Decomposition of 1,1-dimethyl-1-silacyclobutane on a tungsten filament - Evidence of both ring C-C and ring Si-C bond cleavages

Article abstract of DOI:10.1002/jms.1712

The decomposition of 1,1-dimethyl-1-silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum ultraviolet laser single photon ionization time-of-flight mass spectrometry. It is found that the decomposition of DMSCB on the W filament to form ethene and 1,1-dimethylsilene is a catalytic process. In addition, two other decomposition channels exist to produce methyl radicals via the Si-CH₃ bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage of a ring C-C bond and a ring Si-C bond, respectively, to form diradical intermediates, followed by the breaking of the remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C-C bond is dominant over that of propene via an initial cleavage of a ring Si-C bond. When the collision-free condition is voided, secondary reactions in the gas-phase produce various methyl-substituted 1,3-disilacyclobutane molecules. The dominant of all is found to be 1,1,3,3-tetramethyl-1,3-disilacyclobutane originated from the dimerization of 1,1-dimethylsilene. Copyright

Full text of DOI:10.1002/jms.1712

Research Article
Received: 29 June 2009
Accepted: 4 December 2009
Published online in Wiley Interscience: 4 January 2010
(www.interscience.com) DOI 10.1002/jms.1712
Decomposition of 1,1-dimethyl-1-silacyclo-
butane on a tungsten filament – evidence
of both ring C–C and ring Si–C bond cleavages
L. Tong and Y. J. Shi∗
The decomposition of 1,1-dimethyl-1-silacyclobutane (DMSCB) on a heated tungsten filament has been studied using vacuum
ultraviolet laser single photon ionization time-of-flight mass spectrometry. It is found that the decomposition of DMSCB on
the W filament to form ethene and 1,1-dimethylsilene is a catalytic process. In addition, two other decomposition channels
exist to produce methyl radicals via the Si–CH3 bond cleavage and to form propene (or cyclopropane)/dimethylsilylene. It has
been demonstrated that both the formation of ethene and that of propene are stepwise processes initiated by the cleavage
of a ring C–C bond and a ring Si–C bond, respectively, to form diradical intermediates, followed by the breaking of the
remaining central bonds in the diradicals. The formation of ethene via an initial cleavage of a ring C–C bond is dominant over
that of propene via an initial cleavage of a ring Si–C bond. When the collision-free condition is voided, secondary reactions
in the gas-phase produce various methyl-substituted 1,3-disilacyclobutane molecules. The dominant of all is found to be
c
1,1,3,3-tetramethyl-1,3-disilacyclobutane originated from the dimerization of 1,1-dimethylsilene. Copyright ꢀ 2010 John
Wiley & Sons, Ltd.
Keywords: reaction mechanisms; 1,1-dimethyl-1-silacyclobutane; vacuum ultraviolet laser ionization mass spectrometry; tungsten;
chemical vapor deposition
Introduction
The significant ring strain energy in the four-membered ring
structure in SCB molecules[18,19] reduces the decomposition tem-
peraturescomparedwiththoseusingopen-chainalkylsilanes.[14,15]
Furthermore, reactive silene and silylene species produced in the
decomposition process are potentially important thin film precur-
sors. By using DMSCB as a single-source gas in a low pressure
chemical vapor deposition (LPCVD) process, Chiu et al. success-
fully produced amorpho◦us SiC (a-SiC) thin films at temperatures
between 950 and 1100 C.[20] Compared with LPCVD, hot-wire
chemical vapor deposition (HWCVD),[21] also known as catalytic
chemical vapor deposition (cat-CVD),[22] is a new and promis-
ing technique to produce device-quality Si-containing films at
low substrate temperatures. Zaharias et al. reported the forma-
tion of a-SiC using HWCVD with DMSCB at a tungsten filament
temperature of 1550 ◦C on a Si(100) substrate maintained at
250 ◦C.[23] In HWCVD, source gases are catalytically decomposed
to radicals on a heated metal wire, usually made of tungsten.
