REACTIONS OF 40-KEV TRITIATED IONS WITH SOLID ORGANIC COMPOUNDS
427
Reactions of PO-kev Tritiated Ions with Solid Organic Compounds
by Sergio Ascoli, Fulvio Cacace, Giordano Giacomello, and Elvira Possagno Centro d i Chimica delle Radiazioni del C.N.R., Istituto d i Chimica Farmaceutica dell Universitd, Rome, Italy (Receiued September 97, 1966)
The labeled products formed from the reactions of 40-kev tritiated ions with solid organic targets, such as sodium m-iodobenzoate, sodium phenylacetate, and sodium benzoate, have been determined. The results indicate that the radiochemical yield and the i n t m molecular tritium distribution correspond within the analytical errors with the values obtained in experiments involving the use of the much more energetic recoil tritons, as expected on the basis of the current theories on the ‘(hot” chemistry of tritium. In addition, it has been experimentally demonstrated that mono- and polyatomic tritiated ions react with a given target to give the same products, a result consistent with the view that high-energy polyatomic species dissociate into the constituent atoms in their initial collisions with the target molecules, before their energy is sufficiently lowered to make the formation of stable products possible.
Introduction Most of the investigations in the field of “hot” chemistry so far have been carried out using nuclear reactions as a source of the high-energy species. I n 1954, Croatto and Giacomello’ described a different approach, subsequently employed by a number of other based on the use of radioactive ions accelerated with an energy of a few tens kev against solid targets. This “beam technique,” while similar to recoil methods as to the kinetics, analytical procedures, and limitations, offers, in principle, two advantages: the possibility to v : q over a rather extended range the energy of the “hot” species and to reduce the radiation damage to the target, eliminating the unwanted effects of the extraneous radiations invariably associated with the nuclear production of “hot” atoms. According to the prevailing theories,8 no significant difference should exist between the chemical reactivity of a “hot atom’’ produced by nuclear means, with an energy up to several MeV, and the corresponding ion electrically accelerated to several tens kev. In fact, both the recoil “atom” and the ion must collide many times with the target molecules before their energy is sufficiently lowered to make the formation of chemical bonds possible. I n the slowing down process, both the reactive species necessarily cross the same energy region
and therefore the charge-exchange processes should play the same role in both cases. Therefore, independently of the efficiency of such charge-exchange processes,8 one should expect that both recoil tritons and electrically accelerated ions are in the same charge state when they reach the energy region where they can undergo combination, through reactions whose study is the objective of (‘hot” chemistry. From the generally accepted views on the chemically significant energy range and charge state, another consequence follows: polyatomic “hot” species, for instance T2+or T3+ions, should dissociate in their initial collisions with the molecules of a given target to give ((hot” atomic species, whose reactions are expected to
(1) U. Croatto and G. Giacomello, Atti del XLV Congress0 della SIPS, Naples, 1954. (2) B. Aliprandi, et al., Ric. Sei., 26, 3029 (1956). (3) B. Aliprandi and F. Cacace, Ann. Chim. (Rome), 46, 1204 (1956). (4) B. Aliprandi, et al., “Radioisotopes in Scientific Research,” Vol. 2, Pergamon Press Inc., New York, N. Y . , 1958, p 146. ( 5 ) F. Cacace, et al., Energia Nucleare, 5, 387 (1958). (6) R. M. Lemmon, et al., J . Am. Chem. SOC.,78, 6415 (1956). (7) R . M. Lemmon, et al., “Chemical Effect of Nuclear Transformations,” Vol. 2, International Atomic Energy Agency, Vienna, 1961, p 27. (8) R. Wolfgang, Progr. Reaction Kinetics, 3, 106 (1965).
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be identicd with the reactions of monoatomic ions of equivalent energy. No experimental evidence regarding the chemical equivalence of a recoil atom and the corresponding accelerated ion has been so far obtained, nor has the behavior of any polyatomic "hot" species been investigated. It was decided to undertake the comparative study of the reactions of recoil tritons, having a maximum energy of 2.7-3Iev and 40-kev tritium-containing ions, such as T + and H T f , with the same solid targets, at carefully controlled levels of radiation damage. The isotope separator recently built at Rome University9P1Owas used as a source of accelerated ions.
