Techniques radiolysis

The development of pulse radiolysis techniques have led to the determination of a number of the important kinetic processes of the species produced by the irradiation of H2O. The results that have been obtained for a number of the most important reactions are presented in Table II. These results demonstrate that the net effect of radiation is H2O decomposition in the absence of any reactive substrate. 

There is in addition to the pulse radiolysis technique the classical continuous radiation method. This later procedure uses either the 60Co-y sources or in the case of Pu the self a irradiation due to the radioactive decay. 

Asmus et al. unambiguously identified a variety of [R2S.. SR2] radical cations in solution and measured their optical absorption spectra using pulse radiolysis techniques [133]. They proposed that the spectrum of [H2S. .SH2] arises from the transition in the three-electron S.. S... 

The ion-molecule reaction between thiirane and its radical cation to form a thiirane sulfide radical cation and ethylene has been studied by Qin, Meng and WiUiams [134]. ESR studies using a low-temperature sohd-state Freon radiolysis technique provided compeUing evidence that the hemibonded dimer radical cation of thiirane is an intermediate in this so-called sulfur-transfer reaction see Scheme 2. 

Kemp and coworkers employed the pulse radiolysis technique to study the radiolysis of liquid dimethyl sulfoxide (DMSO) with several amines as solutes [triphenylamine, and N, A, A, N -tetramethyl-p-phenylenediamine (TMPD)]. The radiolysis led to the formation of transient, intense absorptions closely resembling those of the corresponding amine radical cations. Pulse radiolysis studies determine only the product Ge, where G is the radiolytic yield and e is the molar absorption. Michaelis and coworkers measured e for TMPD as 1.19 X 10 m s and from this a G value of 1.7 is obtained for TMPD in DMSO. The insensitivity of the yield to the addition of electron scavenger (N2O) and excited triplet state scavenger (naphthalene) proved that this absorption spectrum belonged to the cation. 

The pulse radiolysis technique gives a direct way for measuring the hydrated electron yield. To get the stationary yield, one can simply follow the electron absorption signal as a function of time and, from the known value of the extinction coefficient (Table 6.2), evaluate g(eh). Alternatively, the electron can be converted into a stable anion with a known extinction coefficient. An example of such an ion is the nitroform anion produced by reaction of eh with tetrani-tromethane (TNM) in aqueous solution ... 

In one case pulse-radiolysis techniques were employed to study the effect of pressure on such reactions. The oxidation of [Cu lphenhl by dioxygen proceeds via a Cu1—02 transient in which a copper—oxygen bond is formed, followed by the rapid formation of [Cun(phen)2] and 02 (110). This process is characterized by a AV of 22 cm3 mol1, which is close to the reaction volume expected for the binding of dioxygen. 

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... 

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 importance of track structure, the migration of species, the role of oxygen, the study of model compounds and the use of pulse radiolysis techniques are discussed. 

As Dr. Creutz has mentioned, the pulse-radiolysis technique can be used to reduce the rhodium(III) to rhodium (I I) by the action of various reducing radicals such as the hydrated electron, C09 radical, or the 2-propanol radical. Such reactions... 

Flash photolysis and pulse radiolysis techniques have been developed to study Fe Ru ET in Ru-modified proteins [21,26,27]. A method that allows study of electron transfer from a surfaee ajRu(IIIXhistidine) to a protein redox center is outlined in the Scheme [21]. The ET reaction is initiated by photogenerated... 

Table 6.2. Long wavelength uv absorption bands of benzene radical-cations in water, determined by pulse-radiolysis techniques.

Advances in pulse radiolysis studies in the gas phase have been summarized in several review papers. In a comprehensive review by Sauer [4], a review presented by Firestone and Dorfman [5] in 1971 was referred to as the first review on gas-phase pulse radiolysis. Experimental techniques and results obtained were summarized by one of the present authors [6], with emphasis on an important contribution of pulse radiolysis to gas-phase reaction dynamics studies. Examples were chosen by Sauer [7] from the literature prior to 1981 to show the types of species that were investigated in the gas phase using pulse radiolysis technique. Armstrong [8] reviewed experimental data obtained from gas-phase pulse radiolysis together with those from ordinary steady-state radiolysis. Advances in gas-phase pulse radiolysis studies since 1981 were also briefly reviewed by Jonah et al. [9], with emphasis on an important contribution of this technique to free radical reaction studies. One of the present authors reviewed comprehensively the gas-phase collision dynamics studies of low-energy electrons, ions, excited atoms and molecules, and free radicals by means of pulse radiolysis method [1-3]. An important contribution of pulse radiolysis to electron attachment, recombination, and Penning collision studies was also reviewed in Refs. 10-15. 

The results in this discussion will include those from our laboratory and experiments on electron solvation from other laboratories. The experiments that were done at Argonne made use of the stroboscopic pulse radiolysis technique, which will be discussed below. Experiments from other laboratories have made use of pulse radiolysis and laser photolyis techniques for the measurement of electron solvation. 

Additional experiments were done in mixtures of alcohol alkane [16,17]. The spectra and kinetics were measured in mixtures of 1-propanol n-hexane. Some experiments were done in cyclohexane, where the behavior was qualitatively similar however, the exact concentration where spectra and kinetics changed depended on the alkane [16]. Additional experiments observed the shift of the final spectrum of the solvated electron in supercritical ethane-methanol mixtures. These experiments were done using standard pulse radiolysis techniques and thus we were unable to observe the kinetics [19]. 

