J. Phys. Chem. 1984, 88, 3108-31 10
3108
Selective Hydrogen Atom Abstraction by Tritium and Deuterium Atoms in the Radiolysis of Tritiated Decane at 77 K. Mass Effect on Solid-state Reaction Masakatsu Saeki, Enzo Tachikawa, Department of Chemistry, Japan Atomic Energy Research Institute, Tokai-Mura, Ibaraki 319-1 1, Japan
Tetsuo Miyazaki,* Yoshiteru Fujitani, and Kenji Fueki Department of Synthetic Chemistry, Faculty of Engineering, Nagoya University, Chikusa- ku, Nagoya 464, Japan (Received: October 27, 1983)
The relative yields of five isotopic compounds of hydrogen produced in the radiolysis of partially tritiated deuteriodecane (n-C10D22(T))-protiateddecane (n-Cl&Iz2)mixtures at 77 K were measured by conventional radiogas chromatography. The HT/DT ratio in hydrogen yields is much lower than the HD/D2 ratio. Though D atoms react selectively with n-CloH2,, T atoms do not react as selectively with n-CIoH2*as D atoms do. The ratio (@CIOH22/p~IOD22)T) of the reaction probability = 30) of the T atoms with n-CloHz2to that with n-C10D22 is 4.5-6.3, which is much lower than the ratio (@CIOH21/pCIOD22)D for the D atoms. The ratio for the T atoms in the radiolysis is rather close to the ratio (2.3) for the recoil T atoms. The mass effect on the selective hydrogen atom abstraction reaction in the decane mixtures is discussed.
Introduction Elucidation of the behavior of T atoms in solids is very important in tritium technology relevant to nuclear fusion reactors. Though a number of studies have been reported on the behavior of H and D atoms in solid materials, the studies on T atoms in solids are limited to only a few examples because of the experimental difficulties of tritium handling. When deuterated decane containing a small amount of protiated decane is y-irradiated at 77 K, D atoms produced by the decomposition of the deuterated decane abstract H atoms selectively from the protiated decane.' This reaction was evidenced by analysis of the isotopic composition of the trapped radicals,, the evolved hydrogen gas,'v3and the dimer product^.^ Recently we have studied recoil tritium reactions in n-ClJ122-n-CloH22mixtures at 77 K.5 The ratio of the probability of reaction of the T atoms with n-C,oH22to that for the reaction with n-CloD2, is 2.3, while the corresponding ratio for the D atoms produced by radiolysis amounts to about 30.Ibj4 The remarkable difference in selectivity between the D atoms and the recoil T atoms may be caused by the difference in the initial energy of the atoms or by the difference in the mass of the atoms (mass effect). In order to study the mass effect on the reactions of D and T atoms in the n-CloD22-n-C,oH22system, D and T atoms with the same initial energy must be produced by the same method under the same experimental conditions. Therefore, we have synthesized partially tritiated decane. The radiolysis of this decane produces T and D atoms in the same energy range. Then we have compared the reaction efficiencies of the T and D atoms in the decane at 77 K. Experimental Section
n-CloH22was 99 mol % in purity. Tritiated decane was synthesized as follows by utilizing a recoil tritium reaction. 6Lienriched LiF, in which 6Li/(6Li 'Li) is 0.95, was added to n-CloD2, whose D content was 99%. The mixture was irradiated in the liquid phase by neutrons from the JRR-4 nuclear reactor of the Japan Atomic Energy Research Institute for 5 min at 313 K. The thermal neutron flux and the dose rate of y-rays from
+
(1) (a) Miyazaki, T. Radiat. Phys. Chem. 1977, 10, 219. (b) Miyazaki, T.; Kasugai, J.; Wada, M.; Kinugawa, K. Bull. Chem. SOC.Jpn. 1978, 51, 1676. (2) Gillbro, T.; Lund, A. Chem. Phys. Lett. 1974, 27, 300. (3) Claesson, 0.;Lund, A. Chem. Phys. Lett. 1977, 47, 155. (4) Tilquin, B.; Baudson, Th.; Claes, P.; Lund, A.; Claesson, 0. J . Phys. Chem. 1982,86, 3324. (5) (a) Aratono, Y.; Tachikawa, E.; Miyazaki, T.; Kawai, Y.; Fueki, K. J . Phys. Chem. 1982, 86, 248. (b) Aratono, Y.;Tachikawa, E.; Miyazaki, T.; Fueki, K. Bull. Chem. SOC.Jpn. 1982,55, 1957.
