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The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
for this nonlinear rate ~ 0 n s t a n t . lThis ~ relation suggests k4 = 4 X lo9 M-l s-l a t 1206 K. This is around a factor of 2 larger than that expected from the Arrhenius equation used in Table I. According to our calculation, acceptance of this large k4 value will not significantly affect the simulated k2 of this study, but it does increase k9 to 5 X 1010. In conclusion, due to the dramatic increase of the rate constants of reaction 1 (E, = 88 kcal/mol) and reaction 3 ( E , = 40 kcal/mol) with temperature, methyl radicals and hydrogen atoms take the place of ethyl radicals as the dominant radicals in ethane pyrolysis in shock tubes. Accordingly, the mechanism for the thermal decomposition of ethane varies with conditions. Reactions 1-5 describe the cracking in conventional reactors when the temperature is less than 1000 K. At high temperatures, however, observed product distribution may be explained with a different mechanism. This mechanism is characterized by a fast equilibrium between the ethane molecule and methyl radicals, and by the termination of propagating carriers, hydrogen atoms and ethyl radicals, through the combination of hydrogen atoms and methyl radicals. Ethane from natural gas has been used, in tremendous amounts, to generate ethylene, the most important feedstock in modern industry, through cracking at temperatures over 1100 K. The present mechanism may, hopefully, help give decent insight to this important industrial process. Acknowledgment. The authors gratefully acknowledge the support of the National Science Council of the Republic of China. Assistance of the 1979 class of the chemistry department of National Tsing Hua University on the content of Table I1 is also appreciated. References and Notes (1) F. 0. Rice and K. F. Herzfeld, J . Am. Chem. Soc., 56, 284 (1934). (2) H. G. Davis and K. D. Williamson, World Petr. Congr., Prac., 5fh,
S. K. Tokach and R. D. Koob
(3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30)
1959, 4 (1960); K. D. Williamson and H. G. Davies, Presented at the 169th National Meeting of the American Chemical Society, Phibdelphh, Pa., 1975, abstract no. 195. G. D. Towell and J. J. Martin, AIChE, J., 7, 693 (1961). A. B. Trenwith, Trans. Faraday Soc., 62, 1538 (1966). M. C. Lin and M. H. Back, Can. J. Chem., 44,505, 2357, 2369 (1966). P. D. Pacey and J. H. Purnell, J. Chem. Soc., Faraday Trans. 7 , 68, 1462 (1972). G. B. Skinner and W. E. Ball, J . Phys. Chem., 64, 1025 (1960). G. I. Kozlov and V. G. Knorre, Combust. Flame, 6, 253 (1962). I. F. Miller and S. W. Churchill, AIChE. J., 8, 201 (1962). J. N. Bradley and M. A. Frend, J . Phys. Chem., 75, 1492 (1971). T. C. Clark, T. P. J. Izod, and G. B. K'stiakowsky, J . Chem. Phys:, 54, 1295 (1971). A. Burcat, G. B. Skinner, R. W. Crossley, and K. Scheller, Int. J . Chem. Kinet., 5, 345 (1973). C. P. Quinn, Proc. R. Soc., London, Ser. A , 275, 190 (1963). D. W. Dexter and A. B. Trenwith, Proc. Chem. Soc., 392 (1964). S.W. Benson and H. E. O'Neil, Natl. Stand. Ref. Data Ser., NaN. Bur. Stand., No 21 (1970). A. Lifshitz and M. Frenklach, J . Phys. Chem., 79, 686 (1975); W.-H. Kao and C.-T. Yeh, !bid., 81, 2304 (1977). A. F. Trotman-Dickenson and E. W. R. Steacie, J . Chem. Phys., 19, 329 (1951); J. A. Kerr and A. F. Trotman-Dickenson, Prog. React. Kinet., 1, 105 (1961); R. H. Snow, J. Phys. Chem., 70, 2780 (1966). D. A. Leathard and J. H. Purnell, Annu. Rev. Phys. Chem., 21, 197 (1970). T. C. Clark and J. E. Dove, Can. J. Chem., 51, 2147 (1973). C. J. Chen, M. H. Back, and R. A. Back, Can. J . Chem., 54, 3175 (1976). J.-T. Chen, Y.4. Lee, and C.-T. Yeh, J. Phys. Chem., 81, 667 (1977). J.-T. Chen and C.-T. Yeh, J . Phys. Chem., 81, 1981 (1977). C.-J. Chen, M. H. Back, and R. A. Back, Can. J. Chem., 53, 3580 (1975). K. J. Laidler, "Chemical Kinetics", McGraw-Hill, New York 1965. R. Hiatt and S. W. Benson, J . Am. Chem. Soc., 94, 6886 (1972). D. G. Hughes and R. M. Marshall, J . Chem. Soc., Faraday Trans. 1 , 71, 413 (1975). D. M. Golden, K. Y. Choo, M. J. Perona, and L. W. Piszkiewica, Int. J . Chem. Kinet., 8, 381 (1976). J. Troe, J . Chem. Kinet., 8, 381 (1976). H. E. Van Den Bergh, Chem. Phys. Lett., 43, 201 (1976). K. G l h e r , M. Quack, and J. Troe, Chem. Phys. Lett., 39,304 (1976).
