Radiolysis liquid-phase

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. 

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. 

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. 

Liquid carbon dioxide is decomposed efficiently by ionizing radiation79. The decreased radiation stability of the liquid phase compared to the gas phase has been attributed to the much smaller contribution of ion-molecule reactions to radiolysis in the condensed phase, where an efficient geminate charge neutralization process is likely to minimize the occurrence of such processes. Ion-molecule reactions are probably responsible for the rapid reoxidation observed in the gas phase. The yields of CO, 02 and 03 from the y-radiolysis of liquid C02 can be... 

The evidence for this mechanism is based on mass spectroscopy of the gas-phase radiolysis of isobutylene, which may not be applicable to the typical liquid-phase polymerization system. Initiation in condensed systems may follow the same course as electroinitiation— coupling of radical-cations to form dicarbocations. 

Most publications dealing with the photodecomposition of alkanes discuss the processes in the gas phase several comprehensive works have already been published in this field [14-17]. In the present work, we summarize the results of liquid phase photolytic studies and compare them with those obtained in radiolysis. An early review on liquid alkane photochemistry was published in Ref. 18, a brief overview of the field was given in Ref. 19. 

Horvath, Zs. Ausloos, P Foldiak, G. Comparison between liquid phase photolysis and radiolysis of C3—C4 hydrocarbons mixtures. In Proceedings Fouth Tihany S5unposium on Radiation Chemistry Hedvig, P. Schiller, R., Eds. Akademiai Kiado Budapest, 1977 57 pp. Antonova, E.A. Pichuzhkin, V.I. High Energy Chem. 1977, 11, 201. 

One important difference between the solid and liquid phases is that the isotropy of the liquid is replaced by the anisotropy of the solid. The geometrical correlations that exist in a solid lattice may be expected to play an important part when reaction centers are localized and can react with their neighboring environment. In this section we point out some of these effects in the radiolysis of solid n-hexane the trans/cis hexene yields, the isomeric dimer pattern, and the fragmentation processes. 

Williams83 has recently considered the possibility of ion molecule reactions in alcohols, ketones, ethers, and esters. He postulates that the primary reactions of the parent ion are the inter- or intra-molecular abstraction of a hydrogen atom and the formation of a x-bond between oxygen and the adjacent carbon atom after homolytic scission of a bond to that adjacent carbon atom. After examining some of the liquid phase data on the radiolysis of these oxygen compounds it was concluded that such ion molecule reactions may be of importance in these systems also. 

The electron ejected in the radiolysis reacts with the parent molecule. Optical absorption studies of the radiolysis of CC14 in the liquid phase (see detailed description later) show three absorption lines. These were assigned to the cation CCI4 and to the ion pair CCI3.Cl" or CCl /solvent/CT. However, optical studies only show the existence of inter-... 

In solid-state studies, ESR spectroscopy is the best detection method for studying radical intermediates in radiolysis. It is, however, difficult to apply to liquid-phase studies, and generally, optical methods are favoured. In solid-state work, radicals are trapped (matrix-isolated) and can be studied by any spectroscopic technique at leisure. However, for liquid-phase studies, time-resolved methods are often necessary because the intermediates are usually very short lived. In the technique of pulse radiolysis, short pulses of radiation, followed by pulses of light which explore the UV spectrum, are used. The spectra help to identify the species, but also their kinetic behaviour can be accurately monitored over very short time-scales (from picoseconds to milliseconds). This technique is discussed in Section 3.3. 

Although, in principle, ESR spectroscopy is the most powerful method for detecting radical intermediates, it has not been widely used in conjunction with steady-state or pulse radiolysis, mainly because of technical difficulties. Important early work was done by Fessenden, Schuler and their co-workers (see for example Eiben and Fessenden, 1968 and Fessenden and Schuler, 1971) using steady-state radiation from a Van der Graaf accelerator. In this way, liquid-phase ESR spectra were generated for a range of radicals never previously observed by ESR methods. 

