Rate coefficients for the reactions of Si(3PJ) with C2H2 and C2H4: Experimental results down to 15 K

Article abstract of DOI:10.1063/1.1396855

The kinetic measurements of gas-phase reactions of ground-state silicon atoms with ethylene and acetylene were presented. Pulsed laser photolysis of Si(CH₃)₄ was used to generate Si atoms and laser-induced fluorescence for observing

Full text of DOI:10.1063/1.1396855

Rate coefficients for the reactions of Si ( 3 P J ) with C 2 H 2 and C 2 H 4 : Experimental
results down to 15 K
A. Canosa, S. D. Le Picard, S. Gougeon, C. Rebrion-Rowe, D. Travers, and B. R. Rowe
Citation: The Journal of Chemical Physics 115, 6495 (2001); doi: 10.1063/1.1396855
View online: http://dx.doi.org/10.1063/1.1396855
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 115, NUMBER 14
8 OCTOBER 2001
3
Rate coefficients for the reactions of Si„ PJ… with C2H2 and C2H4:
Experimental results down to 15 K
A. Canosa,a) S. D. Le Picard, S. Gougeon, C. Rebrion-Rowe, D. Travers,
and B. R. Rowe
´
´
UMR 6627: Physique des Atomes, Lasers, Molecules et Surfaces, Equipe Astrochimie Experimentale,
´
Universite de Rennes I, Campus de Beaulieu, F-35042 Rennes Cedex, France
͑Received 1 May 2001; accepted 3 July 2001͒
Rate coefficients for the reaction of ground-state silicon atoms Si(3PJ) with acetylene and ethylene
have been measured at temperatures down to 15 K. The experiments have been performed in a
´
´
continuous flow CRESU ͑Cinetique de Reaction en Ecoulement Supersonique Uniforme͒ apparatus
using pulsed laser photolysis of Si͑CH3͒4 to generate Si(3PJ) atoms and laser-induced fluorescence
to observe the kinetic decay of the atoms and hence determine the rate coefficients. Both reactions
are found to be fast, and the reaction rates show a very mild dependence on temperature. The rate
coefficients match the expressions k(SiϩC2H2)ϭ(2.6Ϯ0.6)10Ϫ10(T/300)Ϫ(0.71Ϯ0.24) exp(Ϫ(29
Ϯ10)/T) cm3 moleculeϪ1 sϪ1 and k(SiϩC2H4)ϭ(3.7Ϯ0.3)10Ϫ10(T/300)Ϫ(0.34Ϯ0.10) exp(Ϫ(16
Ϯ4)/T) cm3 moleculeϪ1 sϪ1 in the temperature range 15–300 K. The nature of the products and the
similarities of the carbon and silicon chemistry are discussed. © 2001 American Institute of
Physics. ͓DOI: 10.1063/1.1396855͔
spectrum and structure are now well known,25–27 but only
five other silicon compounds had a known rotational spec-
trum 10 years ago: SiN, SiO, SiC, SiS, and SiC4.20 Many
experimental and theoretical studies, however, have been
carried out in recent years, so that spectroscopic and struc-
tural data have become available on c-SiC3,28–31 linear sili-
con carbides SiCn , n ranging from 3 to 9 ͑Refs. 32–34͒ as
well as the radio spectra of SiC2H, SiNC, and SiCN,35 the
latter being detected in an astronomical source a few months
later.22 When considering hydrogenated silicon–carbon clus-
ters (SixCyHz), very little is known from a spectroscopic
point of view. Besides SiC2H, the only radicals for which
data are available are silicon methylidine SiCH36–38 and si-
lylidene H2CSi.39 Despite predictions that such clusters
should be detectable19 in circumstellar envelopes and in the
molecular clouds, none of them has been detected yet.39
The second reason for the under-representation of
silicon-containing molecules compared to its cosmic abun-
dance is that, like other refractory elements, silicon is known
to be heavily depleted from the gas phase due to its incorpo-
ration into silicate grains19,40,41 or into nanoparticles.42 It is
possible that such aggregate grains could be responsible of
the formation of complex molecules, among them silicon-
bearing molecules.43
The form taken by the gas phase silicon varies greatly
among the different components of the interstellar medium.
