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Although, as will be explained below, the above hypotheses are not firmly established, they lead to the rough approximations presented in Table 11. These approximations indicate that at 10 eV (1)C picks up hydrogen almost as readily as it inserts in benzene. Such pick up reactions may occur via direct abstraction or by an insertion-decomposition reaction either into the ring, or as has been reported previously for recoil produced carbon atoms, into a C-H bond,1° (2) CH reacts primarily by insertion, but prefers hydrogen pickup over elimination, (3) CH2also reacts primarily by insertion with only a small percentage of the methylene ions losing a hydrogen, and (4)CH, loses one or two hydrogens but also reacts readily to methylate benzene. It is also apparent both from Table I and Table I1 that the most reactive species in producing the insertion products observed at 10 eV is CH2 followed, in order, by CH3, CH, and C. The data of Table I leave no doubt that CH2i s the most efficient species for forming CHT, and that CH is the most efficient species for forming benzene. However, this does not necessarily mean that the bare 14Catom must pick up a hydrogen before the interaction that leads to the labeled benzene product. In fact, our previous studies6J3have shown that the yields of labeled benzene, toluene, and CHT from 14Creactions are essentially constant through the translational energy range of 10-2 eV. Consequently, an alternative interpretation might postulate similar bicycle C7intermediates such as that in (1)arising from the initial reaction of C, CH, or CH2 with benzene. In each case, labeled benzene and biphenyl would be produced following intramolecular hydrogen transfer when needed, as, for example, in reaction 2 or 3. How important the H
H
H
H
(3) labeled
suggested intramolecular hydrogen transfers are may be
determined by future experiments using 14CHZ+ beams at 5 eV, or lower, kinetic energy. A problem with the intramolecular hydrogen shift mechanism lies in the experimental results which support the energetically unfavorable conclusion that C is more readily expelled than CH2. Such a mechanism yields labeled 14CH fragments from C reactions and 14CH2 fragments from CH reactions, but in lower quantities than estimated in Table 11. Here, as in the previous interpretation, there is no mechanistic explanation for the high yield of diphenylmethane in the methyl irradiations.
Acknowledgment. This research was supported by the Division of Biomedical and Environmental Research of the U S . Energy Research and Development Administration. References and Notes (1) Present address: Department of Chemlstry, University of Utah, Salt Lake City, Utah 841 12. (2) W. R. Erwin, B. E. Gordon, and R. M. Lemmon, J. Phys. Chem., 80, 1852 (1976). (3) R. M. Lemmon, Acc. Chem. Res., 6, 65 (1973). (4) H. M.Pohk, W. R. Erwin, F. L. Reynokls, R. M. Lemmon, and M. Calvln, Rev. Sci. Instrum., 41, 1012 (1970). (5) H. Wolff, "Organic Reactions", Vol. 111, R. Adams, Ed., Wiley, New York, N.Y., 1946, p 307. (6) J. Lintermans, W. Erwin, and R. M. Lemmon, J . Phys. Chem., 76, 2521 (1972). (7) R. D. Smith and J. J. DeCorpo, J . Phys. Cbem., 80, 2904 (1976). (8) (a) J. L. Franklin, J. G. Dillard, H. M. Rosenstock, J. T. Herron, K. Drazl, and F. H. Field, Natl. Stand. Ref. Data Ser., Natl. Bur. Stand., NO. 26 (1969). (b) V. I. Vedeneyev, L. V. Gutvich, V. N. Kondrat'yev, V. A. Medvedev, and Y. L. Frankenvich, "Bond Energies, Ionization Potentials, and Electron Affinities", St. Martin's Press, New York, N.Y., 1966. (c) R. Yamdazni and P. Kebarle, J. Am. Chem. SOC.,98, 1320 (1976). (d) "CRC Handbook", Chemical Rubber Co., Cleveland, Ohio, 51st ed, 1970-1971. (9) F. Cacace and P. Giacomello, J. Am. Chem. Soc., 99, 5477 (1977). (10) K. K. Taylor, H. J. Ache, and A. P. Wolf, J. Am. Chem. SOC.,97, 5970 (1975). (11) J. B. Hasted, f r o c . R. SOC. London, Ser. A , 212, 235 (1952). (12) H. B. Gilbody, f r o c . R . SOC. London, Ser. A , 238, 334 (1956). (13) H. M.Pohiit, W. Erwin, T. H. Lin, and R. M. Lemmon, J. Phys. Chem., 75, 2555 (1971).
