Radicals luminol radical anions

The luminol dianion Lum2< > does not exist in appreciable quantities in aqueous solvents hydrogen peroxide and a catalyst such as hemin are required. Thus another mechanism seems to be at work here. Perhaps a hydrogen atom is abstracted from the luminol monoanion Lum( > to yield a luminol radical anion 55 which then reacts with oxygen or a radical ion derived from hydrogen peroxide according to 3,4,109)... 

Aqueous alkaline luminol solutions can be excited to chemiluminescence by pulse radiolysis, the only additional requirement being oxygen 119h The suggested mechanism is that hydroxyl radicals attacking luminol monoanions, followed by reaction of the luminol radical anion thus formed with oxygen ... 

The reaction between hydroxyl radical and luminol results in the quantitative formation of hydroxylated products, which are known to be radical scavengers (Scheme 19) °. The reduction of luminol radical anion by these species decreases the emission intensity, unless HO is generated rapidly in high concentrations . The emission observed under these conditions may be attributed to one-electron oxidation of luminol by hydroxyl radicals, yielding luminol radical anion, or by hydroxyl-mediated generation of potent one-electron oxidants from its reaction with bases. Moreover, the addition of HO to luminol or its monoanion results in an adduct able to react very rapidly with O2, yielding, in addition to hydroxylated luminol, superoxide (Scheme 19) °. 

A similar study has been carried out by the pulse radiolytic production of luminol radical anions and oxygen radical ions simultaneously [77]. 

These results involving the luminol radical anion are compatible with other proposals for the luminol mechanism. Steps after the peroxide intermediate are not specifically detected, and must occur with a rate constant of 10 s ... 

The source of chemiluminescence in the oxidation of luminol was explored by Merenyi and co-workers in detail (153). The oxidation of luminol yields aminophthalate as a final product and the reaction proceeds via a series of electron transfer steps. The primary oxidation product is the luminol radical which is transformed into either diazaquinone or the a-hydroxide-hydroperoxide intermediate (a-HHP). The latter oxidation step occurs between the deprotonated form of the luminol radical and O -. The chemiluminescence is due to the decomposition of the mono-anionic form of a-HHP into the final products ... 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

LH - luminol anion, L - luminol radical, 02"- superoxide anion, L02 --luminol-endoperoxide, and (AP ) - aminophthalate anion in the basic and excited states respectively). 

Irradiation with different types of high energy rays, pulse radiolysis or acoustical cavitation also cause luminol chemiluminescence in aqueous solution. All these physical sources are known to produce HO. radicals by homolysis of the water molecules. The most reasonable assumption is that in aqueous solution the necessary luminol species is a radical anion (39) which rapidly reacts with an appropriate oxidizing species either as the neutral species or after ionization. 

As mentioned above, luminol (and other cyclic hydrazide) radical anions are produced by a wide range of reagents. 

The last dimerisation reaction is probably assumed to resemble the Russell termination step (p. 23) but it must be said that is an unlikely candidate for the typically high quantum yields of the luminol reaction. Direct reaction [76] with oxygen is observed and, instead of diazaquinone, its addition product with oxygen radical anion is assumed. An endoperoxide such as (47) is not proposed as an intermediate. 

Since there is no effect of H2O2 concentration on the integrated chemiluminescence intensity, the conclusion was drawn that oxygen radical anion activates the luminol radical anions to chemiluminescence, forming the non-radical diazaquinone perhydrate (59) ... 

Hydroxyl radicals and/or superoxide radical anions appear to be essential in the luminol reaction in aqueous solution (see p. 101) although not in an aprotic medium. 

Use has also been made of a rather unusual observation, that certain metals appear to inhibit luminol chemiluminescence. There is no explanation available as yet for this phenomenon the suggestion made earlier [158] that these inhibitor metals transform hydrogen peroxide into the non-active 62 " dianion (from which HO-radicals or 02 radical anions cannot arise as easily as from O2H-anion) cannot be correct as in aqueous solution, 02 " dianion is in equilibrium with H02 Among these metals are vanadium [31], zirconium [32], and thorium [34]. 

In the absence of luminol or lucigenin, electronically excited carbonyl groups are thought to be the emitters of native phagocytosis chemiluminescence [117], but the enhanced chemiluminescence obtained in the presence of luminol or lucigenin, stems from 3-aminophthalate and N-methylacridone, respectively, produced very probably by the action of 02 radical anion. 

Other cationic surfactants such as TTAB, DTAB, DODAB, STAC, CEDAB, and DDDAB have been used in CL reactions with less frequency. Thus, tetradecyltrimethylammonium bromide [TTAB] has been used to increase the sensitivity of the method to determine Fe(II) and total Fe based on the catalytic action of Fe(II) in the oxidation of luminol with hydrogen peroxide in an alkaline medium [47], While other surfactants such as HTAB, hexadecylpiridinium bromide (HPB), Brij-35, and SDS do not enhance the CL intensity, TTAB shows a maximum enhancement at a concentration of 2.7 X 10 2 M (Fig. 11). At the same time it was found that the catalytic effect of Fe(II) is extremely efficient in the presence of citric acid. With regard to the mechanism of the reaction, it is thought that Fe(II) forms an anionic complex with citric acid, being later concentrated on the surface of the TTAB cationic micelle. The complex reacts with the hydrogen peroxide to form hydroxy radical or superoxide ion on the... 

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