Nov., 1957
MECHANISMS OF PROTECTION IN RADIOLYSIS OF ORGANIC SYSTEMS
1461
voltage in the mass spectrometer (50-70 v.) it can
TABLEVI1
be assumed that the distribution of ions will be RADIOLYSIS OF PURE METHANE:COMPARISON OF PREDICTED similar to that from high energy irradiation. We AND OBSERVED YIELDS
have weighted the products in our mechanism by the relative abundance of the ions and a listing of the values predicted from this is given in Table VI1 CH4 -2.4 -2.5 together with the values obtained by Lampe6 but Hz 1.6 1.9 recalculated to M / N values by using a value of 30.2 CnHs 0.6 0.7 e.v. as the energy necessary to form an ion pair in CsHe .08 .05 pure methane.’* The formation of CH2+ was asC4HlO .01 .Ol sumed to give two hydrogen atoms and that of CJ34 .Ol .Ol CH+ and C+ to give rise to the formation of two Observations of F. W. Lampe, ref. 5. hydrogen molecules. On this basis the agreein the mass spectral pattern of methane are2*: ment, which could be expected to be scarcely more CH4+, 48%; CH3+, 40%; CH2+, 8%; CH+, 4%; than qualitative, is quite satisfactory. Again, yields can be virtually completely explained without 1%. The fate of the individual ions that constitute assuming any contribution by excitation. 85% of the total ion formation has been discussed in Acknowledgment.-The authors are indebted the preceding sections. In view of the ionizing to Dr. D. P. Stevenson for advice and encouragement and to Dr. Ronald Barker for many helpful (28) American Petroleum Institute, Research Project 44, “Catalog discussions. The mass analyses were performed by of Mass Spectral Data,” Carnegie Institute of Technology, Pittaburgh, Mr. George Young. Pa., 1947-1956. Species
Calcd.
)”‘(
Obsd.’
(I
c+,
MECHANISMS OF PROTECTION IN RADIOLYSIS OF ORGANIC SYSTEMS’ BY MILTONBURTON AND S. LIPSKY Contribution from the Department of Chemistry, University of Notre Dame, Notre Dame, Indiana Received June 10, 1067
d
The addition of relatively small amounts of certain substances to organic systems markedly reduces yields of radiolysis products. Several mechanisms can be invoked to explain this “protective” effect. These can be broadly class5ed into two ty es: first, a scavenger effect of the additives on free radicals produced from primarily excited and ionized solvent molecures; second, more pertinent to our present interest, an effect of additive on the precursors of the first chemical decomposition. The additive is assumed to provide an alternative path of energy dissipation, e.g., by inducing internal conversion in the excited solvent, by excitation transfer or by negative-ion formation. Benzene may act as a scavenger, but it appears also to exercise a strong rotective effect via some form of this second mechanism. I n general, part of the effect of typical radical scavengers may a i 0 involve the second mechanism. Chemical and physical evidences for various protection processes are reviewed.
.
1. Introduction The term “protection” in radiation chemistry has a variety of meanings dependent on the viewpoint of the observer as well as the details of the process investigated. For example, if a reaction M 4 R X yields two radicals either of which can react with a second species N and thus modify or destroy it, an active agent may be added with the objective of intercepting or scavenging the radicals before they can enter into the reaction with N. In this case N is protected from chemical action2 but M is not protected from decomposition. I n this paper our concern is with the true protection of M, not with scavenging of the products of its decomposition. “Real” protection is thus an effect on the precursors of all subsequent chemistry, i.e., on excited and ionized states of the compound primarily affected by the radiation. In any case in study it is frequently difficult to distinguish real protection from scavenger action. In our laboratory we have, in the past, made extensive studies of protection of cyclohexane by ben-
+
(1) Contribution from the Radiation Project operated b y the University of Notre Dame and supported in part under Atomic Energy Commimion Contract No. AT(ll-1)-38. ( 2 ) Such “protection” is, of course, important in biological aystema.