The secondary radical–radical and radical–molecule reactions in
the gas-phase produce the final mix of precursors to react with
the substrate surface leading to thin film formation. These pre-
cursors are believed to be important in determining the film
growth rate and the film properties. Therefore, the nature of
the source gas decomposition on the hot tungsten filament and
subsequent gas-phase reactions is important for an understand-
ing of film growth mechanism. Although the gas-phase pyrolysis
and photolysis of DMSCB have been well studied, little is known
Thermal and photo-induced decomposition of 1,1-dimethyl-1-
silacyclobutane (DMSCB) in the gas-phase have been of long-
standing interest.[1] It has been well established through the
pioneering work of Gusel’nikov and Flowers that gas-phase
pyrolysisofDMSCBisacleanandunimolecularreactiontoproduce
ethene and transient 1,1-dimethylsilene species which can either
react with a chemical trapping agent or dimerize to 1,1,3,3-
tetramethyl-1,3-disilacyclobutane (TMDSCB) in the absence of
trapping agents.[2,3] A theoretical study by Gordon et al. has
suggested that the formation of ethene and silene from the parent
silacyclobutane (SCB) molecules via the formation of a diradical
intermediate by an initial cleavage of a ring C–C bond followed
by the rupture of the central Si–C bond is the most favorable
pathway both kinetically and thermodynamically.[4] However, the
question of whether a stepwise diradical process occurs for the
pyrolysis of DMSCB has not been answered. Experimental studies
of the pyrolyses of various methyl- or phenyl-substituted mono-
SCB molecules have shown that an initial ring C–C bond cleavage
predominates over a ring Si–C breakage.[5–8] Whether this holds
for the pyrolysis of DMSCB is not clear. Ethene and TMDSCB
were also found to be the main products in the gas-phase
photolysis of DMSCB using either a ultraviolet (UV) or a vacuum
UV beam.[9–13] In addition, two minor decomposition pathways
toproducedimethylsilylene/C3H6 (cyclopropaneorpropene)[9–12]
and methyl radicals,[9,11] respectively, were observed in photolysis.
Recently, there has been an increasing interest in the investiga-
tionofalternativesingle-sourceprecursorgasestoreplacethecon-
ventionally used mixtures of separate Si- (silane or chlorosilanes)
and C-containing (various hydrocarbons) gases for silicon carbide
thin film formation using chemical vapor deposition (CVD).[14–17]
∗
Correspondence to: Y. J. Shi, Department of Chemistry, University of Calgary,
Calgary, Alberta, T2N 1N4, Canada. E-mail: shiy@ucalgary.ca
Department of Chemistry, University of Calgary, Calgary, Alberta, T2N 1N4,
Canada
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L. Tong and Y. J. Shi
about the decomposition chemistry of DMSCB on a hot tungsten
filament.
DMSCB (≥97%, Sigma-Aldrich) and TMDSCB (97%, Starfire
Systems) were used without further purification and its purity
was verified in situ by our TOF mass spectrometer. DMSCB-d6
was synthesized by reacting 1,1-dichloro-1-silacyclobutane with
CD3MgI (99% atom% D, Sigma-Aldrich) according to a previously
reported method.[31] Its structure and purity (>98%) were checked
by NMR, GC-MS and TOF mass spectrometer. Isotope purity
of DMSCB-d6 (99% atom% D) was confirmed by TOF mass
spectrometer. The DMSCB, DMSCB-d6 and TMDSCB in He mixtures
were prepared by entraining the room-temperature vapor of each
of the three compounds in He (99.999%, Praxair) after degassing
the liquid sample using several cycles of freeze-pump-thaw.
In this work, gas-phase chemical species produced from the
primary decomposition of DMSCB on a hot W filament and from
the secondary reactions in a reaction chamber using DMSCB were
probed using nonresonant single photon ionization (SPI) with
a 10.5-eV vacuum ultraviolet (VUV) wavelength (λ = 118 nm)
coupled with time-of-flight mass spectrometry (TOF MS). Non-
resonant SPI using VUV wavelengths is generally considered to
be a ‘soft’ ionization method with an enhanced parent ion in-
tensity and minimized fragmentation.[24,25] Its ability to detect
multiple species at one time has proved to be advantageous in
diagnosis of complex reaction systems such as those character-
istic of pyrolysis[26,27] and CVD[28,29] processes. Various reaction
products were detected in this work using VUV SPI/TOF MS. Results and Discussion
The competition between different decomposition pathways was
The room-temperature mass spectrum of DMSCB using 118-nm
discussed. With the help of the experiments on the decompo-
sition of the deuterated isotopomer, 1,1-bis(trideuteromethyl)-1-
silacyclobutane (DMSCB-d6), on the W filament, the mechanistic
details of whether the decomposition of DMSCB to form ethene
and propene occurs via a concerted mechanism or a stepwise
process involving a diradical intermediate and the site of initial
bond cleavage for the different decomposition channels were
provided.
