Tritium Exchange Reactions on Irradiated Silica Gel
The Journal of Physical Chemisty, Vol. 82, No. 20, 1978 2235
Tritium Exchange Reactions on Irradiated Silica Gel. 2.‘ Studies on the Mechanisms of Labeling of Saturated Hydrocarbons and Halocarbons Anthony C. Cottrell, Murray K. Matthews, and A.
L. Odell”
Urey Radiochemical Laboratory, University of Auckland, Private Bag, Auckland, New Zealand (Received April 26, 1978)
The quantity of tritium gas sorbed on to y-irradiated silica gel has been measured by a “H2/T2titration” procedue which utilizes the hydrocarbon-labeling power of tritium sorbed on to color centers in the gel. Some 14% of the sorbed tritium is incorporated into 2-methylbutanewhereas 50% is incorporated into 2-chloro-2-methylbutane. These incorporations take place preferentially in the methyl groups adjacent to tertiary carbon atoms. Tritium sorbed on to silica gel as tritiated water and present as Si-OT shows little exchangeability with hydrocarbons. A mechanism involving tritium activation by a positive hole located on an oxygen atom adjacent to an aluminum atom and carbonium ion production in the hydrocarbon substrate on the aluminum atom is proposed.
TABLE I: Dependence of Yields on Amount of T, Useda T, used, mCi yield, mCi % incorporation 10 1.7 16 25 3.1 12 40 5.1 13 60 7.5 12 150 1.3 5 500 7.8 1.6 1000 1.7 0.8 a Conditions were as follows: 2 g of irradiated SiO,, 1 mmol of 2-methylbutane, 1 h tritium activation time.
Introduction The use of y-irradiated silica gel for the promotion of exchange of tritium gas with aliphatic hydrocarbons has been previously reported.lI2 The irradiation process leads to purple-colored defect centers in the solid, which are bleached by, and appear to “activate”, tritium (or hydrogen) molecules. This activated tritium may then readily exchange with H atoms in subsequently sorbed organic molecules. We now describe more detailed investigations into the mechanism of tritium activation using the tritium labeling of 2-methylbutane and 2-chloro-2-methylbutane as probe reactions. Experiments concerning the influence of the structure of the hydrocarbon on the course of the labeling reaction are also reported and possible mechanisms for the activation and labeling reactions are proposed.
The resulting mixture was injected into the gas liquid chromatograph with the inject port a t 373-398 K. The acetone and acetaldehyde so produced were detected in the effluent carrier gas.
Experimental Section The silica gel used in this work was a BDH Laboratory Grade Reagent containing the following impurities: A1203 (0.054 wt %), NazO (0.15%), TiOp (0.055%), Fez03 (0.029%) (data courtesy of BDH). Silica gel samples (2 g) were degassed to a final pressure of 1mPa by pumping for 1 h a t 973 K, irradiated at room temperature in a 80 Ci cobalt-60 source (dose rate 1000 rd/min), and exposed to tritium gas (admitted initially with the solid a t 77 K) for 1 h at 293 K (the “tritium activation time”) as previously described.2 The organic substrate (1 mmol of liquids, 0.05 mmol of gases) was then admitted by vapor transfer (using liquid nitrogen as coolant) and left on the solid for 1 h a t 293 K before reaction products were removed (by vapor-transfer) and analyzed by radiogas chromatography,2 the radioactivity of the reaction products being monitored by an ion chamber. Hydrogen used in the “H2/T2titration” experiments was dried in a trap cooled in liquid nitrogen before use. Quantities of hydrogen are, for convenience, expressed in “millicurie equivalents” (mCi equiv), 1 mCi equiv being defined as the amount of hydrogen containing the same number of hydrogen molecules as there are tritium molecules in l mCi of tritium (0.017 pmol). ESR measurements were performed a t room temperature using a Varian V4502 spectrometer. In the degradation of 2-chloro-2-methylbutaneto acetone and acetaldehyde the halide was refluxed with sodium ethoxide (2 mL) for 30 min and the resulting alkenes collected in a U-trap at 273 K attached to the top of the reflux condensor. Ozonides of these alkenes were prepared by passing ozone through the alkene mixture at 193 K.
