Solid radiolysis

The energy available in various forms of irradiation (ultraviolet, X-rays, 7-rays) may be sufficient to produce in the reactant effects comparable with those which result from mechanical treatment. A continuous exposure of the crystal to radiation of appropriate intensity will result in radiolysis [394] (or photolysis [29]). Shorter exposures can influence the kinetics of subsequent thermal decomposition since the products of the initial reaction can act as nuclei in the pyrolysis process. Irradiation during heating (co-irradiation [395,396]) may exert an appreciable effect on rate behaviour. The consequences of pre-irradiation can often be reduced or eliminated by annealing [397], If it is demonstrated that irradiation can produce or can destroy a particular defect structure (from EPR measurements [398], for example), and if decomposition of pre-irradiated material differs from the behaviour of untreated solid, then it is a reasonable supposition that the defect concerned participates in the normal decomposition mechanism. 

Kevan and colleagues69 studied the products of the radiolysis of solid diaryl sulfones at room temperature, such as p,p -ditolyl, diphenyl sulfone and dibenzothiophene-S,S-dioxide. The products found for the first two were S02 and the diaryl hydrocarbon. For p,p -ditolyl sulfone the S02 yield is linear with dose upto about 13 Mrad, above which it falls off considerably from linearity. The initial yields give G(S02) = 0.05, which is equal within experimental error to the yield of p,p -bitolyl. The only another organic product observed had a smaller yield by a factor of 7, and could not be identified. The authors pointed out that no polymeric product was found in contrast to what is known on benzene radiolysis. The mass balance suggests that a simple decomposition as shown by equation 50 is the net consequence of radiolysis. 

This sequence of formation of radical cation which is followed by a C—S bond scission into alkyl radical and alkyl sulfonyl cation was previously suggested by the same authors for the radiolysis of polyfolefin sulfonefs in the solid state72 and was confirmed by scavenger studies73. Scavengers are ineffective in crystalline solids such as dialkyl sulfones and hence could not be used in this study. 

The nature of the first type of thermal reactions is as yet only speculative. The two obvious possibilities seem to be (1) reaction of an incomplete molecule (radical) with an unbound nearby ligand, made available by recoil fragmentation, radiolysis, chemical dissociation, or the presence of an external atmosphere and (2) reaction of the moiety with a nearby molecule to abstract a ligand. The first type with an external source of CO has been clearly demonstrated for the case of the Group VI carbonyls which, when heated in an atmosphere of CO (up to 100 atm pressure) showed a marked increase in yield. A much smaller enhancement of yield in vacuo was attributed (99) to radiolytic dissociation, because of the influence of irradiation at various y-fluxes. The alternative possibility—that of equilibrium dissociation of Cr(CO)6 in the solid state—has not been investigated. 

Phase transitions Spin and magnetic transitions Dynamic solid-state phenomena Solid-state reactions (e.g., thermolysis, radiolysis)... 

The second part of the book deals with the use of above method in physical and chemical studies. In addition to illustration load, this part of the book has a separate scientific value. The matter is that as examples the book provides a detailed description of the studies of sudi highly interesting processes as adsorption, catalysis, pyrolysis, photolysis, radiolysis, spill-over effect as well as gives an insight to such problems as behavior of free radicals at phase interface, interaction of electron-excited particles with the surface of solid body, effect of restructuring of the surface of adsorbent on development of different heterogeneous processes. 

We consider, primarily, events in solids since most e.s.r. studies have been carried out on radicals trapped in solids. Only relatively persistent organometallic radicals have been studied by liquid-phase e.s.r. with in situ radiolysis, because of the technical difficulties involved. In most solid systems at low temperature radical centres are physically trapped in the rigid matrix and hence can be studied by e.s.r. without difficulty. However, although radicals as such may be immobile, this does not necessarily apply to electron-gain or -loss centres, particularly if these are charged, since electron-transfer may be facile. 

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... 

Takemura and Shida54 prepared the allene radical ion by /-radiolysis of halocarbon solid solution of allene at low temperatures and showed that the radical cation has a lower D2 structure than the precursor with a skew angle of 30-40°. Kubonzo and coworkers55 56 produced by /-radiolysis in a low-temperature halocarbon matrix several derivatives of the allene radical cation, i.e. the radical cations of 1,2-butadiene, 3-methyl-1,2-butadiene,... 

Radical cations of the most popular spin traps PBN and DMPO have been generated by the methods of ionizing radiolysis and laser flash-photolysis in solid matrices (435-437). As a polar solvent with high solvating ability for... 

