Resonance, among bond structures

It is pointed out that the dissociation of certain substituted ethanes into free radicals is due not to weakness of the carbon-carbon bond in the ethane but to the stabilization of the free radicals resulting from resonance among the structures in which the unpaired electron is located on the... 

Resonance of a Carbon-Chlorine Bond and an Adjacent Double Bond.—We expect the phosgene molecule to resonate among the structures... 

If the radical were restricted to resonance between the KekulA structures A and B, with the free valence on the methyl carbon, resonance would stabilize the radicals to just the same extent as the undissociated molecules, which would then have only the same tendency to dissociate as a hexaalkylethane. But actually the five structures A, B, C, D and E (each with three double bonds) contribute about equally to the structure of the radical, which thus resonates among five structures instead of two and is correspondingly stabilized by the additional resonance energy. 

Another structural feature of importance is the bond angle R—N—N. This angle has the value 116° for structure A (unstrained), 108° for Bf and 180° for C (the value 116° applies also to J , but with the plane of the molecule normal to that for A) (Sec. 4-8). An average value would be expected for resonance among several structures. The observed values, 120° 10° in methyl azide, 114° 3° in cyanuric triazide, and 112.7° 0.5° in hydrogen azide, are in agreement with the value expected for resonance between A arid C, about 112°. 

The discussion of nd orbital participation in Section 6.1 is relevant to the bonding schemes (8)-(15). The examples given for (9)-(13) can all be described in terms of semi-polar bonds, i.e. donation from S to O, and S02 might be placed alongside ozone 03 as an example of scheme (6). A VB description of the S02 molecule would require resonance among the structures ... 

The dissociation of hexaphcnylethane leads to the formation of two triphenylmethyl radicals. With the central carbon atom in the trivalent state eight Kckuld structures are possible the nine carbon atoms in the ortho and para positions in the three rings may also be in a trivalent state and thus in all, 36 valence bond structures are possible for the free radical. As a result of the resonance among these structures there is a gain in energy of the free radical by 50 kcals, with the result that the dissociation energy of hexaphcnylethane is as low as 11-12 kcals. 

At the present time there is no satisfactory theory of these particles. It seems likely, however, that some of them may be classed with the fundamental particles (see the following section), and that others may be described as complexes of two or more fundamental particles, possibly with resonance among several structures, roughly analogous to the resonance of molecules among several valence-bond structures. 

In recent years it has become clear that the structure of metals and alloys may be described in terms of covalent bonds that resonate among the alternative interatomic positions in the metals, and that this resonance is of greater importance for metals than for substances of any Other class, including the aromatic hydrocarbons. Moreover, the phenomenon of metallic resonance of the valency bonds must be given explicit consideration in the discussion of metallic valency it is necessary in deducing the metallic valency from the number of available electrons and bond orbitals to assign to one orbital a special r le in the metallic resonance. 

In this book the discussion has been restricted to the structure of the normal states of molecules, with little reference to the great part of chemistry dealing with the mechanisms and rates of chemical reactions. It seems probable that the concept of resonance can be applied very effectively in this field. The activated complexes which represent intermediate stages in chemical reactions are, almost without exception, unstable molecules which resonate among several valence-bond structures. Thus, according to the theory of Lewis, Olson, and Polanyi, Walden inversion occurs in the hydrolysis of an alkyl halide by the following mechanism ... 

The determination of values of interatomic distances in molecules has been found to provide much information regarding electronic structure, especially in the case of substances which resonate among two or more valence-bond structures. The interpretation of interatomic distances in terms of the types of bonds involved is made with use of an empirical function formulated originally for single bond-double bond resonance of the carbon-carbon bond.1 There are given in this... 

The compound Lajln has Tc = 10.4 K. Because La is hypoelectronic and In is hyperelectronic, I expect electron transfer to take place to the extent allowed by the approximate electroneutrality principle.13 The unit cube would then consist of 2 La, La, and In+, with In+ having no need for a metallic orbital and thus having valence 6 with the bonds showing mainly pivoting resonance among the twelve positions. The increase in valence of In and also of La (to 3 f ) and the assumption of the densely packed A15 structure account for the decrease in volume by 14.3%. Because the holes are fixed on the In + atoms, only the electrons move with the phonon, explaining the increase in Tc. 

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