Resonance constituting structures

Know the meaning of molecular formula, structural formula, structural (or constitutional) isomers, continuous and branched chain, formal charge, resonance, contributing structures, sigma (a) bond, sp3-hybrid orbitals, tetrahedral carbon. 

Alongside electron-pair bonds, odd-electron bonds play an important role in chemistry, and constitute therefore a compulsory test case for any computational method. Odd-electron bonds can be represented as two resonating Lewis structures that are mutually related by charge transfer, as shown in (13) for two-center, one-electron (2c,le) bonds and in (14) and (15) for typical two-center, three-electron (2c,3e) bonds. 

B]onds are theoretical constructs, idealizations, which have aided chemists during the past one hundred years in developing the convenient and extremely valuable classical structure theory of organic chemistry. The theory of resonance constitutes an extension of this theory. It is based upon the use of the same idealizations — the bonds between atoms — as used in classical structure theory, with the important extension that in describing the benzene molecule two arrangements of these bonds are used, rather than only one. [Pauling, 1970,... 

Note the difference between tautomers and resonance forms. Tautomers are constitutional isomers—different compounds with different structures—while resonance forms arc different representations of a single structure. Tautomers have their atoms arranged differently, while resonance forms differ only in the position of their electrons. Note also that tautomers are rapidly interconvertible. Thus, keto and enol isomers are tautomers, but aJkene isomers such as 1-butene and 2-butene are not, because they don t interconvert rapidly under normal circumstances. 

OH ion is denoted iff%. The atoms depicted in the figure are considered as our solute system (5) while the rest of the protein-water environment constitutes the solvent (s) for the enzyme reaction. Although the Ca2+ ion does not actually react, it is included in the reacting system for convenience. As before, we describe the diagonal elements of the EVB Hamiltonian associated with the three resonance structures (t/rf,, t/ff) by... 

A situation in which the electrons can be arranged in more than one way constitutes resonance. In Chapter 3, the resonance structures HF and H+ F were used to describe HF but in the case of S02... 

Ab initio calculations (MP2/6-31G ) of the parent compound of 8 revealed that the most stable arrangement of the dimer adopts Dih symmetry (Fig. 5). Interestingly, the four Li ions and the two phosphorus centers constitute an octahedral skeleton with relatively short Li-Li and Li-P distances of 2.645 and 2.458 A, respectively. Charge analysis (22) undoubtedly supports the electrostatic bonding model for this system because of the high net charges of the natural atomic orbitals (NBO) at Li (+0.768) and P (-1.583), while NBO-Lewis resonance structures support stabilization through delocalization (Fig. 5). 

A solution structure of French Bean plastocyanin has been reported by Wright and co-workers,19 using nuclear magnetic resonance techniques described in Section 3.5 of Chapter 3. The structure, determined from a plastocyanin molecule in solution rather than in a solid-state crystal, agrees well with that of reduced poplar plastocyanin X-ray crystallographic structure reported above. Conformations of protein side chains constituting the hydrophobic core of the French bean plastocyanin are well-defined by the NMR technique. Surface side chains show... 

Figures, h CIDNP spectra (cyclopropane resonances) observed during the electron transfer photoreaction of chloranil with c/s-1,2-diphenylcyclopropane (fop) and ben-zonorcaradiene (.bottom). The opposite signal directions observed for analogous protons in the two compounds constitute evidence that the two radical cations belong to two different structure types.

Molecular orbital descriptions offer a number of significant advantages over conventional resonance structures. For one, they often provide more compact descriptions, e.g., the LUMO in planar benzyl cation conveys the same information as four resonance structures. Second, orbital descriptions are quantitative, compared to resonance structures which are strictly qualitative. Finally, molecular orbital descriptions may be applied much more widely than resonance descriptions. Of course, molecular orbital descriptions cannot be generated using a pencil as can resonance structures, but rather require a computer. It can be argued that this does not constitute a disadvantage, but rather merely reflects a natural evolution of the tools available to chemists. 

In fact, the first saturated pseudoxazolone reported, 4-methyl-2-(trifluoro-methyl)-5(2//)-oxazolone, was incorrectly assigned as the tautomeric 5(4//)-oxazolone and only later did nuclear magnetic resonance (NMR) smdies establish the correct structure. This compound was synthesized from alanine and trifluoro-acetic anhydride (TFFA). This methodology constitutes, under standard conditions, the most general procedure for the synthesis of 5(2//)-oxazolones. 

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