0022-3654/84/2088-3108$01.50/0
the reactor were 8 X 1013n-cm-2-s-1and ~ 1 0R-h-', * respectively. Then, the irradiated mixture was distilled in a vacuum line for purification of tritiated decane, being removed from volatile products (such as DT and CD3T) and heavy products. Tritiated hydrogen, methane, and parent alkane amount to about 95% of the total tritiated products from the recoil tritium reaction in the liquid alkane.6 Thus, tritiated decane, n-CloD2,T,was obtained after removal of D T and CD,T. Since the tritiated decane may contain some impurities, we have checked whether or not the decane can be used for the study of the selective hydrogen atom abstraction reaction (see next section). The radioactivity of nCIoD2,Tin n-CloD,, is about 40 pCi.g-l. The mixture of nCl0D2,Tand n-CIoD2,is denoted hereafter as n-C,,D,,(T). The samples of the n-CloD22(T)-n-CloH22mixtures were irradiated at 77 K by y-rays from 6oCoto a total dose of 1.1 X lo7 rd. The products of hydrogen were measured by conventional radiogas chromatography. A 1.7-m y-alumina column coated with Fe203was used 77 K for the separation of HT, DT, H,, HD, and D2. Neon gas was used as a carrier gas. Figure 1 shows typical gas chromatograms of hydrogen produced in the radiolysis of n-CloD22(T)-n-C10H22 (16.7 mol %) at 77 K. Figure 1A shows a radiogas chromatogram measured by means of a proportional counter. Figure 1B shows a conventional gas chromatogram measured by a thermal conductivity detector. The separation of five isotopic compounds of hydrogen is fairly good.
Results The atomic ratio of T / D of the mixture of n-CloD2,T and n-CloD22(n-ClOD2,(T))is only lo4; Le., nearly all of the molecules are n-CloD2,. Since the tritiated decane, however, was synthesized by a recoil technique and may contain some impurities, we have examined at first the problem whether or not the radiolysis of the tritiated decane (n-CloD,,(T)) is the same as that of neat n-CioD2?. Figure 2 shows the relative yields of D,, HD, and H2 in the radiolysis of the n-CloD22(T)-n-CloH22 mixtures at 77 K. The reported value^'^^^^ of D,, HD, and H2 yields in the radiolysis of the n-CioD,,-n-CioH,, mixtures were averaged and shown by the three curves. The dose of the neat n-CloD22system in ref 1 (9 X lo6 rd) was approximately the same as in the n-Cl0D2,(T) experiments. The present results in the radiolysis of the nCloD22(T)-n-CloH2,mixtures coincide with the reported values of the n-C,oD22-n-CloH22mixtures. The fraction of the H D yields in the total hydrogen yields is much higher than the mole fraction of n-C10H22,indicating the selective formation of HD. Thus, the tritiated decane can be used for the study of the selective hydrogen atom abstraction reaction. (6) Menzinger, M.; Wolfgang, R. J . Phys. Chem. 1968, 72, 1789.
0 - 1984 American Chemical Societv
The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 3109
Radiolysis of Tritiated Decane at 77 K
I.
50
LO 30 Retention time
I
20 min
10
Figure 1. Gas chromatogram of hydrogen produced in the radiolysis of the n-C,oD22(T)-n-CloH22(16.7 mol %) mixture at 77 K: (A) radiogas chromatogram measured by a proportional counter; (B) conventional gas chromatogram measured by a thermal conductivity detector.
.-" lOOr d r
% 3
d
TABLE I: Ratio of Relative Probability of Reaction (pc,oH,,/pc,,o,,) for Hydrogen Atom Abstraction Reaction in Decane at 77 K
T by radiolysis P C I O H ~ ~ / P C I O D ~ 4.5-6.3 Z n-CloH2~ / mol %
Figure 2. Relative yields of D,, HD, and Hz in the radiolysis of the n-C,oD22(T)-n-CloHz2mixtures at 77 K (0)D2; (A) HD; (0)H2. The average values of reported yields1Js4in the radiolysis of the n-CloD22-nCloH2, mixtures at 77 K are shown by three curves: (-) D2;(- - -) HD; ( - * - )H,.
Figure 3 shows the ratios of hydrogen yields, such as HT/DT and HD/D2, against concentration of n-CloHZ2in the radiolysis of the n-CloD2,(T)-n-CloH22mixtures at 77 K.
Discussion Mass Effect on Hydrogen Atom Abstraction Reaction in nC,J)22(T)-n-C,@22Mixtures at 77 K . The HT/DT ratio in figure 3 is much lower than the HD/D2 ratio, indicating that T atoms do not react as selectively with the CIOH22 solute as D atoms do. According to the previous paper,lb HT, DT, HD, and D2 are formed by the following reactions: n-CloD21T C10D21+ T (1)
--
v*n+
CloD2o + D T
(2)
-
+ n-CloDz2
@Cm)T
C I O D ~+, DT
-
+ n-CloD2, D + n-CloHZ2 D
D by radiolysis 27,l 30,4 34'
recoil T 2.35a
0
~
T
n-C10H22 / mol %
Figure 3. Ratios of hydrogen yields in the radiolysis of the n-CloD22(T)-n-CloH22 mixtures at 77 K against n-CloHz2concentration: (A) HD/D2; (0) HT/DT.
