RADIOLYSIS OF ORGANIC LIQUIDS THE RADIOLYSIS OF SOME ORGANIC LIQUIDS BY MRS. A. PREVO~T-BERNAS, A. CHAPIRO, C. COUSIN, Laboratoire de Chimie-Physique, Facultk des Sciences de Paris, Paris, France Received 28th Jauuary, 1952 Y. LANDLER AND M. MAGAT A critical review and comparison is made of the possibilities and limitations of two methods which the authors have used up to the present for the determination of the number of free radicals formed in organic liquids by ionizing radiations (polymerization initiation and reaction with diphenyl picrylhydrazyl radical). Attempts are made to correlate the sensitivity of different compounds with their chemical structure and to evaluate the G-values for the radical formation. This leads to an evaluation of the fraction of energy dissipated which is ultimately used for free radical production.TheP R E V O ~ T - B ~ R N A S , CHAPIRO, COUSIN, LANDLER, MAGAT 99 mechanism of the production of free radicals in liquids irradiated in a pile is discussed and a preliminary evaluation of the contributions of different radiations present is given. The possibilities of the determination of the nature of free radicals formed by radi- ations are briefly indicated and some preliminary results are given. While extensive work has been carried out on the radiation chemistry of water and of aqueous solutions there has as yet been comparatively little attention paid to non-aqueous media. The early work, e.g. Kailan 1 and Lind and his school 2 although giving important indications, suffers from the fact that only final products (and sometimes not all of them) were determined.Therefore, on the basis of their observations, it is very djfficult to draw unambiguous conclusions about the mechanism of the primary steps, It is even more dangerous to try to establish a correlation between the radicals produced and the chemical structure of the compounds irradiated. A systematic study of radiation effects on organic substances can alone lead to generalizations and hence to predictions concerning the behaviour of untested compounds and their likely reactions. We think, in particular, that such system- atic invcstigations is best begun by : (i) the determination of the number of free radicals formed per energy unit (ii) the determination of the nature of primary radicals formed in each case.We suppose at this stage that the subsequent reactions of these radicals are known, at least in principle, from general chemical kinetics. absorbed and this according to the type of radiation used; 1. DETERMINATION OF THE NUMBER OF FREE RADICALS FORMED BY IONIZING RADIATION A. METHODS USED AND THEIR LIMITATIONS.-Jt is Clear that in order to deter- mine quantitatively the number of free radicals formed by radiation, the radicals have to be consumed in some kind of reaction which competes favourably with their recombination. At the same time the direct effect on the reactive material must be either kept small by its low concentration or else must be n:easurable independently, with the assumption that no protective effect through excitation or ionization transfer is operative.3 Let us consider the possibility of trapping the radicals R formed by radiation through addition of some stable free radical F, whose disappearance through the reaction F + R + F R (1) could be easily followed.Such a radical could be, e.g., the diphenyl picrylhydrazyl (DPPH) which is highly coloured even in very dilute solutions, thus making colorimetric measurement of concentration easily feasible. Let us assume that reaction (1) requires an activation energy of 5 kcal, while the recombination re- action require no activation energy whatsoever. Assuming that the collision factors are equal, the concentration of F required to use 99 "/o of R by reaction (1) rather than by the recombination reaction (2) is then determined by the equation : ( 3 ) 2R1 -+A (2) e-5000:RT[F] [R] N 100 [R]2 [F] w 105 [Rl.Should the stationary concentration of R be of the order of 10-12 molelcm3 the concentration of F required would only be about 10-7 mole/cm3; this is low enough to make the direct effect negligible. The concentration of R is not100 RADIOLYSJS OF ORGANIC LIQIJIDS the average overall concentration, but the highest local concentration in the im- mediate neighbourhood of the track. In other words, the above calculation