Time-resolved gas-phase kinetic and quantum chemical studies of reactions of silylene with chlorine-containing species. 1. HCl

Article abstract of DOI:10.1021/jp037630i

Time-resolved kinetic studies of the reaction of silylene, SiH₂, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction was studied in the gas phas

Full text of DOI:10.1021/jp037630i

J. Phys. Chem. A 2004, 108, 3987-3993
3987
Time-Resolved Gas-Phase Kinetic and Quantum Chemical Studies of Reactions of Silylene
with Chlorine-Containing Species. 1. HCl
Rosa Becerra
Instituto de Quimica-Fisica “Rocasolano”, C.S.I.C., C/Serrano 119, 28006 Madrid, Spain
J. Pat Cannady
Dow Corning Corporation, P.O. Box 995, Mail 128, Midland, Michigan, 48686-0995
Robin Walsh*
Department of Chemistry, UniVersity of Reading, Whiteknights, P.O. Box 224, Reading, RG6 6AD, UK
ReceiVed: NoVember 27, 2003; In Final Form: February 19, 2004
Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of
phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction
was studied in the gas phase at 10 Torr total pressure in SF6 bath gas, at five temperatures in the range of
296-611 K. The second-order rate constants fitted the Arrhenius equation: log(k/cm3 molecule-1 s-1) )
(-11.51 ( 0.06) + (1.92 ( 0.47 kJ mol-1)/RTln10 Experiments at other pressures showed that these rate
constants were unaffected by pressure in the range of 10-100 Torr, but showed small decreases in value of
no more than 20% ((10%) at 1 Torr, at both the highest and lowest temperatures. The data are consistent
with formation of an initial weakly bound donor-acceptor complex, which reacts by two parallel pathways.
The first is by chlorine-to-silicon H-shift to make vibrationally excited chlorosilane, SiH3Cl*, which yields
HSiCl by H2 elimination from silicon. In the second pathway, the complex proceeds via H2 elimination (4-
center process) to make chlorosilylene, HSiCl, directly. This interpretation is supported by ab initio quantum
calculations carried out at the G3 level which reveal the direct H2 elimination route for the first time. RRKM
modeling predicts the approximate magnitude of the pressure effect but is unable to determine the proportions
of each pathway. The experimental data agree with the only previous measurements at room temperature.
Comparisons with other reactions of SiH2 are also drawn.
Introduction
cm3 molecule-1 s-1 at 9.5 Torr at room temperature. CBEJ16
pointed out that a weakly pressure dependent reaction implies
incomplete collisional stabilization of the vibrationally excited
SiH3Cl* initially formed by insertion of SiH2 into the H-Cl
bond. They also suggested that decomposition of SiH3Cl* into
SiHCl + H2 could not be too rapid or this would also remove
the pressure dependence. Theoretical calculations, carried out
by Raghavachari, Chandrasekhar, Gordon, and Dykema4 (RCGD),
and Su and Schlegel,17 indicate that the formal insertion process
consists of the initial formation of a donor-acceptor complex,
H2Si‚‚‚ClH, followed by its rearrangement to SiH3Cl. The well
depth of the complex lies in the range of -8 to -29 kJ mol-1
and the barrier to rearrangement in the range of -1 to + 25 kJ
mol-1, depending on the level of calculation. The existence of
a real barrier would imply a positive activation energy for the
reaction. We therefore thought it worthwhile to extend the
kinetic measurements of CBEJ16 to higher temperatures in order
to measure the activation energy and to explore a wider range
of pressures. In order to try to understand the mechanism more
fully, we also decided to reinvestigate the energy surface at a
higher level of theory than the earlier studies of RCGD4 and
Su and Schlegel.17
Silylenes are of importance because they are implicated in
the thermal and photochemical breakdown mechanisms of
silicon hydrides and organosilanes, as well as being key
intermediates in chemical vapor deposition (CVD). Time-
resolved kinetic studies, carried out in recent years, have shown
that the simplest silylene, SiH2, reacts rapidly and efficiently
with many chemical species.1,2 Examples of its reactions include
Si-H bond insertions and CdC and CtC π-bond additions.3
SiH2 also has a great affinity for electronegative elements and
will react with lone pair donor species containing the elements
of N, O, F, P, S, and Cl among others.4 We, and others, have
recently studied the kinetics of its reactions with O-donor
molecules, H2O,5,6 CH3OH (CD3OD),5 Me2O,7,8 Me2CO,9
MeCHO,10,11 N2O,12 CO,13 and CO2.14 Because of the impor-
tance of chlorosilanes and routes to their formation in the
organosilicon industry,15 we have now turned our attention to
reactions of SiH2 with Cl-containing molecules. We report here
the combined results of an experimental kinetic study and
theoretical calculations of the energy surface for the reaction
of SiH2 with HCl.
