Radiolytic methods

The physical and chemical properties of Rh(bpy)32+ generated via flash-photolytic and pulse-radiolytic methods in aqueous solutions are reported and discussed. The reduction potential -0.86 V vs SHE) and electron-exchange rate (> 109 M 1 s "1) for the Rh(bpy)33+/Rh(bpy)32+... 

The second-order rate constant for the reaction of a hydrogen atom with a hydroxide ion to give an electron and water (hydrated electron) is 2.0 x 10 M s . The rate constant for the decay of a hydrated electron to give a hydrogen atom and hydroxide ion is 16M s. Both rate constants can be determined by pulse radiolytic methods. Estimate, using these values, the pA of the hydrogen atom. Assume the concentration of water is 55.5M and that the ionization constant of water is 10 M. 

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. 

The time-resolved studies of the cluster formation achieved by pulse radiolysis techniques allow one to better understand the main kinetic factors which affect the final cluster size found, not only in the radiolytic method but also in other reduction (chemical or photochemical) techniques. Generally, reducing chemical agents are thermodynamically unable to reduce directly metal ions into atoms (Section 20.4) unless they are complexed or adsorbed on walls or dust particles. Therefore, we explain the higher sizes and the broad dispersity obtained in this case by in situ reduction on fewer sites. A classic... 

A further point to consider is the need for AG° or E° values for reactions under the actual conditions used. As an example, a large number of so-called En values for one-electron processes (nitro compounds semiquinones) have been determined in aqueous solution at pH 7 using a pulse radiolytic method (see, e.g. Meisel and Neta, 1975 Rao and Hayon, 1975 Wardman and Clarke, 1976b). Such values may differ somewhat from strictly defined AG° (E°) values listed in standard tables. 

Copper has a direct role in the catalytic mechanism. Complete removal of both copper and zinc destroys the activity of the enzyme which can be restored by addition of cupric ion but not by other metals (12). Pulse radiolytic methods have been used to generate superoxide ion and to follow the enzyme kinetics (24, 25). The reaction is second order with a rate constant of 2.37 X 109 M"1 sec-1 at 25°, independent of pH over the range 4.8-9.5. The following two-step mechanism has been proposed to account for the enzyme activity ... 

Few monomeric rhodium(II) complexes, other than those stabilized by group VB ligands, have anything but a transient existence. Whilst the former can be prepared by reduction of rhodium(III) salts with the group VB ligand, the latter are generated by photolytic or radiolytic methods. The short lifetimes of these species have occasionally been extended by trapping them within a crystal lattice. 

Prior to the quoted investigations, Dorfman had reported rate constants for the reactions of radiolytically generated benzyl- and benzhydryl cations [126] with some alkenes and dienes [127]. Although Dorfman s data on benzhydryl cations agreed well with the results of the other methods, two independent groups came to the conclusion that the corresponding benzyl cation data must be erroneous [128,129], Because only a very limited number of compounds had been studied by the radiolytic method, details of this technique will not be presented here. 

McLendon and Miller employed a combination of photochemical and pulse-radiolytic methods to probe the driving-force dependence of heme-heme ET in this complex. The ET rates exhibit a near-Gaussian free-energy dependence, in... 

The rates of hydrolysis of thiocyanogen, (NCS)2, have been studied by a pulse-radiolytic method, monitoring the optical absorbance due to SCN and the conductance. In acidic solutions the reactions involved are said to be... 

The final size ofthe clusters depends on the metal it decreases as the initial polymer/ ion concentration ratio increases and with PVA lies in the nanometre range (1-10 nm).The radiolytic method has been used for the synthesis ofa great number of noble and non noble metal nanocolloids in various solvents, water, alcohols, liquid ammonia, etc. Their intense colours, due to a Mie resonance in light absorption by the electron pool of the particle, depend on the surrounding medium, on the cluster size and shape, and on the metal (Fig. 4) in water, yellow for Ag spheres of a few nm 380 nm), purple for Au ( max=520nm),... 

Cationic cyclization and its reverse have particular relevance, since ring closure and ring opening have been invoked to account for a large number of biosynthetic pathways.The gaseous cyclopropylcarbinyl-cyclobutyl-homoallyl system represents an extreme instance of a rapid, reversible isomerization of this sort, which has been studied over a period of 20 years both by /1-decay of tritiated cyclobutane and by radiolytic methods. This interconversion is so fast that the three structures... 

The sensitivity of the method is extremely high so that no problems with build-up of products of the radiolysis are likely. Conversely, however, problems with impurities will be serious. The radiolytic method allows the possibility of pre-irradiation as a partial cure. 

Photolytic and radiolytic methods can be used to generate the same intermediate species, or different ones, in the same electron transfer system. The rate-leveling effect of the highly energetic primary radical reactions induced by pulse radiolysis can sometimes result in different distributions of intermediate species than those generated by photolysis. Also, oxidative or reductive radiolytic chemistries can be used interchangeably to approach intermediates from different directions. 

In addition to the studies on reduction and oxidation of metalloporphyrins, radiolytic methods have been used to investigate reactions of radicals with metalloporphyrins that lead to formation of metal-carbon bonds. Formation of metal-carbon bonds has been implicated in various catalytic reactions and in biological systems. Therefore, numerous studies have been carried out on the formation and decomposition of such bonds involving porphyrin complexes of Pe 38.s3,62,68-70co, ° Rh, and other metals, as well as complexes of related macrocycles, such as Co-phthalocyanine and Co-B,2. Certain oxidation states of transition metal ions react with free radicals by attachment to form organometallic products, some of which are stable but others are short-lived. Pulse radiolysis has been used to investigate the formation and decay of such species. 

D.M. Guldi, J. Field, J. Grodkowski, P. Neta, and E. Vogel, One-Electron Oxidation of Metalloporphycenes as Studied by Radiolytic Methods, J. Phys. Chem., 100 (1996) 13609. 

Singh, S., Rath, M. C., Singh, A. K. et al. 2011. CdSe nanoparticles grown via radiolytic methods in aqueous solutions. Radiation Physics and Chemistry 80 736-741. 

Kharazmi, A., Saion, E., Faraji, N., Hussin, R.M., Yunus, W.M.M. Stnictural, optical and thennal properties of PVA/CdS nanocomposites synthesized by radiolytic method. Radiat Phys. Chem. 97, 212-216 (2014)... 

Jovanovic, Z., Jovanovic, Z., Krkljes, A., Stojkovska, J., Tomic, S., Obradovic, B., MiSkovic-Stankovic, V., Kacarevic-Popovic, Z. Synthesis and charaeterization of silver/ polyvinyl 2-pyirolidone hydrogel nanocomposite obtained in situ radiolytic method. Radiat. Phys. Chem. 80, 1208-1215 (2011)... 

The photolytic method has, of course, a long and important history in the formation of photographic images from silver halide emulsions. Over the past twenty years, predominantly from the work of Henglein and of Belloni, a wide variety of colloidal metals has been prepared by this method encompassing both main group metals such as cadmium [87, 88], thallium [89, 90] and lead, [91] as well as other noble [92-98] and non-noble transition metals [99-101]. Radiolytic methods differ in the type of redudng spedes which is formed under irradiation, which is a function of solvent and any added solute. The radiolysis of aqueous solutions of metal ions produces solvated electrons which may either react with the dissolved... 

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