Positron Lifetime Studies in y-Irradiated Organic Crystals Yan-ching Jean and Hans J. Ache* Department of Chemistry, Virginia Polytechnic Institute and Sfate University, Blacksburg, Virginia 2406 1 (Received October 25, 1977)
Four organic solids, adamantane, guanosine, 2'-deoxyuridine, and 5-iodo-2'-deoxyuridine were y irradiated in the dose range from 0 to 200 Mrad and their positron annihilation lifetimes and ESR signals measured, with and without subsequent thermal annealing. No consistent correlation between the positron lifetime data, 11, hz, and I2 and free-radical concentration or absorbed radiation dose could be observed. On the basis of these results, it appears that the positron annihilation method is not a direct technique for the study of paramagnetic centers or radicals in irradiated solid organic materials because the interactions of positron or positronium with these species are frequently obscured by the response of the positron annihilation process to radiation-induced structural changes in these solid materials.
Introduction In previous papers several authors2-5 studied the correlation between ESR or EPR and positron annihilation measurements6-12in y-irradiated organic compounds. In these investigations carried out by Eldrup et al.4 and Hadley et a1.3 with y-irradiated acetyl methionine, a reduction of I2, the intensity of the long-lived component in 0022-3654/78/2062-0656$0 1.OO/O
the positron lifetime spectra, with increasing radiation dose and EPR signal intensity was observed. Similar results were found by Hadley and HSU' in y-irradiated D,L-leUCine, while more recently Ito and Tabata5 investigated y-irradiated eicosane and polyethylene where they observed an increase of the annihilation rate 1 2 associated with the long-lived component and a simultaneous decrease of Iz 0 1978 American Chemical Society
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y-Irradiation of Organic Crystals
with increasing radical concentration. Although these authors could not establish a simple correlation between the EPR centers and the observed changes in the positron lifetimes, they prefer to associate their results with interactions of free radicals with orthopositronium (Ps) atoms. However, since the y irradiation of organic solids is known to cause in addition to the generation of free radicals macroscopic or structural changes in the solid, i.e., changes in free volume (swelling upon irradiation), vacancy formation, etc., to which the positron annihilation technique responds very strongly, we have studied the effect of y irradiation in a variety of organic solids to further assess the origin of the observed changes in the annihilation parameters upon y irradiation and to estimate the contribution made by the reaction of 0-Ps with free radicals to the total effect observed. For this purpose we included in this study four organic solids such as adamantane, guanosine, 2'-deoxyuridine, and 5-iodo-2'-deoxyuridine. Adamantane was chosen because it is unusually stable toward radiation and even after applying massive radiation doses (at room temperature), only relatively small amounts of free radicals can be detected. The other three compounds were selected because they are components of important biological systems and it appeared interesting to investigate whether positron annihilation techniques could reveal details about their behavior upon y irradiation, which could not be detected in ESR studies.