applies to free radicals produced by X-, y-, ,&rays and fast neutrons. For slow neutrons acting through the Szilard-Chalmers effect higher concentrations of F may be required and the possibility of its use must be determined experimentally. For x-rays, the local concentration of R may be so high that the required con- centration of F may preclude the assumption that direct effect on F is negligible.In other words, it is possible to determine the number of free radicals formed by X-, y-, ,&rays and fast neutrons by following the disappearance of DPPH, pro- vided that the dosage rate is reasonably low and that the concentration of DPPH is sufficiently high. At lower concentration of DPPH some of the radicals will escape detection, and the apparent value of the number of free radicals formed by radiation will be too low. This is shown in fig. 1 and 2 ; fig. 1 is based on unpublished data of Boag, Chapiro, Ebert and Gray obtained with the 190 KeV X-rays available at the Hammersmith Hospital, the irradiated liquid being CHC13. Fig.2 is based on data of Chapiro, Corval and Cousin 4 and corresponds to the slow neutron irradiation of CH30H in the Zoe pile at Chatillon. FIG. 1.-Moles DPPH x 108 used per cm3 after 1 min irradiation with 190 KV X-ray as a function of initial DPPH concentration (in 10-8 moles/cm3) at various 0 0 0 27.5 r/min ; This very rapid method gives directly the absolute number of free radicals produced but it has several limitations : (i) DPPH reacts thermally and more or less rapidly with double bonds. It can hence be used safely only with saturated and aromatic compounds with the exclusion of all ethylenic groups. (ii) DPPH reacts with water, so that a high degree of dehydration of the tested compounds and careful control measurements are required. It reacts also with organic hydroxyl groups and other labile hydrogens. Another possibility of counting free radicals formed is to let them initiate an easily measurable chain reaction, such as a polymerization.The necessary assumption is that the rate constants of the successive steps (propagation and termination) are well known from independent measurements. The activation energy for the reaction R* t M -+ RM* (propagation reaction) is known in several instances to be of the order of 8 kcal. We find by an argument analogous to that used previously [MI 2 100 e*OOO/RT. [R] > 108 [R]. dose rates. c) 6 Q) 69 r/min; 0 0 0 17.3 r/min. (4)P R E V O $ T - B ~ R N A S , CHAPIRO, COUSIN, LANDLER, MAG AT 101 In a pure monomer its concentration in mole/cm3 is of the order of 10-2.This method can hence be safely used for the same order of radical concentrations (of 10-11 mole/cm3) as the DPPH method.* A control is afforded by the reaction itself: it can be applied with monomer radical concentrations such that the re- action follows the kinetics previously outlined : 5 I M; + M -+ Mi + propagation M; -l- M:, -3 P termination where M is the monomer, and S the solvent. become If the monomer concentration is too low, the predominant termination will RM; + R* -+ M,R (6) 3oo co X / O A 100 200 FIG. 2.-Moles DPPH x 108 used p x cm3 after 12 h irradiation in the Chatillon pile as a function of initial DPPH concentration (in 10-8 moles/cm3). and the order of the reaction with respect to the monomer concentration will be changed. Fig. 3, taken from the work of Chapiro,s shows the validity of the equations derived from the scheme developed above for mixtures of styrene with some aromatic hydrocarbons.The theoretical curves agree well with the experimental results for solutions containing more than 20 % of monomer. With compounds which, by radiolysis, give a larger number of free radicals than the aromatic hydrocarbons, the agreement between theory and experiment is limited to higher monomer concentrations. The method has the advantage of enabling work to be carried out with highly " radiation sensitive " substances giving a large number of radicals/cm3 (provided that the contribution of the monomer itself is relatively small), since the compound tested can be used at low concentrations (5-10 %). This method has, of course, its limitations : (i) the direct effect on the monomer is by no means negligible, however it can easily be taken into account (provided that excitation and ionization transfer do not interfere) ; * According to Gray (J.Chim. Phys., 1951, 48, 172), the concentration of radicals in the immediate vicinity of a 60 KV electron track becomes about 10-9 mole/cm3 at the time of the first collisions between radicals, and drops probably still further to reach very soon the limiting values of 10--10-10--~~.102 RADIOLYSIS OF ORGANIC LIQUIDS (ii) it is limited to monomers and to compounds which are good solvents of monomer and polymer, since complications arise with precipitants and poor solvents, where the kinetic constants and effective concentrations (or activities) of the monomer are different from the normal ones ; (iii) the determination of the absolute value of the free radicals formed depends, as has already been stated, on the values of propagation and termination rate constants. While at the beginning of our work there was reasonable agreement between the values given by different authors 69 7 concerning these constants for styrene, some doubt recently arose8 introducing an uncertainty of a factor as large as 10.B. BASIC ASSuMPTIONS.-h order to correlate the number of free radicals pro- duced by radiolysis with the chemical structure of the compounds irradiated, we have to assume that we are really measuring (i) all the free radicals produced, (ii) only the free radicals. I 25 50 7;5 loo FIG. 3.-Per cent. polystyrene produced after 250 h irradiation with a y-ray source of 400 mg Ra as a function of dilution.Theoretical curves and experimental points. (0 0 0 benzene; 9 0 48 toluene; Q 8 @ m-xylene and 8 8 eJ ethylbenzene.) Let us discuss these assumptions : (i) the assumption that all the free radicals produced are trapped Cprovided the concentration conditions stated above are satisfied) means that the Franck- Rabinovitch cage is not effective in radiation chemistry. This hypothesis has some experimental and theoretical support that will be discussed elsewhere. At any rate the Franck-Rabinovitch cage effect could probably be little effective in polymerizations where the cage is formed by reactive monomer molecules : (ii) the assumption that only free radicals are measured means that neither ions nor excited molecules are responsible for an appreciable part of the chemical effects observed.While nothing precise is known about the possibility of re- actions between DPPH and organic ions, it was shown that addition of benzo- quinone completely inhibits the radiation-induced polymerization, in this case excluding any contribution of an ionic mechanism. The good agreement found between the radiolysis susceptibilities measured by DPPH and by polymerization indicates that DPPH does not react with the ions produced. The possi- bility of a contribution of excited molecules to the reactions observed cannot be excluded as definitely as the contribution of ions. There seems as yet to be no theoretical estimates of the relative importance of free radical production and of electronic excitation by electron impact.The experimental evidence is also scarce and contradictory. The excitation function for the nitrogen molecule for instance, given by Hanle,g seems to indicate that the cross-section decreases much faster with increasing electron velocity than does the ionization cross-section. Hence the probability of excitation by electrons of a rather wide velocityP RE vo # T - B 6 R N A s , c HAP I R 0, c o u s I N , LAND L E R , M A G AT 103 distribution is expected to be smaller than the probabilities of ionization, of dis- socation or of ionization-dissociation. This is in agreement with the observations of Mme Lousteau 10 who found a photon yield of only 10-7 per ion pair for gaseous nitrogen bombarded with x-rays from polonium.Neither could Boag and Gray find any light emission from HzO bombarded by electrons except, of course, the Cerenkov spectrum.11 The situation is quite different for atoms, where the relative probability of excitation rather increases with the electron velocity.12 A high yield of chemically active emission is further suggested by the recent experiments of Richards and Cole13 and Richards and Dee.14 Since no definite decision can be made, we shall consider for the time being, that all the effects observed (DPPH disappear- ance and polymerization) are due to free radicals only, this point being liable to revision, should further experiments disprove our assumption. RESULTS Various types of radiations (ct-, p-, y-, X-rays fast and slow neutrons) have been used in our experiments but it is only with y-rays and with slow neutrons that there are as yet sufficient data to permit a tentative discussion.