VUV laser ionization recorded when the filament is off shows
predominantly the parent ion at m/z = 100 (base peak) and two
fragment ions at m/z = 72 (78.2 1.7%) and m/z = 85 (3.8
0.3%). Other weak fragment ions were observed at m/z = 43
(0.091 0.009%), 44 (0.094 0.006%), 58 (0.042 0.003%) and 59
(0.20 0.03%). The percentage for each fragment ion represents
the relative ratio to the base peak. The errors are obtained
from the standard deviation of the ratios from more than 10
mass spectra. Under collision-free conditions in our experiments
with DMSCB, mass spectra were collected at different filament
temperatures ranging from 900 to 2400 ◦C at an increment of
100 ◦C. After the filament was turned on, a new mass peak
at m/z = 15 representing methyl radical ion came out and
its intensity increased with increasing filament temperatures, as
shown in Fig. 1(a). This suggests that DMSCB decomposes on the
hot W filament via the cleavage of the Si–CH3 bond to form methyl
radicals.
Experimental Details
The experimental setup used to record the VUV laser SPI TOF
mass spectra of the products from decomposition of a source
gas on a hot filament and from secondary reactions in a reaction
chamber has been described in detail elsewhere.[29,30] Briefly, to
detect species formed from the primary decomposition of source
gases on a hot W filament, the filament (10 cm length, 0.5 mm
diameter) was placed directly in the ionization chamber housing
a linear TOF mass spectrometer (R. M. Jordan). The filament was
resistively heated by a DC power supply and its temperature
was measured by a two-color pyrometer (Chino Works). The
sample gas was introduced into the chamber via a mass flow
controller (MFC) (MKS, type 1179A). The operating pressure of
the ionization chamber is kept at 5 × 10−6 Torr to maintain
a collision-free condition. To examine the chemical products
from the secondary gas-phase reactions between the primary
decomposition products, the heated W filament was placed in
a cylindrical reaction chamber which is incorporated into the
ionization chamber through a 0.15 mm diameter pinhole. The
total pressure in the reaction chamber was maintained at 12 Torr
using the MFC and was monitored by a capacitance manometer
(MKS Baratron, type 626A). This gives a partial pressure of DMSCB
or DMSCB-d6 of 0.48 Torr when using a 4% DMSCB (or DMSCB-
d6)-He mixture. At this pressure, collision-free conditions were
voided, therefore, allowing for the detection of the secondary
gas-phase reaction products. The chemical species produced was
ionized by the 118-nm VUV laser radiation for mass detection.
The 118-nm radiation was generated by frequency tripling the
355-nm UV output from a Nd : YAG laser (Spectra-Physics, LAB
170-10) in a gas cell containing 190 Torr of a phase-matched
10 : 1 Ar : Xe gas mixture. The ions produced were collected by a
microchannel plate (MCP) detector located at the end of a field-
free flight tube. Signals from the MCP detector were preamplified,
averaged over 512 laser pulses and saved into a computer for
analysis.
CH3
Si CH3
Si CH3
CH3
(1)
Figure 1(b) shows the appearance of a new mass peak at m/z = 18
+
representing CD3 from the same experiment performed with
a DMSCB-d6 sample. This confirms the decomposition route
represented by Eqn (1). The temperature-dependent evolution
of the CH3 radical on the W filament shows an Arrhenius behavior
between 1500 and 2100 ◦C. The apparent activation energy for
producing methyl radicals on the W filament can be derived
from the linear fit of the natural logarithm of the methyl radical
intensities against the inverse of filament temperature.[17] The
value is determined to be 61.0 2.2 kJ/mol. No activation energy
data for the formation of methyl radical has been reported in
previousstudiesoneitherthepyrolysisorthephotolysisofDMSCB.
For comparison, we have also determined the activation energy
(Ea = 69.8 2.8 kJ/mol) for the formation of methyl radicals from
the decomposition of TMDSCB on the W filament. The similarity
in the activation energies is due to the fact that both DMSCB and
TMDSCB decompose to form methyl radical via the cleavage of the
Si–CH3 bond and it suggests that the difference in the ring strain
energy of DMSCB (108.7 kJ/mol[32]) and TMDSCB (71.9 kJ/mol[33]
)
does not affect much the activation energy of the methyl radical
formation.