~ e n e . ~A, ~criticism of this work is that our interpretation derived from data on yields of gases (e.g., H2, CHI, C2H4, C2H2, etc.). Non-volatile products were not sought. However, by a variety of indirect means it was shown that scavenger action by the benzene (on free H atoms, for example) was not adequate to account for all the results. More recently, we have made studies of cyclohex~~ ane-benzene mixtures containing also I z , CHJ, C2H61and CeHI1I.6b Studies are also in progress on the effects of various additives on the radiolysis of other pure hydrocarbons and methanol. In all of these cases as well as others from other laboratories, indirect evidence is beginning to accumulate that points to an effect of “real” proteetion. 2. A Classification of Protection Mechanisms Several distinct mechanisms whereby a low-concentration additive confers this type of protection on a radiolyzed system can be considered.8@ We (3) J. P. Manion and M. Burton, THISJOURNAL, 66, 560 (1952). (4) M. Burton and W. M. Patrick, X d . , 58, 421 (1954). (5) (a) M. Burton, J. Chang, 8.Lipsky and M. P. Reddy, J . Chem. Phya., 96, 1337 (1957); (b) forthcoming publication. (6) Cf., M. Burton, Chapter on “Radiolysis of Organio Liquids” in “Actions Chimiques et Biologiques des Radiations,” Troisidme SBrie, M. Haieainsky, Editor, Masson et Cie., Saint-Germain, Paris, 1957.
1482
MILTONBURTONAND S. LIPSKY
classify these mechanisms as follows. 2.1. An Energy Transfer or Sponge Type Mechanism.-If an additive has excited or ionized states lying energetically lower than those of the solvent, energy transfer from solvent to additive may occur either via emission of a virtual photon or by charge transfer. An additive of higher radiation stability than the solvent would then exhibit a protective action. Virtual photon transfer can he imagined as long range transfer (ca. 50 k.)from solvent to additive or, perhaps more likely in a condensed phase, as a series of transfers from one solvent molecule to another until the excitation locates, and is trapped in, the additive.’ This concept of transfer of electronic excitation energy is not recent. It is essentially a resonance transfer of energy, quite similar to the sensitized fluorescence phenomena studied by Cario and Franck many years ago.* The process is capable of a very simple semi-classical description in terms of a coupling between two resonant electronic oscillators. It has been put on a more elaborate quantum mechanical basis first by Kallmann and Londons for monoatomic gases and then later by PerrinlO and more recently by Forsterll for polyatomic molecules. Forster has shown that, if the two participating molecules in the transfer are loosely enough coupled so that the initially excited ( L e . , “donor”) molecule can vibrationally relax before transfer, then the jump frequency of the excitatioa will increase with increasing overlap between the emission spectrum of the initially excited species and the absorption spectrum of the acceptor. The number of donor molecules through which the excitation can wander will, of course, increase with increasing half-life of the excited state. In certain crystals, the coupling between adjacent molecules is strong enough so that transfer can presumably occur even before vibrational relaxation. In these cases energy can migrate over considerably larger distances, perhaps even 5 p . A stationary excited state of the entire crystal is now considered as a standing excitation wave and a movement of localized excitation is expressed in terms of an excitation wave packet or “exciton.”l2 Such extremely rapid transfer or exciton movement might be usefully applied as a mechanism for intramolecular energy transport in polymer molecules.l3 The rapid disappearance of the low-concentration vinylidene groups in polyethylene during the early stages of its radiolysis14 could require such an explanation. At present, a reasonable amount of evidence indicates that the virtual photon transfer process can occur in times relatively short compared to those required for fluorescence. However, fluorescence is a relatively slow process with half-life, even for a strongly allowed transition, rarely less than (7) H.Kallmann and M. Furst, Phys. Rm., 79, 857 (1950). (8) G. Cario and J. Franck, 2. Physik, 17, 202 (1923). (9) H. Kallmann and F. London, 2. phyeik. Chem., B2, 207 (1928). (10) F. Perrin, Ann. phys., 17, 283 (1932). (11) Th. FBrater, Ann. Physik, 2, 55 (1947). (12) J. Frenkel, Phye. Rm., 87, 17 (1931). (13) J. Franck and R. Livingston, Rms. Mod. Phys., 21, 505 (1949). (14) M. Dole, C. D. Keeling and D. G. Roae, J . Am. Chem. Soc., 76,
4804 (1954).
