One-electron mechanisms

Adsorbed layers, thin films of oxides, or other compounds present on the metal surface aggravate the pattern of deactivation of metastable atoms. The adsorption changes the surface energy structure. Besides, dense layers of adsorbate may hamper the approach of metastable atom sufficiently close to the metal to suppress thus the process of resonance ionization. An example can be work [130], in which a transition from a two- to one-electron mechanism during deactivation of He atoms is exemplified by the Co - Pd system (111). The experimental material on the interaction of metastable atoms with an adsorption-coated surface of... 

A nickel-promoted C—S bond cleavage has been reported,860 which occurs when solutions of the Ni1 complex of (330) are electrogenerated. The product was identified by cyclic voltammetry and spectroscopy as [Ni(C6H4S2)2]2. EPR and NMR evidence suggests a one-electron mechanism, involving reduction to a 19-electron Ni1 species, electron transfer, and concomitant C—S bond cleavage, extrusion of ethylene followed by a further one-electron reduction and extrusion of ethylene sulfide. 

The electrochemistry of a range of Ni(n) porphyrins and chlorins has been investigated. All complexes are reduced by a similar one-electron mechanism which appears to involve the formation of anion radicals (Chang, Malinski, Ulman Kadish, 1984). 

Several compounds can be oxidized by peroxidases by a free radical mechanism. Among various substrates of peroxidases, L-tyrosine attracts a great interest as an important phenolic compound containing at 100 200 pmol 1 1 in plasma and cells, which can be involved in lipid and protein oxidation. In 1980, Ralston and Dunford [187] have shown that HRP Compound II oxidizes L-tyrosine and 3,5-diiodo-L-tyrosine with pH-dependent reaction rates. Ohtaki et al. [188] measured the rate constants for the reactions of hog thyroid peroxidase Compounds I and II with L-tyrosine (Table 22.1) and showed that Compound I was reduced directly to ferric enzyme. Thus, in this case the reaction of Compound I with L-tyrosine proceeds by two-electron mechanism. In subsequent work these authors have shown [189] that at physiological pH TPO catalyzed the two-electron oxidation not only L-tyrosine but also D-tyrosine, A -acetyltyrosinamide, and monoiodotyrosine, whereas diiodotyrosine was oxidized by a one-electron mechanism. 

However, in addition to two-electron oxidation by native peroxidase, Compound I can oxidize hydrogen peroxide by one-electron mechanism ... 

The ease of formation of PAH cation-radicals is related to their IP. Above a certain IP, activation by one-electron oxidation becomes unlikely because the removal of one electron by the active forms of P450 or peroxidases is more difficult. A cutoff IP above which one-electron oxidation in not likely to occur was tentatively proposed to be about 7.35 eV (Cavalieri and Rogan 1995). For example, 7,12-dimethylbenz[a]-anthracene has an IP of 7.22 eV and is extremely carcinogenic. Benz[a]anthracene has an IP of 7.54 eV and is very weak in this sense. The active carcinogenicity of dibenz[a,h]anthracene (IP 7.61 eV) is not attributable to the one-electron mechanism. It is worth noting that the one-electron transfer is only one of the operating mechanisms of carcinogenesis. 

Inasmuch as flavins can accommodate two electrons but possess a relatively stable one-electron intermediate, an obvious question which can be asked of any flavin-mediated two electron redox reaction is whether or not the mechanism includes the radical species on a direct line between reactants and products. The mere observation of semiquinones in a reaction mixture is not sufficient evidence for their intermediacy, due to the existence of side reactions such as comproportionation (F -I- FH2 2 FH-) which can generate radicals rapidly. Bruice has discussed this question from a physical-organic point of view and concluded that there must exist a competition between one-electron and two-electron processes and that the actual mechanism should be determined mainly by the free energy of formation of substrate radical and the nucleophilicity of the substrate. Bruice has analyzed a variety of systems which he feels should proceed via one-electron mechanisms among these are quinone and carbonyl group reduction by FH2... 

