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The vacuum ultraviolet spectra of methylated silanes
Volume 13, number 6
15 April 1972
CHEMICAL PHYSICS LETTERS
THE VACUUM ULTRAVIOLET
SPECTRA OF METHYLATED
A.G. ALEXANDER, Department
of Chemistry,
SlLANES
O.P. STRAUSZ
University of Alberta, Edmonton,
Alberta, Canada
and R. POTTIER Department
of Chemistry,
and G.P. SEMELUK
University of Nero Brunswick, Freden-cton, New Brunswick.
Chada
Received 16 February 1972
The vacuum ultraviolet spectra of monomethylsilane, monomethylsilane~~, dirnethylsilane, dimethylsilnne4~, 1, trirnethylmonofluorosilane and tetramethylsilane have been recorded in the trimethylsikme, trimethylsilan~d range 135 nm to 190 nm.
Spectroscopic data on the lower silicon hydrides and their methyl derivatives are sparse. Harada et al. [i] reported low resolution vacuum UV spectra for five of these molecules, namely SiH,, (CH3)2SiH2, (CH,),SiH, (CH,),Si and (CH3)6Si2. The structureless continuum common to all these spectra resembles those observed in the hydrocarbon UV spectra except that they are shifted to the red with enhanced intensi-
WAVELENGTH Fig. 1. Spectra
of monomethyisilane methyIsilan&a
608
ty. In all the silicon hydrides examined, there was no indication of transitions from the bonding orbitals of the CH3 group to the vacant 3d orbitals of the silicon. In connection with current studies [2] on the photochemistry of silicon hydrides we wish to report the vacuum -UV spectra of alI the methylated monosilanes, the deuterium substituted analogs, and trimethylfluorosilane.
WAVELENGTH
(nm)
{solid fine) and mow (broken line).
Fig. 2. Spectra
(nm)
of dimethykilane (solid line) and dimethylsikimxi2 (broken line).
CHEMICAL
Volume 13, number 6
0
”
120
I
”
140
’
n ”
WAVELENGTH
Fig. 3. Spectra
160
’
Ii!
IS0
PHYSICS
15 April
LETTERS
1972
” WAVELENGTH’
(nm)
of trimethylsilane (solid line) and trimethylsilanedl (broken line).
The instrument used was a commercial Jarrell-Ash, one meter 15” Robin vacuum UV double beam spectrophotometer with a spectral range of 1350-3600 a. A detailed description of this apparatus can be found in the literature [3]. The light source was the continuum of a deuterium discharge lamp. Four centimeter cells with 2 mm thick LiF windows were used. The pressure range employed was 0.1-0.5 torr. The principal features of the present spectra, shown in figs. I-4 agree with the earlier data of Harada et al., with the notable distinction that in.the present spectra the maxima around 1400 h\ appear to contain some structure_ This, taken in conjunction with the observation that decomposition of photoexcited silicon hydride molecules is unaffected by pressure up to at least 1000 torr but is strongly suppressed in the condensed phase, places the decomposition lifetime in the order of zIO-~~ sec. With a resolution of 0.15 8, no structure is apparent near the onset of absorption. The appearance of the windows with minima at 1540 a in the tetramethylsilane and at 1500 i%in the trimethylfluorosi!ane spectra would seem to suggest that the main absorber in the 1500 .& to 1600 a region in the partially methylated silanes is the Si-H bond. The longer wavelength transitions are probably
Fig. 4. Spectra
(nm)
of trimethyhnonofluorosikne tetramethylsilane (broken
(solid line) and line).
associated with the Si-C bond absorption and the structured maxima centered at x1400 A with the C-H bond absorption. Deuteration as expected [4] has little effect on the spectra. Substitution of fluorine for the tertiary hydrogen in trimethylsilane has the effects of: shifting the 1400 A maxima slightly to the blue, eliminating the structural features and suppressing the intensity. The effect may be related to d,-p, bonding between silicon and fluorine or to inductive effects. Further studies are in progress. The authors are grateful to the National Council of Canada for financial support.
Research
References Y.Harada, J.N.hlurrell snd H.H.Shcena, Chem. Phys Letters 1 (1968) 59.5. A.G.Alcxander et al., to be published. R.G.Schmitt and R.K.Brchm, Appi. Opt. 5 (1966) 1111. M.Sauer Jr. and L.M.Dorfmen, J. Chem. Phys 35 (1961) 497; H.Okabc and J.R.hlcNesby, J. Chem. Phys. 34 (1961) 668; 37 (1962) 1340.
