THE J O U R N A L O F
PHYSICAL CHEMISTRY Registered i n U.S.Patent Ojfice
0 Copyright, 1980, by the American Chemical Society
VOLUME 84, NUMBER 1
JANUARY 10,1980
Photolysis of Hexamethyldisilane at 147 nm S,, K. Tokach and R. D. Koob" Department of Chemistry, North Dakota State Unlverslty, Fargo, North Dakota 58105 (Recelved March 12, 1979; Rwvised Manuscript Received September 28, 19'79)
Hexamethyldisilane vapor was photolyzed at 147 nm and various additives were used to intercept suspected reactive intermediates. The following primary products (quantum yields) were deduced from the product, distribution of the collected experiments: (CH3),Si (0.99), CH3 (0.51), (CH3)2SiCH2(0.41), (CH3),SiH (0.26), H (0.201, CHI (0.08), and H2 (0.06). The cross disproportionation to combination ratio of CH3and (CH,),Sj is determnned to be 0.22 f 0.06 with no observable dependence on pressure or diluent (N,) concentration. (CH3),Si abstraction of hydrogen competitive with radical recombination rates is also consistent with the results. The rate of reaction of (CH3)SiCH2with methanol is found to be similar to that of (CH3)SiCH2produced by however, the pressure dependence found for this reaction photolyzing (CH3)& and l.,l-dimethyl-it-silacyclobutane; in the last two systems iEi absent for /CH3)2SiCH2produced in the photolysis of hexamethyldisilane.
Introduction Two reactive intermediates encountered frequently in the chemistry of alkylsilanes are the silicon analogue to isobutene, 2-methyl-2-silapropene (MSI'), and the silicon analogue to the tert-butyl radical, the trimethylsilyl radical ((CH3)3Si).Despite the importance of these intermediates in many reaction systems, limited quantitative information is available for these species. Some gas phase reactions of MSP have been characterized quantitatively previously in the thermal work of Gusel'nikov and Flowers1 and later in the photolyines of tetramethylsilane (TMS12and 1,1-dimethylsilacyclobutane (DMSCB)., Trimethylsilyl radical reactions have been investigated in relatively few systems@ and the kinetic data which, have been reported are often conflicting. The range of values from 0.05' to 1.P has been reported for the disproportionation/combination (D/C) ratio for two trimethylsilyl radicals. Recently, trimethylsilyl radical reactions, disproportionation, combination, and abstraction, were characterized in the 147-nm photolysis of TMS,2which generates the (CH3)3Siin high yield. A D/C ratio of f-0.48 was determined and trimethylsilyl radicals were found to be more reactive toward abstraction than methyi raclicals. In a preliminairy investigation of the photolytic decomposition of he~amethyldisilane~ (HMDS), the intramo0022-3654/8Q/2084-0001$01.OO/O
lecular cofragment to the MSP, trimethylsilane, was r e ported as an important product. MSP was inferred as EI major intermediate in the photolytic decomposition, though no direct, evidence for the existence of this species was presented. Results presented here on methanol addition experiments and quantum yield determinations in the photolysici of HMDS confirm the importance of the MSP intermediate to the reaction mechanism. Examination of yields of products whose precursors are trimethylsilyl radicals substantiates the conclusions drawn from the tetramethylsilane photolyses. The production of methyl and trimethylsilyl radicals allows the determination of a D/ C ratio for this radical pair.
Experimental Section HMDS was purchased from Petrarch and purified to 99.9% purity by preparative gas chromatography. Oxygen, ethylene, D2S(Matheson), HI (Linde), and CDBOD(Merck Sharpe and Dohm) were also used without further purification. The photolyses were carried but at 147 nm by using a double-headed, Xe-filled resonance lamp as described in earlier work., Samples were prepared for irradiation by using standard vacuum techniques. Typical sample pressures were 14 torr. 0 1980 American Chemical Society
2
The Journal of Physical Chemistty, Vol. 84, No. 1, 1980
Tokach and Koob
TABLE I: Quantum Yields in the Photolysis of HMDS at 147 nm additive(s) partial press., torr
none
0 2
1.o
MeOHa MeOH,a MeOH, N. C,H, m
HZ S
HI
-~:465
1.*36, 4.0
2.0
0.4
0.12 0.16 0.62 0.19 0.69
0.10 ndb
0.60k 0.08 0.03 f 0.02 1.22 ?: 0.20 0.01 i 0.02
0.59 i 0.05
product
0.12 f 0.02 0.08 f 0.02 CH.4 0.17 f 0.03 CZH, (CH,),SiH 0.26t 0.04 0.62 i 0.08 (CH,),Si 0.18 f 0.02 (CH,),Si( CH,D)OCD, a m MeOH, obtained from plot of product yield as a function termined.
