240
J. Phys. Chem. 1902, 86, 248-251
Temperature Effect on Recoil Tritium Reactions in Solid Alkanes at 20-300 K. Comparison of Recoil T Atoms with H (D) Atoms in y Radiolysis Yasuyukl Aratono, Enzo Tachlkawa, Division of Chemistry, Japan Atomic Energy Research Institute, TokaCMura, Ibaraki 3 19- 11, Japan
Tetsuo Mlyazakl,' Yuklhlro Kawal, and Kenjl Fuekl Department of Synthetlc Chemistry, Faculty of Engineering, Nagoya University, Chikusa-ku,Nagoya 464, Japan (Received: May 4, 198 1; In Final Form: September 29, 198 1)
Hydrogen atom abstraction by recoil T atoms in neopentane and decane-dz2has been studied at 20,77,195, and 300 K by means of ESR spectroscopy and radiogas chromatography. The results are compared with the reaction of H (or D) atoms produced by y radiolysis. When the experiments are conducted at 77 K, the reaction of recoil tritium atoms in the neo-C5H12-i-C4HgD(2 mol 70)and n-ClJ122-n-CloH22(10 mol %) mixtures do not parallel those of H and D atoms generated by y irradiation, whereas the results at other temperatures below and above 77 K (20,195, and 300 K) are more nearly comparable. The different results at 77 K are attributed to the ability of H and D atoms but not T atoms to diffuse and react with iC4H9D (or n-Cldn) solute molecules. The failure of the thermal diffusion of the T atoms at 77 K is explained by a model in which nearly all of the recoil T atoms react either by hot reaction or have a high probability of reacting with a fragment near the end of the path and only a few percent of them diffuse into the bulk matrix.
Introduction Tritium technology in nuclear fusion reactors requires the elucidation of the behavior of hot tritium atoms in solid materials. So far many studies have been undertaken on hot tritium reactions in the gas phase,l but many fewer studies have been reported for solid organic compounds. The studies of recoil tritium reaction in the solid phase at low temperatures were undertaken previously at 195 K in several alkanes,2 77 K in cyclobutane-de3 and neopentanea4 The study at such a very low temperature as 20 K has been limited only to biological substances and was performed by Kusama et al.5 Recently the behavior of H (or D) atoms produced by the y radiolysis of alkanes and the photolysis of hydrogen halides, has been extensively studied in the solid alkanes.66 When H atoms are produced by the radiolysis of n e ~ - C & ~ containing a small amount of i-C4H9Dat 77 K, they migrate through the solid matrix and abstract D atom selectively from the solute, resulting in the remarkable (1) (a) Wolfgang, R. h o g . React. Kinet. 1965,3,97; (b) Rowland, F. S. M T P h t . Rev. Sci., Phys. Chem. Ser. 1972, 9, 109; (c) Urch, D. S. MTP Int. Rev. Sci., Znorg. Chem. Ser. One 1972, 8, 149. (2) Menzinger, M.; Wolfgang, R. J . Phys. Chem. 1968, 72, 1789. (3) Hosaka, A.; Rowland, F. S. J . Phys. Chem. 1971, 75, 3781. (4) (a) Aratono, Y.; Tachikawa, E.; Miyazaki, T. Radiat. Phys. Chem. 1979,13,115; (b) Aratono, Y.; Tachikawa, E.; M i y d i , T.; Sakurai, M.; Fueki, K. Bull. Chem. SOC.Jpn. 1981,54, 1627. (5) Kusama, K.; Fujisawa, K.; Ichihara, Y. The 20th Japanese Conference on Radiochemistry, Shizuoka, Japan, 1976. (6) (a) Wakayama, T.; Miyazaki, T.; Fueki, K.; Kuri, Z. J. Phys. Chem. 1973, 77,2365; (b) Miyazaki, T.; Hirayama, T. J. Phys. Chem. 1975, 79, 566; ( c ) Kinugawa, K.; Miyazaki, T.; Hase, H. Radiat. Phys. Chem. 1977, 10, 341; (d) Miyazaki, T.; Kasugai, J.; Wada, M.; Kinugawa, K. Bull. Chem. SOC.Jpn. 1978,51,1676; (e) Miyazaki,T.; Guedes, S. M. L.; Fueki, K. Bull. Chem. Soc. Jpn. 1980,53,1813; (0Miyazaki, T.;Wakahara, A.; Kimura, T.; Fueki, K. J. Phys. Chem. 1981,85,564. Other related papers are cited therein. (7) (a) Perkey, L.; Willard, J. E. J. Chem. Phys. 1974,60, 2732; (b) Wilkey, D. D.; Willard, J. E. J. Chem. Phys. 1976,64, 3976; (c) Adita, S.; Wilkey, D. D.; Wang, H. Y.; Willard, J. E. J. Phys. Chem. 1979, 83, 599; (d) Wang, H. Y.; Willard, J. E. J. Phys. Chem. 1979,83,2585. Other related papers are cited therein. (8)(a) Iwasaki, M.; Toriyama, K.; Muto, H.; Nunome, K. J . Chem. Phys. 1976,65,596; (b) Iwasaki, M.; Toriyama, K.; Nunome, K.; Fukaya, M.; Muto, H. J. Phys. Chem. 1977,81,1410; (c) Iwasaki, M.; Toriyama, K.; Muto, H.; Nunome, K. Chem. Phys. Lett. 1978,56, 464.