Very few silicon compounds are detected in dense molecular
clouds, as well as in photon dominated regions,40 where sili-
con is expected to be mainly in the form of SiS or SiO.19
Silicon–carbides and more generally, all relatively complex
silicon-containing molecules ͑with the exception of silane͒,
have been found in the circumstellar envelope of the carbon-
rich star IRCϩ10 216. The mapping of silicon dicarbide in
the envelope of this object44,45 has revealed a shell-like dis-
I. INTRODUCTION
Silicon is one of the most abundant elements in the uni-
verse. It is second only to oxygen in its natural terrestrial
abundance ͑Ϸ28% of the earth’s crust͒1 and occurs in a great
variety of silicate minerals and as quartz. During the past
decades, there has been considerable interest in the chemistry
of organosilicon compounds, which have a wide variety of
applications in the field of sensors,2 electronics,3,4
coatings,5,6 biotechnologies,7,8 and nanotechnologies.9,10
More particularly, unsaturated hydrocarbons like ethylene or
acetylene are used as precursor molecules in chemical vapor
deposition ͑CVD͒ of silicon carbide films on silicon surfaces.
In order to better understand the absorption–desorption
mechanism, many experimental11–14 and computational15–17
studies have been made. Although the gaseous phase may
influence the CVD process, the key step is the reaction of the
␲ bonds of the unsaturated hydrocarbons with the silicon
surface.
Silicon is also present in space and its cosmic abundance
is roughly twice that of sulfur. However, when compared to
the 13 sulfur-bearing molecules detected in the interstellar
medium, the silicon-containing molecules are found to be
under-represented. So far, the detected molecules are SiO,
SiS, SiC, c-SiC2, SiC4, SiH2, SiH4,18–20 SiC3,21 and SiCN.22
Atomic Si has been observed only recently23 in the Orion-KL
nebula, a region where Siϩ has already been detected. There
are at least two reasons for this apparent under-
representation.
The first one is that observational study of silicon-
containing molecules relies on the accurate knowledge of
rest frequencies of these species. The Merril–Sanford bands
were assigned to SiC2 more than 40 years ago,24 and its
a͒
Electronic mail: canosa@univ-rennes1.fr
0021-9606/2001/115(14)/6495/9/$18.00
6495
© 2001 American Institute of Physics
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J. Chem. Phys., Vol. 115, No. 14, 8 October 2001
Canosa et al.
tribution which puts additional constraints on the chemistry
responsible for the formation of this species. As hydrocar-
bons, many highly unsaturated, constitute a large part of the
121 molecules detected in space,46 and since acetylene is one
of the major species in IRCϩ10 216 with an abundance of a
few 10Ϫ5 with respect to H2,47–49 it is necessary to measure
the reaction rates of silicon atoms with hydrocarbons.
The chemistry and thermochemistry of silicon are not
very well known. The reaction schemes that are invoked to
account for the formation of the detected neutral organosili-
con compounds in IRCϩ10 216 generally include ion–
molecule reactions, with dissociative recombination as the
ultimate step.18,44,49–51 Some of these ion–molecule reactions
have not been studied in the laboratory, but the temperature
behavior and the 300 K rates can at least be evaluated using
capture theories.18,49 This is not the case for neutral–neutral
reactions, for which their rates and temperature dependence
cannot be predicted up to now. The importance of these re-
actions for astrochemical models has been highlighted,47,52
however, and as it is admitted that they must be important for
organosilicon chemistry,44,49 they are included in some
models.18,53 When the rate constant of interest has not been
measured, the value adopted in the model is estimated from
results concerning the ‘‘analogous’’ reaction with a carbon-
containing species, which is believed to be similar.18,19,53
Measurements of reactions rates of silicon atoms with
acetylene and ethylene, together with other molecules, have
been performed at 300 K54–56 by Husain and co-workers,
who employed time-resolved atomic resonance spectroscopy
to monitor the Si(3P) concentration directly via the
(3p2 3PϪ4s 3P) transition at 252 nm. Although a factor of 2
exists between the different sets of measurements, the au-
thors report rapid room temperature rates, indicating that re-
actions proceed at close to unit collisional efficiency. As far
as we know, the products of reactions of silicon with hydro-
carbon have not been identified.