Photolysis of Tetramethylsilane at 147 nm. Reactivity of (CH&Si and (CH3)2SiCH2 S. K. Tokach and R. D. Koob" Department of Chemistry, North Dakota State University, Fargo, North Dakota 58 105 (Received August 25, 1978; Revised Manuscript Received January 8, 1979)
Results of the photolysis of tetramethylsilane at 147 nm are reported. The observed products, Hz, CH4, C2Hs, (CH3)3SiH,and (CH&Si2, along with accumulated evidence from additive experiments, suggest the primary processes (CH3)4Si CH4+ (CH&3CH2, (CHJ4Si CH3+ H + (CH&3iCH2,and (CH3)*Si CH3+ (CH3)3Si followed by combination, disproportionation, and abstraction reactions of the radical intermediates. The disproportionation/combination ratio for trimethylsilyl radicals is about 0.48. (CH3)$i abstracts hydrogen from a variety of donors approximately 20 times faster than CH3. Both observations were unexpected based on previous work involving (CH3)$i. (CH,)2SiCH2behaves as anticipated based on the previously reported photolysis of 1,l-dimethylsilacyclobutane.
-
Introduction Results of studies of the pyrolysis1,2and radiolysis3 of tetramethylsilane and the photolysis of methyl-4 and dimethyl~ilane~ may be found in the literature. However, the gas or liquid phase photolysis of tetramethylsilane has not been reported. The results of such work are of interest in that a number of intermediates whose reactivity is only partially understood are found as contributors to the total reaction mechanism. In the experiments reported below, 0022-3654/79/2083-0774$01 .OO/O
-
-
special attention is paid to trimethylsilyl free radical and dimethylsilylethylene. Experimental Section Tetramethylsilane (Me,Si) obtained from a commercial supplier (Norell Chemical Co., Inc., 99.5%) was purifed to 99.99% using preparative gas chromatography. DZS, CDBODand CD4 were obtained from Merck Sharpe and Dohme and were checked for isotopic purity by mass 0 1979 American Chemical
Society
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979 775
Photolylsis of Tetramethyisilane
TABLE I: Quantum Yields for the Photolysis of Tetramethylsilane
IR
-
.-
neat
CH, 'CZH, Me,SiH Me,Si,
0.36 i 0.04 0.53 * 0.07 0.12 t 0.02 0.12 * 0.03
4- 0
2
0.32 i 0.02
spectrometry. The appropriate corrections were made to data obtained using these additives. 0 2 was used without further purification. Only in the case of CD4did additive concentrations exceed 10% of the sample. The photolyses were carried out at 147 nm using a double-headed Xe resonance lamp powered by a 2450MHz Raytheon microwave generator. MgF2 windows formeld the interface between the lamp and each sample vessel. Parent to product conversion was held to less than 0.1% to avoid secondary reaction of products. Ethylene actinometry was used in the determination of quantum yields. A &C2H2= 0.90 was assumed6and the ratio of emiission from each window was regularly measured. Analysis of samples was performed by gas chromatography using a flame ionization detector. Detector response was calibrated for each of the observed products. A 26-ft 3% wt/wt squalane column was used to separate trimethylsilane (Me,SiH), Me,%, trimethylmethoxysilane (Me3SiOMe) and hexamethyldisilane (Me6Si2)l. A 25-ft 30% wt/wt squalane column was used to separate methane and ethane. Higher molecular weight products were not eluted. Retention times for each product were verified by authentic samples. Two methods were used to obtain samples for mass spectrlometric analysis. For hydrogens and methanes, the sample reaction vessel cooled to 77 K was attached directly to the mass spectrometer (Nuclide 3-60) inlet system. For trimethylsilanes, the appropriate peak was trapped from the GC stream at 77 K. The cold trap was evacuated and then warmed while attached to the mass spectrometer inlet. All photolyses were done at room temperature. If the slight warming from the discharge lamp is taken into account, the reaction occurred at 30 f 10 "C.