0022-3654/82/2086-0248$0 1.2510
formation of t-C4Hgradicals and HD.6a,bidThese atoms are now termed diffusive atoms. A striking temperature effect was observed in the selective hydrogen atom abstraction. The reaction was suppressed at 4,6c,8 195,6band 300 KtBbwhile it occurred effectively at 77 K.6 A similar temperature effect on the selective hydrogen atom abstraction was also observed in the n-C,oD2z-n-CloH22 mixture^.^*^ The selective hydrogen atom abstraction is effectively caused by both H and D atoms; there exists no defiiite isotope effect between them." Thus, the T atoms with energies similar to those of atoms produced by the radiolysis should also react selectively with alkane solutes at 77 K, while the selective reaction should not take place at 4, 195, and 300 K. The recoil tritium atoms produced by the 6Li(n,a)T reaction have an initial energy of 2.7 MeV. After a sequence of collisional energy losses, they enter the energy range in which they react. However, a certain fraction of them continue to escape reactions and finally become thermalized T atoms. Effects of scavengers, such as O2 and Br2, on the recoil T atom reaction in CH4,10C2H6,'l i-C4H10,1an-C4H10,2 and n-C5H12 systems indicate that 30-40% of the total recoil T atoms are thermalized in the gas phase. In the cooling processes down to thermal energy, T atoms must pass through an energy range of 2-3 eV which corresponds to initial energies of hydrogen atoms formed in the y radiolysis. It may be expected that most of the recoil T atoms in the solid alkane would react either by hot displacement or abstraction reactions or would have a high probability of reacting with a fragment formed near the end of the path, thus precluding diffusion at 77 K such as occurs for nonrecoil H atoms. In this case, little temperature effect on the recoil T atom reaction may be expected. Since there has been no experimental study on the amounts of the diffusive T atoms in the solid alkane, the temperature effect on the recoil T atom reaction has been (9) Claesson, 0.;Lund, A. Chem. Phys. Lett. 1977, 47, 155. (IO) See, D.; Wolfgang, R. J. Chem. Phys. 1967,47, 143. (11) (a) Urch, D. S.; Welch, M . J. Trans. Faraday SOC.1966,62,388; (b) Baker, R. T. K.; Wolfgang, R. Trans. Faraday SOC.1969, 65, 1153.
0 1982 American Chemical Society
Temperature Effect on Recoil Tritium Reactions
The Journal of Physical Chemistty, Vol. 86, No. 2, 1982 249
studied here in the solid phase at 20,77, and 195 K, and in the liquid phase at 300 K in the neopentane and decane-dz2solvents.
