Radiolytic reduction

Radiolytic reduction has been investigated as a means of producing transient Rh(II) porphyrin products, and as in the above study, the observed products were strongly dependent on pH and solvent. Radiolytic reduction of Rh(TMP)Cl in alcohol formed transient Rh(TMP)- which was prevented from dimerization by the bulky TMP ligand. In alkaline 2-propanol the product is [Rh(TMP)r. in weakly acidic 2-propanol the hydride Rh(TMP)H is formed, and in strongly acidic 2-propanol the alkylated rhodium(III) porphyrins Rh(TMP)CH3 and Rh(TMP) (C(CH 3)20H) are observed. The alkyl products result from reaction of Rh(TMP)-with CH3- and C(CH3)20H formed by radiolysis of the 2-propanol solvent. 

Koulkes-Pujo and coworkers studied the radiolytic reduction of Ce in water-DMSO mixtures and found that while in water G(Ce ) is 2.5, the addition of DMSO increases the yield very much and leads to G(Ce )= 18.1 for 1.4 m DMSO. At higher concentrations of DMSO the yields decrease down to 14.4 for pure DMSO. The high value of G(Ce ) was explained on the basis that all species created by ionization of the medium, including H, OH, HjOj and HO2, are rendered reducing in the presence of DMSO. It is not necessary to increase the DMSO concentration beyond 0.5 m to increase significantly the reduction yield. In aqueous solution without DMSO the Ce reduction is accompanied by O2 formation with a yield near 0.8, however in the presence of DMSO, Oj is never formed. The increased yield of reduction and the absence of O2 formation might have implications in radiobiochemistry. ... 

Ru(bipy)3 formed in this reaction is reduced by the sacrificial electron donor sodium ethylenediaminetetra-acetic acid, EDTA. Cat is the colloidal catalyst. With platinum, the quantum yield of hydrogenation was 9.9 x 10 . The yield for C H hydrogenation was much lower. However, it could substantially be improv l by using a Pt colloid which was covered by palladium This example demonstrates that complex colloidal metal catalysts may have specific actions. Bimetalic alloys of high specific area often can prepared by radiolytic reduction of metal ions 3.44) Reactions of oxidizing radicals with colloidal metals have been investigated less thoroughly. OH radicals react with colloidal platinum to form a thin oxide layer which increases the optical absorbance in the UV and protects the colloid from further radical attack. Complexed halide atoms, such as Cl , Br, and I, also react... 

Recently, Chaudhari compared the activity of dispersed nanosized metal particles prepared by chemical or radiolytic reduction and stabilized by various polymers (PVP, PVA or poly(methylvinyl ether)) with the one of conventional supported metal catalysts in the partial hydrogenation of 2-butyne-l,4-diol. Several transition metals (e.g., Pd, Pt, Rh, Ru, Ni) were prepared according to conventional methods and subsequently investigated [89]. In general, the catalysts prepared by chemical reduction methods were more active than those prepared by radiolysis, and in all cases aqueous colloids showed a higher catalytic activity (up to 40-fold) in comparison with corresponding conventional catalysts. The best results were obtained with cubic Pd nanosized particles obtained by chemical reduction (Table 9.13). 

This oxidation state which resembles Cu(II) may be prepared by electrochemical, photochemicalor pulse radiolytic reduction of nickel(II). Nickel(I) macrocycles are powerful reductants and their spectra and redox potentials have been measured. The reactions of the Ni(I) complexes Ni(tmc) 10 and 11 with RX are similar. 

In experiments where ion-radicals are generated by radiolytic reduction or oxidation in solid matrices, the concentration of solute molecules must be at least 10 M to ensure efficient scavenging of the initially generated electrons or holes. At the same time, the upper limit of the solute concentration should not exceed 10 -10 M. It is necessary that the direct effect of radiation on the solute molecule should be ignored. This is one of the major requirements for successful use of radiolytic methods for generating ion-radicals. 