The first and only direct rate study of this reaction was carried
out by Chu, Beach, Estes, and Jasinski16 (CBEJ) who found a
weakly pressure dependent reaction (over the range of 1-10
Torr in He) with a rate constant value of (7.5 ( 1.0) × 10-12
Experimental Section
Equipment, Chemicals, and Method. The apparatus and
equipment for these studies have been described in detail
10.1021/jp037630i CCC: $27.50 © 2004 American Chemical Society
Published on Web 04/13/2004

3988 J. Phys. Chem. A, Vol. 108, No. 18, 2004
Becerra et al.
previously.18,19 Only essential and brief details are therefore
included here. SiH2 was produced by the 193 nm flash
photolysis of phenylsilane (PhSiH3) using a Coherent Compex
100 exciplex laser. Photolysis pulses were fired into a variable
temperature quartz reaction vessel with demountable windows,
at right angles to its main axis. SiH2 concentrations were
monitored in real time by means of a Coherent 699-21 single-
mode dye laser pumped by an Innova 90-5 argon ion laser and
operating with rhodamine 6G. The monitoring laser beam was
multipassed 36 times along the vessel axis, through the reaction
zone, to give an effective path length of 1.5 m. A portion of
the monitoring beam was split off before entering the vessel
for reference purposes. The monitoring laser was tuned to
17 259.50 cm-1, corresponding to a known strong vibration-
rotation transition19,20 in the SiH2 A(1B1) r X(1A1) absorption
band. Light signals were measured by a dual photodiode/
differential amplifier combination, and signal decays were stored
in a transient recorder (Datalab DL910) interfaced to a BBC
microcomputer. This was used to average the decays of between
5 and 20 photolysis laser shots (at a repetition rate of 0.5 or 1
Hz). The averaged decay traces were processed by fitting the
data to an exponential form using a nonlinear least-squares
package. This analysis provided the values for first-order rate
coefficients, kobs, for removal of SiH2 in the presence of known
partial pressures of substrate gas.
Gas mixtures for photolysis were made up, containing
between 1.3 and 5.3 mTorr of PhSiH3, 0-4 Torr of HCl, and
inert diluent (SF6) up to total pressures of between 10 and 100
Torr. Pressures were measured by capacitance manometers
(MKS, Baratron). All gases used in this work were frozen and
rigorously pumped to remove any residual air prior to use.
PhSiH3 (99.9%) was obtained from Ventron-Alfa (Petrarch).
HCl (99.9%) was from Aldrich. Sulfur hexafluoride, SF6, (no
GC-detectable impurities) was from Cambrian Gases.
Ab Initio Calculations. The electronic structure calculations
were performed with the Gaussian 98 software package.21 All
structures were determined by energy minimization at the MP2
) Full/6-31G(d) level. Transition state structures were charac-
terized as first-order saddle points by calculation of the Hessian
matrix. Stable structures, corresponding to energy minima, were
identified by possessing no negative eigenvalues of the Hessian,
while transition states were identified by having one and only
one negative eigenvalue. The standard Gaussian-3 (G3) com-
pound method22 was employed to determine final energies for
all local minima. For transition states four single-point energy
determinations were carried out at the MP2 geometry, viz:
QCISD(T)/6-31G(d), MP4/6-31+G(d), MP4/6-31G(2df,p), and
optimized MP2 ) full/GTlarge, and the values were combined
according to the G3 procedure.22 The identities of the transition
state structures were verified by calculation of intrinsic reaction
coordinates23 (IRC) at the MP2 ) Full/6-31G(d) or B3LYP/6-
31G(d) levels. Reaction barriers were calculated as differences
in G3 enthalpies at 298.15 K. Where required, harmonic
frequencies were obtained from the values calculated at the HF/
6-31G(d) level adjusted by the correction factor 0.893 appropri-
ate to this level.24
Figure 1. Second-order plots for the reaction of SiH2 + HCl: (9)