Vol. 61
see. and more generally of the order of lo-* see. or more. The problem that interests us is not whether this energy transfer to a stable entity can occur before emission but rather whether it can occur before the much faster processes of rupture and rearrangement. Unfortunately, not enough is known about the times required for dissociation in polyatomic molecules. If the initial excitation is to a repulsive state, a fast rupture might possibly occur in a time of the order of second. It can be shown easily that energy transfer to a low-concentration additive (e.g., mole fraction ca. 10-3) cannot compete effectively with so rapid a process.16 On the other hand, the initially excited species may decompose more slowly. If excitation is initially to a level of an attractive state below its dissociation limit, decomposition may occur by a variety of processes’6 including internal conversion to a repulsive state, etc. Predissociation processes of this sort require times much greater than l O - l 3 sec. In times of such order of magnitude, energy transfer might compete effectively with efficiency depending strongly on the total time required for the dissociation, the nature and concentration of the additive, and the physical state of the irradiated material. For the case of electronic excitation transfer, in the classical model one requires an overlap of the electric fields of the variable electronic oscillators of the two participants; in the case of ionization transfer the orbitals involved in the transfer must have an appreciable space overlap. As a consequence, the long range transfer possible for virtual photon transfer becomes forbidden for ionization transfer. The limitation, however, is not severe. For condensed systems, the more likely possibility is a series of resonant ionization transfers” from solvent molecule to solvent molecule until the charge can be trapped by the additive. Competing with transfer will be possible dissociation of the ion and the more likely dissociation attendant upon neutralization. The addition of a second solute characterized by a high electron-capture cross-section could obviously enhance ionization transfer protection by increasing the time available for the transfer process. In general, however, the times involved for all of these competing processes are uncertain and little of quantitative nature can be discussed at the present time. 2.2. A Quenching Mechanism.-When the additive cannot trap the electronic excitation energy of a single molecule, it may, alternatively, promote distribution of the initially localized energy among vibrational-rotational degrees of freedom of neighboring solvent molecules. Processes of this sort are analogous to certain types of fluorescence- and (15) Consider a major component M and a minor component N (mole fraction ca. 10-9 and two possible methods of energy transfer N -r M from M* to N: the one b y direct long-range transfer (M* N*); the other by a succession of short-range transfers from M* to M until finally an M* is created in the neighborhood of an N molecule. If the excitation transfer is to compete effectively with the decornposition, the excitation jump frequency must be of the order of 1014 aec.-l for the long-range process and 1017 sec.-I for the short-range proceen. Such jump frequencies imply coupling interactions between donor and acceptor molecules prohibitively large (ca. 0.1 e.v. at 20 A. or 100 e.v. at 2 A. for long- and short-range transfer, respectively). (16) M. Burton and G. K. Rollefson, J . Chem. Phys., 6 , 416 (1938). (17) E. F. Gurnee and J, L. Magee, ibid., 26, 1237 (1957).
+
+
.