Results (a) Labeling of 2-Methylbutane. (i) Effects of Radiation Dose. Activity yields of labeled 2-methylbutane obtained using 50 mCi of tritium gas and various radiation doses are shown in Figure 1, curve I. The yields increased with dose up to 7 mCi (14% of the total tritium used) at 1Mrd, while above this no further increase occurred and colorbleaching (by the tritium) was not complete. (ii) Effects of Quantities of Reactants. Tritium. With a fixed radiation dose (1 Mrd) yields increased with increasing amounts of tritium up to an incorporation of 7.5 mCi from 60 mCi of T2(see Table I). The fraction of the total tritium present incorporated appears almost constant at 12-16% when up to 60 mCi of T2was used and beyond this the absolute activity yield of labeled 2-methylbutane remained constant (ca. 7.5 mCi). BMethylbutane. Activity yields of 2-methylbutane were found to be independent of the amount of organic substrate used above 0.01 mmol, but below this, yields decreased with decreasing amount used. (iii) Effects of Addition of Hydrogen Gas. Admission of hydrogen gas (up to 1000 mCi equiv) to irradiated silica gel immediately after the admission of tritium (50 mCi) had no significant effect on activity yields of labeled 2methylbutane (both gases being admitted with the solid at 77 or 293 K and left on the solid for 1h at 293 K before admission of 2-methylbutane) as shown in Table 11. When hydrogen was admitted to the solid (radiation dose 1 Mrd) before the tritium, activity yields decreased with increasing amount of hydrogen used (see Figure 2). “H2/T2 titrations” were carried out by admitting a measured quantity of hydrogen followed immediately by
0022-365417812082-2235$0 1.OO/O
0 1978 American Chemical Society
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The Journal of Physical Chemlstry, Vol. 82, No. 20, 1978
A. C. Cottrell, M. K. Matthews, and A. L. O d d
and Endpoints
(mCi equivalents)
I /
A ' -
0
1
rc
2
3
4
S102 I r r a d i a t i o n Dase in Mrad
Figure 1. Irradiation time dependence of the (I) yield of labeled 2methylbutane (mCi, using 50 mCi of T2), (11) H,/T, titration end point values (mCi equiv), and (111) yield of labeled 2-chloro-2-methylbutane ( m a ) (using 400 mCi of T2).
TABLE 11: Effect of H, (Admitted to Silica Gel after T,)" quantity of H, (mCi equiv) and temp of admission yield, mCi none (T, admitted at 77 K) 7.4 1 0 0 mCi equiv, 77 K 6.8 1000 mCi equiv, 77 K 7.3 none (T, admitted at 293 K) 4.5 100 mCi equiv, 293 K 4.2 1000 mCi equiv, 293 K 4.3 aConditions were as follows: 2 g of irradiated SiO,, 50 mCi T,, 1mmol of 2-methylbutane.
tritium and (after these gases had been left on the solid for 1 h at 293 K) admission of 2-methylbutane, and analyzing labeled products in the usual way. The plot of activity of 2-methylbutane against amount of hydrogen added (Figure 2) showed reasonably shart end points at 58 and 63 mCi equiv of hydrogen for gas admissions at 293 and 77 K, respectively. Beyond these end points little labeling occurred. End points from titrations carried out with silica gel samples irradiated with various doses are shown in Figure 1, curve 11. In experiments designed to investigate the adsorption capability of silica gel (for hydrogen) measurements of pressures of hydrogen above the solid, using a mercury manometer, have shown that both irradiated and unirradiated silica gel (2-g samples) adsorbed all gas present at 77 K (up to 10 Ci equiv of H2were used). On warming to room temperature, however, desorption occurred to such an extent that no residual adsorption was detectable on the unirradiated solid, while slight residual adsorption on the irradiated solid was d e t e ~ t e d . ~ This method of measuring amounts of hydrogen retained by the gel at room temperatures is ineffectual and the end point method described above is to be preferred. (b) Labeling of Other Aliphatic Hydrocarbons. (i) Dependence of Activity Yield on Structure of Aliphatic Substrate. It has been reported2previously that straight chain hydrocarbons are labeled less heavily than hydrocarbons containing a tertiary carbon methyl group, and that yields of both types of hydrocarbon decrease with
0
100
300
200
400
500
Amount of H2 (mCi e q u i v a l e n t s 1
Flgura.2. Dependence of activity on amount of H2 sorbed on silica gel before T:, (I) sorption at 77 K; (11) sorption at 293 K.
