Kinetics of dichlorosilylene trapping by methane and mechanism and kinetics of the methyldichlorosilane decomposition

Article abstract of DOI:10.1002/(SICI)1097-4601(1998)30:1<89::AID-KIN10>3.0.CO;2-G

Data relative to methane trapping of SiCl₂ and a rate constant for the SiCl₂ into C-H bond insertion process of k_-1 = 13.4 M^-1s^-1 at 921 K are reported. Results on the decomposition of the trapping product, methyldichlorosilane, are also reported. This decomposition follows first-order kinetics with a rate constant of k = 1.5±0.2×10^-3 s^-1 at 905 K and produces methane, trichlorosilane, methyltrichlorosilane, and tetrachlorosilane. It is argued that the decomposition involves silylene intermediates, is nonchain, and is initiated primarily by the molecular methane elimination process MeSiHCl₂-1→CH₄+SiCl₂. Free radicals and Si-C bond fission may also contribute to the decomposition but are not dominant. The kinetics of MeSiHCl₂ decomposition are shown to be consistent with the kinetics of the reverse SiCl₂/CH₄ trapping reaction and with the overall reaction thermochemistry. Reaction modeling gives product yields, reactant conversions, and rates in reasonable agreement with the data.

Full text of DOI:10.1002/(SICI)1097-4601(1998)30:1<89::AID-KIN10>3.0.CO;2-G

Kinetics of Dichlorosilylene
Trapping by Methane and
Mechanism and Kinetics of
the Methyldichlorosilane
Decomposition
M. A. RING, H. E. O’NEAL, K. L. WALKER
Department of Chemistry, San Diego State University, San Diego, California 92181
Received 25 November 1996
ABSTRACT: Data relative to methane trapping of SiCl and a rate constant for the SiCl into
2
2
Ϫ1 Ϫ1
C9H bond insertion process of kϪ1 ϭ 13.4 M s at 921 K are reported. Results on the
decomposition of the trapping product, methyldichlorosilane, are also reported. This decom-
position follows first-order kinetics with a rate constant of k ϭ 1.5 Ϯ 0.2 ϫ 10Ϫ3
s
Ϫ1
at 905 K
and produces methane, trichlorosilane, methyltrichlorosilane, and tetrachlorosilane. It is ar-
gued that the decomposition involves silylene intermediates, is nonchain, and is initiated
primarily by the molecular methane elimination process MeSiHCl 91 : CH ϩ SiCl . Free
2
4
2
radicals and Si9C bond fission may also contribute to the decomposition but are not dom-
inant. The kinetics of MeSiHCl decomposition are shown to be consistent with the kinetics
2
of the reverse SiCl /CH trapping reaction and with the overall reaction thermochemistry.
2
4
Reaction modeling gives product yields, reactant conversions, and rates in reasonable agree-
ment with the data. ᭧ 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 89–97, 1998.
INTRODUCTION
Ϫ1
ϭ
⇒
SiCl ϩ CH ⇐
CH SiHCl
2
4
3 2
1
In studies of the thermal decompositions of dichloro-
silane and trichlorosilane made in the presence of ex-
cess methane (results reported in a related article [1]),
we observed the formation of methyldichlorosilane in
yields which increased with increasing pressures of
Methane trapping of SiCl is an exciting result since
2
to our knowledge it is the first observation and mea-
surement of a bimolecular silylene/C9H bond inser-
tion process. Intramolecular silylene C9H bond in-
sertion, reported initially by Barton et al. [2] and
subsequently studied by Davidson et al. [3] and by
ourselves [4], is now well established, but the bimo-
lecular insertion process is a new result. Thus, prior
efforts to observe and evaluate the SiH /CH reaction
methane. Methane trapping of SiCl , the dominant in-
2
termediate in both decompositions, as per reaction Ϫ1
was suggested, and data supporting this process are
reported here.
2
4
Correspondence to: M. A. Ring
[5,6] were only successful in establishing an upper
Contract grant Sponsor: National Science Foundation
Contract grant number: Grant CHE-8719843
International Journal of Chemical Kinetics, Vol. 30, 89–97 (1998)
7
limit to its rate constant [6], i.e., k Ͻ 1.5 ϫ 10
Ϫ
1 Ϫ1
M s at 298 K, and the much higher stabilization
energy of SiCl compared to SiH (i.e., 46.3 kcal/mol
᭧
1998 John Wiley & Sons, Inc.
CCC 0538-8066/98/010069-09
2
2

9
0
RING, O’NEAL, AND WALKER
[
7,8] vs. 20.6 kcal/mol [9,10], respectively) nec-
nated silanes) and the higher temperatures of that study
would indeed enhance bond fission initiation as op-
posed to the molecular elimination pathway supported
by this work.
essarily makes the SiCl /CH trapping reaction con-
siderably slower and presumably more difficult to
observe.