609
15 April 1972
CHEMICAL PHYSICS LETTERS
THE VACUUM ULTRAVIOLET
SPECTRA OF METHYLATED
A.G. ALEXANDER, Department
of Chemistry,
SlLANES
O.P. STRAUSZ
University of Alberta, Edmonton,
Alberta, Canada
and R. POTTIER Department
of Chemistry,
and G.P. SEMELUK
University of Nero Brunswick, Freden-cton, New Brunswick.
Chada
Received 16 February 1972
The vacuum ultraviolet spectra of monomethylsilane, monomethylsilane~~, dirnethylsilane, dimethylsilnne4~, 1, trirnethylmonofluorosilane and tetramethylsilane have been recorded in the trimethylsikme, trimethylsilan~d range 135 nm to 190 nm.
Spectroscopic data on the lower silicon hydrides and their methyl derivatives are sparse. Harada et al. [i] reported low resolution vacuum UV spectra for five of these molecules, namely SiH,, (CH3)2SiH2, (CH,),SiH, (CH,),Si and (CH3)6Si2. The structureless continuum common to all these spectra resembles those observed in the hydrocarbon UV spectra except that they are shifted to the red with enhanced intensi-
WAVELENGTH Fig. 1. Spectra
of monomethyisilane methyIsilan&a
608
ty. In all the silicon hydrides examined, there was no indication of transitions from the bonding orbitals of the CH3 group to the vacant 3d orbitals of the silicon. In connection with current studies [2] on the photochemistry of silicon hydrides we wish to report the vacuum -UV spectra of alI the methylated monosilanes, the deuterium substituted analogs, and trimethylfluorosilane.
WAVELENGTH
(nm)
{solid fine) and mow (broken line).
Fig. 2. Spectra
(nm)
of dimethykilane (solid line) and dimethylsikimxi2 (broken line).
CHEMICAL
Volume 13, number 6
0
”
120
I
”
140
’
n ”
WAVELENGTH
Fig. 3. Spectra
160
’
Ii!
IS0
PHYSICS
15 April
LETTERS
1972
” WAVELENGTH’
(nm)
of trimethylsilane (solid line) and trimethylsilanedl (broken line).
The instrument used was a commercial Jarrell-Ash, one meter 15” Robin vacuum UV double beam spectrophotometer with a spectral range of 1350-3600 a. A detailed description of this apparatus can be found in the literature [3]. The light source was the continuum of a deuterium discharge lamp. Four centimeter cells with 2 mm thick LiF windows were used. The pressure range employed was 0.1-0.5 torr. The principal features of the present spectra, shown in figs. I-4 agree with the earlier data of Harada et al., with the notable distinction that in.the present spectra the maxima around 1400 h\ appear to contain some structure_ This, taken in conjunction with the observation that decomposition of photoexcited silicon hydride molecules is unaffected by pressure up to at least 1000 torr but is strongly suppressed in the condensed phase, places the decomposition lifetime in the order of zIO-~~ sec. With a resolution of 0.15 8, no structure is apparent near the onset of absorption. The appearance of the windows with minima at 1540 a in the tetramethylsilane and at 1500 i%in the trimethylfluorosi!ane spectra would seem to suggest that the main absorber in the 1500 .& to 1600 a region in the partially methylated silanes is the Si-H bond. The longer wavelength transitions are probably
Fig. 4. Spectra
(nm)
of trimethyhnonofluorosikne tetramethylsilane (broken
(solid line) and line).
associated with the Si-C bond absorption and the structured maxima centered at x1400 A with the C-H bond absorption. Deuteration as expected [4] has little effect on the spectra. Substitution of fluorine for the tertiary hydrogen in trimethylsilane has the effects of: shifting the 1400 A maxima slightly to the blue, eliminating the structural features and suppressing the intensity. The effect may be related to d,-p, bonding between silicon and fluorine or to inductive effects. Further studies are in progress. The authors are grateful to the National Council of Canada for financial support.
Research
References Y.Harada, J.N.hlurrell snd H.H.Shcena, Chem. Phys Letters 1 (1968) 59.5. A.G.Alcxander et al., to be published. R.G.Schmitt and R.K.Brchm, Appi. Opt. 5 (1966) 1111. M.Sauer Jr. and L.M.Dorfmen, J. Chem. Phys 35 (1961) 497; H.Okabc and J.R.hlcNesby, J. Chem. Phys. 34 (1961) 668; 37 (1962) 1340.
609