TABLE 111: Dependence of Activity Yield on Molecular Size and Structure of the Substrate Hvdrocarbona total tritium radioactivity incorpor- chemical yield, ated, purity,b structure mCi mCi % c-c-c 1.6 2.3 68 c-c-c-c 1.0 1.6 61 c-c-c-c-c 0.1 0.15 67 c-c-c-c-c-c 0.03 0.05 60
F F c-c-c-c F c-c-c-c-c F c-c-c-c-c-c F c-c-c
C-C-C-C-C-C-C
8.5
9.1
93
7.3
7.6
97
4.8
6.3
91
3.4
4.7
73
0.7
6.6
10
a Results presented previously (Table V, ref 2) were obtained under different silica gel degassing conditions, The conditions used were as follows: 2 g of silica gel, 15-h irradiation time, 50 mCi T,, 1mmol of liquid substrates; 0.05 mmol of gaseous substrates. Percentage of incorporated activity occurring in the parent species.
increasing molecular size. We have now shown (in Table 111) that yields of labeled by-products (fragmentation and elimination products) from reactions involving branched-chain substrates increase with increasing molecular size of the organic substrate in such a way that total amount of tritium incorporated in the parent and byproducts is relatively constant (with the possible exception of the 2-methylpropane yield). (ii) T h e Role of Tertiary Hydrogen. Degradative experiments involving gas phase chlorination of 2-methylbutane labeled on irradiated silica gel have indicated2 that the tertiary hydrogen atom in this molecule does not exchange preferentially. This'degradation was, however, conducted at 573 K and at this temperature there may have been some scrambling of hydrogen and tritium atoms.
Tritium Exchange Reactions on Irradiated Silica Gel
The Journal of Physical Chemistty, Voi. 82, No. 20, 1978 2237
A
Percentwe Aotivity Remaining
40
0
Lo
200
400
600
800
l-----l
Exchange Time ( h o u r s )
Figure 3. Exchange reaction of tritium-labeled 2-methylpropanewith sulfuric acid (85%).
We now report a study of the relative extent of labeling of the primary and tertiary positions of 2-methylpropane when this hydrocarbon was labeled on irradiated silica gel. The labeled material was allowed to exchange with 85% sulfuric acid solution. It is known4 that this reagent exchanges only the primary hydrogen atoms of 2-methylpropane leaving the tertiary hydrogen atom unaffected. Results are shown in Figure 3 from which it can be seen that the residual activity after many half-lives is ca. 1% of the original activity, rather than the 10% which uniform labeling of the hydrocarbon would require. We conclude that labeling in the tertiary position is hindered under the conditions used. (c) Labeling of 2-Chloro-2-methylbutane. The tertiary chloride of 2-methylbutane, 2-chloro-2-methylbutane, was found to be more heavily labeled on irradiated silica gel (29 mCi of T2incorporated from 60 mCi of gas, using a 1-Mrd dose) than 2-methylbutane (typical yield under the same conditions, 7 mCi). This yield was not increased significantly by using more tritium gas (32 mCi being incorporated from 400 mCi of T2). Increasing the radiation dose (while using 400 mCi of T.J was found to increase yields in a manner similar to that in which end points of the H2/T2titrations increased with increasing dose (compare curves I1 and 111, Figure l),the absolute yield at any dose being approximately 50% of the corresponding end point value. Distribution of Tritium in 2-Chloro-2-methylbutane Molecules. The distribution of tritium in the labeled alkyl chloride molecule was determined by degradation to acetone and acetaldehyde. Activities of these products were 50 and 10 Ci mol-l, respectively. Correction for the hydrogen atom lost from the acetaldehyde fragment during the initial dehydrohalogenation reaction of the degradation gives a corrected activity of 12.5 Ci if the tritium was originally uniformly distributed over the five positions, or 20 Ci if it was concentrated on the original secondary carbon atom of the halide. It appears, therefore, that 71-80% of the tritium in labeled 2chloro-2-methylbutane molecules is in the two methyl groups attached to the tertiary carbon atom, whereas 54% of the activity would be expected in these groups if labeling were uniform. (d) E S R Studies. Tritium and hydrogen gas, admitted to silica gel after irradiation, were found to quench the ESR signal of the irradiated solid (Figure 4) at room temperature.