Nov., 1957
MECHANISMS OF PROTECTION I N RADIOLYSIS OF ORGANICSYSTEMS
phosphorescence-quenching phenomena. In some cases, however, this quenching process involves chemistry (ie., dissociation of the excited species18 or its disappearance by reaction with the quencherlg). In other cases, particularly in condensed systems, neither the excited species nor the quencher appears to be chemically alteredlg; the latter seems merely to promote conversion from the excited state to the ground state with eventual dissipation of electronic potential energy into heat. Such a quencher protects an excited molecule from decomposition. For protection by quenching, just as for protection by energy transfer, the primary problem is whether the necessary interaction between quencher and excited molecule can compete effectively with relatively rapid rupture and rearrangement processes. Some evidence from studies of high-energyinduced luminescence processes seems to indicate that certain efficient quenchers, such as 0 2 , CHBI and CeHsBr, may depopulate excited states of scintillation solvents at rates perhaps 100 times greater than the rates of radiation emission.20 These times begin to be of the order of magnitude,of the times required for predissociation decomposition from an excited state. Thus, it may be presumed that introduction of certain quenchers into a reaction medium will, at the very least, interfere with the normal course of radiolysis. For a certain class of quenchers, the efficiency of quenching seems to be greatest for those molecules that have either a heavy atom substituent (cf., CHJ, 12,CaH5Br),or a ground state of higher spin multiplicity than singlet (cf., 0 2 , NO), or a considerable admixture of such a higher multiplicity state in the ground state (cf., C2H4). Consequently, it has been suggested frequently that the action of the quencher is to induce non-radiative, spin-prohibited transitions. 21 For molecules with singlet ground states, occurrence of an internal conversion from excited singlet to lowest triplet results in the production of a state slow to lose energy by radiative transition to the ground state; ie., the lifetime of an excited state is thus extended and the molecule is made more viable to a host of competing nonradiative processes with consequent reduction in fluorescence yield. When the effect of a quencher is to convert the direct product of irradiation of the major component to the lowest triplet state, it does not follow that all chemical processes have been inhibited. While the lowest triplet state may not possess enough energy for bond rupture, it may still be labile; ie., it may possess enough energy for rearrangement decomposition or for metathetical reaction. Thus, the effect of a quencher may be to enhance another mode of radiolysis, e.g., a “molecular” decomposition or rearrangement process. An example of such an effect may be afforded by the unusual observation of Bakh and Sorokin that hy(18) P. Pringsheim, “Fluorescence and Phosphorescence,” Interscience Publishers, Inc., New York, N. Y., 1949,pp. 196-206. (19) P. Pringsheim, ref. 18,pp. 322-338. (20) P. J. Berry, 9. L. Lipsky and M. Burton, Trans. Faraday Sac., sa, 311 (1956). (21) E. J. Bowen, “Fluorescence of Solutions,” Longmans, Green and Co.. London, 1953,pp. 25-33.
1463
drogen yield in radiolysis of ethanol is doubled when the alcohol is oxygen saturated.22 In this connection it may be mentioned that the rearrangement type of decomposition from a triplet state M”’
+Products
(1)
may itself be spin-prohibited. I n such case the “quencher” may perform the added function of inducing the decomposition from the triplet state as well as the competing non-radiative internal conversion to the ground state M”‘
+M’(ground)
(2)
The extent to which a quencher might favor one or the other reaction will, of course, determine its effectiveness as a true protective agent. 2.3. A Negative-ion-formation Mechanism.This case has been discussed e x t e n s i ~ e l y . ~The ~,~~ idea involved is that the excited state resultant from a simple neutralization mechanism M++e+M$
(3)
is in general higher than that produced by neutralization involving a negative ion M + + N-+M*
+N
(4)
The excitation energies of MI and M*, E(M$) and E(M*), are related by the expression E(M*)
- E(M*) 2 A(N-)
(1)
where the term on the right is the electron affinity of N-. The negative ion, N-, is formed by a capture process in which an added molecule (or the solvent itself or both cooperatively) capture an electron before it can recombine with its positive hole. If E(M*) is sufficiently low, the chemistry of the subsequent processes may be greatly modified; e.g., rearrangement may become the only decomposition process or, perhaps, only metathetical reactions of the excited molecule may occur or decomposition may be entirely prevented. Negative-ion formation may also affect the probability of various ion-molecule reactions. The latter have recently been shown t o be quite significant in mass spectrometer experiments at slightly elevated pressure^,^^-^^ particularly for smaller molecules. Presumably, the intrusion of an electron capturer might prevent neutralization of the parent positive hole by an electron in a liquid medium and thus increase the probability of ion-molecule reactions of the type M+
+ M +Products
(5)
However, the mass spectrometer results also have shown that a considerable number of saturated sys(22) N. A. Bakh and Yu. I. Sorokin, Sbarnik Rabat Radialsionnoi Khim., Akad. Nauk S.S.S.R., 163 (1955); English translation: Symposium an Radiation Chemistry Acad. Sei. U S S R , 135 (1955). (23) J. L. Magee and M. Burton, J . A m . Chem. SOC.,78, 523 (1951); J. L. Magee, Disc. Faraday Sac., 12, 33 (1952). (24) (a) R. R. Williams and W. H. Hamill, Radiation Research, 1, 158 (1954); (b) R. H. Schuler, THISJOURNAL, 61, 1472 (1957). (25) V. L. Tal’Roze and A. K. Lyubimova, Doklady Akad. Nauk S.S.S.R., 86, 909 (1952). (26) D. P. Stevenson and D. 0. Schissler, J . Chen. Phys., 28, 1353 (1955). (27) D. 0.Schissler and D. P. Stevenson, ibid., 24, 926 (1956). (28) G. C. Meisels, W. H. Ramill and R. R. Williams, Jr., iaid., 26,790 (1956). (29) F. H.Field, J. L. Franklin and F. W. Lampe, J . Am. Chem. Sac., 79, 2419 (1957).
