Valence structures

Zwitterion. A neutral valence structure which incorporates both a formal positive charge and a formal negative charge. 

Scheme 10.4 The catalytic cycle of cytochrome P450. Only one possible valence structure of the oxoferrous species IV has been depicted for clarity. See text for details.

Similar valence structures A, B, etc., may be written for gallium, zinc, and copper and their congeners ... 

In cinnabar, HgS, the sulfur atom has a normal-valence structure, in which it forms two bonds with mercury atoms. The Hg—S—Hg bond angle is 105°. The mercury atom may be described as forming two oppositely directed bonds with use of two sp hybrid bond orbitals the value of the S—Hg—S angle is 172°. The bond length, 2.36 A, leads,... 

The implications of the foregoing concept have profoundly influenced modern trends in polymer research. If polymers owe their differences from other compounds to the extent and arrangement of their primary valence structures, the problem of understanding them is twofold. It is necessary in the first place to provide appropriate means, both experimental and theoretical, for elucidating their macromolecular structures a[Pg.3]

In spite of the proposals of large primary valence structures for rubber by Pickles and somewhat ambiguously for polybutadiene by Lebedev, prevailing opinion favored rings of moderate size for vinyl and diene polymers. Structures similar to those widely accepted for cellulose and rubber were generally assumed. 

If the covalent bonds connect elements of the structure along one dimension only, as in silicon disulfide, it may be desirable to consider the substance as a polymer. This will certainly be appropriate if the valence structure supersedes whatever crystalline arrangement prevails, i.e., if the substance can be melted without seriously disrupting the continuity of the interunit connections. 

Sadler, P.J. (1976) The biological chemistry of gold a metallo-drug and heavy-atom label with variable valency Structure and Bonding, 29, 171—219. 

Complexes 17-19 can be written in one valence structure as a, /3-unsaturated carbonyl compounds in which the carbonyl oxygen atom is coordinated to a BF2(OR) Lewis acid. The C=C double bonds of such organic systems are activated toward certain reactions, like Diels-Alder additions, and complexes 17-19 show similar chemistry. Complexes 17 and 18 undergo Diels-Alder additions with isoprene, 2,3-dimethyl-1,3-butadiene, tram-2-methyl-l,3-pentadiene, and cyclopentadiene to give Diels-Alder products 20-23 as shown in Scheme 1 for complex 17 (32). Compounds 20-23 are prepared in crude product yields of 75-98% and are isolated as analytically pure solids in yields of 16-66%. The X-ray structure of the isoprene product 20 has been determined and the ORTEP diagram (shown in Fig. 3) reveals the regiochemistry of the Diels-Alder addition. The C-14=C-15 double bond distance is 1.327(4) A, and the... 

In a paper on free radicals published in 1929, Ingold and H. Burton suggested that the forces responsible for free radical stability and for the peculiarities of benzene valency arise from the delocalization of electrons permitted by alternative valency structures. 38 Ingold was influenced, he later said, by... 

A bond separation reaction takes any molecule comprising three or more heavy (non-hydrogen) atoms into the set of simplest (two-heavy-atom) molecules containing the same bonds. The only requirement is that bonding must be defined in terms of a single valence (Lewis) structure or set of equivalent valence structures. This in turn guarantees that the bond separation energy is unique. ... 

As previously described in Chapter 6, a bond separation reaction breaks down any molecule comprising three or more heavy (nonhydrogen) atoms, and which can be represented in terms of a classical valence structure, into the simplest set of two-heavy-atom molecules containing the same component bonds. For example, the bond separation reaction for methylhydrazine breaks the molecule into methylamine and hydrazine, the simplest molecules incorporating CN and NN single bonds, respectively. 

More precisely, a bond separation reaction is unique to a particular valence structure. The bond separation reaction for a molecule which can only be represented by multiple (nonidentical) valence stmctures may not be uniquely defined. 

Bond Separation Reaction. An Isodesmic Reaction in which a molecule described in terms of a conventional valence structure is broken down into the simplest (two-heavy-atom) molecules containing the same component bonds. 