296 K, (O) 347 K, (2) 411 K, (0) 486 K, (f) 611 K.
TABLE 1: Experimental Second-Order Rate Constants for
SiH2 + HCl at Several Temperatures ([SF6] ) 10 Torr)
T /K
k /10-12 cm3 molecule-1 s-1
296
347
411
486
611
7.23 ( 0.15
5.39 ( 0.20
5.34 ( 0.18
4.88 ( 0.18
4.66 ( 0.16
TABLE 2: Pressure Dependence of Second-Order Rate
Constants for SiH2 + HCl at 296 and 611 K
P/Torr
k /10-12 cm3 molecule-1 s-1
T ) 296 K
63
32
10
3.2
1.0
6.95 ( 0.70
6.75 ( 0.67
7.23 ( 0.15
6.34 ( 0.63
5.82 ( 0.58
T ) 611 K
56
34
10
3.2
1.6
5.13 ( 0.51
4.65 ( 0.47
4.66 ( 0.16
3.95 ( 0.40
4.17 ( 0.42
required at the higher temperature because signal intensities
decreased with increasing temperature. However, for the
purposes of rate constant measurement at a given temperature,
the PhSiH3 pressure was kept fixed. At each temperature of
study, a series of experiments was carried out to investigate
the dependence of kobs on HCl pressure (at least six different
values). The results of these experiments are shown in Figure
1, where good linear fits were obtained, as expected for second-
order kinetics. The second-order rate constants derived from
the gradients of these plots by least-squares fitting are shown
in Table 1. The error limits are single standard deviations and
are fairly small. It is clear that the rate constants decrease with
increasing temperature (just as has been found in other similar
SiH2 reactions6,9-13,18,25-30).
In addition to these experiments, some runs were carried with
the total pressure (mainly SF6) varied in the range of 1-100
Torr at the lowest and highest temperatures (296 and 611 K,
respectively). The results of these runs are shown in Table 2.
Error limits are higher at pressures other than 10 Torr because
less experimental points were involved in obtaining these fits.
The results show little variation of the rate constant values
beyond reasonable experimental scatter ((10%). At both
Results
Kinetics. Preliminary experiments established that, for a given
reaction mixture, decomposition decay constants, kobs, were not
dependent on the exciplex laser energy (50-70 mJ/pulse, routine
variation) or number of photolysis laser shots (up to 20 shots).
The constancy of kobs (5 shot averages) showed no effective
depletion of reactants. Higher pressures of precursor were

Reactions of Silylene with HCl
J. Phys. Chem. A, Vol. 108, No. 18, 2004 3989
TABLE 3: Ab Initio (G3) Enthalpies for SiH3Cl Species of
Interest in the Reaction of SiH2 with HCl
molecular species
energy /hartree
relative energy /kJ mol-1
SiH2 + HCl
H2Si‚‚‚ClH ylid-anti
H2Si‚‚‚ClH ylid-syn
TS1
TS2
H3SiCl
-751.105067
-751.113650
-751.113666
-751.106188
-751.105838
-751.222552
-751.116999
-751.150589
-751.076150
0
-22.5
-22.6
-2.9
-2.0
-308.5
-31.3
-119.5
+75.7
TS3
H2 + HSiCl
SiH2Cl• + H•
TABLE 4: Molecular and Transition State Parameters for
RRKM Calculations for the Decomposition of SiH3Cla
TS complex
parameter
molecule
TS1
TS3
Figure 2. Arrhenius plot of the second-order rate constants for SiH2
+ HCl.
wavenumber
2172 (2)
2168
937
931(2)
642(2)
510
2115
2105
1354
986
684
643
2170
2050
1530
1001
726
691
524
temperatures the rate constant values for pressures at and above
10 Torr are within the experimental scatter, while those below
10 Torr are slightly lower. Since the two extreme temperatures
show such little effect, pressure dependence studies were not
pursued at the other temperatures. Thus, the rate constants
measured at 10 Torr should represent the values for the true
bimolecular process. These values have been fitted to the
Arrhenius equation, a plot of which is shown in Figure 2. The
fit is reasonably good and the resulting equation is
434
186
498
A /s-1
1.50 × 1014
5.55 × 1013
Eo /kJ mol-1
Z /10-10 cm3
molecule-1 s-1
306
277
4.22
4.22
a Calculated at 296 K.