Experimental Section Mateyials. The inactive compounds used were reagent grade chemicals. The m-iodobenzoic acid was prepared from m-aminobenzoic acid" and crystallized from acetone to a melting point of 187". Tritium gas from C.E.A., France, was employed in the ion source of the isotope separator, neat and mixed with neon. I~radiation. All of the acids were irradiated in the form of the corresponding sodium salt, owing to the necessity of' using compounds of sufficiently low volatility. The solid targets were prepared by evaporation of concentrated alcoholic solutions of sodium benzoate, sodium phenylacetate, and sodium m-iodobenzoate on metallic plates (approximately 10 X 25 cm), in such a way as to obtain a uniform layer about 1 mm thick. The targets were thoroughly dried under vacuum before being mounted in the bombardment chamber of the isotope separator, on a support that could be rotated away from the ion beam a t the beginning of the experiment during the necessary adjustments of the separator. The tritiated ions were obtained from tritium gas or from appropriate tritium-neon and tritium-hydrogen mixtures. I n order to avoid an excessively concentrated beam and therefore locally intense radiation damage to the target, the beam was swept onto the target surface making use of a suitable modulation of the acceleration voltage. The intensity of the beam was adjusted at the beginning of the irradiation and continuously monitored with a model 610 B Keitley electrometer. Typical current intensities ranged from to lo-' amp, with irradiation times from 2 to 60 min, the conditions chosen being, of course, a compromise between the need to introduce into the target an activity sufficient The Journal of Physical Chemistry
S. ASCOLI,F. CACACE, G. GIACOMELLO, AND E. POSSAGNO
for the subsequent analysis and the necessity to keep the radiation damage of the target surface a t the lowest level.
Analysis of the Products Purijcation. After the irradiation, the organic sample was recovered from the metallic support, the appropriate carriers were added, and the purification procedure was undertaken. The benzoic, phenylacetic, and m-iodobenzoic acid sodium salts were dissolved in water, the solution was acidified, and the crude acids were repeatedly crystallized before being converted into the corresponding methyl esters with an excess of diazomethane in ether. The esters were purified by preparative gas chromatography using a Fractovap Model B from SOC. Carlo Erba, Milan, equipped with stainless steel columns. The following conditions were chosen for the purification of the aromatic esters: methyl benzoate and methyl phenylacetate: 1-m long column, packed with Bentone 34 (25Oj, w/w on Celite) at 160" ; methyl m-iodobenzoate : 2-m long column, packed with sodium dodecyl benzyl sulfonate (25% w/w on Chromosorb) at 165". I n order to avoid the mixing of the eluted fractions and their contamination with traces of the stationary phase, separate glass collection traps were directly inserted into the detector block of the gas chromatograph. l 2 Helium was used as the carrier gas in all of the separations. Intramolecular Tritium Dislribufion. The purified phenylacetic acid (I) was degraded with permanganate in alkaline solution to benzoic acid, which was in turn converted to the methyl ester with diazomethane and purified by preparative glpc (11). The difference between the molar activity of I1 and I gave the tritium content of the methylene positions. The benzoic acid was nitrated with concentrated nitric acid at 20-30". The 0- and m-nitrobenzoic acids formed were converted to the corresponding methyl esters. They were separated and purified by preparative glpc using the sodium dodecyl benzyl sulfonate column at 190". The molar activity of the 0- and m-nitrobenzoates, subtracted from the molar activity of the methyl benzoate, gave the tritium content of the ortho and meta positions in the ring. Control experiments were undertaken when required to check that no signifi(9) S. Ascoli, et al., Ric. Sci., 35 (11-A), 15 (1965). (10) S. Ascoli and F. Cacace, Xucl. Instr. Methods, 38, 202 (1965). (11) F. Cattelain, Bull. SOC.Chirn. France, (4)41, 1547 (1918). (12) B. Aliprandi, F. Cacace, and G. Ciranni, Anat. Chem., 36,2445 (1964).
REACTIONS OF 40-KEVTITRIATED IONS WITH SOLIDORGANIC COMPOUNDS
cant tritium exchange occurred in the degradation and purification procedures. Tritium Actiaity Assay. The activity of the purified samples was determined according to the procedure described by Wi1~bach.l~The samples were combusted over zinc and nickel oxide at 650°, obtaining a mixture of HT and CHsT, whose activity was determined with a 250-ml ionization chamber connected to a Victoreen RIodel475 A vibrating-reed electrometer. No difficulty was experienced in the quantitative combustion of the aromatic compounds analyzed according to Wilzbach’s original procedure. Results and Discussion I n order to verify through the analysis of the final products the identical nature of the reactions of two different “hot” species with the same target molecule, it is desirable to conipare the absolute yields of all the labeled products formed and to determine the intramolecular distribution of the radioactive atoms within the major products. Unfortunately, such a comparison is difficult in the present case, owing to the fact that the bombardments with accelerated ions are carried out at pressures of torr and the volatile products are therefore likely to escape from the target, to be lost in the vacuum pumps of the isotope separator. While a technique to trap and recover volatile products from the reaction of accelerated ions, using solid adsorbents and differential pumping has recently been described,14 tho quantitative recovery of the HT, the main gaseous product, is quite difficult and requires the development of different and more elaborate irradiation facilities. The comparison therefore had to be restricted to compounds of low volatility and the logical choice was to measure the radiochemical yield of the labeled parent molecule. Again, the determination of the absolute yield of the products obtained from the bombardment of a solid target with tritiated ions is far from accurate, since the measurement of the ion current impinging on the target is affected by considerable errors, mainly associated with the emission of secondary electrons from the target surface. It was therefore concluded that the ratio of the yields of the major reaction products, instead of the absolute yields, could afford a suitable criterion for a comparison between the reactions Of nucleogenic and electrically accelerated tritons. The results obtained are comDared. in Tables I and values reported by White 11, \?,ith the and Ro~vland’~ and Elatrash and JohnsenlGin their study
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of the interactions of recoil tritons with, respectively, m-iodobenzoic acid and phenylacetic acid.