~-RAYS.-AII our experiments were made with yrays from a Ra source, kindly lent by Mrs. Joliot-Curie. In columns 2 and 3 are given the relative numbers of free radicals produced under identical conditions 15 in the various compounds listed in column 1 as measured by DPPH and polymerization respectively. The values in brackets are to be considered as uncertain because of the perturbation of the reaction by secondary effects ('precipitation or poor solubility of the polymer formed in the polymerization and unusual variations of the reaction with time for DPPH). We sce that in all other cases there is satisfactory agreement between the results obtained by both methods.It appears that the various organic compounds arrange themselves in the following sequence according to increasing susceptibility to radiolysis : aromatic hydrocarbons, aliphatic hydrocarbons, oxygenated compounds, halogenated compounds. Carbon disulphide is the most stable compound so far studied. The particular stability of aromatic hydrocarbons has already been emphasized by Burton and his school,l6 who have also found the sequence: benzene, toluene, ethylbenzene. The NO? group decreases this stability somewhat more than CH3 but much less than chlorine. The introduction of a second C1 atom doubles the effect. This is generally true for an ac- cumulation of chlorine in the compound.Oxygenated compounds are arranged in the sequence : ethers, alcohols, esters, ketones. Some slight stabilization probably occurs in esters containing double bonds. While the results listed in columns 2 and 3 are not dependent on the absolute value of the dose and are hence relatively certain, the results given in columns 4 and 5 are dependent on the dose and are thus only tentative. Absolute dose determination is extremely diffi- cult in our arrangement and the dose was evaluated assuming that the radium source is homogeneously distributed along the axis of a cylinder of the same height as the ampoules whose centres are located on a circle of 4-5 cm radius around the source. This evalu- ation leads to a dosage rate of 1.7 r/min against 3 r/min previously calculated assuming the source concentrated in the centre of the circle.We think that the GR values given now are somewhat closer to reality than the values given previously.lsb The energy utilization yield ( Y %) gives the percentage of the energy absorbed which is used for the ultimate free radical production, assuming that 2 radicals are formed per bond broken and taking for the bond energies the following approximate values : C-C = 80 kcal, C-Cl = 70 kcal, CBr = 65 kcal. Except for acetone, chloroform and carbon tetrachloride, the Y values seem reasonable. A possible explanation for the extremely high values in the last two cases will be proposed at the end of this paper. The agreement between DPPH and polymerization results is less satisfactory if one compares not the relative values but the absolute numbers of free radicals formed in a given compound under identical conditions.A large part of this discrepancy is probably due to the uncertainty in the values of propagation (1,) and termination (k,) rate con- stants of the styrene polymerization. The results are listed in table 1.1 04 compound carbon disulphide benzene styrene toluene m-xylene ethylbenzene acrylonitrile propionitrile nitrobenzene n-heptane iz-octane cyclohexane ether dioxane methanol propanol perdeuteromethanol (CD30D) methyl acrylate methyl methacrylate ethyl acetate vinyl acetate acetone chlorobenzene o-dichlorobenzene ethylbromide 1 : 2-dichloroethane chloroform bromoform carbon tetrachloride RADIOLYSIS OF ORGANIC LIQUIDS TABLE 1 CR relative number of fuee radicals produced per cm3 per unit dose _______~__ (number of free polymerization radicals produced per 100 eV) DPPH 0-7 1.0 1-75 - - - - 2.0 3.5 4.4 5.2 7.4 11.2 13-2 12.5 15.0 13.0 - - 18.4 25.5 12.4 25.4 25.6 33-0 57.0 107.0 (72.0) - - 13.6 15.0 18.7 (50.0) 13.0 27.0 65.0 - - - - (200.0) 0.85 1.8 1.6 3.1 6.3 9.0 2-7 3.9 4.5 9.9 11.4 14.3 24.5 20.0 24.0 30.0 23.0 23.5 27.5 32-0 33.0 50.0 17.5 30.0 28.0 41.0 59.5 57.0 (70.0) Y energy yield in "/, - 3.0 2.8 5.3 11.0 15.0 4.6 6.8 7.8 1742 20.0 24.5 42.0 34-0 41.5 51-5 39.0 40.5 47.5 55.0 57.0 87.0 26.0 45.5 39.0 61.5 88-4 80.0 (105.0) * calculated from molecular weight data.t calculated from polymerization rate data. Table 2 gives the results obtained for styrene from different polymerization data and using the various k,/k,+ values, corrected for 15" C , available in the literature.It also gives the number of free radicals measured with DPPH in benzene which gives nearly the same radical yield as styrene. SLOW NEUTRONS.