Pulse radiolysis electron transfer reactions

A precursor of the studies on electron transfer reactions between short-lived radicals and colloidal particles was the development of a fast pulse radiolysis method to measure. the polarograms of radicals in the 10 s range . After considerable information had been acquired about the electron transfer reactions of a few dozen radicals at the mercury electrode, this compact electrode was replaced by metal colloids somewhat later, by semiconductor colloids These studies led to the detection of the electron-storing properties of certain colloids and of reactions of the stored electrons. 

Packer, J.E., Willson, R.L., Hahnemann, D. and Asmus, K.-D. (1980). Electron transfer reactions of halogenated aliphatic peroxyl radicals measurements of absolute rate constants by pulse radiolysis. J. Chem. Soc. Perkins Transact. II, 296-299. 

The systems that we investigated in collaboration with others involved intermolecular and intramolecular electron-transfer reactions between ruthenium complexes and cytochrome c. We also studied a series of intermolecular reactions between chelated cobalt complexes and cytochrome c. A variety of high-pressure experimental techniques, including stopped-flow, flash-photolysis, pulse-radiolysis, and voltammetry, were employed in these investigations. As the following presentation shows, a remarkably good agreement was found between the volume data obtained with the aid of these different techniques, which clearly demonstrates the complementarity of these methods for the study of electron-transfer processes. 

Application of pulse-radiolysis techniques revealed that the following intramolecular and intermolecular electron-transfer reactions all exhibit a significant acceleration with increasing pressure. The reported volumes of activation are -17.7 0.9, 18.3 0.7, and... 

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... 

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

Intramolecular electron transfer from Ru(II) to Fe(III) in (NH3)3Ru(II) (His-33)cyt(Fe(III)) induced by pulse-radiolysis reduction of Ru(III) in the (NH3)5Ru(III) (His-33)cyt(Fe(III)) complex were investigated [84]. The results obtained differ from those of refs. 77-80 where flash photolysis was used to study the similar electron transfer reaction. It was found [84] that, over the temperature range 276-317 K the rate of electron transfer from Ru(II) to Fe(III) is weakly temperature dependent with EA 3.3 kcal mol 1. At 298 K the value of kt = 53 2 s"1. The small differences in the temperature dependence of the electron tunneling rate in ruthenium-modified cytochrome c reported in refs. 77-80 and 84 was explained [84] by the different experimental conditions used in these two studies. 

Wolfenden BS, Willson RL (1982) Radical-cations as reference chromogens in kinetic studies of one-electron transfer reactions Pulse radiolysis studies of 2,2 - azinobis-(3-ethylbenzthiazoline-6-sulphonate). J Chem Soc Perkin Trans 2 805-812... 

The products of the (e q + RCH=CH2) reaction are RCH—CH2 earbanions. Some of these have been identified by their chemical reactivity. Others have been observed through their absorption spectra by means of pulse-radiolysis techniques. The carbanion of acrylamide, for instance, has been shown to dimerize, to react with other free radicals, inducing anionic polymerization, and to react with oxygen, Ag+ and Fe(CN) - ions, presumably by electron-transfer reactions (Chambers et al., 1967). The absorption spectrum of the product of the (dimethyl fumarate + ey5) reaction has been observed in alkaline solution. The rate... 

Micic, O. I. Nenadovic, M. T. Rajh, T. Dimitrijevic, N. M. Nozik, A. J. Electron transfer reactions on extremely small semiconductor colloids studied by pulse radiolysis, Nato Asi Ser., Ser. C26, 1986. 

Apart from the relevance to the radiation-induced polymerizations, the pulse radiolysis of the solutions of styrene and a-methylstyrene in MTHF or tetrahy-drofuran (THF) has provided useful information about anionic polymerization in general [33]. Anionic polymerizations initiated by alkali-metal reduction or electron transfer reactions involve the initial formation of radical anions followed by their dimerization, giving rise to two centers for chain growth by monomer addition [34]. In the pulse radiolysis of styrene or a-methylstyrene (MS), however, the rapid recombination reaction of the anion with a counterion necessarily formed during the radiolysis makes it difficult to observe the dimerization process directly. Langan et al. used the solutions containing either sodium or lithium tetrahydridoaluminiumate (NAH or LAH) in which the anions formed stable ion-pairs with the alkali-metal cations whereby the radical anions produced by pulse radiolysis could be prevented from rapid recombination reaction [33],... 