Theformationofetheneand1,1-dimethylsilenehasbeenshown
to be the main decomposition route for DMSCB in both pyrolysis
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J. Mass. Spectrom. 2010, 45, 215–222

Decomposition of 1,1-dimethyl-1-silacyclobutane on a tungsten filament
Figure 2. The 118-nm VUV SPI TOF mass spectra of DMSCB in the mass
region of 20–50 amu at different filament temperatures at a chamber
pressure of 5 × 10−6 Torr.
In addition to the two reactions discussed earlier, previous stud-
ies on the photolysis of DMSCB[9,11,12] reported that propene and
cyclopropane were detected as minor decomposition products
with the concomitant formation of dimethylsilylene (Me2Si:).
Me
Si Me
Me2Si
C3H6
(3)
Figure 1. The 118-nm VUV SPI TOF mass spectra of (a) DMSCB; and
(b) DMSCB-d6 in the mass region of 10–20 amu at different filament
temperatures at a chamber pressure of 5 × 10−6 Torr.
In our experiments to detect the direct decomposition products
of DMSCB from the hot W filament, a peak at m/z = 42 was
observed after the filament was turned on. This is clearly shown
in Fig. 2. The peaks at m/z = 43, 44 and 45 are originated from
the photofragmentation of the parent DMSCB ion and present
in the room-temperature mass spectrum. The intensity ratio of
each of the three peaks to that of the parent ion remained
unchanged with increasing temperatures. The peak at m/z = 42
could possibly correspond to propene or cyclopropane. Our mass
spectrometric detection could not distinguish between the two
isomers; however, cyclopropane isomerizes to propene under
our experimental conditions, where the filament temperature is
900 ◦C or higher, therefore, the mass peak at m/z = 42 detected
in our experiment is most probably because of propene. The peak
intensity at m/z = 42 is relatively weak even though the IP of
propene at 9.73 eV is lower than the single VUV photon energy
of 10.5 eV, suggesting a low probability of DMSCB decomposition
to form propene and dimethylsilylene. This is also reflected i+n
the inconsistent increase of the intensity ratio of the :SiMe2
peak at m/z = 58, present in the room-temperature mass
spectrum from the photofragmentation of the DMSCB parent
ion, to that of the parent ion at m/z = 100 with increasing
filament temperatures. Our experiments under the collision-free
conditions clearly demonstrate that unlike the pyrolysis of DMSCB
where only the formation of ethene and 1,1-dimethylsilene was
observed, on a hot W filament, two other decomposition channels
exist to produce methyl/silyl radicals and to form propene/silylene
species.
and photolysis processes.
Me
Si Me
H2C CH2
Me2Si CH2
(2)
In our experiments under collision-free conditions, a new peak at
m/z = 28 representing ethene ion was seen to come out after
the filament was turned on as compared to the room-temperature
mass spectrum. This is shown in Fig. 2. The weak nature of the peak
is due to the high ionization potential (IP: 10.5 eV) of ethene and
hence small ionization cross section when using the 118-nm VUV
photoionization. The weak peaks at m/z = 29 and 31 are from the
chamber background, present even when no gas is introduced.
Examination of the intensity ratio of the [Me2Si CH2]+ peak
at m/z = 72 to that of the parent ion at m/z = 100 showed
an increase when the filament temperature was higher than
1500 ◦C.TheseobservationssuggestthatDMSCBalsodecomposes
on the hot W filament to ethene and 1,1-dimethylsilene. The
apparent activation energy for producing ethene from DMSCB
on the W filament is determined to be 32.6 1.0 kJ/mol in
the filament temperature range of 1500–2400◦C. This activation
energy determined from our experiment is much lower than the
previously reported value of 266.7 2.1 kJ/mol[3] for the pyrolysis
of DMSCB. This implies that the decomposition of DMSCB on the
W filament to produce ethene and 1,1-dimethylsilene is a catalytic
The secondary gas-phase reactions between the primary
decomposition products of DMSCB on the W filament and the
parent molecules were studied by monitoring the species exiting
a reaction chamber housing the hot W filament. Mass spectra
process, consistent with the previous results of the decomposition
[34]
of SiH4
and alkylsilanes[17,23] on the W filament. It is generally
accepted that W filament provides the catalytic site.