I
@Cm)O
@CHn)D
C10D21+ D,
(3)
(7)
+
CloHzl H D
DT is formed by molecular detachment from irradiated n-CloDZ1T and by an abstraction reaction from n-Cl0D2, by T atoms. The probability of the abstraction reaction is represented by p . The ratio of the rate of reaction 6, Le., unimolecular formation of D2, to that of reaction 5 , Le., atomic fragmentation, was previously estimated to be 0.61 from radical yield^.^^,^ The isotopic fraction of hydrogen yields given in Figure 2 also suggests that this ratio is in the range from 0.5 to 0.65. It is assumed here that (7) The ratio of the unimolecular formation of H2 to H from n-CloHZ2is about 0.79 (cf. ref lb).
the ratio of the rate of reaction 2 to that of reaction 1 in the decomposition of n-Cl0D2,T is equal to 0.61. Then, the ratio of @CloH22)T to (PC10D22)T can be estimated to be 4.5 from the relative yields of HT and DT in Figure 3 (cf. Table I). In addition to reactions 1-4 H T and DT may also be formed by the following reactions. H and D atoms produced by the radiolysis of n-CloH2,and n-C,oD22react with n-CIoD2,Tto form H T and DT, respectively.
-
+ n-CloD2iT D + n-CloD2,T
H
@c&~T)H
+ HT
(9)
CloD21+ DT
(10)
CloDzl
@CmT)D
It is assumed here that ( I I c , ~ ~ ~ is , ~nearly ) ~ , ~ equal to ( I I C ~ ~ D ~ ~ ) H , D . Then the ratio of @CIOH22!~CIOD22)T can be estimated by a kinetic analysis similar to that in ref lb. Thus, the value of (PC10H22/ P C ~ ~ D ~is~ obtained )T as 6.3 (cf. Table I). The ratio of @C10H22/pCIOD22) for the T atoms produced by y-radiolysis is compared in Table I with those for D atoms produced by y-radiolysis and for recoil T atoms. The ratio for the T atoms produced by the radiolysis* is much lower than that for the D atoms by the radiolysis but is rather close to the value for the recoil T atoms. Thus, it is concluded that T atoms do not react selectively with the n-CloHz2solute in the n-CioD22-n-CloH22 mixtures as efficiently as D atoms do. The previous study on the recoil T atom reaction in n-CIoD2, at 77 K suggested that only a few percent of the recoil T atoms may diffuse into the bulk m a t r i ~ .This ~ estimation was based upon the assumption that T atoms with low energy react selectively with the n-CIoHzzsolute as eficiently as D atoms do. The low value of ~ c ~ ~ for H T~ atoms ~ produced / ~ ~ by ~the radiolysis ~ D ~ (Table ~ I) suggests that the fraction of the diffusive T atoms in the recoil T atom reaction was underestimated previously. Remarks on Mass Effect. Information on the mass effect of hydrogen atoms in chemical kinetics in the solid phase are too scanty to allow firm rationalization of the difference in selectivity of T and D atoms in hydrogen atom abstraction in the decane mixtures. The tunneling rate constants for abstraction from 3-methylpentane-dl, or 3-methylheptane-d18at 5-29 K are similar for H and D atoms, while H atoms react with methylcyclo(8) The conclusion that T atoms produced in the radiolysis show much less selectivity than D atoms is based upon two assumptions. First, the ratio of T elimination to concerted DT elimination from radiolyzed CIOD21T molecules is the same as the ratio of D to D2eliminatin from C10D22.Second, the energy of ejection of a T atom is the same as that of a D atom.
3110 The Journal of Physical Chemistry, Vol. 88, No. 14, 1984 TABLE II: Effect of Shape of Potential Energy Surface on the Tunneling Abstraction Reaction of T and D Atoms with Decane at 77 K
-3.73 -1.73
8, deg -75 -60
-1.0
-45
-0.58 -0.27
-30 -15
( k C ~ ~ H 2 ~ / k C ~ ~ D z ~ ) (Dk C / ~~H2z)D/ ( ~ C I O H Z ~ / ~ C I O D ~ ~(ICCl~H22)T )T
0.41 0.93
0.72
1.oo 0.92
(~IoDzz)D/ (kC~~D22)T
5 00
207 11
12 1 5 36
0.9 6 33
C = tan 8. C is defined as dRz/dR, (see text). 8 is the angle of the reaction coordinate with respect to the R1 axis.