Experimental Section (A) Purity of Compounds. Adamantane was obtained from Aldrich and 2'-deoxyuridine, guanosine, and 5iodo-2'-deoxyuridine were obtained from ICN Life Sciences Group. They were of the highest purity available (>99%), and no further purifications were made. (B) Preparation o f Samples and Positron Lifetime Measurements. The solids were degassed and sealed under vacuum in quartz tubes. The irradiations were carried out a t room temperature with a I3'Cs y-ray source at Brookhaven National Laboratory at a dose rate of 7.8 Mrad/h for guanosine samples and 1.6-1.9 Mrad/h for others. The irradiated samples were left under vacuum for 14 days, until the amount of free radicals had assumed an approximately constant value. Then, aliquots of the samples were transferred under vacuum into specially designed vials together with a positron source consisting of 3--5 pCi 22Nadeposited on a thin A1 foil, which was placed in the center of the vial. The i.d. of the sample vial was 0.6 cm, which ensured that all of the positrons were annihilated in the sample before they reached the glass wall. All lifetime measurements were carried out at room temperature. The annealing of these samples was carried out under vacuum in an oil bath kept at 100 " C for a predetermined period of time. The positron lifetime measurements and their computational analysis were carried out by conventional methods as described previ0us1y.'~ The resolution of the fast-slow coincidence system was 0.4 ns as measured by the prompt spectrum of 6oCo. (C) ESR Measurements. A Varian E-12 spectrometer was employed to record the first-derivative spectra at a microwave frequency of approximately 9.08 GHz and 100 kHz magnetic modulation. The microwave powers were kept at 30 mW for all measurements. The standard DPPH spectra with known free-radical concentrations were recorded at exactly the same instrumental sensitivities for each sample measurement. The absolute free-radical concentrations were obtained in a standard way by comparing the ESR intensities of samples with DPPH standards. All ESR measurements were carried out at the
The Journal of Physical Chemistry, Vol. 82, No. 6, 1978 657 E&E
RADICAL CONCENTRATION RADIATIDN DOSE
w
+B
Q
ADAMANTANE 0 GUANOSINE V (ANNEALED, 100.C. 20h 0 1 2'-DEOXYURIDINE A (ANNEALED, IO&, ZOh A ) S-IOW-Z'-DEOXYURIDINE
W J
3
u W
RADIATION DOSE ABSORBED (Mrad)
Flgure 1. Free-radical concentration vs. radiation dose.
0
RADIATION DOSE (Mrad) 50 100 150 200
0.85
-'3 a80 I
-
"0
0.75
2 Q70
0
QT (IN
l
I
h
A 1
L
l Y
I
'
2
I
4 3 I
3
FREE RADICAL CONC. IOb RADICALS/MOLECULE)
Figure 2. I2 and X2 vs. free-radical concentrationand absorbed radiation dose in y-irradiated adamantane.
mean time of lifetime measurements at room temperature. For the identification of the ESR signals in the various compounds under investigation, see ref 14 and 15.
Results and Discussion Figure 1, where the radical concentration is plotted as a function of absorbed radiation dose, shows the expected smooth correlation between free-radical concentration and absorbed radiation dose. However, while in the case of adamantane and guanosine, a maximal concentration of about 2.7 and 10.5 radicals/molecule, respectively, is observed at 200 Mrad which, as shown for guanosine, can be almost completely annealed, the corresponding radical concentration in 2'-deoxyuridine and 5-iodo-2'-deoxyuridine is 170 and 100 radicals/molecule and can be, as shown for 2'-deoxyuridine, only partially annealed. From Figures 1-5 it can clearly be seen that the changes in X2 and I2 are relatively small, even after prolonged periods of X irradiation, and do not follow in a consistent way the variation of the free-radical concentration. The obvious divergence of the experimentally observed positron lifetime parameters, X2 and 12, in the four y-irradiated systems as shown in Figures 2,4, and 5 makes a unique interpretation in terms of the previously postulated
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The Journal of Physical Chemistry, Vol. 82, No. 6, 1978
Y. Jean and H. J. Ache
RADIATION DOSE (Wad)
RADIATION DOSE (Mrad) 0
50
I
I
AND ABSORBED RADIATION DOSE ( 0 ) IN y-IRRADIATED OUANOSINE
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I
-
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I
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4 6 8 1 0 FREE RADICAL CONC. (IN Idg RADICALS/MOLECULE)
“5
AND X ~ V SFREE RADICAL CONCENTRATION ( 0 ) AND RADIATION DOSE ( 0 ) I N y-IRRADIATED 2-DEOXY URIDINE
1,
0
Flgure 3. 1, and X, vs. freeradical concentrationand absorbed radiation dose In y-irradiated guanosine. EFFECT OF THERMAL ANNEALING ON 1, AND Ae AND FREE RADICAL CONCENTRATION IN y-IRRADIATED OUANOSINE (20 Yrod)
ae
“y 20
f FREE RADICAL CONC. (IO* RADICALS/MOLECULE)
10.7-2 7-1.2-1.8
0.6
1 0
1
1.8
1
I
I
1 1
I
20 30 40 50 ANNEALING TiME (HOURS AT IOO’C ) IO
Figure 4. Effect of thermal annealing on I2 and X2 and free-radical concentration in y-irradiated guanosine. (Absorbed radiation dose 200 Mrad).