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L. Tong and Y. J. Shi
were observed with the one at m/z = 144 being the dominant
of all. The build up in the intensities of the peaks at m/z = 28
and 42 is more clearly seen from the expanded mass spectrum
in Fig. 3(b). The peak at m/z = 144 is originated from TMDSCB,
one of the only two products from the pyrolysis of DMSCB. The
room-temperature mass spectra of the authentic TMDSCB sample
using the 118-nm VUV radiation confirmed that the parent ion at
m/z = 144 is the dominant mass peak with a weak fragment ion
at m/z = 129 whose intensity is only 3.5% of the parent ion. The
peak at m/z = 129 was also observed in our filament-on mass
spectra together with the peak at m/z = 144 and the intensity
ratio of these two peaks did not vary with the temperatures and
filament-on time. TMDSCB is resulted from the dimerization of
1,1-dimethylsilene produced from the hot-wire decomposition of
DMSCB, which is a reactive intermediate known to dimerize easily
to form TMDSCB.[1,3]
Me
Si Me
Me2Si CH2
2
(4)
Me
Si
Me
Therefore, the observation of a dominant new peak at m/z = 144
after the filament was turned on in the reaction chamber with
DMSCB strongly support that 1,1-dimethylsilene is produced,
which is consistent with previous studies of both pyrolysis and
photolysis of DMSCB. Ethene, the concurrent product of the
transient 1,1-dimethylsilene from the decomposition of DMSCB,
was observed simultaneously with the TMDSCB peak.
The intensities of the two peaks at m/z = 116 and 130
maximized at 1200 ◦C when using DMSCB. From the experimental
results under collision-free conditions, it is known that, aside
from 1,1-dimethylsilene, another very reactive dimethylsilylene
(Me2Si:) species was formed from the decomposition of DMSCB
on the hot W filament. Conlin et al. have shown that Me2Si: can
dimerize to form tetramethyldisilene which then isomerizes to
give 1,3-dimethyl-1,3-disilacyclobutane (I) and 1,1-dimethyl-1,3-
disilacyclobutane (II).[35] Compound (I) can also be formed by
dimerization of 1-methylsilene originating from the isomerization
of Me2Si:, as shown in Scheme 1. Both (I) and (II) can contribute to
the peak intensity at m/z = 116 observed in our experiment.
The cycloaddition reaction of 1,1-dimethylsilene with 1-
methylsilene produced from the isomerization of Me2Si: leads
to the formation of 1,1,3-trimethyl-1,3-disilacyclobutane (m/z =
130), which was indeed observed relatively strong in our
experiments.
Figure 3. (a) The 118-nm VUV SPI TOF mass spectra of 12 Torr of 4%
DMSCB/Hemixtureinareactionchamberatfilamenttemperaturesranging
from 900 to 1500 ◦C; (b) the expanded mass spectra in the mass region of
0–50 amu.
for filament temperatures ranging from 900 to 2000 ◦C at an
increment of 100 ◦C were recorded. Figure 3 shows the mass
spectra collected from 12 Torr of 4% DMSCB/He mixture in the
reactor at different filament temperatures ranging from 900 to
1500 ◦C. At temperatures higher than 1500 ◦C, all the peaks
including the parent mass peak disappeared quickly after the
filament was turned on. As can be seen from Fig. 3(a), the
intensities of the parent ion peak at m/z = 100 and its two
dominant photofragment ion peaks at m/z = 72 and 85 decrease
with increasing filament temperatures. The lowest decomposition
temperature of DMSCB in the reactor is found to be 900 ◦C which
is comparable to that for the parent SCB molecule.[29] This is due
Me
Si H
to the fact that DMSCB (108.7 kJ/mol) and SCB (107.8 kJ/mol[32]
)
Me
Si
H
(5)
CH2
Me2Si
CH2
have similar ring strain energies. When the parent ion decreased
in intensities, new peaks at m/z = 28, 42, 116, 130 and 144
Me
Si
Me
H
H
Me
Me
Me
Si Me
Si
H
2
Si Si
:SiMe2
+
Me
Me
Si
H
Si
Me
Me
Me
(I)
(II)
Si
CH2
H
Scheme 1. Formation of 1,3-dimethyl-1,3-disilacyclobutane and 1,1-dimethyl-1,3-disilacyclobutane.
