RECOIL TRITIUM REACTIONS
3781
ties to accommodate much larger irradiation vessels. With these changes, a more precise evaluation could be made of median kinetic energies of t'ritium atoms to olefins. The basic qualitative conclusion would,
however, remain unchanged-these addition processes are for the most part quite low-energy processes occurring well below the energy region in which substitution processes are observed for saturated positions.
Recoil Tritium Reactions with Cyclobutane-d,. Excitation Energies Accompanying Substitution of Energetic Tritium for Deuterium' by Akio Hosaka and F. S. Rowland* Department of Chemistry, University of California, Zrvine, California 92664
(Received July 1, 1971)
Publication costs assisted by Division of Research, U. S. Atomic Energy Commission
The median residual excitation energy left on c-ChD?T* following the energetic substitution of T-for-D in is essentially the same ( h 0 . 2 eV) as for c-C4H7T*from T-for-H in c-C& and is estimated to be 5 eV. The fraction of excited c-C~D~T* molecules which are collisionally stabilized is higher than for c-C~H~T* at the same pressure because of the fourfold faster decomposition of the protonated species for equal excitation at 1 atm pressure. energies. The per cent collisional stabilization is about 60 for c-C4HiT* and 67 for c-C~D~T* and The per cent stabilization is about 83 in the liquid phase and 95 in the solid at - 196' for both c-C~H~T* c-CkDvT*. The temperature effect on these decomposition reactions is 52% over the range 24-125'. C-Cp8
Introduction Energetic recoil tritium atoms can substitute for hydrogen atoms in all substrate molecules, as in (l),with
T*
+ RI-I +RT* + H
(1)
We have now carried out studies with cyclobutaned8 to complement the earlier c-C4Hs experiments, thereby permitting evaluation of the isotope effects in these substitution reactions. The excited c-C4D7T* molecules formed by the initial substitution reactions can then undergo the competitive reactions of colli-
product yields which show both pressure and phase effects.2-6 The most important source of the yield (1) This research has been supported by AEC Contract No. ATchanges found with variations in gas pressure arise (04-3)-34, Agreement No. 126. (2) F. Schmidt-Bleek and F. S. Rowland, Angew. Chem., Int. Ed. from secondary decomposition of the excited RT* molEngl., 3 , 769 (1964). ecules on a time scale competitive with collisional sta(3) R. Wolfgang, Progr. React. Kinet., 3, 97 (1965). bilization in this pressure range, Le., in most experi(4) R. Wolfgang, A n n . Rev. Phys. Chem., 16, 15 (1965). ments, 0.1-5 atm-roughly 10-g-lO-10 sec. Compari(5) F. 9. Rowland, "+Molecular Beams and Reaction Kinetics," son of recoil tritium results with thermal pyrolysis studAcademic Press, New York, N. Y., 1970, pp 108-138. ies of the same molecules has shown that the primary (6) B.Musgrave, J. K. Lee, and F. S. Rowland, Can. J. Chem., 38, 1760 (1960). RT* products are formed with a broad spread of excitaE. K. C. Lee and F. S. Rowland, J . Amer. Chem. Soc., 8 5 , 897 tion energies in the several electronvolts r e g i ~ n , ~ - ~(7) (1963). while a detailed study of cyclobutane showed that the (8) Y.-N. Tang and F. S. Rowland, J. Phys. Chem., 72, 707 (1968). median excitation energy in this system was approxi(9) C. T . Ting and F. S.Rowland, ibid., 74, 4080 (1970). mately 5 eV.' The T-for-CH3 substitution reaction in (10) C. T. Ting and F. S. Rowland, ibid., 74,445 (1970). (11) Y.-N. Tang and F. S.Rowland, ibid., 74, 4576 (1967). in 1,3-dimethylcyclobutane has shown an even higher (12) Y.-N. Tang, T. Smail, and F. S.Rowland, J . Amer. Chem. Soc., (estimated 6-8 eV) median energy deposition following 91, 2130 (1969). the replacement of this heavier group.'O Further, a (13) C. McKnight and J. W. Root, J . Phys. Chem., 73, 4430 (1969). series of similar experiments has established that high (14) C. McKnight, N. J. Parks, and J. W. Root, ibid., 74, 217 (1970). deposition energies are also commonly observed follow(15) K . A. Krohn, J. N. Parks, and J. W. Root, J. Chem. Phys., in ing the energetic reactions of both 18Fatoms"-l6 and press. 38Clor 39Clatoms,16 with excitation energies in some (16) Y.-N. Tang, W. S. Smith, J. L. Williams, K. Lowery, and cases 2 10 eV.12,14,1E F. S.Rowland, J. Phys. Chem., 75, 440 (1971). The Journal of Physical Chemistry, Vol. 76, No. 86, 1971
AKIO HOSAKA AND F. S. ROWLAND
3782 sional stabilization or decomposition to two molecules of ethylene, as summarized in eq 2-4. Evaluation of
+ C-C~DS+c-C~D,T*+ D c - C ~ D ~ T+* h!I +c - C ~ D ~+T M T*
c - C ~ H ~ T+ * CzD3T
+ CzD4
(2) (3)
(4)
the excitation energies of c - C ~ D ~ Tpermits * comparison of the residual energies left following substitution of Tfor-D us. T-for-H.