2
4
If SiCl /CH trapping does occur at 921 K then the
2
4
reverse reaction (i.e., decomposition of MeSiHCl to
2
methane and dichlorosilylene, reaction 1) should occur
as well. This reasoning prompted the methyldichlo-
rosilane pyrolysis study also reported here. The py-
rolysis study focused on three issues: the probable
mechanism of decomposition, the nature of the pri-
mary dissociation channels (specifically, methane
elimination), and the consistency of the methane elim-
ination kinetics with the SiCl /CH back reaction ki-
EXPERIMENTAL
Methyldichlorosilane and trichlorosilane were pyro-
lyzed in the same apparatus and by the same proce-
dures used to study the chlorinated silane decompo-
sitions [1]. These are described in detail in the article
companion to this. Briefly, a 70.4 cc cylindrical quartz
reactor with direct capillary leak to a quadrupole mass-
spectrometer was employed. Reactor wall condition-
ing with methyltrichlorosilane and reactant prior to
each series of runs was required for reproducible re-
sults. Product concentrations were determined from
mass-spectra obtained as a function of time. All runs
were made with constant reactant and Argon pressures
2
4
netics and thermochemistry.
MeSiHCl has at least two primary dissociation
2
channels other than methane elimination (reaction 1)
which could reasonably initiate its thermal decompo-
sition: HCl elimination (reaction 2) and Si9C bond
fission (reaction 3).
(
0.18 torr MeSiHCl /0.15 torr Argon for MeSiHCl2
2
MeSiHCl 91: CH ϩ SiCl
2
runs and 0.17 torr HSiCl
/0.09 torr Argon for HSiCl
3 3
2
4
runs) and with added N (for MeSiHCl ) or CH (for
2
2
4
9
9
2: MeSiCl ϩ HCl
3: Me ϩ HCl Si
HSiCl ) to give a series of fixed total pressure reaction
3
и
и
conditions. Argon served as the MS analytical stan-
dard, N was a simple diluent and CH was a diluent
2
2
4
and SiCl trapping agent. Runs on both decomposi-
2
Based on analogies in the methylsilane and dichloro-
silane decompositions, reaction 2 should be unimpor-
tions were made at just one temperature, 905 Ϯ 1 K
for methyldichlorosilane and 921 Ϯ 1 K for trichlo-
rosilane.
tant. Thus H (not HCl) elimination is the main pri-
2
mary dissociation channel of dichlorosilane [1], and
methane elimination is only slightly slower (a factor
of six) than H elimination in the shock initiated de-
2
RESULTS AND DISCUSSION OF
METHANE TRAPPING OF SICL2
composition of methylsilane [11]. As for Si9C bond
fission, both methane elimination and bond fission are
competing reactions in the shock initiated decompo-
sitions of dimethylsilane [12] and trimethylsilane [13],
in spite of the 25 to 30 kcal/mol higher activation en-
ergy for bond fission.
Yield data relative to the SiCl /CH trapping reaction,
2
4
taken in the course of investigating the kinetics of the
trichlorosilane decomposition, are given in Table I.
The most reasonable interpretation of these data is that
Therefore, some bond fission initiation in the
SiCl , produced in the primary dissociation process
MeSiHCl decomposition is possible. The question is
2
2
(reaction a), reacts with reactant to produce SiCl (re-
“
how much is some?” Davidson and Dean [14], in an
4
actions b, c followed by d) or is trapped by methane
to give MeSiHCl (reaction Ϫ1). The most likely free
earlier flash pyrolysis investigation of the methyldi-
chlorosilane decomposition, argued for Si9C bond
rupture as the main initiation reaction. Their results
indicated a free radical, HCl catalyzed chain process
for the decomposition. However, the D&D results and
conclusions are not particularly relevant to our inves-
tigation as HCl was not present in our system (see
later) and D & D study temperatures were considera-
bly higher (between 992 and 1139 K). The presence
of HCl in the D&D study suggests reactant hydrolysis
2
radical route to MeSiHCl is considered later (see re-
2
action modeling) and is not competitive.