Another investigation along this line is the pulse radiolysis study of the electron transfer reactions from aromatic radical anions to styrene this type of reaction is commonly used to initiate anionic polymerization of styrene [35], The electron transfer rates from the unassociated biphenyl radical-anions to styrene derivatives in 2-propanol were found to increase along the... 

Pulse radiolysis has been used to measure the bimolecular rate constants of the electron transfer reaction for substituted 2- and 5-nitroimidazoles of interest as antiprotozoal drugs and radiosensitizers [951], The mechanism of inhibition of... 

One particular example of the use of pulse radiolysis to general chemistry was the work of Miller and co-workers on the rates of electron-transfer reactions. These studies, which were begun using reactants captured in glasses, were able to show the distance dependence of the reaction of the electron with electron acceptors. Further work, where molecular frameworks were able to fix the distance between electron donors and acceptors, showed the dependence of electron-transfer rate on the energetics of the reaction. These studies were the first experimental confirmation of the electron transfer theory of Marcus. 

The sub-nanosecond pulse radiolysis techniques have made it possible to study the initial yields of singlets and triplets in the radiolysis of aromatic systems, the decay of electrons in non-polar media, fast electron transfer reactions and solvation experiments. 

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... 

Some electron transfer reactions have been studied in supercritical xenon. Two of them have been shown to be diffusion controlled and two are energy controlled. These reactions have been followed by changes in the optical absorption after the pulse. To carry out these studies requires that the rate of electron attachment to the solute be suffidendy fast to compete with ion recombination, which occurs on the picosecond time scale in pulse radiolysis. The solute hexafluo-robenzene satisfies this criterion the rate constant is sufficiently large (see Fig. 6) that millimolar concentrations will allow formation of anions. The rate constant for attachment to 4,4 -bipyridine (bipy) is also sufficiently large to satisfy this need. ° Another requirement for making these studies is to quench the excimers whose optical absorptions are strong and can interfere with detection of ions. As mentioned under Sec. 2, a small concentration of ethane (0.4%) is sufficient for this purpose. 

Figure 1 shows tJie corresponding optical absorption spectra of these radical cations as taken in the pulse radiolysis of the pure solvents. To avoid the influence of solvated electrons, in the case of alkanes, an electron scavenger such as tetrachloromethane is usually added [see Eq. (5b)]. Alkyl chlorides are internal electron scavengers and do not need further additives. In most of the cases, for the study of electron transfer reactions of type 2 or 3, the solvent derived radicals do not disturb because of their much lower reactivity compared with those of the ions. 

The electron transfer reactions between sulfur-sulfur three-electron-bonded complexes derived from methionine and four hydroxycirmamic acid (HCA) derivatives (caffeic acid, ferulic acid, sinapic acid, and chlorogenic acid) were studied by pulse radiolysis with spectrophoto-metric detection. These HCA derivatives are widely distributed... 

In many cases the product S is itself a free radical (S ), or a hyper-reduced metal ion, which in turn reacts in one-electron gain or loss processes. It is not surprising, then, that radiation-chemical methods are widely used in the study of electron-transfer processes. Of particular value is the technique of pulse radiolysis which permits reactions to be studied on timescales ranging from seconds down to picoseconds, so that even the most reaetive speeies ean be studied. It is this technique and its applications that form the subject matter of this chapter which begins with an outline of the radiation chemistry of water and other solvents. Next there is a historical view of pulse radiolysis, some of the landmark discoveries are discussed, followed by a description of the principal features of a pulse radiolysis facility and the various methods of detecting and measuring transient speeies. The chapter ends with some examples of data capture and analysis, and methods of sample preparation. 