Table I : Irradiation of m-Iodobenzoic Acid with Recoil and 40-kev sH+ Ions 2.73-Mev recoil tritons”
Radiochemical yieldc of the parent Ratio of the benzoic and miodobenzoic acid yields Intramolecular tritium distribution in benzoic acid : ortho positions meta positions para position
IO-kev *H+ ionsb
0.36 f 0 . 3 0.40
2&3 86 f 3 12 f 3
-30d 0.43 f 0.02
6 . 2 f2 82.5 f 2 11.2 f 4
Data from ref 15. Irradiation of sodium m-iodobenzoate. Fraction of the total tritium activity introduced into the system found in the parent molecule. d See text. (I
Table 11: Irradiation of Phenylacetic Acid with Recoil Tritons and 40-kev Tritiated Ions Intramolecular distribution
2.73-Mev recoil tritons“
T ionsb
IO-kev HT + ionab
ortho positions meta positions para position a position
37.0 37.6 23.4 2.0
41.2 f 3 36.8 f 3 18.8 f 5 3.6 f 1
33.8 f 3 36.8 f 3 25.9 f 5 4.2 f 2
IO-kev +
Data calculated from ref 16 without taking into account the labile activity. The accuracy of the activity distribution is probably similar to that given in ref 15. Irradiated as sodium phenyl acetate. 0
Table I shows that the ratios of the yields of the benzoic and m-iodobenzoic acids obtained, respectively, in the experiments with accelerated ions and recoil tritons are very close. Further, the intramolecular tritium distribution in the benzoic acid is practically identical, within the rather large experimental errors involved in this type of analysis. The close correspondence between the tritium content in the meta positions of the labeled benzoic acids (13) K. E, Wilsbach, et al., Science, 118, 522 (1953).
(14) J. M, Paulus and J. P. Adloff, Radwchim. Acta, 4, 146 (1965). (15) R. &I. White and F. 5. Rowland, J. Am. Chem. SOC.,82, 4713 (1960). (16)A. M.Elatrash and R. H. Johnsen, “Chemical Effects of Nuclear Transformations,” Vol. 2, International Atomic Energy Agency, Vienna, 1961,p 123.
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obtained in both types of experiments seems particularly noticeable. I n addition, the peculiar feature of the intramolecular distribution obtained from the recoil experiments, Le., the rather large percentage of activity contained outside the position originally occupied by the iodine atom, is faithfully reproduced. This feature has been explained16 with the “hot” substitution of T for H at an adjacent ring position) followed by intramolecular hydrogen shift and displacement of the iodine atom, a mechanism that is likely to depend to a large extent on the energy of the tritium atoms responsible for the replacement reactions. The similarity of the distribution obtained in the experiments carried out with 40-kev ions provides further evidence that the energy level of the species responsible for the substitution processes is the same in both cases. As to the phenylacetic acid, Table I1 shows that there is a striking similarity (within the rather large experimental errors) in the intramoIecular tritium distribution obtained with recoil tritons by Elatrash and Johnsen’G and in the present irradiation with 40-kev T + ions. The irradiation of sodium phenylacetate with zt biatomic “hot” species, i.e., 40-kev H T + iuns, gdve the distribution shown in the last column of Table 11. A comparison with the second co1ui::ti of the same table shows that the intramolecular tritium distribution is reasonably close in experiments carried out with T+ and H T + ioiis having the same energy. The source of the discrepancies observed cannot, however, be safely traced to experimental errors only, and a more precise determination was therefore deemed necessary in order to establish on a firmer experimental
The Journal of Physical Chemistry
S. ASCOLI,F. CACACE, G. GIACOMELLO, AND E. POSSAQNO
basis the equivalence of mono- and polyatomic accelerated ions. Table I11 shows the results of such experiments carried out with 40-kev T+ and H T + ions accelerated on a sodium benzoate target. A considerable effort was made to improve the accuracy of the analysis, essentially by increasing the activity contained in the labeled benzoic acid. This was achieved combining the irradiated targets from a large number of runs until a sufficient amount of crude material was obtained to
Table I11 : Irradiation of Sodium Benzoate with 40-kev Ttand HTf Ions Intramolecular distribution
T + ions
40-kev HT + ions
ortho positions meta positions para position
38.2 i2 32.4 f.2 29.4 f.2
39.0 f.2 32.6 f.2 28.4 zk 2
40-kev
carry out the subsequent purification and degradation steps without the need to dilute the sample with inactive sodium benzoate. These improved experiments demonstrate beyond any doubt that both types of ions given an identical intramolecular distribution within the parent compound. As a whole, the present results indicate that radioactive ions accelerated at relatively low energy can afford a useful and versatile tool in the study of “hot” atom chemistry.
Acknowledgments. The authors are indebted to B. Aliprandi for the analysis of several irradiated samples and to D. Carrarn for his skillful assistance.