HSiCl 9(a): HCl ϩ SiCl
3
2
(
b, c)
(d)
SiCl ϩ HSiCl ⇐
⇒
Cl HSiSiCl 99 :
2
3
2 3
(
Ϫb, Ϫc)
SiCl ϩ HSiCl
4
(always a major problem when working with chlori-
SiCl ϩ CH 9(Ϫ1): CH SiHCl
2 4 3 2

DICHLOROSILYLENE TRAPPING
Table I Trichlorosilane Decomposition Yield and Loss Data, 921 Ka
91
Reaction Datab
P(torr) HSiCl3(loss%)
Product Yields
Y(SiCl4)
Y(HCl)
Y(MeSiHCl2)
# runs
8
9
3
4
7
7
.5
35.0 Ϯ 14
22.4 Ϯ 5.4
29.1 Ϯ 3.1
30.8 Ϯ 4.6
39.0 Ϯ 3.6
36.9 Ϯ 5.1
0.48 Ϯ .09
0.73 Ϯ .05
0.79 Ϯ .02
0.80 Ϯ .04
0.69 Ϯ .03
0.73 Ϯ .06
0.37 Ϯ .12
0.47 Ϯ .12
0.42 Ϯ .07
0.32 Ϯ .07
0.38 Ϯ .03
0.24 Ϯ .05
0.04 Ϯ .02
0.18 Ϯ .02
0.28 Ϯ .02
0.37 Ϯ .02c
0.30 Ϯ .02
0.37 Ϯ .03
16
11
14
7
5
10
1
4
6
7
9
a
Reactant pressures were constant at 0.17 torr, Argon pressures were 0.09 torr, and the total pressures were
adjusted by additions of CH .
4
b
Data shown are for reaction times of 375 s.
This value appears to be high.
c
A relative product yield expression of Stern-Volmer
The plot is reasonably well behaved with a straight
[
15] form (eq. (I)) is readily generated from the mech-
line intercept of zero and slope of 2.24 Ϯ 0.16 ϫ
Ϫ3
anism and a plot of the product yield data according
to this relationship is shown in Figure 1.
10 . In our related article [1] we derived Arrhenius
expressions for all alphabetically labeled rate con-
stants appearing in the slope expression. Substitution
of the 921 K values of these rate constants yields a
value for the SiCl /CH trapping rate constant of
Y (MeSiHCl2 )
2
4
Y (SiCl4 )
Ϫ
1 Ϫ1
kϪ1 ϭ 13.4 Ϯ 2.5 M s (921 K). The next objective
was to see how well this rate constant correlated with
the rate constant of the reverse decomposition.
(
kϪ )(kϪ ϩ kϪ ϩ kd )
[CH4 ]
1
b
c
ϭ
ϫ
I
(
k )(k ϩ k )
[HSiCl3 ]
d
b
c
Figure 1 Stern-Volmer plot for methane trapping of SiCl2.

9
2
RING, O’NEAL, AND WALKER
Table II MeSiHCl Decomposition Yield and Loss Kineticsc
a
b,c
2
Pressure (torr):
runs:
conversion % :
8
5
34.0
20
5
25.6
47
6
34.3
#
e
Y(CH ):
0.63 Ϯ .03
0.40 Ϯ .04
–
0.46 Ϯ .03
0.37 Ϯ .02
0.64 Ϯ .03
4
Y(m/e ϭ 133)d:
0.54 Ϯ .11
0.055 Ϯ .007
1.64 Ϯ .15
Y(SiCl ):
0.040 Ϯ .003
4
3
Ϫ1
f
kloss ϫ 10 (s ):
1.63 Ϯ .11
1.14 Ϯ .24
a
Initial reactant mixtures were 0.18 torr MeSiHCl , 0.15 torr Argon in enough N to bring the total pressure
2
2
to the values listed.
b
Yields, denoted by Y(X), are moles of product X produced per mole of MeSiHCl reacted.
Yields and reactant loss rate constants are experimental means of runs made at the same total pressure;
2
c
errors are one standard deviation.
d
m/e ϭ 133 yields probably represent a product mixture of HSiCl and MeSiCl (see text).
Conversion % at reaction times of 260 s.
When compared to the 8 and 47 torr data, the 20 torr data appear too low. We attribute this to unknown
3
3
e
f
systematic errors.
RESULTS OF THE
METHYLDICHLOROSILANE
DECOMPOSITION
sequence producing methane exists within the confines
of known silylene chemistry [1,10–13,16]. The non-
chain mechanism of Scheme I, therefore, seems most
likely.
Methyldichlorosilane loss was consistent with first-or-
der kinetics. The major products were methane
Mechanism of the Methyldichlorosilane Decomposi-
tion
ϩ
(
1
m/e ϭ 16, CH ) and trichlorosilane (m/e ϭ
4
ϩ
33, SiCl ). Small quantities of tetrachlorosilane
3
ϩ
(m/e ϭ 168, SiCl ) were also produced. Signifi-
4
MeSiHCl ϩ (M) 91: CH ϩ SiCl ϩ (M)
2
4
2
cantly, HCl, whose presence was readily detectable in
our system (see Table I), was not a reaction product.
