Radiolytic electron transfer

Spectrophotometry has been a popular means of monitoring redox reactions, with increasing use being made of flow, pulse radiolytic and laser photolytic techniques. The majority of redox reactions, even those with involved stoichiometry, have seeond-order characteristics. There is also an important group of reactions in which first-order intramolecular electron transfer is involved. Less straightforward kinetics may arise with redox reactions that involve metal complex or radical intermediates, or multi-electron transfer, as in the reduction of Cr(VI) to Cr(III). Reactants with different equivalences as in the noncomplementary reaction... 

Figure 8 Nucleation and growth of clusters generated by radiolytic radicals at various dose rates, with or without electron donor D. The stabilizing effect of the polymer prevents exclusively coalescence beyond a certain limit of nuclearity, but does not prevent successive ion and electron transfers at low dose rate from the radicals. The donor allows the cluster to become much larger at any dose rate. (From Ref. 19.)...

In many cases, although and M are both readily reduced by radiolytic radicals, a further electron transfer from the more electronegative atoms (for example, M ) to the more noble ions ( °(M /M )electron transfer is also possible between the low valencies of both metals, so increasing the probability of segregation [174]. The intermetal electron transfer has been observe directly by pulse techniques for some systems [66,175,176], and the transient cluster (MM ) sometimes identified such as (AgTl) or (AgCo) [176]. The less noble metal ions act as an electron relay toward the precious metal ions, so long as all are not reduced. Thus, monometallic clusters M are formed first and M ions are reduced afterward in situ when adsorbed at the surface... 

Figure 13 Scheme of the influence of the dose rate on the competition between the inter-metal electron transfer and the coalescence processes during the radiolytic reduction of mixed metal ion solutions. Sudden irradiation at high dose rates favor alloying, whereas low dose rates favor coreshell segregation of the metals because of metal displacement in the clusters. 

One of the important applications of mono- and multimetallic clusters is to be used as catalysts [186]. Their catalytic properties depend on the nature of metal atoms accessible to the reactants at the surface. The possible control through the radiolytic synthesis of the alloying of various metals, all present at the surface, is therefore particularly important for the catalysis of multistep reactions. The role of the size is twofold. It governs the kinetics by the number of active sites, which increase with the specific area. However, the most crucial role is played by the cluster potential, which depends on the nuclearity and controls the thermodynamics, possibly with a threshold. For example, in the catalysis of electron transfer (Fig. 14), the cluster is able to efficiently relay electrons from a donor to an acceptor, provided the potential value is intermediate between those of the reactants [49]. Below or above these two thresholds, the transfer to or from the cluster, respectively, is thermodynamically inhibited and the cluster is unable to act as a relay. The optimum range is adjustable by the size [63]. 

Jhe development of chemistry in the 20th century has been dominated and motivated by the electronic theory of the chemical bond and the role of electrons in chemical reactivity. The electronic structure of the chemical bond could be deduced by more or less direct methods, such as electronic excitation spectra, dipole moments, or paramagnetism but there was no direct indication for the transfer of electrons in chemical reactions. Using isotopic techniques it has been possible to demonstrate bond cleavage and atom transfer reactions, but it is impossible to label an electron and trace its transfer from one molecule to another. It was not until the discovery of the radiolytically produced solvated electron that electron transfer processes could be examined directly and unambiguously. 

One may look upon the research into e aq reactions from two standpoints. One is the standpoint of the radiation chemist or radiation biochemist who is interested in the radiolytic damage caused by e aq as compared with other radiolytic species. The other is the approach of the chemist who may use the reactivity of e aq to investigate the electronic structure of chemical species and test the theories on the role of electron transfer in chemical reactions. The species e aq is important to the chemist from still another angle being the purest and simplest reducing agent it may be used to produce reduced chemical species, some of them only as short-lived transients, which have never before been synthesized. 

Schmidt KH, Flan P, Bartels DM (1995) Radiolytic yields of the hydrated electron from transient conductivity improved calculation of the hydrated electron diffusion coefficient and analysis of some diffusion-limited (e )aq reaction rates. J Phys Chem 99 10530-10539 Schoneich C, Aced A, Asmus K-D (1991) Halogenated peroxyl radicals as two-electron-transfer agents. Oxidation of organic sulfides to sulfoxides. J Am Chem Soc 113 375-376 Schuchmann Fl-P, von Sonntag C (1981) Photolysis at 185 nm of dimethyl ether in aqueous solution Involvement of the hydroxymethyl radical. J Photochem 16 289-295 Schuchmann Fl-P, von Sonntag C (1984) Methylperoxyl radicals a study ofthey-radiolysis of methane in oxygenated aqueous solutions. Z Naturforsch 39b 217-221 Schuchmann Fl-P, von Sonntag C (1997) Heteroatom peroxyl radicals. In Alfassi ZB (ed) Peroxyl radicals. Wiley, Chichester, pp 439-455... 

Fujita S, Steenken S (1981) Pattern ofOFI radical addition to uracil and methyl-and carboxyl-substituted uracils. Electron transfer ofOFI adducts with N,N, Ar, Ar -tetramethyl-p-phenylenediamine and tetranitromethane. J Am Chem Soc 103 2540-2545 Fujita S, Nagata Y, Dohmaru T (1988) Radicals produced by the reactions of SO4 with uridine and its derivatives. Studies by pulse radiolysis and y-radiolysis. Int J Radiat Biol 54 417-427 Fujita S, Horii FI,Taniguchi R, Lakshmi S, Renganathan R (1996) Pulse radiolytic investigations on the reaction of the 6-yl radicals of the uracils with Cu(ll)-amino acid complexes. Radiat Phys Chem 48 643-649... 