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J. Mass. Spectrom. 2010, 45, 215–222

Decomposition of 1,1-dimethyl-1-silacyclobutane on a tungsten filament
ment temperature of 1300 ◦C. The expanded mass spectrum in
the 20–50 amu region is shown in the inset picture of Fig. 4.
By comparison with the mass spectra shown in Fig. 3(b), it is
clear that the two peaks at m/z = 28 and 42 showed signifi-
cant H/D scrambling when using DMSCB-d6. The mass peaks at
m/z = 42–46 corresponding to propene-d0 –propene-d4 species
were observed at all filament temperatures tested. The peaks at
m/z = 47 and 48 were not observed. The observation of signifi-
cant amount of propene-d1, -d2, -d3 and -d4 is inconsistent with
the direct elimination of :Si(CD3)2 from the parent molecule by
simultaneously cleaving two ring Si–C bonds. If the decompo-
sition follows a stepwise mechanism, the diradical intermediate
(III) produces 1,1-bis(trideuteromethyl)silylene (m/z = 64) and
cyclopropane-d0 (m/z = 42), as shown in Scheme 2. However,
1,5-H/D atom transfers in diradical (III) will lead to the formation of
diradicals (IV) and (V). Decomposition of (V) produces deuterated
dimethylsilylene-d5 (m/z = 63) and cyclopropane-d1 (m/z = 43).
A 1,4-H/D transfer in (IV) will yield propene-d1 accompanied by
deuterated dimethylsilylene-d5 via diradical (VI). As there are two
H atoms connected to the β-C and γ -C (β and γ are relative to Si),
respectively, in diradical (III), the 1,5- and 1,4-H/D atom transfers
will produce deuterated cyclopropane and propene with a max-
imum of four deuterium atoms. In our experiments, all formed
cyclopropanes isomerize to propene. The maximum mass shift of
four for the peak at m/z = 42 with DMSCB-d6 confirmed that 1,5-
and 1,4-H/D atom transfers occur in the process, whereas 1,3-H/D
transfer does not. This agrees well with previously reported results
that the barrier height for 1,3-H transfer is much higher than those
for 1,4- and 1,5-H transfers.[36] More importantly, the H/D scram-
bling pattern observed for the peak at m/z = 42 with DMSCB-d6
clearly demonstrated that the stepwise diradical mechanism to
form cyclopropane/propene from DMSCB occurs on the hot W
filament. The stepwise process is initiated by the cleavage of a ring
Si–C bond followed by the breakage of the second Si–C bond in
the diradical intermediate.
Figure 4. The 118-nm VUV SPI TOF mass spectra of 4% DMSCB-d6/He
mixture in a reaction chamber at a filament temperature of 1300 ◦C
(inset: expanded mass spectra in the mass region of 20–50 amu and of
115–150 amu).