This observation is the critical difference between the
present study and the prior D&D study [14]. Methyl-
trichlorosilane, by the mechanism an expected prod-
uct, may have also contributed to the m/e ϭ 133 peak.
4
ϭ
SiCl ϩ MeSiHCl ⇐
⇒
2
MeSiCl SiHCl2
2
2
Ϫ4
MeSiCl SiHCl 95: MeSiCl ϩ HSiCl
2
2
3
9
6: MeSiCl ϩ HSiCl
3
Unfortunately, the parent peak of MeSiCl was not
3
7
ϭ
monitored. Product yield and reactant loss kinetic data
for the methyldichlorosilane decomposition are given
in Table II.
SiCl ϩ MeSiHCl ⇐
⇒
MeHClSiSiCl3
2
2
Ϫ7
MeHClSiSiCl 98: MeSiH ϩ SiCl
3
4
9
9: MeSiCl ϩ HSiCl3
10
MeSiH ϩ MeSiHCl2 ⇐
ϭ
⇒
MeH SiSiCl Me
2
2
DISCUSSION OF THE
METHYLDICHLOROSILANE
DECOMPOSITION
Ϫ10
MeH SiSiCl Me 911: MeSiH Cl ϩ MeSiCl
2
2
2
MeSiH Cl ϩ (M) 912: H ϩ MeSiCl ϩ (M)
2
2
Mechanism
HSiCl 913(wall): HSiCl(w)
MeSiCl 914(wall): MeSiCl(w)
The high methane yields of the methyldichlorosilane
decomposition require either (1) a nonchain reaction
producing methane in the initiation step, or (2) a chain
reaction producing methane in the chain. If Si9C
bond rupture is not the main initiation process, and we
show later that this is probably the case, then a chain
reaction is unlikely since no reasonable silylene chain
2
RSiCl(w) 915(wall): SiCl ϩ SiR (w)
2
2
R ϭ Me or H
SiR (w) 916(wall): Si(w) ϩ RR
2
(assumed to be slow)
Scheme I

DICHLOROSILYLENE TRAPPING
93
The chlorosilylene wall capture reactions 13 and 14
are the critical reactions of Scheme I. They make the
reaction nonchain. Fairly convincing evidence for
HSiCl wall capture and subsequent wall release of
Kinetics
Methyldichlorosilane loss rate constants do not show
a systematic pressure dependence (see Table II). This
is consistent with a decomposition initiated and kinet-
ically controlled by methane elimination (i.e., by re-
action 1). Thus RRKM calculations [19] with k Ϸ
kloss and A1,ϱ ϭ 10
SiCl (as in reaction 15) under conditions of this study
2
comes from our observations on the mono, di, and
trichlorosilane decompositions [1]. We assume here
that MeSiCl is similarly wall scavenged. Results on
the heterogeneous decomposition of dichlorosilane by
Beauchamp et al. [17]. are relevant to reaction 15.
Thus the only gaseous products produced in this hot-
1
1
4
Ϫ1
s
(i.e., both upper limit inputs
acting to push the reaction into fall-off ) indicate no
pressure dependence for the reaction under our study
conditions. By contrast, similar calculations predict a
strong pressure dependence for reaction 3 and this is
a forceful argument against Si9C bond rupture ini-
tiation (see later). Therefore, accepting Scheme I as an
adequate description of the methyldichlorosilane de-
composition, and accepting methane elimination as the
dominant initiation process, the results indicate that
k Ϸ Y(CH ) ϫ k Ϸ (0.59 Ϯ 0.03)(1.5 Ϯ 0.2 ϫ
surface promoted decomposition were SiCl and HCl;
2
H and HSiCl were not formed. Although the nature
2
of the surface bound coproducts of SiCl is speculative
2
(these are represented in reactions 15 and 16 as wall
bound MeSiMe, MeSiH or SiH ), gaseous product re-
2
leases from such coproducts are probably slow and
certainly molecular. Such is the case in the decom-
positions of surface bound products of the MeSiH3
decomposition [18] where H and CH in the ratio
1
4
loss
Ϫ3
Ϫ4 Ϫ1
2
4
10 ) ϭ 8.9 Ϯ 1.8 ϫ 10 s at 905 K.
of 20/1 were desorbed from the walls over times
much longer than the decomposition. Consequently re-
action 16 should not effect reaction rates. It could,
however, have minor effects on methane and hydrogen
yields.