*-~~OW neutrons are capable of producing free radicals by the follow- ing mechanisms. (i) The Szilard-Chalmers effect : RA -k 11 -.* RA' y -> R + A' (7) A' being a stable or radioactive isotope of A. Two radicals ( A and R) are produced per neutron captured. However, fairly early in our work,17 we found that these primary free radicals could not account for more than a few per cent. of the polymerization occurring in pure styrene or in styrene-halogenated hydrocarbon mixtures. By far the largest part of the radicals are hence produced by the secondary effects.(ii) As can be seen from eqn. (7) y-rays are emitted during or immediately after the capture process (capture y-rays). These y-rays of several MeV are emitted either in a single quantum or in a succession of photons of lower energy (cascade) and are partly absorbed, producing a certain number of free radicals. (iii) If A is a radioactive isotope, it will decay to a stable nucleus, either directly or in several steps and usually with the emission of one or several 8- and y-rays. The energies and number of these " decay fi and y " radiations are known from nuclear data. They are also, at least partly, absorbed in the medium. We thank the Com- missariat de 1'Energie Atomique and particularly M. Ertaud for their helpful co-operation. * These experiments were carried out in the Chatillon Pile, Zoe.PREVO$T-B~RNAS, CHAPIRO, COUSIN, LANDLER, MAGAT 105 TABLE 2 method used no.of free radicals produced per cm3/sec inhibition period of styrene polymerization by benzoquinone * absolute value of polymerization rate assuming at 15" C : 6.5 x 1010 1.1 x 1010 (i) k,/k,: = 0.0043 8 1.7 x 1010 (ii) k,/kt4 = 0.0082 6 0.45 x 1010 (iii) k,/k,& = 0.0105 7 0.29 x 1010 DPPH + benzene (corrected for styrene) 1.3 x 1010 * Assuming one benzoquinone molecule is used per free radical ; this probably leads to a too high value for the number of free radicals, because of the possibility of a co- polymerization reaction, benzoquinone-styrene. j- This value is independent of the ratio of propagation and termination constants.Chain transfer was neglected. The correction that would ensue is smaller than the uncertainty concerning the molecular weight as viscosimetrically determined. (iv) The recoil energy of the capture y's is usually much in excess of the dissociation energy E of the chemical bond ruptured. This excess energy which can be very high (30,000 kcal in the case of H) will be carried off as kinetic energy by R and A (" hot " atoms and radicals) and will be consumed in collisions with surrounding molecules. Free radicals (sometimes " hot ") will be formed as a result of some of these collisions in the process. (8) or by activation of RB, leading for instance to an opening of the double bonds. combination of molecular weight and polymerization rate -f A' (" hot ") + R'B --f A' (less " hot " - > 80 kcal) + R' + B - E, H H A' (" hot ") + RC=CH2 -+ A' (" hot " - > 40 kcal) + RC-CH2 (9) I 1 (v) After A' has spent sufficient of its kinetic energy, it will act as a normal free radical; in our case, it will start a polymerization chain and remain fixed to the polymer.However, if it is a radioactive nucleus, it will decay and eventually become another nucleus, say C , of different valency than A , hence, recreate a free radical. (vi) In addition to slow neutrons, y-rays from the fission process and from the fission products,' as well as a few " fast " neutrons are always present in the pile (" pile y-rays ") and also contribute to free radicals production. In order to separate the contributions of these various effects the following procedure was adopted. (i) The amount of free radicals produced by the Szilard-Chalniers effect proper (two radicals per neutron captured) was calculated from the known cross-sections, irradiation