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... 

In a novel kinetic approach, Dorfman et al. developed methods for rapidly generating very reactive carbanions such as the benzyl anion in solvent mixtures containing water and alcohols. With pulsed radiolysis techniques, they have been able to study the fast and very exothermic reactions of carbanions with these solvents. The studies have shown that despite the high exothermicity, the protonation is not diffusion controlled and depends on the nature of the carbanion s counterion. 

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. 

Finally it should be pointed out that the pulse radiolysis technique can be applied to the elucidation of the detailed mechanisms of a variety of catalytic processes. Here two examples for this application will be presented. 

One the other hand, short-lived intermediates formed from styrene by radiations were studied by the pulse radiolysis technique by Metz et al. (43). They observed the anion radicals of styrene as an optical absorption band with the maximum at 370 mp, but could not find cationic intermediates. Shida and Hillma irradiated the 2-methyltetrahydrofuran glass and butylchloride glass, both containing styrene, and observed the absorption bands due to added styrene at 410 mp and 350 mp, respectively. The former band was assumed to be due to the anion-radicals and the latter to the cation radicals (44). 

Nickel(II) complexes with the ligands (76b) and (76d) have been reduced in aqueous solution by solvated electrons with the aid of the pulse radiolysis technique.314 The decay kinetics and the behaviour of these complexes as bases and as reducing agents have also been studied. 

Using pulse radiolysis techniques, the reaction of Ag+ ions with OH- radicals has been studied.534 Formation of silver(II) species were believed to proceed via OH- radical addition, shown by equation (34). Subsequently depending on the pH of the solution, hydrolysis equilibria were established for which the pK values of the reactions given by equations (35) and (36) were calculated as 5.35 and 8.35 respectively.534... 

Let us consider the data on the dependence of the kinetics of et decay at 77 K on the radiation dose. As seen from Fig. 11, over the dose range 3 x 1019 - 3.6 x 102° eV cm 3, the kinetics of et decay is virtually independent of the dose. At the same time, at lower doses, the decay of et is significantly slowed down. For example, for a dose of 1019 eV cm-3, the change in optical density of y-irradiated samples at the maximum (585 nm) of the et absorption spectrum with time is also described by eqn. (5), but the slope of the kinetic curve the coefficient M in eqn. (5)] is smaller by almost a factor of two [28] than for the curve of Fig. 11. Further investigations by pulse radiolysis technique with spectrophotometric recording of et showed that, at a still lower dose (6 x 1017 eV cm"3) no decay of et in water-alkaline matrices is observed at all [43] while at high doses (5 x 1021 eV cm"3) for the same samples, the decay of efr does occur [43]. A decrease in the rate of etr decay via the reaction with O at small doses was also reported in ref. 44. This behaviour of the kinetic curves seems to reflect special features of the spatial distribution of etr and 0 particles in samples irradiated with different doses. 

What predictions for new experiments are provided by the theoretical analysis presented herein It would be extremely interesting to obtain direct spectroscopic evidence regarding the energy levels and charge distribution of the excess electron in liquid helium. Applying the pulse radiolysis technique, recently developed for studying bound electron states in polar solvents (—e.g., H20 and aliphatic alcohols), should make the localized states of an excess electron amenable to spectroscopic study. 

The normally stable oxidation states of the Group 11(b) elements in the periodic table is the +2 state, although in the case of the heaviest member Hg, the +1 state is also stable in a few compounds, where the ion exists in a dimeric form Hg2 +2. The possible role of the + 1 states of Cd, Zn, and Mg as reactive intermediates of transitory existence have often been postulated (14). More definite evidence of the formation of the +1 states of Cd and Zn, but not of Mg, was obtained by Adams, Baxendale and Boag (1) by the pulse radiolysis technique. 

The values of th2o s,h2o in Table II indicate a value for th2o ranging from 10 9 to 10 10 sec., based on a value for s.h,o of 6 X 10 M 1 sec."1 from Debye s equation (11). The constants in Table II are also a measure of the relative reactivity of solute with H20 for any particular form of radiation for which th2o can be considered a constant. The relative reactivities of solute with H20 and e aq for Co60 7-radiation differ slightly but significantly as shown in Table III. Relative reactivities with e aq are based on measurements of absolute rate constants by pulsed-radiolysis techniques (4). 

In the last two or three years the application of the pulse radiolysis technique has provided a wealth of information about rate constants which without pulse radiolysis could have been obtained only with very great difficulty. Lists of rate constants obtained by pulse radiolysis have been given elsewhere (3, 10). The published values for alanine and glycine, including the most recent data, are given in Table I. 

Schuchmann H-P, von Sonntag C (1988) The oxidation of methanol and 2-propanol by potassium peroxodisulphate in aqueous solution free-radical chain mechanisms elucidated by radiation-chemical techniques. Radiat Phys Chem 32 149-156 Schuchmann H-P, Wagner R, von Sonntag C (1986) The reactions of the hydroxymethyl radical with 1,3-dimethyluracil and 1,3-dimethylthymine. Int J Radiat Biol 50 1051-1068 Schuchmann MN, von Sonntag C (1982) Determination of the rate constants of the reactions C02 + OH" -> HC03 and barbituric acid —> barbiturate anion + H+ using the pulse radiolysis technique. Z Naturforsch 37b 1184-1186... 

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