Experimental Section Natura1 LiF, used as a target for 6Li(n,a)T,was purchased from the Johnson Mathey Chemicals. Mass spectroscopic analysis showed that the 6Li/ ('Li + 6Li)ratio in the LiF is 0.04. 6Li-enriched LiF, prepared from metal 6Li, has the 6Li/ (7Li+ 6Li) ratio of 0.95. Neo-C5H12and n-CloH22were 99.9 mol % and 99 mol % , respectively, in purity. The D content of n-CloD22,supplied by Merck Sharp and Dohm, Ltd., is 99%. At least 95% of the isobutane-2-dl (i-C4H9D)was correctly labeled with the deuterium atom at the tertiary position. A sample was sealed into a quartz cell.4 The neutron irradiation at 20 K was performed by a low-temperature irradiation facility of the KUR reactor of Kyoto University.12 Refrigerated helium gas is delivered to the irradiation cryostat. In order to cool the sample sufficiently, helium gas at 0.5 atm was also sealed into the cell. The thermal neutron flux and the dose rate of y rays from the reactor were 8.0 X 1O'O n cm-2 s-l and 1.1X lo6 rad h-l, respectively. The irradiation time was 54 h.13 At the end of the irradiation, the cells were immersed in liquid N2 to make them ready for the ESR measurement. The neutron irradiations at 77,195, and 300 K were done in the JRR-4 reactor of the Japan Atomic Energy Research Institute. The thermal neutron flux and the dose rate of y rays from the reactor were 3 X 1013 n cm-2 s-l and 2 X lo8 rad h-l, re~pective1y.l~The irradiation time was 15 s. The irradiation at 77 K was performed as follows. The sealed cell was inserted into a polyethylene capsule packed with powdery dry ice. Subsequently, the capsule was immersed in liquid N2 to equilibration and then immediately irradiated in the pneumatic tube in the reactor. The sample temperature was kept near 77 K during the irradiation for 15 s.4bThe irradiation of the sealed cell at 195 K was performed in a polyethylene capsule packed with powdery dry ice. The free radicals produced in the irradiated samples were measured at 77 K with a JEOL-PE3X ESR spectrometer at a microwave power level of 0.2 mW, which did not result in saturation of the signals of alkyl radicals. Tritiated products were analyzed by radiogas chromatography. A 5-m ferric oxide y alumina column at 77 K was used for the separation of HT and DT, and a 3-m parapak Q column at 380 K was used for organic products. Results ESR Spectra of Irradiated neo-C&l12-i-C4H& (2 mol %)-LiF Mixtures. There is some possibility that the temperature of the sample may increase during the irradiation in a reactor, though the temperature of the sample holder is kept at 20 K or near 77 K. In order to check the temperature, ESR spectra of the sample were measured. Figure l a shows the ESR spectrum of a neo-C5H12-iC4HgD(2mol %)-LiF mixture irradiated at 20 K in the reactor. The spectrum consists of a triplet with a hyperfine (12) Shibata, T.; Iwata, S.; Yoshida, H.; Nakagawa, M.; Okada, M. Ann. Rept. Res. Reactor Inst. Kyoto Uniu. 1969,2, 89. (13) The y dose to the sample at 20 K was 70 times higher than that at 77 K. Thue, it may be possible that trapped radicala produced by the y irradiation at 20 K effect the reaction of T atoms. The trapped radicals in the neo-C&-i-C,HgD mixtures at 20 K are only neo-C5Hl, radicals. Since the yields of neo-CallT at 20 K are approximately the same as those at 77 K (6. Table I), the recombination reaction between a T atom and the trapped neo-C6H11radical probably plays a minor role in the T atom reactions at 20 K. (14) Aratono, Y.; Tachikawa, E. J. Znorg. Nucl. Chem. 1977,39,555.
U
I!
I) Figure 1. a, ESR spectrum of neo-C,H,,-i-C,H,D (2 mol %)-LiF mixture irradiated in a reactor at 20 K. The dotted line represents a typical spectrum of a neo-C,H,, radical. b, ESR spectrum of neoC,H,-i-C,H,D (2 mol %)-LiF mixture irradiated in a reactor at 77 K. The ESR measurements were performed at 77 K.