time and the neutron flux in the particular channel.An ampoule containing pure styrene was present in each experimental series for eventual monitoring. (ii) The effect of capture y's was assessed from the known number of neutrons captured and from the efficiency of y-rays of Ra. Since the overall contribution of this effect is rather small, the final result is not appreciably influenced by even relatively large errors which may be due to the assimilation of 5 MeV capture y-rays to 1-2 MeV Ra y-rays and to the neglect of a possible cascade emission. (iii) The contribution of decay P- and y-rays to the overall effect which is important if, e.g., Br and Cl atoms are present, was directly determined for bromine by measuring the rate of polymerization of styrene after addition of known amounts of radioactive (pile-irradiated) ethyl bromide.Fig. 4 shows the time variation of the square of the rate of polymerization (which is proportional to the dosage rate), when pre-irradiated ethyl bromide was added to styrene. The figure shows plainly the period of 33.6 h of 82Br and gives an indication of the 4.5 h period of SOBr. The radical yield is rather low as deduced from this experiment pertaining to ,B-rays of 82Br (e.g. GR = 0.05 in a mixture of 60 % styrene + 40 % ethyl bromide). The con- tributions of /3-rays of 80Br and of 36Cl was calculated on the bases of these GR values, taking into account the amount of energy absorbed.The number of disintegretion * It can be safely assumed that the pile P-rays do not penetrate to the irradiated substance in any significant quantity. D106 RADIOLYSIS OF ORGANIC LIQUIDS and of isomerization y-rays was calculated from the cross-section, neutron flux and nuclear data. The contribution of these radiations (particularly the important effect of isomeriza- tion y-rays of 80Br) to free radical formation in mixtures of styrene and organic bromides and chlorides has been calculated solely from values obtained through application of the corresponding absorption coefficients and of GR values listed in table 1 . There are hence rather large uncertainties in this evaluation.(iv) The contribution of free radicals formed by decay of Br to Kr was calculated in a manner analogous to (i). (v) The determination of the contribution of the pile y-rays and of fast neutrons present in minute amounts became possible only through the development of the DPPH tech- niaue. It is based on the following idea.4 If one irradiates in the pile a solution of For CI their number is very small and was neglected. DPPH in a solvent containing, for ';example, some FIG. 4.-Time dependence of the rate of polymeri- zation of styrene induced by radioactive ethyl- bromide at 37" C. (Correction for thermal poly- merization was made.) hydrogen atoms besides carbon and oxygen, the discoloration of DPPH is produced by all the radicals formed in the processes (i), (ii), (iii) and (v).If, however, hydrogen is replaced completely by deuterium, the effect observed is due entirely to pile y's and fast neutrons. Assum- ing that the y-ray effect is wave length independent, one can hence express the effect of pile y-rays and of fast neutrons as equivalent to the effect of Ra y-rays in some standard arrangement. Knowing the relative radical yields listed in table 1, one can then calculate the number of free radicals produced by pile y-rays and fast neutrons (counted as y-rays) in any particular mixture. In our experiments, we used CH30H and CD30D as solvents," because the latter compound was available in the laboratory. The measurements were fairly reproducible but we had to introduce a correction for the thermal reaction between alcohols and DPPH which is responsible for an appreciable part of the total effect with CD30D and hence decreases the accuracy of determination.A perdeuterated compound which is expected to show no thermal reaction with DPPH is now being prepared and the pile y-effect will be rede- termined. (iv) The difference between the number of free radicals required to explain the ob- served polymerization and the number of free radicals due to the effects (i) to (v) is, for the time being, assumed to arise from collisions of " hot " recoil atoms ; of course, the errors involved in the evaluation of all the other effects accumulate in this value. Only with pure styrene, where no contribution