In this mechanism no distinction is made between the syn
and anti forms of H2Si‚‚‚ClH. Investigation of the surface
between these forms showed a low maximum (TS) of 0.8 kJ
mol-1 which means almost free rotation around the Si‚‚‚Cl bond
which connects the two forms. This also means that the IRC
calculations which connect the complex to TS1 and TS2 cannot
distinguish which form (conformer) links to which transition
state. The structures of the intermediate species, products, and
reaction transition states are shown in Figure 3 and the enthalpy
values are listed in Table 3 as well as being represented on the
PES in Figure 4. In addition we have also calculated the energy
of the potential Cl-abstraction reaction to give chlorosilyl radical
and a hydrogen atom (•SiH2Cl + H•). This is included in the
data in Table 3 but not shown in Figure 4 since it not competitive
with the other processes identified here.
log(k/cm3 molecule-1 s-1) ) (-11.51 ( 0.06) +
(1.92 ( 0.47 kJ mol-1)/RTln10
The slight pressure dependence observed for this reaction
hints at the possibility of a third-body assisted association
process. However, its small magnitude suggests that other
processes may also be occurring. An explanation for this and
the full mechanism lies in the underlying potential energy
surface, calculations of which are described in the next section.
Ab Initio Calculations. Possible species on the SiH3Cl
potential energy surface (PES) were explored in some detail at
the G3 level of theory. A fairly straightforward surface and set
of species has been found. This comprises three local minima
(i.e., stable intermediates or products), apart from the reactants,
SiH2 + HCl, and three transition states. The reaction pathway
may be described as follows. The initial step forms the donor-
acceptor complex (ylid), H2Si‚‚‚ClH (in syn and anti configura-
tions), which can then either undergo a chlorine-to-silicon
H-shift to form chlorosilane, SiH3Cl* (vibrationally excited) or
decompose to give chlorosilylene by H2 elimination (a 4-center
process). The SiH3Cl* can then be stabilized by collisions or
alternatively decompose to give HSiCl by H2 elimination from
silicon (a 3-center process). This may be summarized in the
following mechanistic scheme:
In order to throw more light on the mechanism and the issue
of a third-body assisted association (i.e., the fate of SiH3Cl*),
we have carried out RRKM calculations31 described in the next
section.
RRKM Calculations. The pressure dependence of an as-
sociation reaction corresponds exactly to that of the reverse
unimolecular dissociation process provided there are no other
perturbing reaction channels. Although the ab initio calculations
strongly suggest that there are such channels, we have carried
out a calculation on the unimolecular dissociation of SiH3Cl as
a point of reference. Despite the existence of some data on the
thermal decomposition of SiH3Cl,32 the complexity of the
experimental system led us to base our calculations on param-
eters obtained from the ab initio calculations. Table 4 shows
the vibrational wavenumbers for the molecule and the transition
state TS1, for its dissociation to SiH2 + HCl. Also listed are
parameters for transition state TS3, to be discussed later. For
each transition state the corresponding Arrhenius A factor and
critical energy Eo are given. Although there may be uncertainties
associated with these assignments, provided the A factors are
reasonable and the Eo values thought to be correct, the RRKM
SiH2 + HCl
H2Si‚‚‚ClH
H2Si‚‚‚ClH
SiH3Cl*
9
18 H2Si‚‚‚ClH
28 SiH3Cl*
38 HSiCl + H2
48 HSiCl + H2
58 SiH3Cl
9
9
9
SiH3Cl* + M
9

3990 J. Phys. Chem. A, Vol. 108, No. 18, 2004
Becerra et al.
Figure 3. Ab initio MP2 ) full/6-31G(d) calculated geometries of local minimum structures and transition states on the SiH2 + HCl energy
surface. Selected distances are given in Å and angles in degrees.
TABLE 5: Predicted Pressure Dependences for the
Third-Body Assisted Association Reaction SiH2 + HCl f
SiH3Cl (in SF6)
∞ a
P/Torr
k1/k1
T ) 296 K
100
30
10
3
0.964
0.902
0.787
0.590
0.378
1
T ) 611 K
100
30
10
3
0.599
0.409
0.254
0.130
0.060
1
a Falloff relative to the high-pressure limit.
greater, the rate constants being 25% at 10 Torr and 6% at 1
Torr (of the high-pressure limit). These calculations thus predict
far greater pressure dependence than is observed.