interactions of positrons or 0-Ps with free radicals rather difficult. For example, in the adamantane system where the free radical concentration even after absorption of relatively high radiation doses is rather small, no change of I z can be observed as a function of free-radical concentration (Figure 2). On the other hand X2 shows a decrease with free-radical concentration, which is clearly the opposite of the effect one would expect from the most likely interaction between 0-Ps and paramagnetic species, namely the ortho-para spin conversion of the Ps, which should lead to a shortening of the positron lifetime, i.e., an increase of Xz.6-12 Thus, the observed effect, i.e., the decrease of Xz, cannot be associated with this process but may be most likely understood as the result of an increase of the free volume of the compound in which the 0-Ps can reside and experience a longer lifetime.16 This enlargement of the free volume, caused by the radiation-induced swelling of the solid, and its effect on the positron annihilation process clearly outweigh at these low radical concentrations the consequences of Ps reactions with the radicals present. In y-irradiated guanosine (Figure 3) the initial drop in Xz at low radiation doses or free-radical concentrations may have the same origin as discussed above for adamantane, whereas the subsequent drop of I2 and increase of Xz at
50
IO0
I50
FREE RADICAL CONCENTRATION ( lo6 RADICALSIMOLECULE) Figure 5. I , and X2 vs. free-radicai concentration and absorbed radiation dose in y-irradiated 2‘-deoxyuridine.
higher free-radical concentrations may now be the result of the interaction of “hot”2-4J7J8and thermalized 0-Ps, respectively, with the radicals present, which at these higher radical concentration overcompensate the freevolume effect. The decrease in I z could, however, also be explained in terms of the simple spur reactionlgor modified spur reaction mode120 by assuming a reduction in the number of available electrons for Ps formation due to fast reactions between radiation-produced species (radicals) and electrons in the positron spur, thus competing for available electrons with the Ps formation process. The thermal annealing of a sample which had received 200 Mrad leads to an almost complete elimination of the paramagnetic centers, (Figure 4) from initially 10.7 to 1.2-1.8 radicals/molecule. This is accompanied by a change in the positron annihilation data, which assume values coinciding with those obtained at about 30 Mrad; Le., where positronium-radical reactions seem to have no significant impact. Thus, it appears that while thermal annealing at 100 “C can eliminate most of the paramagnetic species produced, it cannot restore the initial structure of the organic solid. It would be difficult to suggest such a simple interpretation for the 2’-deoxyuridine system, (Figure 5) where I2 and hz,both increase with free-radical concentration or absorbed radiation dose. This compound is very sensitive to radiation and the damage caused by the large radiation doses applied in this study has probably generated some profound structural changes in the compound and produced a considerable amount of stable radiolysis products whose interactions with Ps lead to the observed variations in the positron annihilation data. This is also supported by the fact that annealing of the compound (100 “C for 20 h) does not restore the initial Iz or X2 values, nor does it lead to a complete elimination of the paramagnetic centers. An interpretation of the positron lifetime data in this system would require a much more detailed knowledge of the various species present after irradiation and is, because of lack of such data, presently not feasible. The analysis of the positron lifetime spectra of 5iodo-2’-deoxyuridine, is complicated by the fact, that the long-lived component has an intensity of only 1-2%, which introduces a large experimental error in the evaluation of X2 and Iz. The measurements reveal only slight increases, even after large radiation doses have been absorbed, in Iz
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y-Irradiation of Organic Crystals