Results The photolysis of 30 torr of pure tetramethylsilane yields four products with the quantum yields of which are given in Table I. The formation of all products with the exception of CM4 is completely suppressed with the addition of oxygen. The quantum yield of CH4 decreases to 0.32 f 0.02 in thle presence of 0,. A solid polymeric material is deposited on the MgF2 window during the photolysis which causes a decrease in the transparency of the window. In order to correct for this piolymeric deposition, window emission ratios are determined before and after each quantum yield determination. The MgF, windows can be cleaned either mechanically or with trichloroethene. In the latter case, much care in taken in drying the sample cell and MgF, window to ensure removal of the solvent since the trichloroethene could act as a radical scavenger. Several additive studies have been carried out in the photolysis of tetramethylsilane. With the exception of CD4, the additives used in these studies absorb strongly at 147 nm and additive concentrations must be kept small. Of the additives used, ethene has the largest extinction coefficient. The quantum yield of acetylene formation from ethene a t 147 nm is high (0.90). Monitoring the appearance of acetylene formation serves as a convenient check to indicate what percentage of the light is being
Figure 1. Plot of the ratio of the yield of trimethyisiiane to hexamethyldisilane (HMDS) vs. the square root of reciprocal hexamethyldisilane concentration.
TABLE 11: Quantum Yields for Abstraction and Combination Products of Me,Si and CH, for Selected Concentrations of D,$,CD,OD, and CD, [XD] X 1 0 5 M 8.6
4.1
11.8
7.5
534
160
0.014
0.007
Quantum Yields CH,D Me,SiD CZH, Me,Si, kdk,
0.59 0.45 0.86 0.74 0.33 0.39 0.01 0.02 23.4 i 3.2
0.06 0.08 0.52
0.04 0.06 0.56 0.10 0.12 8.5 t 1.1
0.12 0.12 19.3
absorbed by the ethene. If additive concentrations are kept 57%, we find that absorption of light is primarily by the tetramethylsilane and that direct photochemical reaction of the additive does not contribute to the observed product distribution. When ethene is added in small concentrations, changes in the Me3SiH/Me6Si2ratio are observed. Results are presented in Figure 1 as a function of the absolute concentration of Me6Siz. Addition of D2S and CD30D to the photolysis mixture yielded CH3Dand Me3SiD. A mixture of tetramethylsilane and excess CD4when photolyzed also produced Me,SiD, however, in very low yields. Results for these additive experiments are presented in Table 11. In the case of CD30D and D2S,the results shown are for representative concentrations. Only the two concentrations of CD4shown were used as yields of Me3SiD were very small. Thus, the uncertainty of this measurement is high. When using the perdeuterated additives D2S and CD30D, much care is taken in deuterating the sample cell walls since in the absence of this precaution some exchange between the additive and cell wall could be observed, The mass spectrum of each additive was taken periodically to determine the percent deuteration. Corrections were made in cases of incomplete deuteration. The ratio Me3SiD/ Me3SiH is determined by monitoring m / e 59 and 60. These m / e values correspond to the parent - 15 ion. Corrections are made for contributions to peak heiights from naturally occurring isotopes. The contribution of CH3DC decomposition to m / e 16 is taken into account when determining the CH3D/CH4 ratio. For every sample where a D/H ratio is measured, a total quantum yield of product is also measured. No unexpected
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The Journal of Physical Chemistty, Vol. 83, No. 7, 1979
S. K. Tokach and R. D. Koob
ne0pentane.I The silicon analogue of isobutene is not a stable product, however, and the yield of the methane cofragmentb) which actually is produced as (CH3)2SiCH2 must be estimated by indirect methods. These experiments will be discussed in later paragraphs. Reactivity of Trimethylsilyl Radical. When tetramethylsilane is photolyzed in the absence of additives, ethane, trimethylsilane, and hexamethyldisilane are observed in addition to methane. These products are apparently the result of free-radical reactions. Both ethane and hexamethyldisilane are explained simply as the result of well-known radical combination reactions: I
I 25
I
I
IO
IS
20
2CH3
I
I
I
3,O
35
40
x lo-'
1
rcoPo1 ,-
Figure 2. Plot of reciprocal quantum yield of trimethylmethoxysilane (TMMS) vs. reciprocal concentration of CD,OD at a pressure of 25 torr.
