J. Phys. Chem. 1985,89, 1155-1 156
1155
Methylene-Methyl Exchange In the Reaction of Triplet-State CD, with Methylfluorosllanes T. N. Bell,* A. G. Sherwood, and G. Soto-Garrido Department of Chemistry, Simon Fraser University, Burnaby, British Columbia, Canada V5A IS6 (Received: August 21, 1984; In Final Form: October 25, 1984)
Ketene, photolyzed at 3 13 nm, in the presence of methylfluorosilanes, leads to the reactions of singlet and triplet methylene with these compounds. Of particular interest here are the reactions of 'CD2 and the subsequent formation of acetylene, ethane, and ethylene. From an analysis of the isotopic components and their dependence on silane concentration, we suggest an isotopic scrambling exchange process via a five-coordinate intermediate to explain the results: CD2H
-/ CD2
3CD2
+
CH,Sif
S 'c i
or CDH, or
CH3
H + \"C-Sif
x = O to 2
/
02-#
CH3
A similar exchange process is also proposed involving the hydrogen atom.
C D 2 C 0 (=1.8 torr) was photolyzed with 313-nm radiation (producing both singlet state ('CD,) and triplet-state (3CD2)under these conditions), in the presence of varying amounts of dimethyldifluorosilane ((CH3)2SiF2). Addition of NO (=0.1 torr) was sufficient to totally suppress triplet and radical reactions, as evidenced by the absence of acetylene and ethanes. The reactions of 'CD2 were thus studied in the presence of NO and wefe as expected: namely, insertion into the C-H bond of the methylsilane leading to CD2HCH2Sif, together with a minor amount of insertion into the Si-F bond. CzD4 was also formed 'CD2 + CDzCO C2D4 + CO +
Of interest in this article are reactions carried out in the absence of NO which thus include the reactions of 3CD2, leading to acetylene, ethanes, and some of the ethylene formed. Analysis of these hydrocarbon products was by gas chromatography and mass spectroscopy, with the results shown in Figures 1 and 2. These show equal amounts of CzD2and C2D6 in the absence of silane and a significant decrease in these products as the silane pressure increases. The total ethane, however, increases slightly with silane pressure and, thus, C2D6 is replaced by various mixed deuterioethanes. Quantitative estimates were made of the ethanes CzD6, C2D,H, C2D4HZ,and C2D3H3from the mass spectrometric cracking patterns. This was not possible for the m / e 32 parent corresponding to C2D2H4,though all indications are that this is formed. The only acetylene formed is C2D2and it is generally accepted that this arises from triplet 3CDz + 3CD2 C2D2+ (2D or D2) (1) Ethane is generally accepted to arise from methyl recombination CD3 + CD3 C2D6 (2) The equivalence of the yields of C2D6 and C2D2observed is in agreement with Rowland's proposal' that hydrogen atoms resulting from reaction 1 lead to methyl radicals (reaction 3). D + CD2CO CD, + CO (3) The decrease in the C2D2as the silane pressure increases requires some competitive reaction of 3CD2,involving the silane. Further, the formation of C2D5H(reaction 4) requires the forCDZH CD3 C2DSH (4)
-
+
+
-
(1) Russell, R. L.; Rowland, F. S.J. Am. Chem. SOC.1970, 92, 7508. (2) Laufer, A. H. Rev. Chem. Intermed. 1981, 4, 225. (3) Canmamas, C. E.; Frey, H. M.; Walsh, R. J . Chem. SOC.Faruday Trans. 2 1984, 80, 561.
mation of the CD2H radical. C2D4H2would arise from either or both of the recombinations, (5) or (6), and C2D3H3from (7) and/or (8). 2CD2H -* CzD4Hz CD3 + CDH2 C2D4HZ
-
+
+ CH3 CD2H + CH2D CD3
C2D3H3
+
C2D3H3
(5)
(6) (7)
(8)
Ethylene formed from 3CD2has been suggested4v5to occur through its reactions with methyl. These triplet and radical reactions are not observed in the presence of NO. Thus, from the methyl radicals participating in reactions 5-8, we account for the observed ethylenes (Figure 2) through reactions 9-14.
+ CD3
-
+D 3CD2+ CD2H CD2CD2+ H CD2 CDH + D 3CD2+ CDH2 CD2CH2 + D CD2CDH + H 'CD2 + CH3 CDzCH2 + H 3CD2
+
-
- -
C2D4
(9) (10) (1 1) (12) (13) (14)
An examination of the ethanes (Figure 1) suggests that CD2H is likely to be an important methyl radical and, thus, if CH3 were not formed, a considerable imbalance would be expected in the olefins produced from reactions 9-13, with CD2CDH >> CDzCH2. Reference to Figure 2 suggests this is not so and, thus, reaction 14 is likely to be of consequence. The importance of the CD2H radical would be accounted for through the H atoms, resulting from olefin formation (reactions 10, 13, and 14) reacting with ketene, thus H + CD2CO CD2H + CO (15) +
The competitive reaction of H or D with ethylene, leading to ethyl radicals, is equally as fast as reactions 3 or 15. This competition is not included, however, because of the ethylene/ketene ratio, initially zero and never exceeding 1/40 in those experiments of maximum conversion. In order to account for the decrease in C2D2and in C2D6 and to sustain the total ethane through the formation of additional (4) Laufer, A. H.; Bass, A. M. J . Phys. Chem. 1975, 78, 1635. (5) Rlling, M. J.; Robertson, J. A. Chem. Phys. Lett. 1975, 33, 336.