The observation of the two peaks at m/z = 116 and 130, therefore,
provides strong evidence that dimethylsilylene was produced
from the decomposition of DMSCB on the hot W filament and
the isomerization between dimethylsilylene and 1-methylsilene
occurs in the reaction chamber where the pressure of DMSCB is
0.48 Torr.
Both our experiments under collision-free conditions and in the
reaction chamber have demonstrated that DMSCB decomposes
on the hot W filament to produce ethene/1,1-dimethylsilene and
to form propene (or cyclopropane)/dimethylsilylene. However,
whether the production of ethene and propene occurs via the
concerted fission or stepwise cleavage of ring Si–C/C–C bonds
and the site of the initial ring bond cleavage are not clear. In
order to gain better insight into the mechanistic details, the same
experiments have been performed using a DMSCB-d6/He mixture
as a source gas. If the decomposition of DMSCB to ethene and
propene is via the stepwise process which involves the formation
of diradical intermediates, the H/D scrambling of the peaks at
m/z = 28 and 42 is expected. Figure 4 shows the mass spectra
of 12 Torr of 4% DMSCB-d6/He mixture recorded at a W fila-
Detailed kinetic study of the pyrolysis of DMSCB to ethene
and 1,1-dimethylsilene has argued that concerted scission of ring
C–C and Si–C bonds is unlikely and the reaction is a stepwise
process in which one ring bond is cleaved first to form a diradical
intermediate, followed by the breaking of the remaining central
bond.[3] However, the question of whether the diradical is formed
by an initial cleavage of a ring C–C or a ring Si–C bond has
not been answered. For DMSCB-d6, the initial cleavage of a
CD3
Si CD3
Si-C
(CD3)2Si
(CD3)2SiCH2CH2CH2
(III)
cleavage
(m/z = 42)
H
D
D
CD2 CH2 1,5-H/D
D2C
CH2
CH2
D2C
CHD
CH2
1,5-H/D
transfer
HD2C
D3C
D3C Si
CH2
Si
D3C Si
D3C Si
CHD
C
C
transfer
C
H2
H2
(III)
H2
(m/z = 43)
(IV)
(V)
H
C
H
1,4-H/D
transfer
D2C
Si
D3C
D2C
Si
HD2C
D3C
Si
CH CH2D
HC CH2D
CH2=CH-CH2D
(m/z = 43)
C
D3C
H2
H2
(IV)
(VI)
Scheme 2. Stepwise ring-opening reactions to form cyclopropane/propene.
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L. Tong and Y. J. Shi
CD3
Si-C
Si CH2CH2CH2
cleavage
CD3
CD3
(III)
Si CD3
H2C CH2
(D3C)2Si CH2
CD3
H2C Si CH2CH2
CD3
C-C
cleavage
(VII)
Scheme 3. Formation of ethene via a stepwise ring-opening process.
ring Si–C and C–C bond, respectively, forms diradical (III) and
(VII), as shown in Scheme 3. Direct decomposition of both will
form ethene-d0 (m/z = 28) and 1,1-bis(trideuteromethyl)silene
(m/z = 78).
to produce the deuterated 1,1,3-trimethyl-1,3-disilacyclobutanes-
d12 –d8 peaks of similar intensities. The observed intensity pattern
for the peaks at m/z = 138–142 shown in the inset of Fig. 4
confirms the assignment of the peak at m/z = 130 obtained
with DMSCB to 1,1,3-trimethyl-1,3-disilacyclobutane and its for-
mation mechanism as cycloaddition of 1,1-dimethylsilene and
1-methylsilene. The peak at m/z = 116 observed with DMSCB
is expected to be shifted to m/z = 120–128 from the dimer-
ization of deuterated dimethylsilylene-d6, d5, -d4, -d3, and -d2
(m/z = 64–60). These peaks were observed in Fig. 4. Part of the
peak intensity at m/z = 125 was from the impurity (1.10 0.04%)
in the DMSCB-d6 sample and this gives the high intensity of this
peak in the clusters.
It should be noted that propene can also be formed by the
reaction between methyl radical and ethene,[37] both of which
were produced in the decomposition process of DMSCB on the
W filament. If this is the case, the peak at m/z = 42 with DMSCB
should be shifted to m/z = 45–47 with DMSCB-d6. The fact that
the peak at m/z = 47 was not observed with DMSCB-d6 suggested
that the contribution of the reaction between CH3 and C2H4 to
the propene signal intensity is negligible. In the previous studies
of the photolysis of DMSCB where the decomposition channel to
form propene existed, the amount of propene was found to be
no more than 0.03 times that of ethene.[9,10] An examination of
the intensity ratio of the peak at m/z = 28 to that at m/z = 42
observed in our experiments showed that it ranged from 0.4 to
3.8 depending on the filament temperature and filament-on time.