In this paper, we present the first measurements of reac-
tions rates of silicon atoms with acetylene and ethylene down
to 15 K. The experimental work has been achieved using the
CRESU technique ͑French acronym for ‘‘reaction kinetics in
a uniform supersonic flow’’͒, which is now well established
in the field of low temperature rate constant measurements.57
This technique has been recently used in Birmingham ͑UK͒
to study the reaction rates of carbon atoms with molecules of
astrochemical interest. C(3P) atoms were generated by pho-
tolysis of C3O2 at 193 nm, and their decay was followed
either by chemiluminescence58 or laser-induced fluorescence
͑LIF͒ in the VUV.59 These sets of measurements will be
compared to the results obtained for silicon atoms, and this
comparison will be the basis of the discussion about the
similarities of atomic carbon and atomic silicon reactions.
studies of neutral–neutral reactions. The device is exten-
sively described in Ref. 61, so the description below will
focus on details specific to the work reported here.
In the CRESU technique, the low temperatures are
achieved by the isentropic expansion of a gas mixture ͑con-
sisting predominantly of a carrier gas, here helium, argon, or
molecular nitrogen͒, through a Laval nozzle. A supersonic
flow is generated in which the temperature, density, and ve-
locity are uniform along the flow. This temperature is a true
thermodynamic temperature as the density of the flow ͑typi-
cally 1016 to 1018 cmϪ3͒ is high enough to ensure thermal
equilibrium by collisions. The nozzle is mounted on a mov-
able reservoir kept at room temperature, into which the car-
rier gas, the reagent gas ͑ethylene or acetylene͒, and the pre-
cursor of the silicon atoms ͓tetramethylsilane: Si͑CH3͒4͔ are
introduced, after passing through mass flow controllers
͑Tylan͒, and mixed together prior to expansion.
As in a previous study concerning the reaction of Si(3P)
with O2,62 silicon atoms were produced in the supersonic
flow by multiphoton photolysis of tetramethylsilane ͑TMS
hereafter, Aldrich, 99% purity, used without further purifica-
tion͒. TMS, which is a liquid at room temperature, has a high
vapor pressure, however: 785 mbar at 296 K. A gas mixture
containing about 500 mbar of TMS and 2 bar of argon was
thus prepared and stored into a stainless-steel tank. A small
concentration of this mixture, typically 0.1% of the carrier
gas, was sufficient to generate a significant amount of silicon
atoms. Photolysis of TMS was achieved using the fourth
harmonic ͑266 nm͒ of a YAG pulsed laser beam ͑Spectra
Physics, GCR 190, 10 Hz, 50 mJ͒ propagated along the flow
and mildly focused at the nozzle exit by means of a converg-
ing lens ͑37 cm focal length͒ located inside the reservoir. A
burst of silicon atoms was produced around the focal point,
and flowed for several tens of microseconds before reaching
the detection region. Here, a tunable pulsed laser ͑Spectra
Physics, MOPO 730 with frequency doubling option, 10 Hz,
ϳ10 ␮J͒ propagated perpendicularly to the flow and excited
the 3p2 3P0 –4s 3P1 transition of Si at 251.43 nm. The reso-
nant fluorescence signal was collected using a UV-enhanced,
optically fast telescope–mirror combination mounted inside
the vacuum chamber and set at right angles to both the flow
axis and the probe laser beam. It was then imaged through a
slit, onto the photocathode of a UV-sensitive photomultiplier
tube ͑Thorn EMI, 9813QSB͒ after passing through a narrow-
band interference filter ͑centered at 250 nm͒ to reduce the
II. EXPERIMENT
A. The apparatus
Originally designed in the 1980’s for ion–molecule stud-
ies at very low temperatures ͑down to 8 K͒,60 the CRESU
apparatus has been associated with the pulsed laser photoly-
sis ͑PLP͒ laser-induced fluorescence ͑LIF͒ method for the
FIG. 1. Schematic diagram of the CRESU apparatus.