of decay radiation is liable to occur, can this number be assessed with a reasonable margin of error.According to the par- ticular set of propagation and termination constants adopt M it is found that the " hot " atoms form respectively about 2000 or 500 secondary free radicals per neutron captured. If one assumes that only mechanism (8) is operative, the energies necessary to produce these numbers of free radicals would be 85 x 103 kcal and 20 x lo3 kcal respectively while the recoil energy available is of the order of 30 x lo3 kcal. The energy necessary for the production of these numbers of free radicals would be of course smaller if * We have ascertained that the radical yield was the same for CH30H and CD30D (table 1).P R E V O S T - B ~ R N A S , CHAPIRO, COUSIN, LANDLER, MAGAT 107 mechanism (9) contributes to their formation, but, at any rate, the energy utiiization yield remains rather high.Fig. 5 shows for a few mixtures the relative contribution of the different mechanisms to the free radical formation. In pure hydrocarbon systems (styrene and styrenefcyclo- hexene) it is the contribution of the " hot '' recoil atoms which is by far the most pre- dominating. The situation is quite different in styrene+ halogenated hydrocarbon mixtures, where the contribution of decay radiations may become decisive. This effect becomes very important even at low concentrations of halogens. (1) (2) ( 3 ) (4) FIG. 5.--Origin of free radicals in pile irradiated systems. (I) pure styrene ; (2) 60 % styrene + 40 % cyclohexane ; (3) 60 % styrene + 40 % ethyl bromide ; (4) 60 % styrene t 40 % chlorobenzene ; (5) 60 % styrene + 40 % dichlorobenzene.(a) contribution of the Szilard-Chalmers effect. (b) ,, ,, capture y-rays. (c) ,, ,, decay p-rays. (d) ,, ,, hot atoms. (e) ,, ,, disintegration and isomerization y-rays. (f) ,, ,, pile y-rays and fast neutrons. (8) ,, ,, thermal centres. For dichlorobenzene the values of c is the maximum value according to the GR values listed in table 1 for the corresponding compounds. 11. DETERMINATION OF THE NATURE OF FREE RADICALS PRODUCED BY RADIOLYSIS WITH 7-RAYS Our knowledge of the nature of free radicals produced by radiations in liquids is even scantier than our knowledge of their number. Even for water the problem is far from being settled.18 There could a priori be some hope that mass-spectro- metry would at least tell us what free radicals are formed by the mechanism ABww+A+ + B + 2e.Unfortunately, a closer examination of the data shows that in many instances, the reaction goes through an intermediate state, probably an excited- ABf ion of considerable life-time. We think less of relatively rare " metastable " ions than of ions whose formation requires a rather profound reorganization of the original molecule : e.g. C4H9+ ions produced from the 3 : 3-diethylpentane and which give rise to the strongest peak in the mass-spectrum of this compound.19 In the liquid state these intermediate excited ions, of life-time at least 10-10-10-12 sec and probably more, will suffer a large number of collisions which may influence their mode of decomposition. This mechanism of free radical formation is of course not the only one and the overall spectrum of free radicals will in general be different from the one deduced from mass-spectrographic data and will depend on the composition and the state of the surrounding medium.A striking example is given by Hamill, Williams and Voiland20 who found that in the gas phase, at 500 mm Hg pressure, n-pentane on radiolysis yields108 RADJOLYSIS OF ORGANIC LIQUIDS preferentially methyl radicals ; in the liquid phase the main products are vinyl and sec-amyl radicals. The mass spectra would suggest ethyl radicals as main products. The general way to solve the problem of the nature of radicals formed is the same as used for the determination of their total number, i.e. trapping them with reactive entities, such as " stable " free radicals or monomers, and analysis of the final products.This method was successfully used by Dainton21 to show the formation of OH radicals in the radiolysis of water and in an elegant way by Hamill and his co-workers who tagged the radicals with radioactive iodine and bromine. We are now trying to trap the radicals produced with DPPH and to separate and identify the products formed. Fig. 6 reproduces the absorption spectra of mixtures of DPPH derivatives formed by the irradiation of different compounds (e.g. CHC13 and CCl,) in presence of DPPH. With chloroform and methanol at least two substances could be separated by paper chromatography of the irradiated products. We hope to be able to identify them with micro infra-red spectroscopy and eventually to achieve further separations.FIG. 6.