Figure 1. ESR spectra of isobutyric acid following gamma radiolysis in the solid state at (A) 77 K (B) 195 K.

Again the close correspondence between the measured radical and carbon dioxide yields for 7-radiolysis of the N-acetyl amino acids in the solid state suggests that the mechanisms for radical production and carbon dioxide formation are closely related, as they were for the aliphatic carboxylic acids. The following mechanism has been proposed (5.) in order to account for the major degradation products and observed radical intermediates. 

POLYCARBOXYLIC ACIDS The gamma radiolysis of the homopolymers of acrylic, methacrylic and itaconic acids have been investigated in the solid state at 303 K, and in each case the yields of carbon monoxide, carbon dioxide and of radical intermediates have been measured. These are reported in Tables VII and VIII respectively. 

POLYAMINO ACIDS Aliphatic polyamino acids irradiated in the solid state have been reported to undergo N-Ctf, main-chain, bond scission on gamma radiolysis (9) and the stable radical intermediates formed following radiolysis at 303 K are alpha carbon radicals, as observed in the N-acetyl amino acids. 

Other complexities are revealed when frozen solutions of spin trap in methanol are irradiated, and the solution is then melted. The proportions of spin adducts are markedly dependent on radiolysis temperature. One contributory factor is undoubtedly the reaction of MeO with neighbouring methanol in the solid matrix, to produce HOCH2, before diffusion to reach spin-trap molecules is possible. 

Reduction by pulse radiolysis of Mn04 under acidic conditions has allowed a study of the UV-visible spectra of the unstable ion 03Mn (0H) and the determination of the pAa of this ion, 7.4 0.1." Abrasive stripping voltammetry has been used to characterize solid barium and... 

Racemizations in the crystalline state have a long history. It is known that L-a-amino acids slowly racemize in the solid state [62]. As this also happens in solid proteins the implications are manifold, not only in pure chemistry but also in biochemistry, nutrition, food technology, and geology. Therefore, techniques have been developed to determine the dl ratio of amino acids down to 0.1% and inversion rate constants have been determined under acid hydrolysis conditions [63]. One could think of very slow deamination and readdition of the amine or an enolization mechanism. However, such reactions can also be induced by photolysis or radiolysis from natural sources [64]. 

Figure 11 Effect of temperature on the yield of OH radicals from gamma radiolysis. Experiments (A) Kent and Sims [100], ( ) Elliot et al. [101] Calculation (solid line) IRT modeling using track structure simulation.

Figure 6 Comparison of experimental and predicted values of G(Pi) from electron scavenging as a function of the scavenger power fcio[Si]. Si = N2O ( ) CH3CI (A) C(N02)4 ( ) glycylglycine ( ) NO3" (O). The broken line is predicted from direct observation of the time dependence of G e ) in pulse radiolysis experiments. (From Ref. 49.) The solid line is the fit obtained by Pimblott and LaVerne [43] with the restriction that G°(e q) = 4.80 molecules (100 eV) ...

By comparison with G e, relatively few independent measurements of G( OH) have been made. In contrast to, only the relative change in G( OH) with time has been reliably measured by pulse radiolysis [51]. In practice, absolute values of G( OH) have been obtained from scavenger studies or by material balance (reaction (7)). Fig. 7 shows data for aerated solutions of formate ion [52] and hexacyanoferrate(II) [53] taken from Fig. 1 of Ref. 54. The data for formic acid, which were included by LaVerne and Pimblott [54], have been omitted here because they were obtained at low pH where the primary yields are different (see Section 3.4). The solid line shows the best fit obtained using Eqs. (16) and (17) and the broken line is the best fit when the term u[5]/2 is omitted from Eq. (17). The respective sets of parameters are a = 1.64 and 1.69 nsec, g( OH) = 2.53 and 2.50 molecules (100 eV) and G°( OH) = 4.48 and 4.86 molecules (100 eV) These values differ significantly from those obtained by LaVerne and Pimblott [54], which were a = 0.258 nsec, g(" OH) = 2.66 molecules (100 eV) and G°( OH) = 5.50 molecules (100 eV) The reason for the difference is that LaVerne and Pimblott [54] chose G°( OH) = 5.50 molecules (100 eV) ... 

Figure 6 The production of HO2 in the proton radiolysis of water as a function of initial ion energy [75]. The slope of the solid line (0.030) is the track average yield while the slope of the dotted line (0.023) is the track segment yield for 10-MeV protons.

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