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Reactions of Si (3PJ)with C2H2 and C2H4
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J. Chem. Phys., Vol. 115, No. 14, 8 October 2001
FIG. 2. Silicon atoms, Si(3P0), fluorescence signal as a function of acety-
lene density at 15 K for a time delay set to 130 ␮s.
scattered light from the photolysis laser. A sketch of the ap-
paratus is shown in Fig. 1.
B. Measurement of the rate constant
The time delay between the photolysis pulse and the
probe laser pulse was set equal to the time required for the
silicon atoms to flow from the focal point to the detection
zone. During this time, the silicon atoms react with the hy-
drocarbon molecules according to the reaction
SiϩCnHm→products,
FIG. 3. First-order rate coefficients, k1st , as a function of the time delay for
͑a͒ Si(3PJ)ϩacetylene at 15 K and ͑b͒ Si(3PJ)ϩethylene at 23 K.
with a rate constant k. The LIF signal of the silicon atoms
was recorded and averaged over several hundreds of laser
shots, and the measurement was repeated for different hydro-
carbon flow rates. The decrease of the LIF signal as a func-
about 30 ␮s. On the other hand, using helium as a carrier
gas, the Si(1D2) state turned out to relax slowly on the time
scale of the experiment. For all Laval nozzles working in
helium, it was therefore necessary to inject a quencher into
the flow to electronically deexcite the silicon atoms. N2 was
a good candidate for this purpose, as its reaction with Si (3PJ
tion of hydrocarbon density C H was well fitted by the
͓
͔
n
m
exponential function exp(Ϫk C H ), where k is ex-
͓
͔
Ј
Ј
n
m
pressed in cm3/molecule. ͑Fig. 2͒. As the nozzle can be
moved, the delay ⌬t between production and detection of
Si(3P) can be varied using a delay generator, and since k
Ј
1
and D2͒ to yield SiN is endothermic. Its quenching effi-
ϭkϫ⌬t, the plot of k versus ⌬t is a straight line whose
Ј
ciency, moreover, was found to be high so that addition of a
small amount of N2, corresponding to about 1% of the total
density, ensured complete deexcitation of the electronic state
within 50 ␮s. Although higher electronic states were not
probed in this study, it is likely that they are also produced
and we assume that they are relaxed efficiently by argon or
nitrogen.
slope is the rate constant k, at the temperature and density of
the nozzle used ͑Fig. 3͒.
C. State of the Si atoms
As discussed in Ref. 62, because of laser focusing, pho-
tolysis of Si͑CH3͒4 generates silicon atoms in a non-
Boltzmann spin–orbit distribution, and the first excited elec-
tronic state Si(1D2) which lies 0.78 eV above the ground
state has been detected together with the (3P) states of in-
The spin–orbit relaxation of the Si(3P) states was
checked by probing the three levels (Jϭ0,1,2) by exciting
the transitions 3p2 3P0 –4s 3P1 ͑251.43 nm͒, 3p2 3P1
–4s 3P2 ͑250.69 nm͒, 3p2 3P2 –4s 3P2 ͑251.61 nm͒, respec-
tively. For all nozzles, the relative intensities of these transi-
tions were found to be constant after about 40 ␮s, indicating
that an equilibrium had been reached.
In all cases, therefore, the kinetics of the reactions of Si
with ethylene and acetylene was carried out in conditions
where both deexcitation reactions were completed ͑⌬t
Ͼ50 ␮s; see Table I͒. As a consequence, the rate coefficients
obtained in our experiment are for Si(3PJ) atoms displaying
1
terest. The efficiency of the quenching of the D2 state de-
pends, however, on the nature of the carrier gas. The relative
population of this state was measured by exciting the
3p2 1D2 –4s 1P1 transition at 288.16 nm with the probe la-
ser; resonant photons were collected through the previously
described optical system in which a narrow-band filter cen-
tered at 289 nm was used. When the carrier gas was argon,
and in the absence of any reactant gas, measurements indi-
1
cated that quenching of D2 was always completed within
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6498
J. Chem. Phys., Vol. 115, No. 14, 8 October 2001
Canosa et al.
