Pulse radiolysis transient species

Since 1962, when it was first characterized by pulse radiolysis transient absorption measurements in water, the solvated electron has been widely studied in numerous solvents. The solvated electron, denoted by e, is a thermodynamically stable radical, but like most free radicals, it has a short lifetime due to its high chemical reactivity. The solvated electron is a unique chemical moiety whose properties may be compared in many solvents and are not dependent on the method creating the solvated electron. The solvated electron is an important reactive species as it is the simplest electron donor, its reactions correspond to electron transfer reactions and its reactivity may be used to probe electron transfer properties of acceptors. During the last 40 years, due to its optical absorption properties, the... 

Figure 5. Pulse radiolysis transient absorption data for Co-Piq-V, showing the decay at 600 nm due to oxidation of the viologen radical cation moiety and the bleach at 308 nm due to the reduction of the Co -tris-bipep species. The initial rapid increase at 600 nm is due to the reaction of CO2 with viologen.

The species and their associated stability constants are consistent with those of uranium(VI) and are retained in the present review. Schmidt et al. (1980) used a pulse radiolysis transient conductivity technique to study the hydrolysis of neptu-nium(VI) in a self-medium (very low ionic strength). They could only determine a stability constant for Np020H and obtained a stability constant of between log Pi = -5. 4 to —5.5. This value is considered consistent with that obtained by Lemire et al. (2001) at zero ionic strength from the earlier data of Cassol et al. (1972a). 

The absorption of radiation produces unstable species. Flash photolysis does so by interaction of light with a solute. The transient may be a photoexcited state or a molecular fragment. Pulse radiolysis starts with highly reactive entities formed by dissociation of the solvent (e.g., H, eaq, and HO from H20) and consists of a study of their reactions or of reactive transients derived from them. In either case one monitors the ensuing reactions by luminescence (for excited states), light absorption, or conductivity changes. 

Baxendale, J. H., Busi, F. (eds.) The study of fast processes and transient species by electron pulse radiolysis, D. Reidel Publishing Co., Dordrecht 1982... 

Monomeric platinum(III) complexes have been observed frequently as transient species in electrochemical or pulse radiolysis studies and they are proposed as an intermediates in reductive elimination and oxidative addition reactions of platinum(IV) and platinum(II) respectively.382-388... 

Finally, radical cations can be generated in solution by different types of pulse radiolysis225. Like PET, this is inherently a method for transient spectroscopic observations, but it has proved to be invaluable in investigations of dimer cations, e.g of polyenes, which form spontaneously upon diffusion of radical cations in the presence of an excess of the neutral parent compound, but a discussion of the electronic structure of such species is beyond the scope of this review. Pulse radiolysis is of interest in the present context because it allows the observation of large carotenoid radical cations which are difficult to create in solid-state or gas-phase experiments... 

In some cases pulse-radiolysis techniques were employed to study the effect of pressure on inorganic reactions. For instance the oxidation of [CuI(phen)2] by dioxygen via the formation of a C -C transient species was studied using this technique (see Section III,A). Other examples include the formation and cleavage of metal-carbon (7-bonds, which formally involve a change in the oxidation state of the metal. A typical example of a volume profile for the formation and cleavage of a Co-CH3 bond is reported in Fig. 21 for the reaction (162)... 

The solute benzene radical cation was formed on pulse radiolysis of an acidic aqueous solution of benzene. The transient optical absorption bands (A-max = 310, 350-500 nm) were assigned to the solute benzene radical cation which is formed on acid-catalysed dehydration of the OH adduct. The radical cation is able to undergo an electron-transfer reaction with Br and was found to be a strong electron oxidant. Pulse radiolysis has been used to study the complex reaction that follows electron addition to hydroxybenzophenones (HOBPs). The various radical species involved have been characterized spectrally and their p/fa values evaluated. The differences... 