MILTONBURTON AND S. LIPSKY
1464
tems do not seem to exhibit ion-molecule reactions. 30 In general, although ion-induced-dipole forces between M + and M can increase the collision cross-section greatly (e.g., to ca. 150 chemical processes may not ensue because of the high activation energies required. In a liquid, new possibilities arise which appear to be energetically allowed. According to one of the current theories of the mechanism of radiolysis of liquids, the electron produced in the initial ionization process may be slowed down and captured by the parent positive hole in a time short compared with lO-’3 sec.31s32 However, in a liquid all molecular (as well as ionic) species enter into collision with frequency of the order of 1013sec.-l so that the possible ion-neutralization processes may be represented not only by reaction 3 but also by e
+ M + + M -+- Products
(elss
When an additive is present which can capture an electron before it recombines with its positive hole, it interferes with the possible reaction 6 as well as with reaction 3. 2.4. Ionization Transfer and Ion-Molecule Reactions.-In the previous section, reference is made to a newly postulated type of reaction, reaction 6. When an additive (e.g., B) can capture the ionization before neutralization can occur M+
+B+M
+ B+
(7)
the subsequent molecule-ion-electron reaction involves M, B+ and the electron and the energy available for excitation and resultant chemical reactions can be greatly reduced. Thus, the entire course of the ensuent decomposition may be modified. From another point of view, the neutralization process can be written e
+ (M+ + B ) +
M ’ + B’
(7%)
where M’ represents a state of lower energy than M$ in reaction 3 and B‘ is in excited state of B (which, in the case of benzene, may not decompose).34 3. Examples 3.1. Self-protection.-The phenomenon of protection frequently is treated as if all so-called examples are essentially speculative and as if no authenticated cases of protection have ever been observed. It is accordingly fitting to recall that the familiar phenomena of low-quantum yield processes in photochemistry are examples of cases in which substances possess an inherent mechanism for energy dissipation without decomposition. Cases of low quantum yield approaching zero are afforded by b e n ~ e n and e ~ ~by~ ~cr0tonaldehyde.~7 ~ So far as can be determined, benzene is not affected by absorption of light in its first absorption band (ca. 2500 A.) although bond breakage processes (30) R. Barker, private aommunication. (31) M. Burton, J. L. Magee and A. H. Samuel, J . Chem. Phys., S O , 760 (1952). (32) A. H. Samuel and J. L. Magee, ibid., Sl, 1080 (1953). (33) This same mechanism may be employed in explanation of
radiation-induced cross-linking phenomena in polymers; forthcoming publication. (34) The auggestion of this type of procem is due to Reed Bell. (35) W. West, J . A m . Chem. ~ o c . ,67, 1931 (1935). (36) J. E. Wilson and W. A. Noyes, Jr., h?l 83, ., 3025 (1941). (37) F. E. Blaoet and J. G. Roof, ibid., 58, 73 (1936).
