Resonance structures Various Lewis

Resonance (Section 1.9) Method by which electron delocalization may be shown using Lewis structures. The true electron distribution in a molecule is regarded as a hybrid of the various Lewis structures that can be written for a molecule. 

Figure 3 shows 13c MAS spectra of acetone-2-13c on various materials. Two isotropic peaks at 231 and 227 ppm were observed for acetone on ZnCl2 powder, and appreciable chemical shift anisotropy was reflected in the sideband patterns at 193 K. The 231 ppm peak was in complete agreement with the shift observed for acetone diffused into ZnY zeolite. A much greater shift, 245 ppm, was observed on AICI3 powder. For comparison, acetone has chemical shifts of 205 ppm in CDCI3 solution, 244 ppm in concentrated H2SO4 and 249 ppm in superacid solutions. The resonance structures 5 for acetone on metal halide salts underscore the similarity of the acetone complex to carbenium ions. The relative contributions of the two canonical forms rationalizes the dependence of the observed isotropic 13c shift on the Lewis acidity of the metal halide. 

This brings us to a new term, resonance. A molecule shows resonance when more than one Lewis structure can be drawn for the molecule. In such a case, we call the various Lewis stmctures resonance structures. 

Novel lanthanide fi-diketonate complexes have been synthesized, Their properties include thermal, hydrolytic and oxidative stabilities, volatility, Lewis acidity, and unusually high solubility in nonpolar organic solvents. Various combinations of these properties make lanthanide complexes useful as NMR shift reagents and fuel antiknock additives and in other applications. NMR spectral studies revealed that the Pr(III), Yb(III), and Eu(III) complexes of 1,1,1,2,2,3,3,7,7,7- decafluoro-4,6-heptanedione have sufficient Lewis acidity to induce appreciable shifts in the proton resonances of weak Lewis bases such as anisole, acetonitrile, nitromethane, and p-nitrotoluene. Data from single-crystal structure determinations indicate that the NMR shift reagent-substrate complexes are not stereochemically rigid and that effective axial symmetry may exist by virtue of rapid intramolecular rearrangements. 

To deal with circumstances such as the bonding in ozone, yet retain Lewis formulas as a useful tool for representing molecular structure, the notion of resonance was developed. According to the resonance concept, when two or more Lewis structures that differ only in the distribution of electrons can be written for a molecule, no single Lewis structure is sufficient to describe its true electron distribution. The true structure is said to be a resonance hybrid of the various Lewis formulas, called contributing structures, that can be written for the molecule. In the case of ozone, the two Lewis formulas are equivalent and contribute equally to the resonance hybrid. We use a double-headed arrow to signify resonance and read it to mean that the Lewis formulas shown contribute to, but do not separately describe, the electronic structure of the molecule. 

Writing the various Lewis formulas that contribute to a resonance hybrid can be made easier by using curved arrows to keep track of delocalized electrons. We can convert one Lewis structure of ozone to another by moving electron pairs as shown ... 

The ab initio calculations of various three-electron hemibonded systems [122, 123] indicated that the inclusion of electron correlation corrections is extremely important for the calculation of three-electron bond energies. The Hartree-Fock (HF) error is found to be nonsystematic and always large, sometimes of the same order of magnitude as the bond energy. According to valence bond (VB) and MO theories, the three-electron bond is attributed to a resonance between the two Lewis structures... 

This process of constructing functions for the various resonant formulae, followed by an adequate combination of them, is mathematically more complex than the mathematics of molecular orbital theory. It is therefore understandable that, after the initial preference of chemists for the v.b. bond theory which has a closer relation to Lewis structures - especially due to the contribution of Linus Pauling - m.o. theory became increasingly popular. In addition, m.o. theory leads directly, not only to fundamental states (through the occupied m.o.), but also to excited states (through vacant m.o.) of molecules. In recent years, however, a new form of valence-bond theory has been developed that is more amenable to computation (spin-coupled valence-bond theory) in which the molecular wavefunction is expressed as a linear combination of all the coupling schemes of the various electrons corresponding to the same resultant spin (ref. 97). 

Measuring the acidity of the Bronsted and Lewis acid sites is problematic. The adsorption and desorption of various amines has been used, but there is some disagreement about what it means.160 Some workers prefer to use isopropylamine, which desorbs only from Bronsted sites. Solid-state nuclear magnetic resonance (NMR) has also been used in the study of reactions on zeolites.161 Many zeolites crystallize into crystals that are too fine for conventional X-ray analysis. A new method that uses synchrotron X rays on microcrystalline powders promises to make it much easier to determine the structures of zeolites and related materials.162... 

Write a Lewis structure for each of the following simple molecules. Show all bonding valence electron pairs as lines and all nonhonding valence electron pairs as dots. For those molecules that exhibit resonance, draw the various possible resonance forms. 

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