Radicals contain impaired electrons

As all the metals in the first row of the d-block in the Periodic Table with the sole exception of zinc, iron contains impaired electrons and can thus qualify as a radical (Halliwell and Gutteridge 1989, p. 15), and ferrous ions can participate in electron transfer reactions with molecular oxygen. 

A diradical is an atom or molecule containing two impaired electrons. The properties of diradicals are for the most part like those of monoradicals. They are paramagnetic and show paramagnetic resonance absorption. Although they are very reactive chemically, this is not a reliable criterion for the diradical state. Spectroscopically the diradical will probably be a triplet state if a double bond structure coupling the two electrons is geometrically possible. But when the two electrons are fairly well isolated from each other the state is probably a double doublet, like two independent radicals. 

The electrocyclic reactions of n systems containing an impaired electron are difficult to interpret using the above simple theories. The symmetry of the HOMO of the radical system corresponds to that of the corresponding anion. Thus the allyl radical would be expected to cyclize in the same manner as the alkyl anion i.e., in a conrotatory manner. In fact the interconversion takes place in a disrotatory manner. Theoretical calculations based on Huckets theory also give ambiguous or incorrect predictions. And therefore more sophisticated calculations are required to obtain reliable results. 

Since 02 exists in a "triplet" state with two impaired electrons, it reacts rapidly only with transition metal ions or with organic radicals (Chapter 16). For this reason, most oxygenases contain a transition metal ion, usually of iron or copper, or contain a cofactor, such as FAD, that can easily form a radical or act on a cosubstrate or substrate to form a free radical. 

The term vitamin C refers to ascorbic add (the fully reduced form of the vitamin) and to dehydroascorbic acid. Removal of one electron from ascorbic acid yields semidehydroascorbic add (ascorbate radical). This form of the vitamin is a free radical it contains an impaired electron. The structures of free radicals are written with large dots. The removal of a second electron yields dehydroascorbic add. Conversion of ascorbate to dehydroascorbate, via the removal of two electrons, can occur imder two conditions (1) with use of ascorbic acid by ascorbate-dependent enz5mies and (2) with the spontaneous reaction of ascorbate with oxygen. Semidehydroascorbate is an intermediate in this conversion pathway. 

The products of homolysis are radicals, which maybe atoms or molecules, and contain an impaired electron. 

Photopolymerization systems, like thermally initiated systems, contain initiator, monomer, and other additives that impart desired properties (color, strength, flexibility, etc) (6). The reaction is initiated by active centers that are produced when light is absorbed by the photoinitiator. One important class of active centers includes free-radical species, which possess an impaired electron (5,7). The highly reactive free-radical active centers attack carbon-carbon double bonds in imsaturated monomers to form pol5nner chains. Although the kinetic treatment of photopoljnner systems is similar to that in thermal systems, significant differences arise in the description of the initiation step, which in turn affect the... 

Step 1 A one-electron reduction of the alkyne gives an alkenyl radical anion [i.e., an ion containing an impaired electron on one carbon and a negative charge on an adjacent carbon (note that we use a single-headed arrow to show the repositioning of single electrons)]. 

Addition polymers form when monomers such as ethylene join end to end to make polyethylene. Reactions of this type can be initiated by a radical—a species that contains an impaired electron [M4 Section 8.8]. The mechanism of addition polymerization is as follows ... 

It has been suggested that P BChl (where BChl is one of the two monomeric or "accessory BChls that are not part of P) is a transient state prior to P "I (14,16,19), although the evidence supporting this view has been criticized (23, 24) Recent subpicosecond studies find no evidence for P "BChl (8,9) These new results do not preclude some involvement of a monomeric BChl in the early photochemistry, only that P BChl apparently is not a kinetically resolved transient state Perhaps P itself contains some charge-transfer character between its component BChls, or between P and one or both of the monomeric BChls (8,9,25-27) One of the two monomeric BChls apparently can be removed by treatment of the reaction center with sodium borohydride (28) and subsequent chromatography, with no impairment of the primary electron transfer reactions (29) Thus, at present it appears that P I is the first resolved radical-pair state, and it forms with a time constant of about 4 ps in Rps sphaeroides ... 

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