Consistency of the Thermochemistry and
Kinetics of Reactions 1, Ϫ1
Reaction 1 thermochemistry and the observed rate
constant for the reverse trapping reaction at 921 K are
shown below.
1
ϭ
Ϫ
1 Ϫ1
MeSiHCl2 ⇐
⇒
CH ϩ SiCl
kϪ1 ϭ 13.4 M s (921 K)
4
2
Ϫ1
a
7
20
d
0
⌬
H : Ϫ94.2 Ϯ 1
Ϫ17.9
44.5
Ϫ37.8 Ϯ 1.5
⌬H (298 K) ϭ 38.5 Ϯ 1.8
f
0
b
4
20
24
0
S :
79.6
67.2
12.1
⌬S (298 K) ϭ 32.1
b
c
8.520
11.1
15.0
17.2
24
C (298 K) :
21.7
27.5
32.4
34.5
p
(
(
500 K):
800 K):
1000 K):
13.1
13.6
13.7
(
a
0
⌬
H units are kcal/mol; ⌬H units are kcal.
f
b
c
d
0
0
S and C units are cal/deg · mol; ⌬S units are cal/deg.
Estimated by bond additivity based on MeSiH , SiCl , and SiH data.
An average of values from references listed under ref. [8].
p
3
4
4
Ϫ
1
13.0
Ϫ
67,900
Ϯ
1800/RT
Ϫ1
With heat capacity corrections, the thermochemistry
predicts reaction enthalpies and entropies of
s
(i.e., from k ϭ 10 ϫ e
s ), with
1
an estimated error of a factor of 2.7. This is 40% of
the experimental kloss value, well within the errors, and,
hence, could be interpreted as support for exclusive
reaction 1 initiation.
⌬H(920 K) ϭ 36.5 Ϯ 1.8 kcal and ⌬S(920 K) ϭ 28.8 cal/
deg. Assuming an A-factor for methane elimination of
1
3
Ϫ1
A Ϸ 1.0 ϫ 10 s (a reasonable assumption for the
1
kind of tight transition state expected [4]) and using
the calculated thermochemistry and measured kϪ1 val-
ues at 921 K yields trapping reaction parameters of
Possible Free Radical Participation
6
Ϫ1 Ϫ1
AϪ1 ϭ 5.1 ϫ 10 atm s , E ϭ 31.4 kcal/mol
Ϫ
1
(
P ϭ 1.0 atm standard state) as well as an activation
It’s possible to write an apparently reasonable free rad-
ical mechanism for the trichlorosilane decomposition
that is consistent with all the observed products, in-
energy for methane elimination of E ϭ 67.9 Ϯ 1.8
kcal/mol. The rate constant for methane elimination at
9
1
Ϫ4
05 K is therefore estimated to be k ϭ 4.0 ϫ 10
cluding MeSiHCl , the product attributed here to the
1
2

9
4
RING, O’NEAL, AND WALKER
SiCl /CH4 trapping reaction. However, modeling
Reaction Modeling
2
studies on the HSiCl decomposition [1] show that this
3
Modeling of the methyldichlorosilane decomposition
reaction was done with a mechanism containing all
relevant reactions of Schemes I and II. These are listed
with their Arrhenius parameter assignments in Table
III. For reactions involving silylene intermediates (re-
actions 4–11, 13–15, 22), Arrhenius parameters and
rate constants of our companion article [1] were
adopted with minor changes required by reaction path
degeneracy differences due to the assumption, Me ca.
H. Arrhenius parameters for the free radical metathesis
reactions (reactions 18, 19, 23, and 24) were assigned
by analogy [22] with the reasonable assumption that
radical abstraction of H from Si9H is comparable to
abstraction from a tertiary C9H. Parameters of re-
action 1 are those calculated here. For reaction 3, two
sets of Arrhenius parameters were employed: set 1
based on the reaction thermochemistry [7] and kinetics
mechanism is totally unable to account for either the
extent of reactant conversion or for the MeSiHCl2
yields observed.
Free radical participation in the MeSiHCl decom-
2
position, however, is not as easily dismissed. Thus by
the results of D&D [14] and others [21] on this and
related systems (e.g., MeSiCl ) Si9C bond rupture
3
could compete with molecular methane elimination. In
this case free radical processes as in Scheme II could
occur.