Aftercalibrationagainsttheionizationcrosssectionsofetheneand
propene when using the 10.5 eV photoionization,[29] the amount
of ethene produced was determined to be 14.4 to 136.8 times
that of propene. The low yield of propene demonstrates that the
decomposition of DMSCB to form propene on the W filament is a
minor pathway as compared to the formation of ethene, which is
similar to the observations in the photolysis processes. The high
yield of ethene relative to that of propene suggests that an initial
ring C–C cleavage is favored over an initial ring Si–C cleavage
whenDMSCBdecomposesonthehotWfilament. Thisisconsistent
with the results obtained from pyrolyses of 1,1,2-trimethyl-1-
silacyclobutane,[5,6] 1,1,3-trimethyl-1silacyclobutane,[8] and 1,1-
dimethyl-2-phenyl-1-silacyclobutane.[7]
According to the previous discussions, 1,4- and 1,5-H/D atom
transfers have been shown to occur in the decomposition process
of DMSCB-d6 on a hot W filament. As there are four H atoms
in diradical (III) and two H atoms in (VII) that can be exchanged
via a 1,4- or 1,5-H atom transfer, the maximum mass shifts for
the peak at m/z = 28 should be four and two, respectively,
for the decomposition of DMSCB-d6 initiated by the cleavage
of a ring Si–C bond via diradical (III) and a ring C–C bond via
(VII). In our experiments with DMSCB-d6, peaks at m/z = 28–30
representingethene-d0,-d1 and-d2,wereobserved.Theweakpeak
at m/z = 31 is from the background and its intensity remained
unchanged with increasing filament temperatures. Therefore,
ethene-d3 was not formed, nor was ethene-d4. The observation
of signals from ethene-d0, -d1, -d2 and the absence of signals
from ethene-d3, -d4, therefore, provide strong evidence that the
formation of ethene from DMSCB is initiated by a ring C–C bond
cleavage.
The corresponding deuterated 1,1-dimethylsilene products
accompanying ethene-d0, -d1, -d2 are methyl deuterated 1,1-
dimethylsilene-d6, -d5 and -d4, respectively. As the intensity of
the ethene-d0 peak at m/z = 28 is always much stronger than
those of the ethene-d1 and -d2 peaks, the intensity of the methyl
deuterated 1,1-dimethylsilene-d6 at m/z = 78 should be dom-
inant over those for methyl deuterated 1,1-dimethylsilene-d5
(m/z = 77) and -d4 (m/z = 76). Therefore, the predominant
dimers of methyl deuterated 1,1-dimethylsilene should be 1,1,3,3-
tetra(perdeuteromethyl)-1,3-disilacyclobutane (m/z = 156) origi-
nated from CH2 Si(CD3)2 (m/z = 78). This is indeed the case from
our experiments, as shown in Fig. 4. Their isotope peaks ascribed
mainly to 29Si and 30Si were observed at m/z = 157–160. The
peaks at m/z = 152–155 were weak due to the weak nature
of the methyl deuterated 1,1-dimethylsilene-d5 (m/z = 77) and
-d4 (m/z = 76). As discussed earlier, the peak at m/z = 130
comes from 1,1,3-trimethyl-1,3-disilacyclobutane formed by cy-
cloaddition of 1,1-dimethylsilene and 1-methylsilene. The latter
species originates from the isomerization of dimethylsilylene, the
concomitant decomposition product of propene. When using
DMSCB-d6, signals from propene-d0, -d1, -d2, -d3 and -d4 were ob-
served with relatively similar intensities. This indicates that signals
from the concurrent products, deuterated dimethylsilylene-d6,
d5, -d4, -d3, and -d2 (m/z = 64–60), should also have simi-
lar intensities. As 1,1-bis(trideuteromethyl)silene, CH2 Si(CD3)2
(m/z = 78), is the predominant silene produced with DMSCB-
d6, the cycloaddition between this molecule and each of the
dueterated 1-methylsilene-d6 − d2 (m/z = 64–60) is expected
In previous studies on the gas-phase pyrolysis of DMSCB, the
dominant and only decomposition route is found to be the pro-
duction of ethene and 1,1-dimethylsilene.[2,3] Our study on the
decomposition of DMSCB on a hot W filament has clearly shown
thattwootherchannelsexisttoproducemethylradicalsviaSi–CH3
cleavage and propene/silylene via diradical intermediates. In addi-
tion, it has been demonstrated that the formation of ethene from
the hot W filament is a catalytic process with its activation energy
(32.6 1.0 kJ/mol) much lower than that of the DMSCB pyrolysis.
c
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J. Mass. Spectrom. 2010, 45, 215–222

Decomposition of 1,1-dimethyl-1-silacyclobutane on a tungsten filament
Conclusion
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VUV wavelength of 118 nm (10.5 eV) was used as a soft ionization
source in tandem with TOF mass spectrometer for detection
of the products from direct decomposition of DMSCB on a
heated W filament and from secondary gas-phase reactions
in a reaction chamber. Three decomposition pathways exist
to produce ethene/1,1-dimethylsilene, propene/dimethylsilylene
and methyl radicals. The similarity in the activation energies for the
methyl radical formation via Si–CH3 bond cleavage from DMSCB
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