Ethylene was used as an actinometer and a quantum yield of 0.90 for acetylene formation was assumed.1° The ratio of light intensities through the MgFz windows was measured before and after each quantum yield determination since some polymer deposition on the window occurred as noted in other organosilane photolyses.2 Polymer deposition was suppressed in the presence of both oxygen and methanol. Sample analysis was performed by FID gas chromatography with a 25-ft 30% squalane column at room temperature to separate CHI, C2H6,TriMS, and TMS. A 25-ft 3% squalane column at 50 "C was used to determine trimethylmethoxysilane. A Nuclide isotope-ratio mass spectrometer was used in the analysis of H2 and trimethylmethoxysilane. The sample cell was attached directly to the mass spectrometer inlet system. The volatile gases at -196 "C were quantitatively transferred into the mass spectrometer by Toepler pump and the H2/CH4ratios measured. The mass spectral response to Hz and CHI was calibrated by admitting equal pressures of the compounds and monitoring the m / e values of 2 and 16, respectively. The TMMS was trapped from the GC stream in a trap cooled at -196 "C. The cold trap was evacuated and then warmed while attached to the mass spectrometer inlet. The ratio TMMS-d4/TMMS-d3 was determined from the ion beam intensity ratio of m / e 93 and 92. These masses correspond to the (P- 15)+ion for the d4 and d3 compounds, respectively. Each methyl was considered equivalent and a statistical correction was applied to obtain a value for [TMMS-d4]/[TMMS-d3]. In addition, the isotopic purity of the CD30D was regularly measured. For the data reported the methanol used was 90% d4 and 10% d3. The upper limit of I(93)/I(92) is then 1.51. The average value found was 1.47 f 0.05. Within experimental error, we conclude that reaction 1 is the only source of TMMS. The possibility of a dark reaction between CD30D and (CH3)6Sizwas investigated by allowing the two compounds to stand together in a reaction vessel for 24 h. Analysis revealed no TMMS. Results
Quantum yields of products measured in the presence and absence of various additives are presented in Table I. The products observed in the 147-nm photolysis of 14 torr of pure HMDS are hydrogen, methane, ethane, trimethylsilane (TriMS), and tetramethylsilane (TMS). 02 completely suppresses the formation of ethane and TMS and decreases the yields of hydrogen, methane, and TriMS. Those products whose yields are affected by the O2 addition are assumed to have radical precursors. The persistence of methane and TriMS in the presence of 0 2 indicates that an intramolecular elimination contributes to their production.
0.12 0.16 0.62 0.18 0.69
0.28 0.04 0.41
of methanol concentration, Figure 1.
- I
1.27 * 0.18 nd denotes not de-
p'
zoy 0
IOL I
0
I5 0
I 30 0
'
LC40D],rrole/l fer
q5 0
x
t 60 0
-4
IO
Flgure 1. A least-squares fit to a plot of reciprocal TMMS quantum yield vs. reciprocalmethanol concentration. Intercept = 1.45. 0 refer to ylelds at a total sample pressure of 14 torr. A refer to yields at a total sample pressure of 460 torr.
Addition of methanol-d4 to the HMDS photolysis leads to formation of trimethylmethoxysilane-d4(TMMS). This product was monitored as a function of CD30D concentration. These results are presented in Figure 1. In all experiments the HMDS was held at a constant pressure of 14 torr and CD30D concentrations are kept low (510%). At these concentrations, it is assumed that direct photochemical reaction of the additive does not contribute to the reaction mechanism. Mass spectral analysis of the TMMS confirm this product to be the expected (CH3)zSi(CHzD)OCD3. Increasing the total sample pressure to 465 torr with Nz led to no observable changes in product yields. Addition of CzH4 to the HMDS/CD30D mixture, at a given CD30D concentration, decreases the TMMS yield. At 9.5% CD30D, the quantum yield of the TMMS is reduced to 0.41 by the addition of C2H4.l' The quantum yields of the TriMS and TMS were also monitored in these experiments and were reduced to 0.28 and 0.04, respectively. Though D2Shas been reported as an unsuitable radical scavenger for the (less reactive) tert-butyl radical,12 addition of H2S to the HMDS photolysis results in a marked reduction of CzH6 and TMS yields and enhancement of the CH4 and TriMS yields. For completeness, HI was added to the HMDS. As can be seen from Table I, the results from these two additional experiments do not differ within experimental error. Quantum yields for production of CH3and Me3Si radicals are measured by these additions to be 0.52 f 0.08 and 0.99 f 0.24, respectively. The extinction coefficients of CD,OD, DzS, and HI have all been measured.13 The largest of these at 147 nm is 1200 compared to a value of 25000 for HMDS. Since no additive exceeded 10% of HMDS the largest fraction of light
The Journal of Physical Chemistty, Vol. 84, No. 7, 1980 3
Photolysis of Hexamethyldisilane
absorbed by any additive was