A Free Radical Mechanism for the MeSiHCl Decom-
2
position
MeSiHCl ϩ (M) 93: Me
и
ϩ HCl Si
и
ϩ (M)
2
2
HCl Si
и
ϩ (M) 917: H
и
ϩ SiCl ϩ (M)
2
2
[21] calculated by Allendorf, et al., and set 2 based on
kinetics of Davidson [14] and thermochemistry of
H
и
ϩ MeSiHCl 918: H ϩ MeCl Si
и
2
2
2
0
Walsh [23]. In both cases, E ca. ⌬H and transition
3
3
states were assumed to be ‘loose.’ Reaction 3 is in its
pressure fall-off regime under study conditions and
RRKM calculations [19] fit to the high pressure pa-
Me
и
ϩ MeSiHCl 919: CH ϩ MeCl Si
и
2
4
2
MeCl Si
и
ϩ (M) 920: Me ϩ SiCl ϩ (M)
и
2
2
rameters predict k/k at the 8 and 47 torr conditions
ϱ
of 0.12 and 0.25 (set 1), and 0.34 and 0.55 (set 2). One
would expect the pressure dependencies of reaction 3
2
MeCl Si 921: (MeCl Si) 922:
и
2
2 2
MeSiCl ϩ MeSiCl
3
to be reflected in pressure dependent MeSiHCl loss
2
rate constants, and this is the case (see later).
The critical reaction for free radical participation is
the chain propagation step (reaction 20) which, be-
Me
и
ϩ MeSiHCl 923: CH ϩ H CSiHCl
и
2
4
2 2
иH2CSiHCl ϩ MeSiHCl 924:
cause of SiCl stabilization, probably proceeds via a
2
2
2
MeSiHCl ϩ MeCl Si
и
tight transition state with a back activation energy
comparable to radical addition to an olefin (ca. 5 kcal/
2
2
Scheme II
1
4
Ϫ1
mol). In this case, assuming A Յ 10
s
and with
ϱ
E ϭ 54.3 or 57.5 kcal/mol depending on the ther-
mochemistry assigned (i.e., Allendorf [7] or Walsh
[23], respectively) one obtains an upper limit estimate
While reaction 3 is much slower than reaction 1 at 905
K, reactions 19 and 20 constitute a chain which could
be long and fast. Thus, if reaction 19 were the rate
determining step of the chain, a chain length ca.
ϱ
for the radical decomposition rate constant of k
ϭ
0
2
3
4
Ϫ1
k [MeSiHCl ]/k ca. 10 to 10 would be possible.
5 or 1 s at 905 K. Chain length is then controlled by
this decomposition. On the other hand, if a loose tran-
sition state with zero back reaction activation energy
is assumed for reaction 20, faster rates and longer
chains are obtained. However, by RRKM calculations
the reaction is then strongly pressure dependent. The
critical point here is that if reaction 20 does operate
near its upper rate limit, its strong pressure dependence
should be reflected in pressure dependent reactant con-
versions and rate constants.
19
2
3
This chain is consistent with the products. It produces
methane directly and branches, via SiCl formation,
2
into Scheme I reactions which generate the observed
chlorinated reaction products. This branching essen-
tially guarantees product yields that are initiation re-
action independent. Reactant conversions and reaction
rates, however, should be sensitive to the initiation
process. We therefore modeled the methyldichlorosi-
lane decomposition to see if predicted reactant con-
versions and product yields could distinguish between
the two most likely initiation modes: methane elimi-
nation, reaction 1, or bond rupture, reaction 3.
The MeSiHCl decomposition was first modeled
2
according to Scheme I alone, i.e., exclusive initiation
via methane elimination was assumed. Results are

DICHLOROSILYLENE TRAPPING
95
Table III Reactions and Arrhenius Parameter Assignments of Modeling
Reaction
log A
E(kcal/mol)
A 91: CH ϩ SiCl2
13.0
17.2
7.7
13.0
14.1
14.1
8.0
13.5
14.0
13.5
9.3
67.4
95.1
15.4
48.3
52.5
52.5
17.4
50.4
54.0
50.5
1.2
4
A 93: CH · ϩ HCl Si·
P : ϱa
3
2
SiCl ϩ A 94: Cl HSiSiCl Me
2
2
2
Cl HSiSiCl Me 9(Ϫ4): A ϩ SiCl
2
2
2
Cl HSiSiCl Me 95: HSiCl ϩ MeSiCl
2
2
3
Cl HSiSiCl Me 96: MeSiCl ϩ HSiCl
2
2
3
SiCl ϩ A 97: Cl SiSiHClMe
2
3
Cl SiSiHClMe 9(Ϫ7): A ϩ SiCl
3
2
Cl SiSiHClMe 98: SiCl ϩ MeSiH
3
4
Cl SiSiHClMe 99: HSiCl ϩ MeSiCl
3
3
MeSiH ϩ A 910: MeH SiSiCl Me
2
2
MeH SiSiCl Me 9(Ϫ10): MeSiH ϩ A
14.3
14.3
13.8
3.0
52.7
54.4
67.0
0.0
2
2
MeH SiSiCl Me 911: MeH SiCl ϩ MeSiCl
2
2
2
MeSiH Cl 912: H ϩ MeSiCl
2
2
RSiCl 913,14: RSiClwall R ϭ Me or H
RSiClwall 915: RSiRwall ϩ SiCl2
HCl Si· 917: H· ϩ SiCl
2
(fast)
15.5
10.0
8.0
47.7
8.8
7.9
P : ϱa
P : ϱa
2
2
H· ϩ A 918: H ϩ MeCl Si·
2
2
Me· ϩ A 919: CH ϩ MeCl Si·
4
2
MeCl Si· 920: Me· ϩ SiCl
17.3
10.0
14.0
8.5
49.3
0.0
52.5
11.6
8.9
2
2
2
MeCl Si· 921: (MeCl Si)
2 2
2
(
MeCl Si) 922: MeSiCl ϩ MeSiCl
2
2
3
Me· ϩ A 923: CH ϩ HCl SiCH ·
4
2
2
HCl SiCH · ϩ A 924: A ϩ MeCl Si·
7.3
2
2
2
a
See article for k and k/k values at experimental pressures.