Vol. 61
are energetically feasible. Simple explanations of the phenomenon have been given.38 Light of X 1849 8.does affect benzene but the quantum yield of decomposition is still very low (y = to 10-2).36 When benzene is exposed to ca. 1.8 MeV. electrons or to Coaoy-rays the yields of decomposition into gas3are G(HJ = 0.036; G(C2H2) = 0.22 and the yield of benzene molecules converted into polymer is G(C8Hs --+ P) = 0.76. Evidence has been given that a fixed fraction of excited benzene molecules and only such molecules enter into polymer formation.3s It is interesting that determination of free radical yield in benzene by either the Iz,~O the p~lymerization~l or the DPPH41technique give values not far from 0.75. One implication is that to a significant extent excited benzene molecules may enter into reactions similar to those exhibited by free radicals but, judging from the HP yields, they do not significantly decompose. The whole matter of the possibility of hydrogen atom scavenging by benzene is discussed elsewhere6b; it is sufficient to note here that the scavenger mechanism is inadequate to account for low H-atom yield in this case. Alkyl substituted benzenes are, like benzene, resistant both t o ultraviolet light and to high-energy radiation but in these cases the yields are somewhat higher. 42,43 When ultraviolet light is used the compound dissociates preferentially at a bond p to the ring; the indication is of a strong interaction between the ring and the substituents. The low yield in radiation chemistry is attributed to an energy transfer from the alkyl groups to the ring, resultant in substantial protection of the former. This is similar to the type of protection responsible for quantum yield approaching zero in photolysis of crotonaldehydes7at wave lengths between ca. 3700 and 2600 A. Here, the energy is absorbed (in the classical sense) in the carbonyl group and dissipated in the chain without chemical effect. The protective effect of the aromatic ring has been shown also in the well-known experiments of Alexander and C h a r l e ~ b ywhich ~ ~ demonstrate that benzene nuclei substituted into polymer chains (cf’., polystyrene) decrease the 100 e.v. yield of crosslinks; the naphthyl group substituted into dodecane also protects it from radiolysis and its effectiveness is a maximum when it is substituted near the middle of the chain ( i e . , when the average distance which the energy must be transmitted from the primarily affected portion of the molecule is a minimum). 3.2. Protection in Mixtures.-The first indication of possible protection was in the classical observation of Schoepfle and that a mix(38) 8. Gordon and M. Burton, Disc. Faraday Soe., 18, 88 (1952). (39) W. N. Patrick and M. Burton, J . A m . Chem. SOC.,78, 2626 (1954). (40) E. N. Weber, P. F. Forsyth and R. H. Schuler, Radiation Research, 8 , 68 (1955). (41) L. Bouby, A. Chapiro, M. Magat, E. Migirdicyan, A. Prevot.
Bemas, L. Reinisch and J. Ssbban, Proc. Con/. Peaceful User 01 Atomic Energy, 7 , 526 (1955), published: United Nations, New York, N. Y., 1956. (42) R. R. Hents and M. Burton, J . A m . Chem. SOC.,7 8 , 532 (1951). (43) R. R. Henti, T. J. Sworski and M. Burton, ibid., 78, 1998 (1951). (44) P. Alexander and A. Charlesby, Natwe, 173, 578 (1954).