0
3.6
--
CzHG
2Me3Si MeGSi2 The cross combination product, tetramethylsilane CH3 + Me3Si Me4Si
(2) (3)
-
(4) is not observable as it is identical with the starting material. The appreciable yield of trimethylsilane is not easily explained based on the limited information presently in the literature for trimethylsilyl radicals. Strausz e t al. suggests that disproportionation is unimportant, i.e., k 5 / k 3 = 0.046.8 2(CH3),Si (CH,),SiH + (CH,),SiCH2 (5) Morris and Thynne have suggested that an alternate source of trimethylsilane, abstraction of a hydrogen atom by the trimethylsilyl radical, is ~ n i m p o r t a n t . ~ (CH3),Si + (CH3),Si (CH3)3SiH+ CH,Si(CH,), (6) The rate constant for abstraction of H from CHI by trimethylsilyl is calculated from ref 8 to be only 8.6 X cm3mol-' s-I at 300 K. Even lowering the activation energy by 5 kcal/mol [the difference between Ea,,(CH3+CH4) and Ea,,(CH3+(CH3)4Si)]'oonly increases the rate constant by 4 orders of magnitude, still leaving k6 too small to account for any observed (CH,),SiH. To rationalize the presence of Me3SiH in good yield it is necessary to examine the importance of reactions 5 and 6 in detail. The oxygen scavenged experiments indicate that both Me6Sizand Me3SiH are excusively bimolecular in origin. If it is further assumed that only reactions 3, 5 , and 6 are important in the generation of these molecules then the following relationship can be derived:
-
0
20 1
I
I
1
IO
20
3D
40
I
50
I
I
60
70
Figure 3. Plot of reciprocal quantum yield of trimethylmethoxysilane vs. reciprocal concentration of CD,OD. Total sample pressure is 460 torr with N,:Me,Si = 17.4.
behavior in the total quantum yield was found that would change the interpretation of D / H ratio for any given product. Methanol yields trimethylmethoxysilane as a product when added to a Me4Si photolysis mixture. This species was confirmed by comparison of GC retention time with commercial trimethylmethoxysilane and by trapping at 77 K and examining the fragmentation pattern by mass spectrometry. From Figure 2, it can be seen that MeaSiOMe exhibits a methanol concentration dependence. Studies where the tetramethylsilane-methanol mixture is diluted with nitrogen to a total pressure of 460 torr show that the trimethylmethoxysilane yield increases with pressure a t a given methanol concentration. Results are presented in Figure 3. The quantum yields of all other products remain unchanged at higher pressures. Discussion
Products without Radical Precursors. When tetramethylsilane is photolyzed in the presence of oxygen as a radical scavenger only methane is observed as a product. This suggests methane is produced in the primary photoprocess (CH3I4Si.-%CH4 + [(CHJ2SiCH2] (1) The quantum yield for methane and thus for reaction 1 under these conditions is 0.32 f 0.02. [(CH3)2SiCH2]is the empirical composition of the methane cofragment (or cofragments) written in analogy to the isobutene formed in a reaction similar to reaction 1 in the photolysis of
-
t is the total photolysis time. [Me3SiH]/[Me6Siz] is constant in the neat photolysis, however, a change in the ratio can be induced by use of a radical interceptor. Ethene has been demonstrated to react efficiently with (CHJ3Si radicals,'' however, this reaction is not as efficient as (CH,),Si plus oxygen. Ethene can be varied over a convenient concentration range to change [Me3SiH]/ [Me6Si2]without actually completely eliminating either product. Figure 1presents the results of such experiments. The data were plotted using h3 = 1014.26 cm3 mol-' s-l as measured by Cadman et This gives k 5 / k 3 = 0.48 and h6 = 1.6 X 10: cm3 mol-' s-l. Both of these results are orders of magnitude higher than anticipated based on earlier work and bear further examination. k 5 / k 3 is conveniently discussed later along with (CH3)2SiCH2,thus we turn immediately to a more detailed examination of h@ Since CH3 is generated in abundance in the photolysis of tetramethylsilane and since the concentration of methyl radical is conveniently monitored by the ethane yield,
Photolysis of Tetramethylsilane
The Journal of Physical Chemistry, Vol. 83, No. 7, 7979 777
reaction 2, we have examined the relative reactivity of (CH3)i3i and CH,. This procedure has the advantage that the relative measurements are easily made. Moreover, the reactions of CH3 have been studied extensively and comparison can be made based on absolute rate constants for CH[3if desired. Using reactions 2 and 3 to monitor CH3 and (CH,),Si, respectively, and writing the generalized abstraction reactions CH3 + RD CH3D + R (7) (CH3)3Si RD (CH3),SiD + R (8)