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

In this chapter, firstly, a very brief survey is given of recent advances in such studies as classified according to the detection technique of transient species in pulse radiolysis. Secondly, examples are chosen from our recent investigations, with special emphasis on the important contributions of pulse radiolysis methods to gas-phase collision dynamics one is electron attachment, the other is Penning ionization and related processes. The detection techniques and corresponding reaction processes, together with major references, are given below ... 

Sauer, M.C., Jr. In Study of Fast Processes and Transient Species by Electron Pulse Radiolysis Baxendale, J.H., Busi, F., Eds Reidel Dordrecht, 1982 601 pp. 

Copper(III)-aquo and-amine species have been generated by pulse radiolysis [194] but these are transient species that undergo rapid decay. Electrochemical generation of Cu(III) complexes of macrocyclic amines have been reported in acetonitrile [195] but they are also unstable and undergo spontaneous reduction to Cu(II). Nonetheless, it is presumed that copper(III) intermediates are generated as transients in a number of reactions, particularly those involving copper catalysis of multielectron transfer reactions. 

The absorption band at 320-330 nm seen in Fig. 4.4.10B was reported in a Agl particle formed by pulse radiolysis technique. The unstable species thus formed was attributed to initial aggregates of Agl molecules whose lifetime was on the order of milliseconds. Therefore in the present experiment, these transient species were successfully stabilized with RSH and can survive for several days before further... 

A method for preparing a-methylstyrene to investigate its radiation-induced polymerization yields samples which exhibit reproducible kinetics. The kinetic results are interpreted as indicating that free radicals, carbonium ions, and carbanions can all propagate simultaneously, the relative importance of each species depending upon the dryness of the monomer and all associated glassware. This viewpoint is further supported by data from a preliminary investigation of the transients formed in a-methylstyrene, as studied by the pulse radiolysis technique. 

The transient absorption spectrum obtained in the pulse radiolysis of polystyrene solution in CC1 is shown in Figure 13. The spectrum is very similar to the charge transfer radical complex (PS4+C14-) species. The lifetime is about 200 ns. Consideration of the absorption spectrum and the lifetime suggest that this species is (PS4+C14-)-. The processes leading to formation of this species in liquid CC14 can be written as follows (4,7). 

Redox reactions involving the nickel(IV) complex are also subject to divalent metal ion catalysis (170, 171). Oxidations of the two-electron reductant ascorbate (40) and the one-electron reductant [Fe(CN)6]4-(172) have been examined in some detail. Both reactions have as the rate-determining step the transfer of one electron from the reductant to nickel(IV) in an outer-sphere process to give an undetected nickel(III) transient. Spectroscopic properties of the nickel(III) species have been determined by pulse radiolysis (41). 

There is an extensive chemistry of the nickel(I) ion generated by pulse radiolysis which is beyond the scope of this review. Complexes with saturated amines such as 1,2-diaminoethane have been studied by this method and by the y radiolysis of aqueous glasses, but the species formed have no more than a transient existence. The imine ligands phen and bpy offer a more attractive environment for nickel(I) by allowing electron delocalization over the ligand n system (178,179). A number of complexes of these ligands have been reported in y-radiolysis studies. The EPR spectra indicate that reduction is primarily metal centered with a significant orbital contribution. Electrochemical reduction of [NiH(bpy)3]2+ in anhydrous acetonitrile results in [Ni (bpy)3] +, which can be detected by EPR methods. The reduction potential is reported to be —1.55 V but the complex is thermodynamically unstable with... 

The pulse radiolysis method has been described in detail in some of the early papers (22, 22), in a brief review of the subject (23), and in a current comprehensive review (14). It is, in brief, a fast reaction method in which the external perturbation applied to the system is a microsecond pulse of electrons. The current is sufficiently high to produce an instantaneous concentration of transient species high enough to be observed by fast measurement of the optical absorption. Spectra may be recorded either photographically or spectrophotometrically. The kinetics are studied by fast spectrophotometry. Since a perturbing pulse as short as 0.4 /xsec. has been used, the time resolution has approached 10-7 sec. The flash photolysis method used in some of the other studies (27, 15) has been reviewed in detail (24). 

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