Resonance structures improbable

For silatrane, however, containing a penta-coordinate silicon atom, such a resonance structure is improbable. Both the enhanced cr-donor (an increase in 5C ) and lowered Tt-acceptor (a decrease in 6Cp) effects of the silatranyl grouping result m not only a decrease of double bond polarization (SC ) but also in a chang of the sign. The 8C p values for trimethylsilylvinylsilatrane and the model triethoxysilane as calculated by an additive scheme with the use of 5Qp = 11.0 ppm for 1-vinylsilatrane... 

The selective UV absorption of imidazole-4-carbaldehyde has been referred to resonance structures such as (92), which are improbable in the cation. In addition, a hydroxymethylene form can be excluded. 

Figure 14.5. Improbable Resonance Structure. The structure contributes little to the terminal part of ATP, because two positive charges are placed adjacent to each other.

In graphite each carbon atom is bound to three others in the same plane and here the assumption of inversion of a puckered layer is improbable, because of the number of atoms involved. A probable structure is one in which each carbon atom forms two single bonds and one double bond with other atoms. These three bonds should lie in a plane, with angles 109°28 and 125°16,l which are not far from 120°. Two single bonds and a double bond should be nearly as stable as four single bonds (in diamond), and the stability would be increased by the resonance terms arising from the shift of the double bond from one atom to another. But this problem and the closely related problem of the structure of aromatic nuclei demand a detailed discussion, perhaps along the lines indicated, before they can be considered to be solved. 

In the above discussion we have considered that the bond between two atoms A and B may be described as a resonance hybrid of the covalent A—B and ionic A B+ structures. Theoretically we should also consider the alternative ionic structure A+ B and resonance between the three structures would further increase the energy of the bond. In general, however, the second ionic state is so improbable as to contribute little stability to the molecule. Thus for example, it will be appreciated readily that the state Na " C1+ or H C1+ are most improbable and do not play a part in determining the behaviour of the molecules. The contribution of such states to the resonance of the molecule is, therefore, in general neglected. 

Structures (A) and (B) can be eliminated immediately because they are not electrostatically feasible. These two structures are cis and trans forms of the same compound which juxtaposes positive charge - an unlikely occurrence. The peroxide linkage (0-0) has low thermodynamic stability and makes structure (C) improbable as the arrangement of the dimer. Structure (D) seems to be the correct form for the dimer in that it has resonance stabilization and is electrostatically feasible. The electron arrangement, however, does not make the geometry of the molecule clear. 

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