and X, (0 Mrad: 1, 1.2%; X2 0.62 X lo9 s-l; 100 Mrad: I2 2.7%, X2 0.72 X lo9 s-’). The explanations suggested above are essentially based on the model that the long-lived component in the lifetime spectra, with the decay rate X2 and an intensity 12,is the result of the thermalized 0-Ps formed in the substance and its subsequent interactions with its environment. These explanations may, however, have to be qualified if one considers the results of recent experiments by Hogg , ~ seem ~ to indicate et a1.21p22and Chuang and T ~ owhich that the long lifetime component in positron annihilation lifetime spectra for organic solids may not be due to 0-Ps. These authors23prefer to associate this component with the annihilation of “free” positrons trapped in void-type defects of large size in the solid, in which case the size of the void would control the positron annihilation X2 and a correlation should exist between the number of the positrons annihilating in these voids and 12. Since it can be assumed that the size and number of these voids depend on the radiation conditions, it is not suprising that X2 and 1, are affected by the irradiation. In summary, the experimental results obtained in this study seem to confirm the contention that values I , and h2 extracted from positron annihilation lifetime measurements in y-irradiated substances cannot always be directly related to the presence of free radicals. It seems that whatever interaction between paramagnetic species and Ps or positrons in irradiated solid organic compounds occurs, it could be overshadowed by other parameters such as free volume and other structure changes introduced as a result of the irradiation, to which the positron annihilation process responds more directly. This appears to be especially true in cases where only a relatively small amount of radicals is formed as a result of massive radiation, as in the adamantane system, or where a rather complicated radiation chemistry results as a consequence of the irradiation as in 2’-deoxyuridine. Thus, one would have to conclude that the positron annihilation technique
The Journal of Physical Chemistry, Vol. 82, No. 6, 1978 659
cannot compete with ESR methods in the detection of paramagnetic species in irradiated solids because of its relative insensitivity, its susceptibility to interferences caused by structural changes in the substances, and its apparent failure to distinguish between different types of radicals. Its usefulness will, therefore, most likely be limited to the detection of paramagnetic centers in gaseous systems, where such interferences do not exist and where its sensitivity is dramatically increased, as recently shown by Brandt et al.24
References and Notes Work partially supported by the U S . Energy Research and Development Administration. J. H. Hadley, Jr. and F. H. Hsu, Chem. Phys. Lett., 7, 465 (1970). J. H. Hadley, Jr. and F. H. Hsu, Chem. Phys. Lett., 12, 291 (1971). M. Eldrup, E. Lund-Thomsen, and 0. Mogensen, J . Chem. Phys., 56, 4902 (1972). Y. Ito and Y. Tabata, Bull. Chem. SOC. Jpn., 48 808 (1975). J. Green and J. Lee, “Positronium Chemistry”, Academic Press, New York, N.Y., 1964. V. I. Goldanskii, At. Energy Rev., 8, (1968). J. D. McGervey in “Positron Annihilation”, A. T. Stewart and L. 0. Roellig, Ed., Academic Press, New York, N.Y., 1967, pp 143-154. J. A. Merrigan, S. J. Tao, and J. H. Green in “Physical Methods of Chemistry”, Vol. I, Part IIID, A. Weissberger and B. W. Rossiter, Ed., Wiley, New York, N.Y., 1972, pp 501-586. H. J. Ache, Angew. Chem., Int. Ed. Engl. 11, 179 (1972). J. H. Green, MTPPnt. Rev. Sci., 8, 251-290 (1972). V. I. Goldanski and V. G. Firsov, Annu. Rev. Phys. Chem., 22, 209 (1971). See, e.g., T. L. Williams and H. J. Ache, J . Chem. Phys., 50, 4493 (1969). D. R. Gee, L. Fabes, and J. K. S. Wan, Chem. Phys. Left., 7, 31 1 (1970). A. Muller, Prog. Biophys. Mol. Biol., 17, 99 (1967). W. Brandt and I. Spirn, Phys. Rev., 142, 231 (1966). L. J. Bartal and H. J. Ache, J. Phys. Chem., 76, 1124 (1972). L. J. Bartai and H. J. Ache, Radiochim. Acta, 19, 49 (1973). 0. E. Mogensen, J . Chem. Phys., 60, 998 (1974). S. J. Tao, Appl. Phys.. 10, 67 (1976). D. P.Kerr, S. Y. Chuang, and B. G. Hogg, Mol. Phys., 10, 13 (1965). G. Deblonde, S. Y. Chuang, B. G. Hogg, D. P. Kerr, and D. M. Miller, Can. J . Phys., 50, 1619 (1972). S. Y. Chuang and S. J. Tao, Appl. Phys., 11, 247 (1976). W. Brandt and D. Spektor, Phys. Rev. Lett., 38, 595 (1977).