ϱ
given in Table IV, column 3. Note that the predicted
product yields, methane excepted, agree well with the
data and that the calculated reactant conversion at
We next modeled the decomposition with exclusive
initiation via reaction 3, i.e., Si9C bond rupture as
in Scheme II. Results are shown in Table IV, columns
6–8. As expected, product yields are essentially the
same as those obtained for initiation by reaction 1
alone, however, reaction conversions differ according
to the nature of the reaction 20 transition state (‘tight’
or ‘loose’). If tight (most likely case), predicted con-
versions are much too low irrespective of the ther-
mochemistry assumed in setting the rate constants. If
loose (less likely case) conversions fall in the plausible
range but, contrary to observation, they increase a fac-
tor of two as pressures change from 8 to 47 torr. Since
no such increase was observed, we conclude that while
bond fission initiation and free radical processes are
2
60 s is almost exactly as observed. Further, the lower
and upper k limits permitted by the estimated errors
1
give conversion levels which easily bracket observed
levels: 13% and 62% conversions at 260 s for the
lower and upper k limits, respectively.
1
It’s interesting to note that higher yields of methane
can be rationalized by different interpretations of un-
characterized aspects of reactions 15 and 16. Thus wall
bound MeSiCl may not disproportionate on the walls
like HSiCl. In this case SiCl generation via reaction
2
1
5 would be reduced and this in turn would lower
reactant conversions and raise products yields. Mod-
eling predictions for this condition are shown in col-
umn 4 of Table IV. Higher methane yields also result
if methane desorptions from wall bound methylated
species (as in reaction 16) occur at rates comparable
to reaction times. Modeling results maximizing this
possibility are shown in column 5 of Table IV. Taken
in total, its evident that the data and modeling are con-
sistent in all respects with initiation via reaction 1
alone.
possible in the MeSiHCl decomposition, they are not
2
important under our study conditions.
CONCLUSION
The decomposition of MeSiHCl is mainly initiated
2
by molecular elimination to methane and SiCl . The
2

9
6
RING, O’NEAL, AND WALKER
Table IV Modeling Results for the MeSiHCl Decomposition (905 K)
2
ۙ
ۙۙۙۙReaction 1ۙۙۙۙ
ۙۙۙۙReaction 3ۙۙۙ
Initiation Reaction:
Pressure Condition (torr):
Reaction 20 Transition State:
ۙۙۙۙall pressuresۙۙۙۙ
not applicable
ۙۙۙۙۙcalculatedۙۙۙۙ
47
tight
8
47
loose
loose
Experimentala
31.3
ۙۙۙۙcalculatedۙۙۙۙ
26.0c
0.48c
0.27c
0.14c
0.41c
0.057c
31.5d
0.62d
0.32d
0.17d
0.49d
0.067d
2.5e
11.1e
35.2
0.49e
0.47
0.29
0.30
0.16
0.16
0.45
0.46
0.06
21.3
71.7
e
conversion %:
31.5
0.38
f
f
f
3
.0
e
f
e
f
(CH4):
0.58
0.44
0.50
0.52
0.29
0.28
0.16
0.15
0.45
0.43
0.07
0.07
f
0
.34
e
e
e
Y(HSiCl ):
0.32
0.17
0.49
0.067
0.31
3
f
f
f
0
.31
e
e
e
Y(MeSiCl ):
0.18
3
f
f
f
0
.20
Y(m/e ϭ 133)b:
0.44
0.49
e
e
e
f
f
f
0
.51
e
e
e
Y(SiCl ):
0.048
0.06
4
f
f
f
0
.06
0.06
a
Average values (see Table I); errors are Ϯ2 ␴.
