Concentrated solutions, 7-radiolysis

Platzman (1967) has emphasized that most direct ionizations in molecules leave the positive ions in an excited state. Based on crude DOSD, he estimated that in water the average positive ion will have about 8 eV excitation energy. Later, the less approximate calculation of Pimblott and Mozumder (1991) reduced that figure to about 4 eV The chemical role of this excitation energy is unknown, although it may have some effect in the radiolysis of highly concentrated solutions. 

An interesting extension of aqueous solution radiolysis involved solutions of sodium dodecyl sulphate in the presence of MNP. Spin adducts of secondary alkyl radicals were detected provided that the critical micelle concentration of the surfactant was exceeded. Whilst it was rather loosely concluded that there is a marked catalytic effect of micelles on the rates of reaction of radicals with nitroso spin traps , no single origin of this effect could be clearly identified (Bakalik and Thomas, 1977). 

Figure 5. Pulse radiolysis of deoxygenated solutions of polyvinylpyrrolidone showing peaks at 250 and 400 nt/x, and a broad absorption ascribed to electrons up to 700 inf, in low concentration solution...

Ionizing radiation can act in two distinct ways on organic substances. In the absence of water, in condensed systems or in concentrated solution, the predominant effects occur directly on the organic molecule and produce electronic excitations or ionizations which may lead to chemical modification. In dilute solution (1% or less) the major effects are the result of reactions between the solute and reactive species produced by the radiolysis of water. These indirect effects are the subject of this article. 

However, this mechanism does not explain the chain reaction. Tabata and coworkers measured the optical spectrum of the dimer cation radical, by pulse radiolysis of benzonitrile solution of the dimer immediately after the pulse. They found only a peak at 770 nm without other peaks, except for a possible small shoulder at 740 nm (which is within the limit of experimental error). Addition of cation scavengers leads to elimination of this spectrum, while oxygen does not remove it, suggesting that the spectrum is due to a cation. This 770-nm peak of the cation of the cyclodimer of VC reminds one of the 770-nm peak found 1.6 jus after the pulse in the case of 1 M VC solution. It should be noticed that while in this second paper the authors also mentioned this shift from 790 nm to 770 nm, the data in their figure show a peak at 790 nm both immediately and 1.6 jus after the pulse. Consequently, Tabata and coworkers suggested that the observed spectrum in pulse radiolysis of aerated solution of VC in benzonitrile is a composite of the spectrum of VC cation together with that of the cation of the cyclodimer of VC. The contribution of each intermediate to the observed spectrum depends on the concentration of VC and how long after the pulse the spectrum was taken. In a dilute solution, the dimer cation will be produced as time proceeds, but it is absent immediately after the pulse. In concentrated solutions, both cations coexist even immediately after a pulse. 

Lam KY, Himt JW. (1975) Picosecond pulse radiolysis. VI. Fast electron reactions in concentrated solutions of scavengers in water and alcohols. Int J Radiat Phys Chem 7 317-338. 

Radiolysis of the aqueous nitrate system is discussed in terms of (a) indirect effect in dilute solution, and (b) concurrent indirect and direct effects in concentrated solution. Analysis of energy fractionation breaks down (b), gives G(N02 )no3- = 4.0, and demonstrates stoichiometry for direct effect according to... 

Scavening with H> and n-propyl alcohol quantitatively supports this. Pulse radiolysis shows NO. and N02 as intermediates, NO > being characteristic of concentrated solutions. Formation and decay of NO. is discussed. Primary process of direct radiolysis is formulated as... 

We have investigated in detail the parameters affecting the continuous y-radiolysis of concentrated solutions (intensity, pH, 02, scavengers, etc.). Transients were investigated by pulsed electron beam radiolysis and kinetic spectroscopy, and the reactions of the optically accessible excited states of nitrate were investigated by conventional photolysis. This paper represents a survey of our recent results which, taken in conjunction with the work of others, allows the construction of a model whereby the main features of this system may be understood and may even be predicted. Literature review is necessarily selective for the present purpose (because of doubtful relevance to liquid state processes, low temperature radiolysis, and hence ESR work has been omitted from... 

The understanding which we now have of the mechanism of radiolysis in dilute solution provides an adequate quantitative base for progressing to consideration of the effects in concentrated solution. However, some current gaps exist in our knowledge. As indicated in Reactions 1 and 4, we are still ignorant of the molecular structures of the species denoted by N (IV) interesting ionic relations and differences of reactivity may exist between their various forms. The mechanism of Reaction 5 is somewhat obscure in fact, actual evidence for it is sparse. These are of course suitable topics for pulse radiolysis experiments, and we expect to be able to clarify them in the near future. Such experiments may also evaluate the extent of Reaction 3 at high intensities and its rate constant. 

The immediate problem in considering the radiolysis of concentrated solutions is how to account for the water radiolysis process which still undoubtedly accounts for the major part of the energy dissipation. The simplest approach is to assume that the phenomena characteristic of concentrated solution originate independently of water radiolysis hence,... 

In summary, then we have a reasonably satisfactory working model for the description of the radiolysis of concentrated solutions and the interrelationship of indirect and direct effects. However, we must consider the remaining problems. 

A major chemical effect of y-rays on simple peptides such as the N-acylamino acids under oxygen-free conditions, both in the solid state and in concentrated aqueous solution, leads to formation of labile amidelike compounds which are readily degraded on mild hydrolysis to yield ammonia as a characteristic product. Several classes of nitrogen-deficient products are formed concomitantly with the ammonia. Earlier communications have discussed certain limited aspects of the radiolytic lability of simple peptides in the solid state and in concentrated solutions (9, 10, 18). The radiation chemistry of these systems is more complex than that involved in the radiolysis of simple peptides in dilute oxygen-free aqueous solution under which conditions main-chain degradation is of minor importance (10). In this paper we report detailed experimental evidence... 

As is seen, the direct action of radiation on HNO3 leads to the formation of only one radical NO3 (reaction (Equation 4.7) per one acid molecnle destroyed in the reaction (Equation 4.19), whereas the pulse radiolysis of less concentrated solutions produces two radicals NO3 according to reactions (Equation 4.15)-(Equation 4.18). The evidence for the reaction (Equation 4.19) is the absorption band of NO at 400 nm [14] in an optical spectrum appearing in 15 M HNO3 immediately after advancing the electronic pulse. Such absorption was not observed for diluted 3M solutions. The approximate calculation of the NO3 radiation yield for 5-8 M solutions of HNO3 gives a minimum value of 10 radicals per 100 eV [10]. The yield of direct radiation decomposition of nitrate anions was estimated to be 8-17 ions per 100 eV [11]. 

Reactions such as these are of interest in themselves. Beyond that, one can use the pulse radiolysis experiment as a preparative technique for other species. Thus, the reactions of numerous aliphatic, carbon-centered radicals have been evaluated.22 If one employs a reasonably high concentration of solute, say 0.1-1 M CH3OH, the formation of CH2OH is complete within the electron pulse. Following that, reactions such as the following can be studied ... 

Koulkes-Pujo and coworkers5 5 studied the formation of methane in the reaction of OH radicals and H atoms with aqueous DMSO in acidic media. In the radiolysis of deaerated acidic aqueous solution of DMSO they found that G(CH4) increases monotonously with CH4 concentration up to 0.8 m DMSO. Similar results were obtained for C2H6 but the yields of C2H6 are much lower than that of CH4. 

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