Odd-electron

For odd electron systems in the absence of spatial symmehy H Eq. (12) becomes... 

The structure of nitrogen dioxide contains an unpaired (odd) electron and the molecule is consequently paramagnetic. The odd electron is not localised on any atom and the structure can be best represented as a resonance hybrid of the structures ... 

Unlike nitrogen monoxide, nitrogen dioxide has properties more typical of an odd electron molecule. It is a coloured (brown), reactive gas which dimerises to the diamagnetic colourless gas dinitrogen tetroxide, N2O4. in which the odd electron is paired. The structure of dinitrogen tetroxide can be represented as a resonance hybrid of ... 

The aqueous solution is safe to handle, the dissolution being essentially physical. On standing in sunlight the solution slowly decomposes to a mixture of acids. In alkaline solution a mixture of chlorate(lll), CIO2, and chlorate(V), CIO J, ions is rapidly produced. Chlorine dioxide is paramagnetic, the molecule containing an odd electron and having a structure very like that of NOj (p. 231). 

In practice side reactions intervene to reduce the efficiency of the propagation steps The chain sequence is interrupted whenever two odd electron species combine to give an even electron product Reactions of this type are called chain terminating steps Some commonly observed chain terminating steps m the chlorination of methane are shown m the following equations... 

FIGURE 10 3 (a) The spin density (yellow) in allyl radical is equally divided between the two allylic carbons There is a much smaller spin density at C 2 hydrogen (b) The odd electron is in an orbital that is part of the allylic tt system... 

Structural formulae of ions in which it is desired to indicate the localization of an odd electron can, alternatively, be written without the parentheses. Thus,... 

Radical ion. An ion containing an unpaired electron that is thus both an ion and a free radical. The presence of the odd electron is denoted by placing a dot alongside the symbol for the charge. Thus, and SF are radical ions. 

In principle, this degradation can continue until the residual radical contains only hydrogen or methyl groups attached to the carbon with the odd electron. Those radicals which stiU contain a carbon—carbon bond can form an olefin via reaction 23 (or sequence 2, 24). Methyl radicals are a special case with limited options. 

Bonds may also be broken symmetrically such that each atom retains one electron of the pair that formed the covalent bond. This odd electron is not paired like all the other electrons of the atom, i.e. it does not have a partner of opposite spin. Atoms possessing odd unpaired electrons are termed free radicals and are indicated by a dot alongside the atomic or molecular structure. The chlorination of methane (see later) to produce methyl chloride (CH3CI) is a typical free-radical reaction ... 

Weak interactions may occur between molecules (intermolecular association) as well as within a molecule (intramolecular) for chalcogen-nitrogen ring systems. This behaviour is especially significant for odd electron species, e.g., [EsNa]" (4.14, E = S, Se) and [PhCNaEa] (4.15, E = S, Se), both of which are seven r-electron molecules. As mentioned in the previous section, it also occurs for the eight r-electron dithiatriazines 4.10... 

Nitric oxide is the simplest thermally stable odd-electron molecule known and, accordingly, its electronic structure and reaction chemistry have been very extensively studied. The compound is an intermediate in the production of nitric acid and is prepared industrially by the catalytic oxidation of ammonia (p. 466). On the laboratory scale it can be synthesized from aqueous solution by the mild reduction of acidified nitrites with iodide or ferrocyanide or by the disproportionation of nitrous acid in the presence of dilute sulfuric acid ... 

Processes occurring upon ionization and notations used for odd-electron ions (4) and even-electron ions ( + ) are illustrated in Equations 1, 2, and 3. 

I Initiation The polymerization reaction is initiated when a few radicals are generated on heating a small amount of benzoyl peroxide catalyst to break the weak 0-0 bond. A benzoyloxy radical then adds to the C=C bond of ethylene to generate a carbon radical. One electron from the C=C bond pairs up with the odd electron on the benzoyloxy radical to form a C-O bond, and the other election remains on carbon. 

For such odd electron species (sometimes called free radicals) it is impossible to write Lewis structures in which each atom obeys the octet rule. In the NO molecule, the unpaired electron is put on the nitrogen atom, giving both atoms a formal charge of zero ... 

A similar reaction occurs when an equimolar mixture of NO and N02 is cooled. Two odd electrons, one from each molecule, pair off to form an N—N bond ... 

Radical ion (odd electron ion) An ion containing an unpaired electron that is both a radical and an ion. 

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