Canonical structure

In valence bond terms the pyrazine ring may be represented as a resonance hybrid of a number of canonical structures (e.g. 1-4), with charge separated structures such as (3) contributing significantly, as evidenced by the polar character of the C=N bond in a number of reactions. The fusion of one or two benzene rings in quinoxaline (5) and phenazine (6) clearly increases the number of resonance structures which are available to these systems. 

The electronic structure of nitrile A-oxides may be represented as a resonance hybrid of the canonical structures (335a-e). The structure (335a) is commonly used to represent this reactive species. 

C Chothia, AM Lesk. Canonical structures for the hypervariable regions of immunoglobulins. J Mol Biol 196 901-917, 1987. 

The setpin fold comprises a compact body of three antiparallel p sheets, A, B and C, which ate partly coveted by a helices (Figure 6.22). In the structure of the uncleaved form of ovalbumin, which can be regarded as the canonical structure of the serpins, sheet A has five strands. The flexible loop starts at the end of strand number 5 of p sheet A (plS in Figure 6.22), then... 

The N-Lim classification does not eliminate the possibility of borderline cases between these two categories, but it leads to the suggestion that no sharp distinction can be made between the possible intermediates in these mechanisms and that perhaps all solvolyses proceed via an intermediate. The mechanistic category of a particular solvolysis then depends upon the relative weights of the canonical structures 3, 4, and 5 to the transition state resonance hybrid. 

The nature of the sulfur-oxygen bonds in sulfoxides and sulfones has been much discussed over many years. One would expect the actual structures to be intermediate between the canonical structures 3 and 6, and 8-11, respectively, and that discussion would be about the relative contributions of the various canonical structures. In practice the emphasis has been on the extremes is the sulfur-oxygen linkage in sulfoxides and sulfones essentially a coordinate bond or a double bond Various lines of evidence have been considered to bear on this question and these will now be examined briefly6. 

In the ionization of the p-nitroanilinium ion, the free base is stabilized by delocalization of electrons involving the canonical structure 19. An analogous structure is not possible for... 

For canonical structures the sign is positive, the Coulomb coefficient being l/2n i. In drawing the superposition pattern for canonical structures the arrows may be replaced by lines. 

Fig. 2. The five canonical structures for n = 3, and some of their superposition patterns.

The benzene molecule can now be treated very simply by the Slater method, with the help of the rules formulated by one of us4 for finding the matrix elements occurring in the secular equation. The bonds between the six eigenfunctions can be drawn so as to give the independent canonical structures shown in Fig. 1. Any other... 

Fig. 1. The five canonical structures contributing to the normal state of the benzene molecule.

With naphthalene the calculation is very much more involved. The number of canonical structures is here forty-two (Fig. 2), so that the... 

Fig. 3. Unexcited and first-excited canonical structures of sym.-diphenylethylene.

A possibly more accurate value for the double bond character of the bonds in benzene (0.46) id obtained by considering all five canonical structures with weights equal to the squares of their coefficients in the wave function. There is some uncertainty aS to the significance of thfa, however, because of- the noii -orthogOnality of the wave functions for the canonical structures, and foF chemical purposes it fa sufficiently accurate to follow the simple procedure adopted above. 

The comparison of these results with the simple theory of conjugated systems [Pauling and Sherman, J. Chem. Phys., 1, 679 (1933)] not straightforward because of non-orthogonality of the canonical structures. If we assume that the double bond character... 

A possibly more reliable prediction can be made on the basis of Sherman s wave function for naphthalene,16 by considering all 42 canonical structures. The fractional double bond character of a bond can be considered to be given approximately (neglecting non-orthogonality of the canonical wave functions) by the expression... 

In contrast to pyridine, the unshared pair in canonical structure A in pyrrole is needed for the aromatic sextet. This is why pyrrole is a much weaker base than pyridine. 

Matsumoto et al. reported some reactions of diruthenium complexes containing a bridging disulfide ligand with unsaturated compounds such as olefins and ketones [135]. These diruthenium complexes show unique reactivities towards double-bond systems since the S-S bond has some double bond character from the contribution of the canonical structure B in Scheme 36. 

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