Experimental Section Irradiations. The energetic recoil atoms were formed by standard thermal neutron irradiation procedures :2-5 (a) condensed phases, 6Li(n,ar)T in capillaries containing LiF plus the substrate; (b) gas phase, 3He(n,p)Ton mixtures of 3He, substrate, and scavenger molecules in glass ampoules. The irradiations generally lasted for about 1hi in a thermal neutron flux of 10" n sec-l. Most irradiations were carried out in the dry exposure room of the Hawthorne TRIGA reactor under conditions for which absolute yield determinations were not possible. The 125" samples mere irradiated in an oil bath kept at a temperature of 125 k 5". The duplicates for this comparison were irradiated in an unheated oil bath at the same time, in order to keep neutron irradiation conditions as comparable as possible. The -196" samples were frozen and then irradiated while inside a large dewar of liquid nitrogen. The nitrogen level was depleted a t an accelerated rate in the radiation field but visibly lasted to the end of the irradiation in some cases. No yield differences were observed for the others, and we believe that the sample temperatures remained at - 196" for essentially the entire period of irradiation. Cyclobutane contained in capillaries with LiF remained visibly transparent when cooled to -78" with Dry Ice and is presumed to have been liquid during irradiation. (Literature values for the melting point vary from -50 to -goo.> Chemicals. Cyclobutane (Merck Sharp and Dohme) contained less than 1% n-butane and 'was used without further purification. Cyclobutane-de was supplied by Merck Sharp and Dohmc and was stated to be 98% isotopically pure in deuterium. The most important impurities observed in c-C4D8 by gas chromatography were n-butane (-2%) and propane (-0.2%). The material was used with these hydrocarbon impurities present. Sample Preparation. Samples were filled by the standard vacuum line procedures described earlier. 2--6 While initial compositions of all samples were monitored by measurement of input gas pressures, the relative ratios of c-C4H8,c-C4D8, and CH, in mixtures with each other were determined by thermal conductivity measurements during the postirradjation analysis. The relative thermal conductivities used in this analysis The Journal of Physical Chemistry, Vol. 76,No. $6,1971
were determined as follows (n-C4Hlo = 1.00) : c-C4H8, 0.936; C-C4D8,0.867; CH4,0.459. Chromatographic Separation and Analysis. The various radioactive products were separated by gas chromatography and analyzed with an internal flow proportional counter in the standard manner.17 Most samples were analyzed with a 50-ft dimethyl sulfolane column or a 50-ft safrole column for the hydrocarbon peaks, while H T and/or D T was separated from methane-t with a second aliquot on a 50-ft PCA column (propylene carbonate on activated alumina). Isotopic components were separately measured in samples containing both protonated and deuterated components by the following columns: (a) H T from DT, 8 ft of activated alumina preceded by a 12-ft buffer column of Chromosorb P ; (b) c-C4H7Tfrom c-C4D7T, these peaks separate readily on the 50-ft safrole column; (c) CHz=CHT from CD2=CDT, 200-ft column of AgNO3 in ethylene glycol. No corrections have been made to the observed data for the estimated 16% c-CID~Hin c-C4D8.
Results Samples with Only One Hydrocarbon Present: c-C4He or c-C~DB. The distribution of radioactivity among the chief volatile products following recoil tritium reaction with c-C4Deis shown in Table I, together with some concurrently measured samples of c-C4He. In each case, the yields have been expressed relative to the sum of c - C ~ D ~ TCzD3T (or c-C4H7T CZHaT) as 100. I n the presence of O2 scavenger in the gas phase, the only products with important yields are the abstraction product D T (or HT) and the complementary pair of cyclobutane-t plus ethylene-t. The addition of HzS raises the yields of CH3T and CzH6T,indicating the presence of moderate yields of CHzT and C2H4T radicals. The very small yield of n-C4H9T indicates that a negligible yield of n-C&HgTand sec-C4H8Tradicals can be found in the gas-phase samples. In the condensed phasesin the absence of a scavenger molecule-several products have increased yields, most notably that of n-butane-t in the solid phase. The much shorter time prior to stabilizing collisions substantially reduces the secondary decomposition of both c - C ~ D ~ Tand * c-C4H7T* in the condensed phases, as shown by the diminished relative yields of C2D3T and C2HaT. The pressure and phase dependence of the yields of the three major products are summarized in Table I1 for c - C ~ Dand ~ c - C ~ Hin~concurrent experiments. The relative yield of cyclobutane-t for each of these experiments is displayed in Figure 1, together with the earlier c-C4Hsdata of Lee and Rowland7 and some mixture experiments described below. The two sets of c-CdHs data were measured in completely different laboratories,
+
+
(17) J. K. Lee, E. K. C. Lee, B.Musgrave, Y.-N. Tang, J. W. Root, and F. S . Rowland, Anal. Chem., 34, 741 (1962).
RECOIL TRITIUM REACTIONS
3783
Table I: Observed Tritiated Products from Recoil Tritium Reactions with c - C ~ Hand ~ c-C~D~ 7 -
---
7-
266 gas 19 0% 10aHe
7 -
24
Product
C -
61.8 38.2 150 0.6 0.2 0.3 1.7
1.3 0.7 1.5
Irradiation conditionsac-CdDs-258 gas Liquid Other------19 0 1 LiF 10 SHe Temp, oC-----125 24 Re1 yieldsd-----
61.7 38.3 159 0.2 0.1 0.4 1.8 1.1 0.6 1.4
83.7 16.3 199 4.4 9.5 1.9 13.0 0.9 2.2 8.8
-----
7
----.--c-C,H 277 gas
SolidC
Irradiation conditions'---------------------E
610 gas
7 -
66 HzS 14 *He
15 02 12 'He
LiF
c
- 196
LiF
LiF
_ _ _
24
24
Productb
57.0 43.0 259 16.0 15.1 0.4 1.o 1.3 1.0 2.3
52.9 47.1 154 0.1 0.1 0.6 0.4 1.5 1.0 1.5
95.2 4.8 215 5.0 8.3 1.9 54.1