A comprehensive series of oxidation-reduction potential measurements have shown the FAD moiety to have the following one-electron couples PFl/PFIH = = —290 mV and PFIH 7PFIH2 = —365 mV while the FMN moiety exhibits the following PFl/PFl- = -110 mV and PFIH /PFIH = -270 mV. The FMN and FAD smiquinones were found to both be the neutral form as judged from absorption and ESR spectral data. The overlap of oxidized/semiquinone potential of the FAD moiety withkhe semiquinone/hydroquinone couple of the FMN moiety demonstrates the thermodynamic facilitation of flavin-flavin electron transfer via a one-electron mechanism. Stopped-flow kinetic data are also consistent with this view in... 

Diarylpyrylium ions also are dehydrogenating agents.232,250 Thus 2,2 6,6 -diaryl-4,4 -bis-4//-pyrans 163 were dehydrogenated by such agents to 4,4 -dipyranylidene derivatives 164 by a one-electron mechanism involving radical cation intermediates like 378.232 2,2 4,4 6,6 -Hexamethyl derivative 163a was oxidized with perbenzoic acid, but the reaction products were not identified.218... 

Optical activity arises from the coupling of given electric-allowed transitions with a chiral orientation (coupled oscillator mechanism or two-electron mechanism) or from the electric or magnetic moments of a transition being pertubed by a chiral static field (asymmetrically perturbed field mechanism or one-electron mechanism) in the given one molecule. A similar mechanism of the optical activity can be expected for molecular assemblies which are composed of chiral and achiral ones. This type of optical activity is called induced optical activity and depends on types of inter-molecular interaction modes. 

This mechanism is rather probable 02 is an electron donor, and organyl halogenides can, in principle, accept one electron. With the one-electron mechanism, the inversion of configuration is determined by 02 attack on the side, which is opposite to the halogen atom in RHal (Morkovnik Okhlobystin 1979). The direction of this reaction depends on the nature of the solvent. In pyridine, benzene, and DMF, the main product is alkyl peroxide. In DMSO, an alkyl carbinol is the main product (Sawyer Gibian 1979). Obviously, the aforementioned intermediary product ROO- reacts faster with the solvent Me2SO than with the substrate RHal, Scheme 1-78 ... 

The proposition of a one-electron mechanism for the electron-transfer reduction of dioxygen and the associated conclusions present significant ramifications relative to the development of improved fuel cells and metal-air batteries. To date the practical forms of such systems have used strongly acidic or basic electrolytes. Such solution conditions normally cause atom transfer to be the dominant reduction process for molecular oxygen at metal electrodes. Hence, the search for effective catalytic materials should be in this context rather than in terms of a one-electron-transfer process. 

The One-Electron Mechanism. Both the electric and magnetic dipole transitions reside in the same chromophore. The rest of the dissymmetric molecule acts as a perturbing field which partially breaks down the symmetry of the chromophore, and therefore mixes the two transitions. The one-electron theory is also known as the Condon, Altar, and Eyring theory. 

Strangely, these small molecules are also used industrially as oxidation catalysts, however, small changes in their structures lead to dramatically different behaviour. In these model compounds, unlike SOD2, the metal is bound by two oxygen and two nitrogen atoms in a square planar orientation with axial positions occupied by water or reactive oxygen species. The superoxide dismutase catalytic cycle has been proposed to occur by a one electron mechanism ... 

Boyd JA, Eling TE (1984) Evidence for a one-electron mechanism of 2-aminofluorene oxidation by prostaglandin H synthase and horseradish peroxidase. J Biol Chem 259 13885-13896... 

In real situations (Sections 3.1 and 3.5) sequential one-electron transfers precede the formation of electron-rich or electron deficient multi-electron catalytic complexes. Thus, such systems may be considered as devices for switching processes from the multistep one-electron mechanism to the multi-electron mechanism. 

According to Eq. 3.1, for the reaction to occur by the one-electron mechanism of hydrogen abstraction from an alkane in a free state (for instance, from a terminal methyl... 

Analysis of the cyclic voltammograms in the positive potential range from 0.0 to 1 500 mV indicates a simple one-electron mechanism for the quasi-reversible oxidation processes at 840 mV and 1310 mV. These oxidation processes were attributed to the equilibrium ... 

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