It is now apparent that nitrenium ions occur in a variety of contexts including the so-called Bamberger rearrangement (i.e., acid-catalyzed isomerization of N-hydroxyaniline 14 to 4-aminophenol 15, Fig. 13.9), and the aforementioned catabolism of arylamines. The earlier workers, however, for the most part did not explicitly consider the possibility of such a species. Heller et al. were the first to present kinetic evidence for such an intermediate. Interestingly, they chose to represent it as the iminocyclohexadienyl cation (16), With the customary ellipsis of allowing a single valency structure to represent mesomeric systems. ... 

Finally, some diradicals can be made in situ by an internal hydrogen-transfer reaction from a suitable hydrogen donor to a carbene or nitrene. In benzene derivatives, this is a well-tested route to o-quinoid compounds, which are not biradicals, although a biradical valence structure probably makes a significant contribution to their electronic structure. However, if the donor and the carbene or nitrene are... 

Abstract—A quantum mechanical treatment of the hydrogen bond by the method of valence structures is described. The interaction of the group A—K and the atom B is characterized by terms which include a donor-acceptor interaction between H and B, the decrease of repulsion between the non-bonded atoms as well as the Coulomb-type attraction. The treatment yields also the explanation of some spectroscopic phenomena of the hydrogen bond. 

In 1947 the present writer published a paper [1] in which he studied the interaction between the polar covalent diatomic molecule A—X and the atom B which has a complete electronic shell. The atom X, in particular, may be a hydrogen atom. To simplify the problem only four electrons were considered two electrons from the bond A—H, and the unshared pair of electrons from atom B, viz. A H B. The respective problem was solved approximately by the method of valence structures. The following structures were adopted ... 

N. D. Sokolov (Moscow) The application of valence schemes (structures) is justified if corresponding calculations are made. If calculations for a certain system are made it is possible of course to draw conclusions in the simplest cases about the properties of other similar systems on the ground of comparisons of the valence structure. However, in general it is risky to make such conclusions. I should like to note that even in simple cases we draw conclusions about the properties of a. molecule not simply on the ground of what structures formally may be attributed to the molecule but comparing experimental data. Structures may be only a means of the visual representation of a molecule naturally they cannot serve the aim of explaining its properties. 

The valence theory (4) includes both types of three-center bonds shown as well as normal two-center, B—B and B—H, bonds. For example, one resonance structure of pentaborane(9) is given in projection in Figure 6. An octet of electrons about each boron atom is attained only if three-center bonds are used in addition to two-center bonds. In many cases involving boron hydrides the valence structure can be deduced. First, the total number of orbitals and valence electrons available for bonding are determined. Next, the B—H and B—H—B bonds are accounted for. Finally, the remaining orbitals and valence electrons are used in framework bonding. Alternative placements of hydrogen atoms require different valence structures. 

Fig. 6. The valence structures and styx numbers of BbH9, B4H10, and B6H...

The possible number of valence structures for a given boron hydride has been defined ... 

It is important to distinguish tautomerism from resonance, a term used to indicate that the properties of a given molecule cannot be represented by a single valence structure but can be represented as a hybrid of two or more structures in which all the nuclei remain in the same places. Only bonding electrons move to convert one resonance form into another. Examples are the enolate anion, which can be thought of as a hybrid of structures A and B, and the amide linkage, which can be represented by a similar pair of resonance forms. 

However, there are systems for which it is not clear how to write down the structure formula. Such situations occur when all valences cannot be filled in a natural way. Several valence structures are then possible. It is not unexpected to find that the RHF model then also runs into similar problems. Consider as an example the ozone molecule 03. The most important valence structures for this molecule are ... 

The two last of these structures have an oxygen-oxygen double bond, at the expense of moving one electron to the end atom. The first structure corresponds to a di-radical where two of the tc electrons are unbound. The near degeneracy between these valence structures can in MO theory be traced to the near degeneracy between the two upper tc molecular orbitals tc2 and Jt3 (the molecule is labelled 0B0A0C) ... 

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