Liquids, radiolysis

TJeactions of specific hydrocarbon ions in the liquid phase are difficult to study directly. Ions may be produced in the liquid by direct liquid radiolysis, but the situation is complicated because many other reactive species such as electrons, radicals, and excited states are produced simultaneously. This complex situation may be simplified by producing specific ions in the vapor phase and injecting them by means of an electric field into a liquid or solid matrix. Under such conditions the positive ion is separated from its concomitant electron and is accelerated into the liquid or solid alone. We call this the ion injection method. It shows considerable promise for studying specific ion-molecule reactions in the liquid phase and should allow new types of studies on positive ion trapping in inert matrices to be made. 

Gamma Radiolysis of Liquid Dinitrogen Tetrox-ide , PATR 3072 (1963) 20) Anon,... 

Hayon23 studied the yields of ions and excited states in pulse radiolysis of liquid DMSO using anthracene as a solute to determine the yield of free ions and naphthalene as a solute to measure the yield of triplet excited states. Anthracene is known to react with solvated electrons to give the anthracene radical anion, A T... 

Cooper and coworkers30 measured also the absorption spectrum of transient species produced in the radiolysis of pure liquid DMSO-d6 and found the same absorption of the first two bands, however, the intensity of the absorption is about 30% larger in the case of the deuterated compound for both of the absorption bands. The intensity of the absorption is given by Ge, but as the same change was found for both bands it seems most reasonable that the 30% difference arises from a change in G rather than in e. This is similar to water, where the fraction of ions which become free ions is substantially larger for the deuterated compound32. 

Table III shows that in the gas phase at a pressure of 40 torr the relative rates of the H2 transfer reactions from the cyclopentane ion to the various additives differ drastically from those derived from liquid phase radiolysis experiments. This indicates that the changes in density may profoundly affect the relative rates of the two competitive reactions, Reactions 22 and 28. Experimental results, which will be described in a later publication, indicate that in the liquid phase an increased importance of the H2 transfer reaction to some of the additives occurs at the expense of the H atom transfer reaction, Reaction 23.

More common in the liquid phase is pulse radiolysis . In this technique, electron accelerators which can deliver intense pulses of electrons lasting a very short time (ns up to fis) are used. Each single pulse can produce concentrations of intermediates which are high enough to be studied by methods such as light absorption spectroscopy or electrical conductivity. 

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 SS technique has also proved to be informative for studying intermediate particles in radiolysis of organic mixtures in gaseous and liquid media [20]. 

It is now clearly demonstrated through the use of free radical traps that all organic liquids will undergo cavitation and generate bond homolysis, if the ambient temperature is sufficiently low (i.e., in order to reduce the solvent system s vapor pressure) (89,90,161,162). The sonolysis of alkanes is quite similar to very high temperature pyrolysis, yielding the products expected (H2, CH4, 1-alkenes, and acetylene) from the well-understood Rice radical chain mechanism (89). Other recent reports compare the sonolysis and pyrolysis of biacetyl (which gives primarily acetone) (163) and the sonolysis and radiolysis of menthone (164). Nonaqueous chemistry can be complex, however, as in the tarry polymerization of several substituted benzenes (165). 

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. 

A number of terms in this area will be unfamiliar to most chemists. Cavitation is the formation of gas bubbles in a liquid and occurs when the pressure within the liquid drops significantly below the vapor pressure of the liquid. Cavitation can occur from a variety of causes turbulent flow, laser heating, electrical discharge, boiling, radiolysis, or acoustic irradiation. We will be concerned... 

LaVerne, J. A. (1988), Bibliography of Studies of the Heavy Particle Radiolysis of Liquids and Aqueous Solutions, Special Report SR-124 of the Notre Dame Radiation Laboratory, Notre Dame, Indiana. 

According to Ludwig (1968), there is a some similarity between UV- and high-energy-induced luminescence in liquids. In many cases (e.g., p-ter-phenyl in benzene), the luminescence decay times are similar and the quenching kinetics is also about the same. However, when a mM solution of p-terphenyl in cyclohexane was irradiated with a 1-ns pulse of 30-KeV X-rays, a long tail in the luminescence decay curve was obtained this tail is absent in the UV case. This has been explained in terms of excited states produced by ion neutralization, which make a certain contribution in the radiolysis case but not in the UV case (cf. Sect. 4.3). Note that the decay times obtained from the initial part of the decay are the same in the UV- and radiation-induced cases. Table 4.3 presents a brief list of luminescence lifetimes and quantum yields. 

TABLE 4.5 Excited State Yields in the Radiolysis of Liquids... 

When averaged over the distribution of energy loss for a low-LET radiation (e.g., a 1-MeV electron), the most probable event in liquid water radiolysis generates one ionization, two ionizations, or one ionization and excitation, whereas in water vapor it would generate either one ionization or an excitation. In liquid water, the most probable outcomes for most probable spur energy (22 eV) are one ionization and either zero (6%) or one excitation (94%) for the mean energy loss (38 eV), the most probable outcomes are two ionizations and one excitation (78%), or one ionization and three excitations (19%). Thus, it is clear that a typical spur in water radiolysis contains only a few ionizations and/or excitations. 

With the advent of picosecond-pulse radiolysis and laser technologies, it has been possible to study geminate-ion recombination (Jonah et al, 1979 Sauer and Jonah, 1980 Tagawa et al 1982a, b) and subsequently electron-ion recombination (Katsumura et al, 1982 Tagawa et al, 1983 Jonah, 1983) in hydrocarbon liquids. Using cyclohexane solutions of 9,10-diphenylanthracene (DPA) and p-terphenyl (PT), Jonah et al. (1979) observed light emission from the first excited state of the solutes, interpreted in terms of solute cation-anion recombination. In the early work of Sauer and Jonah (1980), the kinetics of solute excited state formation was studied in cyclohexane solutions of DPA and PT, and some inconsistency with respect to the solution of the diffusion equation was noted.1... 

TABLE 2. Yields of final products from radiolysis of liquid aliphatic dienes (G, molecule/100 eV)... 

TABLE 3. Radiolytic yields of the final products of the radiolysis of cyclic oligoenes I. Liquid phase... 

Cyclopentadiene behaves differently than the cyclohexadienes in that its radiolysis leads to high molecular weight polymer via a cationic mechanism89, whereas such compounds are not formed in high yield from cyclohexadienes irradiated in the liquid phase. 

Radiolysis of CO, both in the liquid and gaseous states, also leads to C02 and C302 formation, the latter product appearing mainly as a polymeric solid23-27. A scheme of reactions consisting of the initial production of a carbon and an oxygen atom, followed by (10)-(12) and... 

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