hexane-d14more rapidly than D atomse9 Some of the possible explanations are considered below. If it is assumed that the hydrogen atom abstraction reaction by T and D atoms is caused by quantum-mechanical tunneling at 77 K, the mass effect on the reaction can be estimated by the method used in a previous paper.1° The rate constant ( k ) for the tunneling abstraction is given by
where G( W ) is the permeability of the particle with the kinetic energy of W1and can be obtained exactly for the unsymmetrical Eckart potential.12 A and R are the frequency factor and the gas constant, respectively. The effective mass (m) in the linear triatomic system, a-x-b, such as n-CloH2,-H-T, is generally expressed by m = (mamb(l+ Q2+ mbmxC + m,m,)/((m,
+ m, + mb) X
m1
(1 + where mi represents the mass of the partical i.13 The potential energy surface is represented by two interatomic distances, R1 and Rz, which are interatomic distances of a-x and x-b, respectively. C is defined as dRz/dR1, which indicates the direction of the reaction coordinate on the potential energy surface. C was taken as -1 in the previous calculation10based upon the assumption that the potential energy surface is symmetrical. Since the real potential energy surface for the decane-tritium (or deuterium) system is unsymmetrical, C has been changed here from -3.73
Saeki et al. to -0.27. The theoretically calculated potential energy surface for the CH3-H-T system shows that C is about -0.5.14 Using the same potential parameters for the decane-hydrogen system as those for the ethane-hydrogen systern,l0 we have calculated the ratios of rate ~ 0 n s t a n t s . l ~The calculated results are summarized in Table 11. Since the ratio of the isotope effect for D atom reaction to that for T atom reaction, Le., (kCloH2t/ ~ C , ~ D U ) D / ( ~ C I O H ~ ~ / ~ C ~isOless D ~ ~than ) T ’ or equal to 1, kCl&I,/kC&2 for the T atom reaction should be larger than or equal to that for the D atoms. The experimental results in Table I, however, are contrary to the calculated results. Thus, the experimentally observed mass effect is not related directly to the tunneling reaction. It is noted that ( ~ c ~ ~ H ~ ~ ) D / and ~ c ~( ~ H c ~~ ~~ D) T ~ ~ ) D / are ~ c ~ ~ D ~ ~ larger than 1 unless C, Le., tan 8, is equal to -1 (cf. Table 11). This indicates that the tunneling abstraction from decane by D atoms takes place much faster than that by T atoms, if the potential energy surface of the decane-hydrogen system is unsymmetrical. The mass effect on the selective hydrogen atom abstraction reaction may be related to the diffusion rates of D and T atoms. The diffusion coefficient of a particle is proportional to l/m1/2, where m is the mass of the particle. The diffusion rate of D atoms is (3/2)’12 times as fast as that of T atoms. If quantum-mechanical tunneling plays a role in the diffusion, the difference of the diffusion rate between D and T atoms may be greater. The decay rate of D atoms at 18 K in solid methane, where the atoms diffuse and recombine other atoms, is much slower than that of H atoms.16 If the diffusion of T atoms in the n-CloDzzmatrix is significantly slower than that of D atoms, the encounter of the T atoms with the solute in the n-CloDzzmatrix may be difficult and thus the reaction of the T atoms has the character of a diffusion-controlled reaction. Then the selective reaction of T atoms with the solute in the decane mixtures would not be expected to be as pronounced as that of D atoms, resulting in the low value of (PC10H22/pC,0D22)T as observed. Acknowledgment. This work was done under the Collaboration Program between the Japan Atomic Energy Research Institute and Nagoya University. This work was partially supported by a Grant-in-Aid for Scientific Research from the Japanese Ministry of Education, Science, and Culture. Registry No. n-CloD2,T,90320-94-8;n-CloD22, 16416-29-8; n-CloHzz, 124-18-5; T, 15086-10-9;D, 16873-17-9.
(9) Wang, H. Y.; Willard, J. E. J . Phys. Chem. 1979,83, 2585.
S.;Fujitani, Y.; Fueki, K. J . Phys. Chem. 1983,87, 1201. (11) For a review, see: Caldin, E. F. Chem. Rev. 1969, 69, 135. (12) (a) Eckart, C. Phys. Rev. 1930,35,1303. (b) Johnston, H. S.J. Phys. Chem. 1962, 62, 532. (13) Johnston, H. S. “Gas Phase Reaction Rate Theory”; Ronald Press: New York, 1966; p 51. (10) Aratono, Y.; Tachikawa, E.; Miyazaki. T.; Nagaya,
(14) Raff, L. M. J . Chem. Phys. 1974,60, 2220. (15) The barrier heights for reactions 3, 4,7, and 8 are taken here as 9.7, 8.5,9.7, and 8.5 kcal/mol, respectively. (Cf.: Kagiya, T.; Sumida, Y.; Inoue, T.; Dyachkovskii, F. S.Bull. Chem. SOC.Jpn. 1969, 42, 1812.) (16) Bhattacharya, D.; Wang, H.; Willard, J. E. J . Phys. Chem. 1981,85, 1310.
) T