Contributors to the m/e ϭ 133 peak are thought to be HSiCl and MeSiCl (see text). The yield of the 133 peak is based on the HSiCl
b
3
3
3
sensitivity, i.e., MeSiCl is assumed to have the same MS sensitivity as HSiCl at the m/e ϭ 133 peak.
3
3
c
d
e
MeSiCl wall reactions to generate gaseous SiCl are excluded.
2
Fast CH release from all wall bound methylated species is assumed.
4
Kinetics and thermochemistries of reactions 3 and 20 are based on Allendorf, et al. [7,22]: k ϭ 1018.3 ϫ eϪ97,500/RT sϪ , k
1
ϭ
(8
3,ϱ
20,ϱ
49,300/
Ϫ
1
14.0
Ϫ
54,300/
Ϫ
1
Ϫ
7
Ϫ1
1
016.5 ϫ eϪ
RT s (loose Þ), k
ϭ 10 ϫ e
RT s (tight Þ). RRKM calculations [19] at 905 K give k Ϸ 7.0 ϫ 10
s
20,
ϱ
3
Ϫ7
Ϫ1
Ϫ
1
Ϫ1
Ϫ1
torr) and k Ϸ 14.5 ϫ 10
s
(47 torr); k20 (loose Þ) Ϸ 190 s (8 torr) and k20 Ϸ 660 s (47 torr); k20 (tight Þ) Ͻ 5 s (47 torr).
3
f
Kinetics and thermochemistries of reactions 3 and 20 are based on Davidson [14] and Walsh [23]: k ϭ 10 ϫ e 87,700/RT sϪ
16.5 Ϫ
1
,
3,ϱ
k2 ϭ 101 ϫ e
6.5
Ϫ
52,800/
RT s (loose Þ), k
Ϫ
1
ϭ 10 ϫ e
14.0
Ϫ
57,500/
RT s (tight Þ). RRKM calculations [19] at 905 K give k Ϸ 7.0 ϫ 10
Ϫ1 Ϫ1 Ϫ1
Ϫ1
Ϫ6
0,
1
ϱ
20,
ϱ
3
Ϫ
Ϫ
6
Ϫ1
s
(8 torr) and k Ϸ 12 ϫ 10
s
(47 torr); k20 (loose Þ) Ϸ 46 s (8 torr) and k20 Ϸ 159 s (47 torr); and k20 (tight Þ) Ͻ 1 s (47 torr).
3
kinetics of this elimination are completely consistent
with the kinetics of the SiCl /CH trapping reaction.
Free radical reactions and Si9C bond fission initia-
7. M. D. Allendorf and C. F. Melius, J. Phys. Chem., 97,
20 (1993).
. JANAF Thermochemical Tables, 2nd Edition: NSRDS,
970; P. Ho and C. F. Melius, J. Phys. Chem., 94, 5120
1990); C. L. Darling, and H. B. Schlegel, J. Phys.
7
2
4
8
1
(
tion are not important in the 905 K decomposition.
Chem., 97, 1368 (1993).
9
. S. R. Gunn and L. G. Green, J. Phys. Chem., 65, 779
(
6
1961); S. R. Gunn, and L. G. Green, J. Phys. Chem.,
8, 946 (1964).
The authors thank the National Science Foundation for fi-
nancial support of this work through Grant CHE-8719843.
1
1
1
0. H. E. O’Neal, M. A. Ring, W. H. Richardson, and
G. F. Licciardi, Organometallics 8, 1968 (1989).
1. B. A. Sawrey, H. E. O’Neal, M. A. Ring, and D. Coffey,
Int. J. Chem. Kinetics, 6, 7 (1984).
2. S. F. Rickborn, D. S. Rogers, M. A. Ring, and H. E.
O’Neal, J. Phys. Chem., 90, 408 (1986).
13. M. A. Ring, H. E. O’Neal, S. F. Rickborn, and B. A.
Sawrey, Organometallics, 2, 1891 (1983).
14. I. M. T. Davidson and C. E. Dean, Organometallics 6,
966 (1987).
15. J. G. Calvert and J. N. Pitts, Jr., Photochemistry; John
Wiley & Sons: New York, 1966; O. Stern, and M. Vol-
mer, Physik. Z. 20, 183 (1919).
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1
2
3
4
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2
9. P. J. Robinson and K. A. Holbrook, Unimolecular Re-
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