V
44 gauss
Figure 4. ESR spectra of y-irradiated silica gel (B) and silica-alumina5 (A).
In a previous publication2 this ESR spectrum was attributed to the presence of F centers in the solid. Further study, however, has revealed a similarity between this spectrum (Figure 4) and that reported by Hentz and Wickended for radiation-induced positive holes in silica alumina (to which the radiation-induced coloration is also attributed). These positive holes are thought to be trapped on silicon-aluminum bridging oxygen atoms5 and this conclusion is supported by the sextet structure of the band (IAl= 5/2). The color centers should therefore be described as V center^.^ The intensity of this part of the spectrum has been shown4 to depend on the aluminum content. The silica gel used in this work contained roughly 5 X 1019A1 atoms/2 g of SiOz). The population of holes in silica gel (subject to a dose of 1Mrd), determined by spin counting (using a standard sample of coal containing 3.46 X 10“ spins for calibration), was aproximately 2 X 1017holes/2 g of SiO,; that is of the same order as the number of tritium atoms (1.5 X incorporated in 2-methylbutane when exposed in 2 g of irradiated silica gel to 50 mCi of tritium gas. Uncertainties in both these determinations prevent us from claiming a precise equality. (e) Exchangeability of Silica Gel Surface Hydroxyls. The exchangeability of surface hydroxyl groups was investigated using “tritiated’ silica gel prepared by soaking the solid (10 g) in tritiated water (10 mL, 0.3 Ci/mL) for 10 days at room temperature before drying and degassing (at 773 K). This silica gel was found after degassing to have specific activity of 4 mCi/g (measured by scintillation counting), and when a 2-g irradiated sample of this gel (dose 1Mrd) was used in a run involving 2-methylbutane and no T2 gas, a small activity yield (2 pCi) of labeled 2-methylbutane was produced. This yield is insignificant, however, when compared to that obtained in a normal run, namely, 1.5 mCi (when using 4 mCi of tritium gas per gram of SiOz). (The total number of hydroxyl groups on the solid under the degassing conditions used was ca. 1021/2 gJ6
Discussion (i) Activation of Tritium toward Exchange with 2Methylbutane. The activity yield of labeled 2-methylbutane was not lowered by the addition of hydrogen gas to the irradiated silica gel immediately after adsorption of the tritium at 77 K, although both gases were quantitatively adsorbed at this temperature. On the other hand,
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The Journal of Physical Chemistry, Vol. 82, No. 20, 1978
the yield was greatly reduced if hydrogen gas was allowed to adsorb before admitting tritium. These two specimens of gas thus retain their identity throughout the sorption and labeling processes and the observed lack of scrambling between them implies that sorption of tritium, in such a way as to lead to exchange with 2-methylbutane, must occur on specific sites (even at 77 K when the color of the solid is not bleached during sorption). The total amount of tritium (or hydrogen) which the irradiated silica gel is capable of adsorbing on such specific sites is measured by the “titration end points” which represent the amount of hydrogen (added before tritium) necessary to supress the subsequent labeling of 2methylbutane. The yield of labeled 2-methylbutane (Figure 1, curve I) is always less than the corresponding end point value at any radiation dose (Figure 1, curve 11). The radiation dose (ca. 1 Mrd) above which a constant yield (7 mCi or 14% of the total T2 used) of labeled 2methylbutane is observed (Figure 1, curve I) coincides roughly with that dose necessary to induce sufficient “specific adsorption” sites for the adsorption of the total amount of tritium used (50 mCi). Furthermore, this maximum level of incorporation into 2-methylbutane (14% of total T2used) is in rough agreement with the fraction of the sorbed tritium incorporated when less than 50 mCi of T2 are used in experiments involving a 1-Mrd dose (Table I). The constancy of this yield, at 14% incorporation, even when all tritium present is adsorbed and adsorption sites are present in excess, leads us to suggest that the sorbed species may react by alternative pathways, one, type A, leading to labeled parent and other(s) (type(s) B) leading to other labeled products. Some 14% of the total sorbed tritium reacts by path A when the organic substrate is 2-methylbutane. When “end points” from hydrogen titration experiments are plotted against radiation dose a two-limb curve (Figure 1, curve 11) is obtained. This extrapolates to zero at zero dose indicating that the tritium adsorption sites are indeed radiation induced. The resolution of this plot into two straight lines suggests that two processes may be involved in the production of these sites, perhaps involving defects already present in the solid during early stages of irradiation and radiation-induced defects in later stages, as in the formation of color centers in ionic crystals.’ (ii) Activation of Tritium toward Exchange with 2Chloro-2-methylbutane. The amount of tritium incorporated into 2-chloro-2-methylbutaneis about 50% of the total amount of tritium absorbed on the silica gel (as measured by titration end point values), for all radiation doses (compare curves I1 and 111, Figure 1). This relationship holds (with a dose of 1 Mrd) even when excess tritium (400 mCi) is used. (iii) Reaction of the Hydrocarbon with Silica Gel. The structural requirements for efficient labeling of alkanes on irradiated silica gel appear to be similar to those needed for hydrogen exchange reactions on silica-alumina cracking catalyst^.^^^ It has been proposed8 that hydrogen exchange reactions with these cracking catalysts involve exchange of acidic hydrogen on the solid with carbonium ions, more exchange being observed with branched-chain substrates than with straight-chain ones because of the greater stability (and ease of formation) of tertiary carbonium ions. Cracking reactions are also known to be favored by the presence of a tertiary carbon atom and by large molecular size.l” Our observations that hydrocarbons with a tertiary carbon atom give higher activity yields of labeled alkanes and that yields of fragmentation (cracking) by-products
A. C. Cottrell, M. K. Matthews, and A. L. Odell
increase with increasing substrate molecular size suggest that carbonium ions may well be involved in the exchange of activated tritium with alkanes. It has been suggested8 that carbonium ions are formed by abstraction of H- ions from the tertiary-positions of organic substrate molecules by “Lewis acid centers” (involving three-coordinate aluminum atoms)ll on silica-alumina. This abstraction need not be complete8but may result in distortion of the C-H bond to form a pseudo-carbonium ion as shown in section v. The aluminum content of the silica gel used in the present study (0.054% or 2.5 X 1019 A1 atoms/g of SO2) is more than sufficient to account for the levels of labeling reported. It is noteworthy, however, that all the branched-chain alkanes studied show similar levels of total tritium incorporation if incorporation into fragmented products is included, regardless of molecular size, whereas the activity yields of parent hydrocarbon decrease with increasing molecular size. The heavier labeling of 2-chloro-2-methylbutane, compared to 2-methylbutane, may be ascribed to the greater ease of abstraction of C1- than H-(because of the greater polarizability of the C-C1 bond), resulting in a more complete charge separation than is obtained in forming the pseudo-carbonium ion of 2-methylbutane. There will also be a higher degree of hyperconjugation with @-related hydrogen atoms in the case of the carbonium ion formed from the chloro-substituted compound, facilitating the release of @-methylhydrogen atoms as H+ ions. The high abundance of tritium in the two methyl groups adjacent to the positive carbon atom of the carbonium ion may then be ascribed to this effect. (iv) T h e Nature of Activated Tritium. From analogy with the silica-alumina reactions it appears that tritium, activated by irradiated silica gel toward exchange with organic substrates, is a positive ionic species, Tt, which may exchange with acidic hydrogen atoms of carbonium ions. The hindered exchange with the tertiary hydrogen of 2-methylbutane then arises because T+ would be repelled from the positive tertiary carbon atom of the pseudo-isobutylcarbonium ion. For ionization of the adsorbed tritium to occur it is necessary to involve a positive center in the activation process. We assign this function to the center responsible for the coloration and which has been detected by the ESR method. This center has been described6as a positive hole trapped at a bridging oxygen atom adjacent to a substitutional aluminum atom. We suggest that this positive center is responsible for both adsorption and ionization of the tritium molecule shown in reactions 1 and 2. The absorption (at 7 7 K )
T:T j.
.+
.+
0 I \
I \
T,(g)+ Si
0 AI-. Si
AI
ionization (at 298 K)
T:T
T+
J.