-Absorption spectra of DPPH, diphenylpicryl hydrazine and the. products ob- tained by irradiating DPPH in CCI4 and CHC13. Initial DPPH concentration 2x 10-8 moles/cm3. Finally, some indications can also be gained from a detailed study of the kinetics of the disappearance of DPPH during the irradiation. For instance, one observes (with CHC13 and with CC14) a post irradiation effect, the DPPH continu- ing to disappear after irradiation has ceased. This " after-effect " is much more important with CCl4 than with CHC13. From the work of Gunther and his co-workers,23 it is known that CHC13 and CCl4 yield HCl and Cl;! respectively on irradiation. We attribute the after-effect to a relatively slow reaction (as com- pared with the free radical reaction) of HCl and Cl2 with DPPH.Gunther and Cronheim 23 attributed the HCl production in chloroform to a chain reaction of the type : C1* + CHC13 -+ HCl + CCl; CCl; + CHC13 -+ C1* + CC13CHC12 Cl; + CHC13 -+ HCl + CCl., and similar reactions with H atoms.PREVOST- BERNAS, CHAPIRO, COUSIN, L A N D L E R , MAGAT 109 Such a chain would be impossible in the presence of DPPH which is known to act as a chain terminator.24 We have hence to assume that C12 and HCl are also produced in one step, either through CC14 % x ~ % + C12 + CCl2+ -t 2e (1) and CHC13 -xxxx%+ HCl + CC12+ + 2e, (2) respectively or by electron capture,257 26 or else, competing with the " normal reactions ',, CC14 + e + CC1 + Cl2 + C1- cc13 \%A++- --f cc12 + CI2 CC14 \-+V -+ CCl3+ + C1 + 2e CHC13 A\W+ CHC12+ + C1 4- 2e CC14 xxxxxb--f CC13 + C1, (3) (4) reactions (l), ( 3 ) and (4) being relatively more important as compared with the reactions (5), (7), etc., for CC14 than the corresponding reactions with CHC13.Experiments are being carried out to establish this relative importance of the two types of reactions in CCl4 and CHC13. It is important to notice that the energy required for reactions of the type (l), (2), (3), (4) is only slightly larger than that necessary for the reactions (5), (6), (7) (the difference is only of about 13-15 kcal for reactions (4) and (7)), while twice as many radicals are produced. This would explain the abnormally high values of the energy yield Y for these compounds as seen in table 1 which were calculated on the assumption that only (5), (6) and (7) type reactions are occurring. Much further work is of course necessary before these ideas could be con- sidered as more than convenient hypotheses. 1 Kailan, Ber. Wen. Akad., from 1910 onwards. 2 Lind, Chemical Efects of Particles and Electrons (New York, 1928). 3 Burton, J. Chem. Educ., 1951, 404. 4 Chapiro, Corval, Cousin, Compt. rend. (in press). 5 Chapiro, J. Chim. Pltys., 1950, 47, 747. 6 Bamford and Dewar, Faraday SOC. Discussions, 1947, 2, 310. 7 Melville and Valentine, Trans. Faraday Sac., 1950, 46, 210. 8 Matheson, Auer, Bevilacqua and Hart, J. Amer. Chem. Soc., 1951, 73, 1700. 9 Hanle, Physik. Z., 1932, 33, 245. 10 Mme Lousteau, Thesis (Paris, 1951), as yet unpublished. 11 Gray, J. Chim. Plzys., 1951, 48, 172. 12 Bethe, Handbuch der Physik, 2nd ed., 1933, 24/I, 519. 13 Richards and Cole, Nature, 1951. 14 Dee and Richards, Nature, 1951. 15 for details of geometrical arrangement and experimental conditions, see (a) Chapiro, J. Chim. Phys., 1950, 47, 747 ; (b) Chapiro, Compt. rend., 1951, 233, 792. 16 Hentz and Burton, J . Amer. Chem. SOC., 1951, 73, 532. 17 Landler and Magat, Bull. SOC. Chim. Belg., 1948, 57, 381. 18 e.g. Haissinsky, this Discussion. 19 Tables published by Nat. Bur. Stand. (Washington). 20 Hamill, Williams and Voiland, Brookhaven ConJ (Chemistry, 1950). 21 e.g. Dainton, J . Physic. Chem., 1948, 52, 490. 22 Gunther, Von der Horst and Cronheim, Z. Elektrochem., 1928, 34, 616. 23 Cronheim and Gunther, 2. physik. Chem. B, 1930, 9, 201. 24 Bartlett and Kwart, J. Amer. Chem. SOC., 1950, 72, 1051. 25 Warren, Hopwood and Craggs, Proc. Physic. SOC., 1950, 63, 180. 26 Vought, Physic, Rev 1947, 71, 93.