In one study, it has been reported that radiolytic reduction of the Fe -... 

Due to the accurate knowledge of the dose used, a control of the progressive extent of the radiolytic reduction was achieved. However, quite often, puzzling data were reported when the zero-valent metal was formed, such as an induction time for precipitation, a sensitivity of the radiolytic yields to the initial presence of added particles, and an unusually weak reproducibility [9,10]. Moreover, oxidation of silver atoms by molecular oxygen was observed [7], although the process was thermodynamically improbable for a noble metal such as silver. 

The spectra of silver and gold nanoclusters are intense and distinct (Table 4). They are thus particularly suitable to detect the evolution of a cluster composition during the construction of a bimetallic cluster in mixed solution. The system studied by pulse radiolysis was the radiolytic reduction of a mixed solution of two monovalent ions, the cyano-silver and the cyano-gold ions Ag(CN)2 and Au(CN)2 (Fig- 7) [66]. Actually, the time-resolved observation demonstrated a two-step process. First, the atoms Ag and Au are readily formed after the pulse and coalesce into an alloyed oligomer. However, due to... 

Actually, the kinetics study of the redox potential of transient clusters (Section 20.3.2) has shown that beyond the critical nuclearity, they receive electrons without delay from an electron donor already present. The critical nuclearity depends on the donor potential and then the autocatalytic growth does not stop until the metal ions or the electron donor are not exhausted (Fig. 8c). An extreme case of the size development occurs, despite the presence of the polymer, when the nucleation induced by radiolytic reduction is followed by a chemical reduction. The donor D does not create new nuclei but allows the supercritical clusters to develop. This process may be used to select the cluster final size by the choice of the radiolytic/chemical reduction ratio. But it also occurs spontaneously any time when even a mild reducing agent is present during the radiolytic synthesis. The specificity of this method is to combine the ion reduction successively ... 

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. 

The characteristics of intermediates MMOH-Q and RNR R2-X continue to converge. The radiolytic reduction of MMOH-Q affords an Fe(III)/Fe(I V) form designated Qx, which has Mossbauer properties similar to RNR R2-X [25]. This work and the recent EXAFS characterization of MMOH-Q and RNR R2-X [26-28], revealing that the two intermediates have very similar core structures, provide strong support that the 02 reaction chemistry in these two enzymes is analogous. This notion very likely extends to the other diiron oxygen activating enzymes. 

Early experimental studies of cryogenic radiolytic reduction of heme proteins in frozen solutions demonstrated that in most cases reduced proteins cannot undergo... 

Aguila, A. O Shea, E. Kamat, P. V. Radiolytic reduction and oxidation of diethyl benzylphosphonate. A pulse radiolysis study, Advance Oxidation Technology 1998, in press. 

Davydov, A., Davydov, R., Gr%oslund, A., Lipscomb, J. D., and Andersson, K. K., 1997, Radiolytic reduction of methane monooxygenase dinuclear iron cluster At 77K6EPR evidence for conformational change upon reduction or binding of component B to the diferric state, J. Biol. Chem. 272 702267026. 

The methyliron(lll) corrphycene complex has been prepared and is stable in the absence of dioxygen the methylcobalt(lll) complex is stable even in the presence of dioxygen. 153 Radiol)ftic reduction occurs at the metal center to produce Fe(ll) and Co(l) corrphycenes, which are also stable upon further radiolytic reduction in neutral solutions they yield species with chlorin-like spectral signatures. In alkaline solutions, species with phlorin-anion-like characteristics are produced. 153 ... 

Palladium nanostructures formed by nanowires, nanoplates or having a flower-like shape were synthesized using y-radiolysis, by the slow reduction of palladium acetylacetonate in propan-2-ol under CO atmosphere.By a fast radiolytic reduction, at much higher dose rate with electron beams, 2-nm nanoparticles were obtained, showing that the dose rate, which determines the reduction kinetics, has an influence... 

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