TABLE I. Rate coefficients for the reaction of Si(3PJ) atoms with acetylene. For each temperature the main
experimental conditions are also given and more particularly the acetylene density range. The number of
measurements represents the number of time delays ⌬t for which an LIF decay signal was recorded as a
function of the acetylene density.
T
͑K͒
Buffer
gas
Number of
Total density
Rate constant
C H
⌬t range
͑␮s͒
͓
͔
2
2
measurements (ϫ1016 cmϪ3
)
(ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1
)
(ϫ1013 cmϪ3
)
15
23
36
49
52
72
97
168
295
He
He
He
He
Ar
N2
Ar
Ar
Ar
5
5
5
5
5
6
5
6
5
5.05
4.73
5.28
10.0
10.3
5.79
15.45
9.01
9.23
1–27
1–23
1–22
1–81
1.5–71
1–40
0.1–8.9
0.1–27
0.4–16
75–200
60–115
60–190
75–105
100–400
50–150
100–450
100–500
100–500
3.49Ϯ0.15a
4.17Ϯ0.12
4.53Ϯ0.28
5.69Ϯ0.26
4.90Ϯ0.17
5.18Ϯ0.25
4.20Ϯ0.32
3.10Ϯ0.25
2.26Ϯ0.22
aErrors quoted are Ϯt␴ errors, where t is the value of the student’s t-distribution for the 95% point.
the Boltzmann-type spin–orbit distribution at the tempera-
ture of the flow. Determination of the individual state-to-state
relaxation rate coefficients will be the subject of a forthcom-
ing article.
stants that were not significantly higher than the rate con-
stants measured without N2. This is interpreted by the fact
that ethylene is an efficient quencher of the 1D state. A simi-
lar behavior was noted for O2.62 At the opposite, as acetylene
seems to be a poor quencher for the D state, the reaction
rate constants obtained with N2 have been found to be about
30% higher than without N2.
Si(3P1) and Si(3P2) lie at 111 and 321 K above
1
3
Si(3P0), respectively, so that at 15 K, only the P0 level is
populated ͑Fig. 4͒. This was obvious on the LIF spectra, on
which at 15 K only the signal of the Jϭ0 state was present
and at 23 K, the signal arising from the Jϭ1 state was hardly
detectable ͑Fig. 5͒. Incidentally, the measurements at 15 and
23 K are relative to the single (Jϭ0) state.
The reaction rates were fitted by the relations
III. RESULTS
Our experimental results are listed in Tables I and II, and
log(k) is plotted as a function of log(T) in Fig. 6 for both
reactions. Only statistical errors are quoted. Some systematic
errors due to flow control inaccuracies or inaccuracies in the
determination of the total buffer gas density should, however,
also be taken into account. Every effort was made to mini-
mize these, and we estimate that the additional error does not
exceed 10%.
3
1
These results concern the P states of Si, the D state
being quenched by the addition of N2. It is worth noting that
in the case of ethylene, the addition of N2 led to rate con-
FIG. 5. LIF spectra of the Si 4s(3P )–3p2(3P ) transitions recorded in
͓
͔
J
J
the CRESU apparatus at two different temperatures: 141 K ͑upper spectrum͒
and 23 K ͑lower spectrum͒. The transitions are labeled according to the
upper and lower spin–orbit levels involved (J ,J ). In both cases, the delay
Ј
Љ
times between the photolysis and probe laser pulses were made sufficiently
FIG. 4. Relative spin–orbit populations ͑calculated͒ as a function of T.
long ͑290 and 95 ␮s, respectively͒ to allow total spin–orbit relaxation.
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Reactions of Si (3PJ)with C2H2 and C2H4
6499
J. Chem. Phys., Vol. 115, No. 14, 8 October 2001
TABLE II. Rate coefficients for the reaction of Si(3PJ) atoms with ethylene. For each temperature the main
experimental conditions are also given and more particularly the ethylene density range. The number of mea-
surements represents the number of time delays ⌬t for which an LIF decay signal was recorded as a function
of the ethylene density.
