it is more stable than the others and contributes the most to the resonance hybrid. Therefore, the actual structure most resembles this resonance structure (rule 4). In addition, the actual energy of the compound is also closer to the energy of the most important resonance structure. In other words, this compound has only a small resonance stabilization. However, even though structures (b) and (c) make... 

In this reaction, a proton is transferred from the acid to the base. The unshared pair of electrons on the base is used to form the new bond to the proton while the electrons of the H—A bond remain with A as an unshared pair. Previously, curved arrows have been used to show electron reorganization in resonance structures. Organic chemists also use these arrows to show electron movement in reactions. The arrows are a kind of bookkeeping device that helps us keep track of electrons as the Lewis structures of the reactants are converted to the Lewis structures of the products. Remember, an arrow always points from where the electrons are to where they are going. It does not point from where the hydrogen (or other atom) is to where it is going. 

Three resonance structures can be written for naphthalene. Note that the C-l —C-2 bond is a double bond in two of these structures and a single bond in one, while the C-2—C-3 bond is a single bond in two structures and a double bond in one. This explains why the C-l —C-2 bond is shorter than the C-2—C-3 bond. 

In discussing the resonance concept (p. 54), which allows a molecule to be represented by two or more structures, it was seen that a bond represented as a single bond in one of the structures might well be represented as double bond in another structure. In benzene (one of the simplest cases) the three carbon-carbon double bonds in one Kekule structure become single bonds in the other. The bond number here is between 1 and 2, but a more exact specification of bond number would depend on how this number is described, and agreement is by no means universal on this point. Since, however, the two Kekule structures of benzene are equivalent, it is quite reasonable to assign a number of 1.5 to the carbon-carbon bonds. Similarly, for the carbonate ion,... 

Table 1 summarizes these parameters characterizing the keto-enol equilibria, where A refers to the difference between the enol and keto forms. The enol forms are significantly more stable, consistent with the inclusion of an intramolecular hydrogen bond in the structures and concurrent resonance stabilization. The low frequency torsional vibration of the keto forms can account for their significantly greater relative entropy. 

All resonance structures must have identical geometries. Otherwise they do not represent the same molecule. For example, the following structure (known as Dewar benzene) is not a resonance form of benzene because it is not planar and has two less ir electrons. Because molecular geometry is linked to hybridization, it follows that hybridization also is unchanged for the atoms in resonance structures. (Note If it is assumed that the central bond in this structure is a bond, then it has the same number of electrons as benzene. However, in order for the p orbitals to overlap, the central carbon atoms would have to be much closer than they are in benzene, and this is yet another reason why Dewar benzene is an isolable compound rather than a resonance form of benzene.)... 

In many molecules, the choice of which atoms are connected by multiple bonds is arbitrary. When several choices exist, all of them should be drawn. For example, as shown in Figure 3-1, three drawings (resonance structures) of C03 are needed to show the double bond in each of the three possible C — O positions. In fact, experimental evidence shows that all the C — O bonds are identical, with bond lengths (129 pm) between double-bond and single-bond distances (116 pm and 143 pm respectively) none of the drawings alone is adequate to describe the molecular structure, which is a combination of all three, not an equilibrium between them. This is called resonance to signify that there is more than one possible way in which the valence electrons can be placed in a Lewis structure. Note that in resonance structures, such as those shown for in Figure 3-1, the electrons are drawn in different places but the atomic nuclei remain in fixed positions. 

Using the same sequence of atoms, it is possible to have more than one correct Lewis structure when a molecule or polyatomic ion has both a double bond and a single bond. Consider the polyatomic ion nitrate (N03 ) shown in Figure 9-11a. Three equivalent structures can be used to represent the nitrate ion. Resonance is a condition that occurs when more than one valid Lewis structure can be written for a molecule or ion. The two or more correct Lewis structures that represent a single molecule or ion are often referred to as resonance structures. Resonance structures differ only in the position of the electron pairs, never the atom positions. The location of the lone pairs and bonding pairs differs in resonance structures. The molecule O3 and the polyatomic ions N03, N02, 803 and C03 commonly form resonance structures. 

Empirical Valence Bond Methods. - To examine some important questions relating to enzyme action (e.g. to analyse the causes of catalysis, i.e. why an enzymic reaction proceeds faster than the equivalent, uncatalysed reaction in solution), it is necessary to use a method that not only captures the essential details of the chemical reaction, but also includes the explicit effects of the enzyme and solvent enviroment. One notable method in this area is the empirical valence bond (EVB) model.143 In the empirical valence bond approach, resonance structures (for example ionic and covalent resonance forms)... 

Another very important facet of theoretical considerations is interpretation of the phenomena under study. This is accomplished by quantum mechanical models, which yield simplified but quintessential picture of the molecular behavior [32]. In order to rationalize ortho-para directional ability of OH group let us make an inspection of the valence bond (VB) resonance structure of the benzenium ion (Fig.2). It appears that a depletion... 

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