splitting constant of 22.1 G, indicated by arrows (l), and an unsymmetrical spectrum, indicated by broad arrows the latter is due to a color center of irradiated quartz. The broad triplet lines disappear on annealing the irradiated sample at 195 K, while the unsymmetrical spectrum does not change during the annealing. The spectrum of the broad triplet coincides with the typical spectrum of the neo-C5H11radical, shown by a dotted line, and thus it is ascribed to that of the neo-C5Hll radical. A similar ESR spectrumkPsb is obtained by y irradiation of the neoC5Hl2-i-C4Hl0(2 mol %) mixtures at 4 K. The similarity is due to the fact that the sample in the reactor is exposed to a high dose of y rays as well as recoil T atoms with 2.7 MeV and a particles which also cause radiation chemical effects. The neo-C5Hfl radicals in Figure l a are formed in the following reaction sequence neo-C5H12 neo-C5Hll + H (1) H neo-C5H12 neo-C5Hll + H2 (2) and amount to more than 95% of the total radicals formed. The H atoms do not migrate at 20 K, but react promptly with neo-C5H12. Figure l b shows the ESR spectrum from the reactor irradiation of neo-C5H12-i-C4H&(2 mol %) at 77 K. The spectrum is quite different from that irradiated at 20 K. The octet with a splitting 'constant of 22.6 G, indicated by arrows (j), is attributed to t-C4Hgradical. The t-C4Hg radicals are formed by the selective hydrogen atom abstraction reaction by mobile H atoms produced from the radiolysis of neopentane (Reactions 1 and 3).6 H + i-C4H& HD + t-CdHg (3) Therefore the temperature change from 20 to 77 K in the reactor causes a drastic effect on the reaction of the H atoms. Yields of Tritiated Products. In order to examine the temperature effect on the recoil T atom reaction, tritiated products were analyzed by radiogas chromatography. 6LiF was used as a T atom source at 77, 195, and 300 K for a quantitative measurement of DT. There was no appreciable difference in the product distributions between 6LiF and natural LiF. Figure 2 shows a typical radiogas chro-
(4);
+
--
-+
250
The Journal of Physical Chemisfry, Vol. 86, No. 2, 1982
Aratono et al.
TABLE I : Yields o f Tritiated Products o f Recoil Tritium Reactions in Neopentane" relative yields, % sample neo-C,H12b neo-C,H,,-i-C,H,D neo-C,H,,-i-C, H, D neo-C,H,-CC,H,D neo-C,H,,-i-C,H,D
( 2)b (2)' (2)" (2)"
temp, K
HT
DT
20 20 77 195 300
43 42 47 46 40
0.33d 1.1 0.24 0.33d
C H A T C,H,T
0
16 17
11 11 11
0.4 0.4 0.1 0.1 0.1
'
C,H,T
C,H,T
C,H,T
C,H,T
C,H,T
C,H,,T
1.0
0.7 0.8 0.2 0.3 0.3
0.9 0.5 0.2 0.2 0.2
6.2 6.3 4.2 6.3 6.1
2.5 2.8 4.1 4.4 4.0
29 29 32 32 37
1.1 0.4 0.5 0.6
Natural Concentration of i-C,H,D is 2 mol %. The yields are mean values of three runs and t h e errors are about 10%. LiF (0,05 g i g of solvent) and helium gas a t 0.5 a t m are added t o t h e sample. The DT yields are mean values of two runs.
'
TABLE 11: Yields of Tritiated Products o f Recoil Tritium Reactions in Decane-d,,' relative yieldb sample
temp, K
DT
HT
~-C,oD2,;~-C1oH,, (10)" n-C,oD,, n-C,o D,,-n-C,oH,, (10 1 n-C10D2zd ~-C,0D,,-~-C,0H,,(10) n-C10D2,d n-C10D2,-~-C10H,, (10)
20 77 77 195 195 300 300
1.00 1.00 1.00 1.00
0.23
1.00
0.28
1.00 1.00
0.04
"
0.05 0.29
0.04 0.27
CD,T 0.11 0.09 0.09 0.07 0.09 0.05 0.05
C,D,T 0.10 0.15 0.15 0.05 0.07 0.04 0.05
C,D,T 0.09 0.10 0.10
C A T 0.08 0.06 0.06
0.06
0.05
0.09 0.04 0.04
0.05 0.03 0.04
Concentration of n-CloH,, is 10 mol %. Natural LiF (0.04 g/g of solvent) is added to all samples. The yields are mean The yield of DT is normalized to 1. Helium gas a t 0.5 a t m is added t o values of 2-4 runs and errors are about 10%. n-CloDZ2 contains the H equivalent to 1%C,,H,, distributed as partially hydrogenated decanes, such as the sample. sample. Cl0D2,H. Thus a small amount of HT is formed in the n-CloD22
'
paper will be confined only to the abstraction reaction which is one of the main processes in the T atom reactions and is closely related to the reaction by H (or D) atoms in the radiolysis. The hydrogen atom abstraction reaction by recoil T atoms is caused by both hot T atoms and thermal T atoms. The reaction in the solid phase takes place in the tracks of the recoil T atoms and in the bulk matrix by diffusive thermal T atoms. Ignoring this complication, the apparent overall abstraction reactions will be represented here by
-
T + neo-C5H12 (n-C1oD22) 10
1
I
I00
150
200
Retention time / mln
Figure 2. Radlogas chromatogram of the neo-C,H,,-i-C,H@ (2 mol %F%F mixtures irradiated in a reactor: -, irradiated at 195 K; ---, irradiated at 77 K.