T
͑K͒
Buffer
gas
Number of
Total density
Rate constant
C H
⌬t range
͑␮s͒
͓
͔
2
4
measurements (ϫ1016 cmϪ3
)
(ϫ10Ϫ10 cm3 moleculeϪ1 sϪ1
)
(ϫ1013 cmϪ3
)
15
23
52
72
97
He
He
Ar
N2
Ar
Ar
Ar
5
5
7
5
6
5
6
5.05
4.73
10.3
5.79
15.45
9.01
9.23
1–24
1–27
0.1–12.9
1–50
0.2–5.6
0.6–6.7
0.4–12.4
80–200
60–120
50–350
50–125
200–450
200–400
100–800
3.52Ϯ0.10a
4.29Ϯ0.07
4.77Ϯ0.19
5.08Ϯ0.18
4.45Ϯ0.22
4.30Ϯ0.20
3.46Ϯ0.32
141
295
aErrors quoted are Ϯt␴ errors, where t is the value of the Student’s t-distribution for the 95% point.
Ϫ͑0.71Ϯ0.24͒
IV. DISCUSSION
T
k SiϩC H ͒ϭ 2.6Ϯ0.6͒10Ϫ10
͑
͑
ͩ ͪ
2
2
300
A. What are the products?
Our experimental technique does not allow us to deter-
mine the nature of the products, so the following discussion
is based on energetic considerations. Heats of formation of a
number of organosilicon compounds are available in the
litterature,63 but most of the data are for saturated stable
compounds such as methylsilanes, and few data are available
for unsaturated molecules. Among the compounds of inter-
est, only the heat of formation of cyclic silicon dicarbide
c-SiC2 has been determined experimentally64 and theoreti-
cally ͑see Ref. 27 and references therein͒, all other heats of
formation are calculated.27,64–67 The available thermochemi-
cal data are summarized in Table III.
29Ϯ10͒
͑
ϫexp Ϫ
cm3 moleculeϪ1 sϪ1
,
ͩ
ͪ
T
and
Ϫ͑0.34Ϯ0.10͒
T
k SiϩC H ͒ϭ 3.7Ϯ0.3͒10Ϫ10
͑
͑
ͩ ͪ
2
4
300
16Ϯ4͒
͑
ϫexp Ϫ
cm3 moleculeϪ1 sϪ1
,
ͩ
ͪ
T
where errors are quoted as Ϯ2␴, ␴ being the standard error.
The variation of the Gibbs energy of reaction ⌬G°R tells
us if a reaction proceeds or not. Such data are scarce for
organosilicon compounds, and when the entropy of forma-
tion of a compound is not available, we shall consider that
the reaction must be at least exothermic. Endothermic reac-
tions are ruled out as our measurements are carried out at
low temperatures. It should be noted that the lower the tem-
perature, the closer ⌬G°R and ⌬HR° .
According to these constraints, two main routes are open
for the reaction of silicon with acetylene or ethylene
SiϩC2Hn→SiC2Hn
→SiC2HnϪ2ϩH2.
For the Si(3P)ϩacetylene reaction, their energetics at 298 K
are the following:
SiϩC2H2→c-Si͑CH͒ ⌬H°1ϭϪ320 kJ/mol,
͑1͒
2
FIG. 6. Rate constants for the reaction of Si(3PJ) with acetylene and eth-
ylene as a function of temperature plotted on a log–log scale. The open
circles ͑᭺͒ show the results from the present work for Si(3PJ)ϩC2H2 and
the filled circles ͑᭹͒ for Si(3PJ)ϩC2H4. The results of other workers at
room temperature for Si(3PJ)ϩC2H2 are shown as an open triangles ͑Ref.
56͒ ͑᭝͒ and an open diamond ͑Ref. 54͒ ͑छ͒. The results of other workers at
room temperature for Si(3PJ)ϩC2H4 are shown as a filled triangle ͑Ref. 56͒
͑᭡͒ and a filled diamond ͑Ref. 55͒ ͑ࡗ͒. The dashed line is a fit to all the
CRESU data for for Si(3PJ)ϩC2H2, yielding kϭ(2.6Ϯ0.6)
→HSiCCH
⌬H°1 ϭϪ238 kJ/mol
Ј
⌬G°1 ϭϪ208 kJ/mol,
͑1Ј͒
Ј
→c-SiC2ϩH2 ⌬H°2ϭϪ61.7 kJ/mol
⌬G°2ϭϪ61.2 kJ/mol,
ϫ10Ϫ10 (T/300)Ϫ(0.71Ϯ0.24) exp(Ϫ(29Ϯ10)/T) cm3 moleculeϪ1 sϪ1
.