Nov., 1957
MECHANISMS OF PROTECTION IN RADIOLYSIS OF ORGANIC SYSTEMS
ture of cyclohexane and benzene exposed to cathode ray bombardment behaves, so far as gas production is concerned, more like benzene than like a non-interacting mixture of the two. Extensive studies of this phenomenon have led to the conclusion that a scavenger effect by benzene is inadequate to account entirely for the low gas yields f ~ u n d . ~Alternatively, ,~~ it has been suggested that energy is transferred from cyclohexane to the less radiolytically reactive benzene leading to a protection of the cyclohexane and a slight sensitization of the benzene (cf., G(C2H2)). Some results from a study of the effect of oxygen on highenergy-induced luminescence from cyclohexanebenzene-p-terphenyl mixtures seem to agree with this type of interpretati0n.~7,48 The effect of iodine on cyclohexane radiolysis is, in some respects, similar to the action of benzene.6 Addition of small quantities of either benzene or iodine markedly reduces G(H2). For pure cyclohexane, G(H2) = 5.9. At lo-* M solute, G(H2) 1! 5.3 and 4.7 for benzene and iodine, respectively. With iodine as solute the reduction in G(H2)cannot be attributed to H atom scavenging by iodine; G(H1) is far too small in comparison with the reduction in G(H2) produced by a given iodine concentration. A striking difference between the effects of iodine and benzene is evidenced by the dependence of G(H2) on concentration of either additive. In the case of benzene, G(H,) continues to fall rather rapidly with increasing benzene concentration; in the case of iodine, G(H2) reaches an approximate plateau value of about 3.8 at [I2] N M . Methyl iodide and cyclohexyl iodide seem to act in a manner similar to that of iodine. If benzene is added to a cyclohexane solution containing a plateau concentration of iodine, G(H2) is found to increase at first, reach a maximum of about 4.0 at M , and then rapidly decrease to [CeHe] = values below the plateau value. The situation is complex and only a skeleton of a mechanism has been so far suggested.6 The mechanism involves several types of reaction discussed in earlier sections of this paper. It is exceedingly speculative and satisfies only the minimum requirement of consistency with known observation. However, considerable interest derives from the seeming necessity (45) C. S. Schoepfle and C. H. Fellows, J . Ind. Eng. Chem., 2S, 1396 (1931). . (46j W. N. Patrick and M. Burton, THISJOURNAL,68, 424 (1954). (47) M.Burton, P. J. Berry and 6. Lipsky, J . chim. phye., 62, 657 (1955). (48) P. J. Berry and M. Burton, J . Chem. Phye., 2S, 1969 (1955).
1465
of now having to attribute efficient quenching action t o a notorious radical scavenger-like iodine. Recent experiments with DPPH seem also to require its ability to remove excited states as well as radicds.49 The work of Bouby and Chapiro60 on radical yields from organic mixtures as measured by DPPH technique also hints a t functions of DPPH other than simple scavenging. The effects of iodine and inorganic iodides on methanol radiolysis is currently being investigated.61 As in the case of the cyclohexane system, simple radical scavenging is unable to account for the totality of observed results. Again, it has been found necessary t o postulate a quenching type action of iodine on excited states that would otherwise have rearranged to yield formaldehyde and hydrogen. Part of the observed yield of ethylene glycol is postulated to come from the reaction CHtOH+
+ CH30H + e ---f (CH2OH)z + HZ
(8)
Iodine presumably also interferes with this process by electron capture (cf., section 2.3). The addition of 10% di-m-tolylthiourea to polymethacrylate has been shown by Alexander and C h a r l e ~ b yto ~ ~increase by a factor of four the amount of energy required to cause one main chain break in the polymer. Both aniline and allylthiourea also raise this energy requirement. The interpretation suggested by Alexander and Charlesby is that energy absorbed in the polymer is efficiently siphoned into the additive by some sort of energy transfer mechanism. As evidence, the authors point t o the greater sensitivity of aniline to radiation damage when irradiated in a mixture with polymethyl methacrylate than when irradiated by itself. 4. Conclusion A number of evidences now exist that energy transfer plays a significant role in the radiation chemistry of liquids. When transfer is to a lowconcentration solute, protection of the solvent occurs, occasionally at the sacrifice of the solute but, in many important cases, without damage to either solvent or solute. The mechanisms of such processes cover a rather wide range which in turn may have a variety of significant effects in modification of radiation chemical reactions. We may consequently expect numerous studies of the details of such transfer mechanisms. (49) L. R.Griffith, W.D. thesis, University of California, Berkeley, 1956. (50) L. Bouby and A. Chapiro, J . chim. phys., 62,644 (1955). (51) G. Meshitsuka and M. Burton, forthcoming publication.
