Lewis structures dominant

Self-Test 2.11A Calculate the formal charges for the two Lewis structures of the phosphate ion shown in (27). Which structure is dominant ... 

Select from each of the following pairs of Lewis structures the one that is likely to make the dominant contribution to a resonance hybrid. Explain your selection. 

The dominant feature of the NBO description of B5H9 is the large delocalization that remains even for the best possible three-center Lewis structure (3.250). The non-Lewis density for this structure is found to be 1.2625c, or fully 3.71% of the total of 34 electrons, even higher (on a percentage basis) than in such highly... 

Although the generalization to localized three-center bonds inevitably involves certain complications, important simplifications of the localized Lewis-structure picture remain. In the case of 3c/4e co-bonding, the corrections can often be adequately described in terms of two-term resonance between alternative two-center bonding patterns. In the case of 3c/2e T-bonding, the resonance-theoretic description (although possible in principle) becomes unwieldy. However, in terms of the single best three-center localized Lewis-structure description (or the resonance mixture of symmetry-equivalent structures), one can still identify specific localized donor-acceptor interactions (e.g., of type) that dominate the delocaliza-... 

HC1 addition usually proceeds through a carbocation intermediate, with the dominant product resulting from the most stable cation. Compare energies for the possible cations that can be obtained from isoprene (isoprene+H+). Rank them from most to least stable. Examine the geometries of the cations, and sketch one or more Lewis structures for each which are consistent with its geometry. Be sure to point out any unusual features. What factors might be responsible for the ordering of cation stability ... 

Why does the charge on the central alkyl group reduce the resonance energy of Sn2 transition states The answer is straightforward, once it is realized that the X A 1 X structure commonly contributes to the two Lewis structures. Thus, the higher the contribution of the triple ionic structure, the more similar the two Lewis structures, and the lower the resonance energy becomes. In the theoretical limit, where the X A+ X structure becomes the dominant structure in the TS, say 100% of the TS wave function, then the resonance energy should go to zero. It is seen that the semiempirical expression in Equation 6.20 mimics this limit. Other semiempirical expressions make similar predictions (5,53), but Equation 6.20 is the simplest one. 

In cases of strong resonance delocalization, there is no longer a single dominant Lewis structure. Instead, the properties (p)of the system may be pictured as a weighted average... 

We see that Lewis structures are a simple yet powerful way of predicting covalent bonding patterns in molecules. In addition to the octet rule, we see that the concept of formal charge can be used to identify the dominant Lewis structure. 

If we can draw several Lewis structures for a molecule, the concept of formal charge can help us decide which is the most important, which we shall call the dominant Lewis structure. One Lewis structure for CO2, for instance, has two double bonds, as we saw on page 298. However, we can also satisfy the octet rule by drawing a Lewis structure having one single bond and one triple bond. Calculating formal charges in these structures, we have... 

Note that in both cases the formal charges add up to zero, as they must because CO2 is a neutral molecule. So, which is the more correct structure As a general rule, when more than one Lewis structure is possible, we will use the following guidelines to choose the dominant one ... 

The dominant Lewis structure is generally the one in which the atoms bear formal charges closest to zero. 

Thus, the first Lewis structure of CO2 is the dominant one because the atoms carry no formal charges and so satisfy the first guideline. The other Lewis structure shown (and the similar one that has a triple bond to the left O and a single bond to the right O) do contribute to the actual structure but to a much smaller extent. 

The cyanate ion, NCO, has three possible Lewis structures, (a) Draw these three structures and assign formal charges in each, (b) Which Lewis structure is dominant ... 

We sometimes encounter molecules and ions in which the experimentally determined arrangement of atoms is not adequately described by a single dominant Lewis structure. Consider ozone, O3, which is a bent molecule with two equal O—O bond lengths ( FIGURE 8.12). Because each oxygen atom contributes 6 valence electrons, the ozone molecule has 18 valence electrons. This means the Lewis structure must have one O — O single bond and one 0 = 0 double bond to attain an octet about each atom ... 

There is no reason for one of these Lewis structures to be dominant because they are equally valid representations of the molecule. The placement of the atoms in these two alternative but completely equivalent Lewis structures is the same, but the placement of the electrons is different. Lewis structures of this sort are called resonance structures. To describe the structure of ozone properly, we write both resonance structures and use a double-headed arrow to indicate that the real molecule is described by an average of the two ... 

Notice that the arrangement of atoms is the same in each structure—only the placement of electrons differs. In writing resonance structures, the same atoms must be bonded to each other in all structures, so that the only differences are in the arrangements of electrons. AH three N03 Lewis structures are equally dominant and taken together adequately describe the ion, in which all three N— O bond lengths are the same. 

In some instances, all the possible Lewis structures for a species may not be equivalent to one another. Instead, one or more may be dominant. We will encounter examples of this as we proceed. 

The S03 ion has 26 electrons, which leads to a dominant Lewis structure in which all the S—O bonds are single ... 

Which of the Lewis structures for NO is dominant based on analysis of the formal charges ... 

Chemists are still debating which of these two structures is dominant for P04 . Recent theoretical calculations based on quantum mechanics suggest to some researchers that the left structure is the dominant one. Other researchers claim that the bond lengths in the ion are more consistent with the right structure being dominant. This disagreement is a convenient reminder that, in general, multiple Lewis structures can contribute to the actual electron distribution in an atom or molecule. 

The first structure is expected to be the dominant one because it has the lowest formal charges on each atom. Indeed, the molecule is usually represented by this single Lewis structure. 

Sometimes a single dominant Lewis structure is inadequate to represent a particular molecule (or ion). In such situations, we describe the molecule by using two or more resonance structures for the molecule. The molecule is envisioned as a blend of these multiple resonance structures. Resonance structures are important in describing the bonding in molecules such as ozone, O3, and the organic molecule benzene, CgHg. 

Calculate formal charges from Lewis structures and use those formal charges to identify the dominant Lewis structure for a molecule or ion. (Section 8.5)... 

The first structure shows each atom with a zero formal charge and therefore it is the dominant Lewis structure. The second one shows a positive formal charge for an oxygen atom, which is a highly electronegative atom, and this is not a fevorable situation. 

For example, the higher energy p-orbital (instead of an sp hybrid) parallel to the vicinal axial acceptor maximizes the hyperconjugative anomeric n o j, Y interaction in tetrahydropyrans (see Chapter 6). NBO analysis which determines the best hybrids describing a Lewis structure finds two lone pairs of different hybridization in tetrahydropyran a purely p-orbital and an sp hybrid. The deviation from sp hybridization predicted by the idealized model is readily explained by Bent s rule, - where atoms direct orbitals of different hybridization towards neighboring atoms based on electronegativity and size. When the dominant delocalization is intermolecular, the preferred hybridization can change. For example, NBO finds two sp hybrids for the two lone pairs of the H O molecule that serves as H-bond acceptor in the water dimer. 

---