This result is not surprising since it is clear from the ab initio
calculations that SiH3Cl* can also decompose via TS3 (step 4
of the mechanism). Therefore we have also investigated the
more complex system:
Figure 4. Potential energy (enthalpy) surface for the reaction of SiH2
+ HCl. All enthalpies (kJ mol-1) are calculated at the G3 level.
calculations are known to be reliable.31 The calculations were
carried out in conjunction with a collisional deactivation process
employing a weak collisional (stepladder) model,31 because there
is considerable evidence against the strong collision assump-
tion.33 The average energy removal parameter, ∆E down, which
determines the collisional efficiency was taken as 12.0 kJ mol-1
(1000 cm-1), similar to that used in previous systems with SF6
bath gas.6,9,11,13 The calculations were fairly insensitive to this
value, since in the operational temperature range this cor-
responded to close to strong collisions. The results of these
calculations are shown in Table 5 in terms of the so-called
degree of fall-off, viz, the value of k1/k1∞ where k1∞ is the true
bimolecular rate constant for SiH2 + HCl. Thus, the predictions
here indicate that, at 296 K and 10 Torr pressure, the rate
constant should be at 79% of its high-pressure limit falling to
38% at 1 Torr. At 611 K, the predicted fall-off is significantly
In this scheme the RRKM calculations have been employed to
find the average rate constants for k-1 and k4 ( k-1 and k4 ,
respectively) under experimental conditions.34 For k4 this
required the assignment shown in Table 4 for TS3. In these
calculations the strong collision assumption was used. The rate
constants obtained are shown in Table 6 at each of the two
temperatures. It can be clearly seen that at both 296 and 611 K,

Reactions of Silylene with HCl
J. Phys. Chem. A, Vol. 108, No. 18, 2004 3991
TABLE 6: RRKM Calculated Values for the Average Rate
Constants for the Decomposition of SiH3Cl* under
Experimental Conditions
TABLE 8: Lennard-Jones Collision Efficiencies at 298 K for
Selected Reactions of SiH2
a,b
reaction partner
ka
ZLJ
efficiency(%)
P/Torr
k-1 /s-1 a
k4 /s-1 a
ω /s-1 b
SiH4
GeH4
C2H4
C2H2
Me2CO
N2O
4.60c
3.06d
3.5e
4.60
4.14
4.92
4.90
5.54
4.12
4.10
4.04
100
74
71
82
T ) 296 K
10
3
1
4.14 × 107
4.14 × 107
4.14 × 107
1.00 × 109
1.00 × 109
1.00 × 109
1.38 × 108
4.14 × 107
1.38 × 107
4.0f
4.2g
76
0.019h
0.46
T ) 611 K
CO2
HCl
<4.2 × 10-5i
<1.02 × 10-3
10
3
1
5.74 × 108
5.74 × 108
5.74 × 108
3.03 × 109
3.03 × 109
3.03 × 109
7.48 × 107
2.24 × 107
7.48 × 106
0.0723j
1.79
a High-pressure limiting values, units: 10-10 cm3 molecule-1 s-1
.
b The procedure for calculating ZLJ is described in ref 29. Parameters
were taken from refs 29, 38, and 39. c Reference 18. d Reference 29.
e Reference 26. f Reference 25. g Reference 9. h Reference 12. i Refer-
ence 14. j This work.
a Average values over the molecular energy distribution. b Collision
frequency () Zp).