+
--
one may write
_k8 -k,
[(Cb),SiDI [(CH3D)l
(
k3[CzHd )'Iz k2[Me&~l
The use of perdeuterated additives allows direct determination of the relative importance of abstraction by (CH3)3Siand CH3. Table I1 displays the data and resulting values for k8/k7 when RD = D2S and CD,OD and kz = cm3 mol-l s-l,13 Note that the occurrence of other abstraction reactions involving trimethylsilyl or the molecular production of CH4 will not interfere with these measurements since they yield nondeuterated products. In both cases D abstraction by (CH3)3Siis found to be more efficient than by CH3. This is in marked contrast to a ratio for k8/k7= based on the values in the literature for abstraction of H from methane by trimethylsilyl and CH3,respectively. As al test of the conclusion that (CH3I3Si abstracts hydrogen readily we photolyzed tetramethylsilane in excess CD, (which does not absorb light at 147 nm) and found that (CH3),SiD is produced. If reactions 3 and 8 [RD = CD,] are considered k8 =
(-")
'I2 [ (CH3),SiD] [Me6Si21 [CD,]
One calculates k8 = 3.21 f 0.58 x io3 cm3mol-1s-l using the data of Table I (converted to units of mol/cm3) and k3 = 1014.25c1n3 molm1as before. This may be compared with a value of h, = 1.66 X lo2 for the abstraction of D from CD, by CH3 calculated at 313 K from Dainton and McElcheran's evaluation of the Arrenhius parameters for that r e a ~ t i 0 n . lk8/k7 ~ is then found to be 19.3, 5;imilar to ratios found for D2S and CD30D. Measurements using four different additives C2H4, D2S, CD30D, and CD, and two different techniques (varying radical concentrations with C2H4 and observing directly the abstraction products with the labeled D2S, CD,OD, and CD4) dl indicate abstraction of hydrogen by trimethylsilyl generated by the photolysis of tetramethylsilane is an important r e a ~ t i 0 n . l ~ To test whether the reactivity of trimethylsilyl radical was the result of excess energy carried by it from the primary photodissociation, determinations of CH3D, (CH,),SiD, C2Hs, and MesSi2 were made with the tetramethylsilane diluted by a factor of 15 with nitrogen and total pressures up to 460 torr. No effect on k8/k7 outside of experimental error could be detected. We are inclined to believe, based on this observation, that the reactivity of the trimethylsilyl radical we have determined is charac1,eristic of thermal and near-thermal radicals and comparable values should be seen in other systems. It is perhaps useful to note that at 20 "C trimethylsilyl radicals add to sthylene over 10, times faster than methyl radicals while ethyl and n-propyl radicals, which abstract from a given hydrocarbon substrate more slowly than CH,, also
add more slowly to ethylene." While the analogy is weak, at least the higher rate of addition of (CH3),Si relative to CH, to ethene is consistent in this context with the more facile abstraction of H or D by (CH3),Si relative to CH3 from various substrates. In the pyrolysis of (CH3),Si, Clifford et al. find trimethylsilane to be one of their two highest yield products.' They ascribe a high activation energy to reaction 6 (18 kcal/mol) and attribute the yield entirely to disproportionation, k!,/k3 = 1.2 at 900 K. In a related system, the pyrolysis of hexamethyldisilane, Davidson and Howard, determine Arrhenius parameters for the abstraction of H from Me,&? by trimethylsilyl radical to be log A = 13.2 and E A = 17.2 over the temperature range 770-872 K.16 However, they consider this reaction to be the only important source of trimethylsilane and do not conE;ider disproportionation as a significant reaction in direct contrast to the work of Clifford et al. In the radiolysis of tetramethylsilane, Mains and Dedinas, attribute all observed trimethylsilane to reaction 6 but offer no evaluation of k6, 1,I-Dimethylsilaethene.A plausible cofragment in the intramolecular photoproduction of CH4 is 1,l-dimethylsilaethene ((CH3),SiCH2). (CH3)2SiCH2is not stable, however, and its reaction products in room temperature photolyses are unknown. Methanol may be used as a titrant and produces trimethylmethoxysilane in a1 1,2 addition to (CH3)2SiCHz.17Unfortunately, small amounts of methanol (