matogram of HT and DT in the neo-Ca12-i-C4H$ (2 mol %) systems. Radioactivity is expressed in a logarithmic scale. Irradiations at 20 and 300 K give the same radiogas chromatogram as that at 195 K. The DT/HT ratio is much higher at 77 K than at other temperatures. The relative yields of the volatile tritiated products in the neopentane systems are shown in Table I. The yields are mean values of three runs and the errors are about 10%. As polymeric products are produced in a low yield in a butane system,2most of the radioactivity in the neopentane system may also appear in volatile form. Thus the yields in Table I represent roughly the overall reaction products. Table I1 shows the relative yields of the volatile tritiated products in decane-dz2systems. The yield of DT is normalized to 1. The yields are mean values of 2-4 runs and the errors are about 10%. Discussion Comparison of Hydrogen Atom Abstraction Reaction by T Atoms with That by H (D) Atoms in Radiolysis. Though recoil T atoms cause abstraction, replacement, and fragmentation reactions with alkanes, the discussion in this
Pmht
H T + neo-C5Hll (DT + C10D21)
T + i-C4H9D2DT + t-C4Hg (n-CioHzz) (HT + C10H2J
(4) (5)
where i-C4H$ or n-Cl,,H22are the solute in the neo-C5H12 or n-CloDZ2matrices, re~pectively;'~ p is a relative probability of reaction. Recent studies of radiation chemistry have elucidated that thermal H atoms easily abstract H or D atom from alkanes at low temperature by a quantum mechanical t ~ n n e l i n g . ' ~ ! ~Thus, J ~ both hot and thermal T atoms can abstract hydrogen atom in reaction 4 and 5. The ratio of relative probabilities for the reactions 4 and 5 is approximated by the yield of H T and DT in Tables I and 11. For example, in the neo-C5H12-i-C4HgD(2 mol %) system K = - psOlute - [DTI [neo-C5H1,I Psolvent [HT][i-CdHgD]
(6)
In hot T reactions, an activation energy is no longer an important controlling factor determining the preference of a reaction. Most of hot T atoms with high energy react (15)The possibility of the reaction of diffusive T atoms, such as T + i-C,HJl- HT + i-C4Ha-,can be diemissed on the basis of the following result. When H atoms are produced by the photolysis of HI in the nec-C6Hl2-i-CIH& mixtures at 77 K, only t-C&,. radicals are produced. The isobutyl radicals (i-C4H8D.),which should be formed in the above reaction, were not observed by ESR spectroscopy in the reaction of H atoms with i-C4HJl at 77 K. (16)LeRoy, R.J.; Sprague, E. D.; Williams, F. J.Phys. Chem. 1972, 76,546.
The Journal of Physical Chemistry, Vol. 86,No. 2, 7982 251
Temperature Effect on Recoil Tritium Reactions
TABLE 111: Ratio of Relative Probability of Reaction @solute/?solvent 1 for Hydrogen Atom Abstraction Reactiona neo-C,H,,-i-C,H,D ( 2 mol %) temp, K
n-C,,D,,-n-C,,H,, (10 mol %)
H by recoil T radiolysisb recoil T
2 0 (4)d
0.39