Ito T, Shinohara H, Hatta H, Nishimoto S-l (1999) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers product and laser flash photolysis studies on the oxidative splitting mechanism. J Phys Chem A 103 8413-8420 ItoT, Shinohara H, Hatta H, Fujita S-l, Nishimoto S-l (2000) Radiation-induced and photosensitized splitting of C5-C5 -linked dihydrothymine dimers. 2. Conformational effects on the reductive splitting mechanism. J Phys Chem A 104 2886-2893 ItoT, Shinohara H, Hatta H, Nishimoto S-l (2002) Stereoisomeric C5-C5 -linked dehydrothymine dimers produced by radiolytic one-electron reduction of thymine derivatives in anoxic solution structural characteristics in reference to cyclobutane photodimers. J Org Chem 64 5100-5108 Jagannadham V, Steenken S (1984) One-electron reduction of nitrobenzenes by a-hydroxyalkyl radicals via addition/elimination. An example of an organic inner-sphere electron-transfer reaction. J Am Chem Soc 106 6542-6551... 

JellinekT, Johns RB (1970) The mechanism of photochemical addition of cysteine to uracil and formation of dihydrouracil. Photochem Photobiol 11 349-359 Jiang Y, Lin W-Z, Yao S-D, Lin N-Y, Zhu D-Y (1999a) Pulse radiolytic study of electron transfer reaction for fast repair of the one-electron oxidized radicals of dAMP and dGMP by hydroxycinnamic acid derivatives. Radiat Phys Chem 54 349-353... 

Jovanovic SV, Steenken S, Simic MG (1991) Kinetics and energetics of one-electron-transfer reactions involving tryptophan neutral and cation radicals. J Phys Chem 95 684-687 Kagiya T, Kimura R, Komuro C, Sakano K, Nishimoto S (1983) Promotion effect of 2,2,6,6-tetrameth-ylpiperidine-1-oxyls on the radiolytic hydroxylation of thymine in deaerated aqueous solution. Chem Lett 1471-1474... 

Most free-radical reactions of synthetic value are chain reactions, the key steps of which are illustrated in Scheme 4.1. In the initiation step, a reactive radical is generated from a nonradical precursor (initiator). In many cases, this can be accomplished thermally. For instance, peroxides possess a weak oxygen-oxygen bond and, consequently, undergo homolytic dissociation upon heating ROOR —> 2RO . Free radicals can also be generated photochemically, radiolytically, or by electron transfer from appropriate precursors. 

Electron-Transfer Reduction of 02. Within aqueous solutions the most direct means to the electron-transfer reduction of dioxygen is by pulse radiolysis. Irradiation of an aqueous solution by an electron beam yields (almost instantly 10-12 s) solvated electrons [e (aq)], hydrogen atoms (H-), and hydroxyl radicals (HO-)- If the solution contains a large excess of sodium formate [Na+ 0(0)CH] and is saturated with 02, then the radiolytic electron flux efficiently and cleanly reduces 02 to superoxide ion (O ) 21-25... 

In related model complex studies, Isied and coworkers, have examined photo-induced (or pulse-radiolytically initiated) electron-transfer processes in which a polypyridine-ruthenium(II) complex is linked by means of a 4-carboxylato,4 -methyl,2,2 -bipyridine ligand and a polyproline chain to a [Co(NH3)5] + or [(-NH-py)Ru (NH3)5] acceptor. Chains composed of from zero to six cis-prolines have been examined. The apparent distance dependence of the electron-transfer rate constant, corrected for variations in the solvent reorganizational energy, seems to exhibit two types of distance dependence, 0.7-1A for short chains and /3 a0.3 A for long chains. A very detailed theoretical analysis of electron transfer in the complexes with four proline linkers has indicated that the electronic coupling is sensitive to conformational variations within the proline chain. ... 

Recently, new techniques such as laser-driven photocathode accel-erators have increased the time resolution available for radiation-chemical studies. They have been of great use in studying fast electron-transfer processes, but are not the one-to-two orders of magnitude improvement that would be needed to explore some of the fundamental questions of electron-precursor reactions and initial distribution of radiolytically produced species. Newer techniques, such as the laser-wakefield accelerator, have the potential to answer these sorts of questions however, they have not reached their maximum potential. ... 

The formation of hydrated electrons (in the water-saturated zeolites X and Y) has been identified through their absorption spectra, their short lifetimes distinct from the long-lived cation cluster-trapped electrons, and their reactivity towards typical hydrated electron quenchers such as methylviologen. Based on these spectra (Fig. 5), yields between 4 x 10 mol and 6 x 10 mol were measured for electrons in fully hydrated NaY. These high radiolytic yields were also explained by electron transfer from the ionized zeolitic skeleton to the water clusters [Eqs. (1) and (7)]. 

Fundamental studies on the radiolytic oxidation of aromatics Rao) and radiolytic redox reactions as seen in electron transfers Brede and Naumov) are reviewed in Chapters 14 and 15. 

Figure 2. Dependence of rate constant on -AG° for electron transfer from B to A in A-Sp-B " generated pulse radiolytically in 2-methyltetrahy-drofuran. B = 4-biphenylyl, A = 2-naphthyl (1), 9-phenanthryl (2), 1-pyr-enyl (3), hexahydronaphthoquinon-2-yl (4), 2-naphthoquinonyl (5), 2-benzoqui-nonyl (6), 5-chlorobenzoquinon-2-yl (7), 5,6-dichlorobenzoquinon-2-yl (8). (Adapted from Ref. [31]).

More recent work has investigated the bleaching of dyes, including Methylene Blue and Thionine, by electron-transfer from radicals such as CH2OH generated under radiolytic conditions. There is a strong dependence of the bleaching reaction, both with respect to efficiency and rate, upon the redox potential of the dye. 

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