I
I +
-Si
0
0
I1 \
I / \
I
Al---Si I
I
Al- t T. /
(2)
tritium atoms so liberated may recombine in pairs to produce T2or react with some other center. Reaction 1may occur at 77 K without color bleaching, while reaction 2 represents the thermally-activated bleaching process2 which occurs during warm-up after
Trltlum Exchange Reactions on Irradiated Silica Gel
The Journal of Physical Chemistry, Vol. 82, No. 20, 1978 2239
tritium admission at 77 K (with room temperature admission of T2 the lifetime of the absorption complex formed in reaction 1 would presumably be very short causing the observed rapid bleaching). The sorbed T+ formed in reaction 2 represents the exchangeable tritium; the maximum amount exchangeable being ca. 50% of the absorbed tritium (one T+ per Tzmolecule) as observed in the yields of labeled 2-chloro-2-methylbutane.The remaining nonexchangeable tritium atom may pair with another to produce T2or may be bound to the solid as a - group Si-OT (reaction complex species (I)or as a hydroxyl T ~
i
0
/ .\
Si
A1
I
2), the latter being known not to be exchangeable (see paragraph e). In support of this model it is known12that silica gel releases hydrogen gas when irradiated with y rays. (v) The Exchange Reaction. The behavior described above may be rationalized by the mechanism (tertiary aliphatic) shown in eq 3 in which the reactive center is the C
c-& +-c I
C
l
T+ .t. -0-AI-O-Si-
c-c-c
l
i
H H
/
I H ei T+
I t -0-AI-0-Si-
exchange of T+a n d H
(3)
e l i m i n a t i o n of T’ a n d HS-as HT leaving carbonium ion
olefins, fragments, etc.
aluminum atom with a T+sorbed on an adjacent oxygen atom. The acidic aluminum polarizes the tertiary C-H
bond forming a pseudo-carbonium ion intermediate. Two alternative paths for this intermediate are (i) exchange of T+with C-H on the carbon atom adjacent to the tertiary center followed by desorption, or (ii) loss of H T from the surface formed by union of Hb-and T+. Those carbonium ions having higher stability will approach closer to the surface and will also be longer lived and both these effects will favor exchange rather than loss of HT. For different sorbed materials there will clearly be different ratios of exchange and loss of HT. The loss of H T will lead to formation of a true carbonium ion and this in turn will lead to olefinic and fragmented products. The mechanism by which such products become labeled remains obscure but it must involve some mobility of either the carbonium ion or the tritium T+ over the surface. Acknowledgment. We thank the Research Committee of the University Grants’ Committee for a scholarship (for M.K.M.) and for a grant for equipment. We thank the Director of the Physics and Engineering Laboratory D. S.I.R. for the use of ESR facilities. We thank Mr. P. C. Crossley and Mr. A. A. T. Bailey for technical help. References and Notes (1) B. J. Coweli, K. M. Matthews, and A. L. Odeli, Chem. Commun., 1264 (1971). (2) K. M. Matthews and A. L. Odeli, J. Chem. SOC.,Dalton Trans., 1145 (1973). (3) G. B. Pariiskli, Y. A. Mischenko, and V. 8. Kazanskil, Klnet. Kafal., 6, 625 (1965). (4) (a) 0. Beeck, J. W. Otros, D. P. Stevenson, and C. D. Wagner, J . Chem. Phys., 1, 418 (1949); (b) A. L. Odeil, A. Rosenberg, and R. Wolfgang, b i d . , 40, 3730 (1964). (5) R. R. Hentz and D. K. Wickenden, J. Phys. Chem., 73, 617 (1969). (6) W. K. Hall, H. P. Leftin, F. J. Chesekke, and D. E. O’Reiliy, J. Cafal., 2, 506 (1963). (7) R. B. Gordon and A. S. Nowick, Phys. Rev., 101, 977 (1956). (8) S. G. Hindin, G. A. Mills, and A. G. Oblad, J . Am. Chem. Soc., 73, 278 (1951). (9) R. C. Hansford, P. G. WaMo, L. C. Drake, and R. E. Honig, I d . Eng. Chem., 44, 1108 (1952). (10) H. H. Voge, “Catalysis-Alkylation, Isomerization, Polymerization, Cracking, and Hydroreforming”, Vol. 6, P. H. Emmett, Ed., Reinhoid, New York, N.Y., pp 341-406. Catalytic cracking by H. H. Voge. (11) M. W. Tamete, Discuss. Faraday Soc., 8, 270 (1950). (12) H. Ogura, Y. Tachika, Y. Suzuki, C. Nakazato, and M. Kondo, J. Nucl. Sci. Technol., 12, (3), 167 (1975).