0022-3654/85/2089- 1155%01.50/0 0 1985 American Chemical Society
Bell et al.
1156 The Journal of Physical Chemistry, Vol. 89, No. 7, 1985
reaction. Under the low-intensity, steady-state photolysis conditions employed, it is not conceivable that this sequence could account for the amounts of duterioolefins and -ethanes under question. We therefore propose a ’CD2silane reaction involving a radical scrambling exchange reaction as a more likely precursor process, thus
-
3CD2 -t CH3Sif
+
CH,Sif
CD2H
-
P SILANE (TORR)
CD2 )Si+
CD2H
S ‘c i
(17)
/
CH2
Figure 1. Ethane formation as a function of PMo2SiF2. (Ketene pressure constant at 1.8 torr.) CDHSif 80\
+
CH2D
-
CDH
\ S i c
/
CH2D
S O i
The proposal of (1 7) would also imply a similar reaction involving H or D atoms (D)(H)
H or D
+
CHPif
- lie /
(H)(D)Sif i-
(18)
CH3
CH3
An estimate6s7of the enthalpy of reaction 17 is -12 kcal/mol and of (18) is -23 kcal/mol, the former being similar to an estimate for that for the system 3CH2+ CHI 2CH3. The suggestion of a five-coordinate intermediate has been made to explain radical exchange in group 4 alkyl systems* and in the reaction of methylflu~rosilanes~ with I17 NO. Unpublished calcu1ationsl0 on the latter system indicate an exothermic process (4 kcal/mol) with no barrier to the formation of the intermediate. In the reaction of H and D atoms with silanes, Potzinger” suggested the possibility of a five-coordinate SiHSintermediate radical which would decompose to H2 + SiH3 as an explanation for a “paradoxical” result. A further study of this latter reaction by Michael et a1.I2 showed the formation of deuterated silane. These results were explained in terms of a vibrationally hot molecule cascade process, resulting in sequential deuteration of the silane. The scrambling exchange scheme proposed in this present paper could also account for such deuteration and such exchange would not produce deuterated silyl radicals, only molecules. This would account for the puzzling fact that the only observed disilane was SizH6,resulting from the combination of SiH3 from the fast H abstraction reaction. Similarly, in the reaction of D with the MexSiH4-xsystem, the formation of H atoms and CH3 radicals was observed. Again, an exchange process of the type proposed here would explain this observation.
-
V
0
I
10
1
1
1
I
20
30
40
50
P SILANE (TORR)
Figure 2. Ethylene formation as a function of PMc2SiF2. (Ketene pressure constant at 1.8 torr.)
isotopically mixed ethanes, some conversion reaction of ’CD2 to methyl radicals is required. The obvious process is the H abstraction process 3CD2+ CH3Sif
-
CD2H + CH2Si+
(16)
however, not so easy to explain is a process leading to CH3 and/or CH2D, required to account for C2D3H3through (7) or (8). The reaction of H or D with the CH,Sif radical resulting from (16) could be considered, Le. H or D
+ CH2Sif
-
CH2D or CH3 + S i f
While this reaction is the~nodynamically6.~ attractive (AH= -12 kcal/mol), the resultant olefins or ethanes from CH2D or CH3 would require a subsequent consecutive bimolecular radical-radical ( 6 ) J . Phys. Chem. Ref. Data, Suppl. 1982, 11. (7) Bell, T. N.; Perkins, K. A.; Perkins, P. G. J . Chem. SOC.Faraday Trans. 11981, 77, 1779.
Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada for grant support. Registry No. (CH3),SiF2, 353-66-2; CHI, 2465-56-7; CH,.,222907-4; CD2, 14863-68-4.
(8) Bell, T. N.; Platt, A. E . J. Chem. Soc. D 1970, 325. (9) Bell, T. N.; Soto-Garrido, G.; Shewood, A. G. Can. J. Chem. 1982, 61, 946. (10) Bell, T. N.; Perkins, P.G.; Perkins, K. A., personal communication. (1 1) Glasgow, L. C.; Olbrich, G.; Potzinger, P. Chem. Phys. Lett. 1972, 14, 466. (12) Cowfer, J. A.; Lynch, K.P.; Michael, J. V.J. Phys. Chem. 1975,79, 1139.