The electrons ejected from molecules by the passage of ionizing radiation through condensed media can be solvated very soon after the primary ionizing event and the solvated electron, e q, so formed can undergo chemical reactions with solute and solvent molecules. The main evidence for the existence of solvated electrons in the liquid phase has been obtained by the use of pulse radiolysis in conjunction with optical spectroscopy (Hart and Boag, 1962). Very recently the e.s.r. spectrum of the solvated electron has been obtained by a similar method (Avery et ah, 1968). The solvated electron is not located on one solvent molecule but is associated with an assembly of molecules which form a potential well around the electron by virtue of dipolar and polarization forces. There is a close similarity between this system and the blue solutions obtained by dissolving alkali metals in liquid ammonia. 

In summary, in this first era of radiation chemistry it was discovered that the medium absorbs the energy and the result of this energy absorption leads to the initiation of the chemical reactions. The role of radium in these systems was not as a reactant or as a catalyst, but instead as a source of radiation. Most quantitative work was done with gases. It was learned that there was a close correspondence between the amount of ionization measured in a gas and the yield of chemical products. Solid and liquid-phase radiolysis studies were primarily qualitative. 

The radiolysis of liquid propane has been examined recently by Koob and Kevan with emphasis on the radical reactions and the non-radical reactions . The radiolysis mechanism is essentially the same as that in the gas phase, but the liquid-phase radiolysis gives a reduced G-value for propane loss. It is noted that the dis-proportionation/combination ratio for various radicals is increased in the liquid phase. 

The rare-gas sensitized radiolysis of propane has been studied in the gas phase and in the liquid phase". Charge transfer from the rare-gas ion to propane is followed by ionic reactions. In particular, the decomposition of C3H8 to give CH4 by reaction (4) is important, as is the ion-molecule reaction of C2H5 with propane to give ethane by reaction (7). 

The radiolysis of liquid ethylene shows many of the same reactions as the gas-phase radiolysis, with the ethyl, vinyl, and hydrogen atoms being the predominant radical species . The vinyl radical plays a more important role in the liquid phase, as does the butenyl radical. 

The radiolysis of benzene has been the subject of many studies in both the liquid phase and the gas phase. In all cases the radiolysis leads mainly to polymer formation, but hydrogen and acetylene as well as other minor products are formed in much lower yields. 

In the liquid-phase radiolysis benzene is particularly resistant to decomposition with the major product, polymer, having a G-value of 0.75 (refs. 468-470). The polymer has been found to contain biphenyl, phenylcyclohexadiene, phenylcyclo-hexene, Cjs compounds and higher-molecular-weight material A brief and rather uncertain reaction scheme for the radiolysis of liquid benzene is... 

A recent study using a pulse-radiolysis technique on liquid benzene solutions has shown that there is an abundant yield of excited singlet ( B2 ) and triplet ( B) states of benzene. The effect of ionic scavengers in this study shows that ions are precursors of both singlet and triplet states. It is concluded that the excited states arise from ion-electron recombination. This recombination is particularly facile in benzene solution owing to the numerous excited states of low energy in benzene, which rapidly thermalize the electrons. The formation of excited species which are relatively unreactive, as implied by the observation of fluorescence from these states, may account for the low (7-value of benzene decomposition in the liquid phase. 

The gas-phase radiolysis of cyclohexane shows an increase in products resulting from breaking of the cyclohexane ring . The major products of radiolysis in inert gas matrices are the same as for the liquid-phase radiolysis ° . 

The low G-values suggest that recombination of initially formed radicals in the liquid phase is fairly efficient , and this is supported by the much larger magnitude of the exchange yield (G 6) when radiolysis is carried out in the presence of radioactive iodine ... 

Among the products of the radiolysis of 2-pentanone and 2-hexanone, ethylene and propene were found in considerable amounts . The distribution of the butenes in the radiolysis of 4-methyl 2-hexanone (both in the vapour and in the liquid phase) was very much similar to that observed in fhe photolysis at short wavelengths . 

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