TABLE 7: Comparison of Arrhenius Parameters for
Selected SiH2 Reactionsa
can be made in terms of collision efficiency. We have calculated
the Leonard-Jones collision numbers, ZLJ, of SiH2 with the
molecules of all these reactions at 298 K and used them to work
out collision efficiencies. The results, shown in Table 8, indicate
the same two groupings of reactant molecules as in Table 7,
with the high A factor group having collision efficiencies close
to 100% whereas the low A factor group have much lower
efficiencies. CO2 is exceptionally low because it also has
(unusually for SiH2 reactions) a positive activation energy. These
divisions do not per se depend on the reaction type. The efficient
group includes bond insertion processes (SiH4, GeH4), π-type
additions (C2H4, C2H2), and reactions with lone pair donors
(Me2CO). The less efficient group involves (at the moment)
only reactions with lone pairs (N2O, CO2, and HCl). The
underlying feature which distinguishes between these two groups
is the secondary barrier to reaction. In SiH2 + CO2 (and now
SiH2 + HCl) the barrier to rearrangement of the initial complex
is high enough to make the second step rate determining. (For
SiH2 + N2O the energy surface is unknown.) The secondary
barrier does not have to be positive to do this (although it is for
CO2) since the secondary processes involve tight transition states
which therefore cause bottlenecks (sometimes called entropy
bottlenecks). To verify this for the present case, we have used
the ab initio calculations to estimate the entropies of TS1 (270
J K-1 mol-1) and TS2 (264 J K-1 mol-1) from their calculated
structures and vibrational wavenumbers. We have then used the
transition state theory expression (A ) e2(kT/h) exp(∆Sq/R))35
to calculate log(A/cm3 molecule-1 s-1) values of -11.82 and
-12.13 for reaction via TS1 and TS2, respectively (using also
calculated entropies for SiH2 and HCl). Although these values
do not quite match the experimental one (-11.51) they are
sufficiently close (a factor of 2 for TS1) to support the idea
that passage through one or the other (or both) of TS1 and TS2
are rate controlling. The measured negative activation energy
is also close to the calculated values for these transition states.
Thus it appears to us that the donor-acceptor complex (in either
syn or anti form) does not affect the kinetics of this reaction,
except as a stepping stone to the transition states. This is partly
a consequence of its weak binding energy and stands in complete
contrast to the SiH2 and H2O reaction where the complex is the
final product.5,6
log(A/cm3
reaction
molecule-1 s-1
)
Ea /kJ mol-1
ref
SiH2 + SiH4
SiH2 + GeH4
SiH2 + C2H4
SiH2 + C2H2
SiH2 + Me2CO
SiH2 + N2O
SiH2 + CO2
SiH2 + HCl
-9.91 ( 0.04
-9.88 ( 0.02
-9.97 ( 0.03
-9.99 ( 0.03
-10.17 ( 0.04
-12.09 ( 0.04
-11.89 ( 0.13
-11.51 ( 0.06
-2.1 ( 0.2
-3.3 ( 0.3
-2.9 ( 0.2
-3.3 ( 0.2
-4.5 ( 0.3
-2.0 ( 0.3
16.4 ( 1.2
-1.9 ( 0.5
18
28
25
24
9
12
14
this work
a High-pressure limiting values (for pressure dependent reactions).
k4 is significantly larger than k-1 , thus indicating that
redissociation of SiH3Cl*, which is the cause of pressure
dependence in this system is, at best, a minor process at both
temperatures. The calculations suggest a 3-4% pressure
dependence (viz reduction from k1∞) at 296 K and a 15-16%
pressure dependence at 611 K. If a full weak collision
mechanism were invoked these effects would be slightly less.31
These effects are relatively small but comparable with those
observed. It should be noted that this model does not include
the direct process of H2 elimination from the association
complex (step 3 of the mechanism).
Discussion
General Comments and Rate Constant Comparisons. The
main experimental purpose of the present work was to study
the temperature and pressure dependence of the rate constants
for the reaction of SiH2 with HCl. This has been accomplished,
and the reaction found to have a moderately high A factor and
small negative activation energy. This is discussed in more detail
below. Comparison with the previous study of CBEJ16 shows
that our value of the room temperature rate constant in 10 Torr
SF6, shown in Table 1, agrees well with theirs ((7.5 ( 1.0) ×
10-12 cm3 molecule-1 s-1 in 9.5 Torr He). CBEJ16 also obtained
a value of (4.3 ( 0.6) × 10-12 cm3 molecule-1 s-1 in 1.0 Torr
He. This suggests a rather greater pressure dependence (in He)
than obtained by us (in SF6). This is plausible since He is a
weaker collision partner than SF6. However, the effects of
pressure are still small. The possible mechanistic implications
are discussed later.
To try to put this study into the perspective of the range of
kinetic behavior exhibited by SiH2, a comparison of Arrhenius
parameters of a variety of its reactions is shown in Table 7.