In the following section some pulse radiolysis investigations are described in which the radiolysis of nonaqueous solvents has been used to generate one-electron redox reagents and, in some cases, to characterize their involvement in electron-transfer reactions. 

The radiation chemistry of 2-propanol is analogous to that of methanol, that is, the main reactive species are Cs and (CH3)2 COH. In alkaline solution, (CH3)2 COH deprotonates to (CH3)2CO . In the presence of N2O or acetone, es is converted to (CH3)2 C0H/(CH3)2C0 by the reactions in Eqs. 30 and 18, or the reaction of Eq. 20, respectively. The solvated electron in 2-propanol has been utilized to study electron-transfer reactions between aromatic radical anions (donor) and aromatic molecules (acceptor) [16]. The donor-acceptor pairs studied were pyrene-anthracene, pyrene-9,10-dimethylanthracene and w-terphenyl-/ -terphenyl. In the first two cases an equilibrium was established and the parameters forward and kback were measured this was the first example of the measurement of an equilibrium constant by use of pulse radiolysis. The rate constants for the electron-transfer reactions were examined in terms of the Marcus theory [17]. 

One-electron oxidants can be generated by pulse radiolysis of acetone containing appropriate solutes. For example, it has been shown that solutions of KSCN and KBr produce (SCN)2 and Br2 , respectively [19], and, similarly, NO3 and Cl2 are formed in solutions of LiN03 and LiCl [20], These oxidants are formed in significant yields, presumably by reaction of the corresponding anion with the solvent radical cation (CH3)2CO +, and are sufficiently long-lived to permit their electron-transfer reactions to be studied in this solvent. 

The discovery in 1962 of the intense absorption band of eaq (Amax 720 nm, Cmax 1900 m mor ) [56] in pulse radiolysis experiments on aqueous solutions was made almost simultaneously at Mount Vernon Hospital [57] and Manchester [58], and provided an extremely useful method for measuring the rate constants for the reaction of this species with a variety of compounds. As mentioned in the Introduction, reactions of the hydrated electron are electron-transfer reactions. The first paper dealing with this type of measurement appeared in 1963 [59] and contained the rate constants for the reactions of Caq with H, H2O2, and O2. Many other rate constants for the reactions of Caq were determined in the following years. The NDRL/ NIST Solution Kinetics Database, Version 3, which covers the literature up to 1994, contains nearly two thousand entries for this type of reaction, almost all of them obtained by means of pulse radiolysis [7a]. In many cases, the rate constant for a given reaction has been determined more than once for example, the rate constants for the reaction of Caq with H+, NOs , C6H5NO2, Ag+, Cu +, and MV + (l,l -dimethyl-4,4 -bipyridinium) have been determined 19, 16, 14, 11, 10, and 8 times, respectively [7a]. 

In the context of electron-transfer reactions, the contribution to the body of knowledge on the kinetics and mechanisms of these processes from pulse radiolysis studies is very significant. As has been shown above and elsewhere (see the list of Further Reading ), pulse radiolysis is particularly well suited to the study of fast electron-transfer reactions involving free radicals and metal ions in unusual oxidation states. 

Pulse Radiolysis Studies of Organic Electron Transfer Reactions . P. Neta, A. Harriman in Photoinduced Electron Transfer, Part B. Experimental Techniques and Medium Effects (Eds. M.A. Fox, M. Chanon), Elsevier, Amsterdam, 1988, Chap. 2.3, pp. 110-162. 

The photoinduced cleavage of metal-metal bonds is now a general reaction that finds many synthetic applications e.g., many heteronuclear metal-metal bonded complexes are most conveniently prepared by irradiating solutions containing a mixture of two homonuclear compounds. A discussion of such reactions is given in 13.3, organized according to the type of metal involved. This chapter then closes with discussions of photoinduced electron-transfer reactions and pulse-radiolysis techniques. 

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