The clear distinction can be drawn between the high A factor
processes (log(A/cm3 molecule-1 s-1) in the range of -9.8 to
-10.2) of SiH4, GeH4, C2H4, C2H2, and Me2CO and the low A
factor processes (log(A/cm3 molecule-1 s-1) in the range of
-11.5 to -12.1) of N2O, CO2, and HCl. Another comparison
Ab Initio Calculations, RRKM Calculations, and the
Mechanism. The early theoretical calculations of RCGD4
revealed the donor-acceptor complex at -8 kJ mol-1 and TS1
at +25 kJ mol-1 relative to SiH2 and HCl. Su and Schlegel17
obtained well depths of between 6.3 and 19.7 kJ mol-1
(including ∆ZPE) and barriers (TS1) in the range of -4.6 to
+28.9 kJ mol-1, depending on the level of calculation and basis
set used. Our calculations are in quite close agreement with

3992 J. Phys. Chem. A, Vol. 108, No. 18, 2004
Becerra et al.
TABLE 9: Quantum Chemical Calculated Energy Barriers
(in kJ mol-1) for Chlorosilane Decomposition Pathways
particular silylenes implicated are SiCl2 and MeSiCl, the
importance of the work presented here is that it provides a
framework of reference for the kinetics and mechanisms of
reactions of silylene with chlorinated species.
decomposition
pathway
this Raghavachari Su and Walch and
work
et al.a
Schlegelb
Dateoc
SiH3Cl f SiHCl + H2 277
SiH3Cl f SiH2 + HCl 305
a Reference 4. b Reference 17. c Reference 36.
280
318
279
322
Acknowledgment. R.B. and R.W. thank Dow-Corning for
a Grant in support of the experimental work. R.B. also thanks
the Spanish DGI for support under project BQU2002-03381.
314
those of Su and Schlegel17 at the MP2/6-311++G(3df,3pd)
level, with a relatively deep well and a small negative barrier.
The crucial point is that TS1 lies 3 kJ mol-1 below the reactants.
A positive energy for TS1 would not be consistent with the
observed reaction kinetics. A new feature of this PES is the
direct route (step 3) from complex H2Si‚‚‚ClH) to products
(HSiCl + H2) via TS2, a pathway not previously identified
(either theoretically or experimentally) in this system. The
analogue of this pathway, however, has been shown to exist in
the SiH2 and H2O reaction leading from H2Si‚‚‚OH2 to HSiOH
+ H2.5,6 It is not possible to say from the results of the present
study whether TS1 or TS2 is the dominant rate determining
step, as they are extremely close in energy. TS2 has the tighter
structure (lower entropy) which suggests that step 2 of the
reaction scheme (via TS1) may be the more important pathway.
For product determination it hardly matters since step 2 (via
TS1) leads to chemically activated SiH3Cl at circa 28 kJ mol-1
above the threshold barrier (TS3) for formation of HSiCl and
H2. The RRKM calculations indicate that the slight pressure
dependence found in our experiments is not consistent with that
of a third-body assisted association since the calculated extents
of fall-off (at both 296 and 611 K) were much greater than those
found. When the RRKM calculations were extended to include
decomposition of SiH3Cl* via TS3, a much diminished pressure
dependence was found more in keeping with the experimental
findings and also those of CBEJ.16 This arises because the large
bulk of SiH3Cl* decomposes via TS3 (step 4) and very little
redissociates to reactants via TS1. The calculated trend is still,
however, to show increased pressure dependence with increasing
temperature. The measured effects do not support this, although
the scatter in the data could mask it. These calculations still
leave open to some extent the question of the contribution of
step 3. Although the direct process of H2 formation by this
pathway is pressure independent, the ab initio calculations show
that it could compete. The difficulty of the results is that if this
pathway was dominant there would be no pressure dependence,
whereas for it to provide a best fit would require the importance
of step 3 to increase with temperature in order to offset the
increased pressure dependence predicted (from steps 1, -1, 4
and 5) at higher temperature. The energy surface and the A factor
calculations do not support this. Thus, we are uncertain of the
relative importance of the two routes leading to final products.
References and Notes
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Our calculations may also be compared with those of other
4,17,36
studies
concerned with the thermal decomposition of
(22) Curtiss, L. A.; Raghavachari, K.; Redfern, P. C.; Rassolov, V.;
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pathways. For both available channels the values for the
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Pople, J. A. J. Chem. Phys. 1998, 109, 7764.
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.
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There seems to be strong evidence that silylenes are involved
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Reactions of Silylene with HCl
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(34) The average rate constants for each pathway i, ki were computed
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microcanonical rate constant of the jth level of pathway i.31
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