The
͑2͒
continuous line is a fit to all the CRESU data for Si(3PJ)ϩC2H4, yielding:
kϭ(3.7Ϯ0.3)ϫ10Ϫ10(T/300)Ϫ(0.34Ϯ0.10) exp(Ϫ(16Ϯ4)/T͒ cm3 mole-
and for Si(3P)ϩethylene
culeϪ1 sϪ1
.
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Canosa et al.
TABLE III. Thermochemical data for organosilicon compounds.
aReference 63.
bReference 27.
cReference 64.
dReference 67.
eReference 66.
fReference 65.
gReference 79.
⌬G°4 ϭϪ67 kJ/mol.
͑4Ј͒
SiϩC2H4→SiC2H4Ϫ287.7
р⌬H°3рϪ33.8 kJ/mol
Ј
From a mechanistic point of view, reactions ͑1͒, ͑1Ј͒, and ͑3͒
are three-body associations whose apparent second-order rate
constant is measured. In the conditions of our experiment,
only the buffer gas can play the role of the third body, and in
this case, the calculated three-body association rates are
͑depending upon the isomer͒,
͑3͒
͑4͒
⌬G°3ϭϪ214 kJ/mol for H3SiCCH,
→c-Si͑CH͒2ϩH2 ⌬H°4ϭϪ143 kJ/mol,
greater than 10Ϫ27 cm6 moleculeϪ2 Ϫ1. Such a calculated
s
→HSiCCHϩH2 ⌬H°4 ϭϪ63 kJ/mol
rate could correspond to a saturated reaction, but its fairly
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Reactions of Si (3PJ)with C2H2 and C2H4
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J. Chem. Phys., Vol. 115, No. 14, 8 October 2001
FIG. 7. Reaction scheme for Si(3PJ)ϩC2H2.
FIG. 8. Reaction scheme for Si(3PJ)ϩC2H4.
high value suggests that if three-body associations occur,
they are competing with bimolecular channels.
The reaction of sillylene (SiH2) with acetylene has been
studied experimentally68,69 and theoretically.66,70 This reac-
tion could have some similarities with SiϩC2H4, as products
are isomers with the formula SiC2H4. Experimentally, it was
found that the pressure dependence of this reaction was such
that it suggested the coexistence of a bimolecular and a ter-
molecular channel.69 Ab initio calculations coupled with ki-
netic analysis have been performed on the association chan-
nel, and have suggested a reaction scheme, where the
we measure true bimolecular rate constants is the insensitiv-
ity of the rate constants to the nature of the carrier gas, which
is required to stabilize the adduct.
The possible reaction schemes are summarized in Figs. 7
and 8.
3
B. Comparison with C„ PJ…¿acetylene or ethylene
reaction
intermediate
is
excited
silacyclopropene
1. Reaction rate constants
(c-H2SiC2H2), which can be either collisionally stabilized, or
undergo isomerization to silylacetylene (HCCSiH3) or vinyl-
silylene (H2CCHSiH). These rearrangements have an activa-
tion barrier of 172 and 154 kJ/mol respectively, which is less
than the exothermicity of the reaction, and do not require
more than two chemical bonds to be broken and reformed.
If a cyclo-addition of Si to ethylene occurred in our ex-
periment, then the adduct would be silacyclopropylidene
c-Si͑CH2͒2. This reaction is rather exothermic if the adduct is
formed in its ground electronic state, but if one considers the
possibility of forming c-Si͑CH2͒2 in the first excited (3B1)
state, a reaction which is spin-allowed, the energy excess is
much less,
The reactions of C(3PJ) atoms with hydrocarbons,
among them acetylene and ethylene, have been studied re-
cently by Chastaing et al.58,59 The reaction kinetics was mea-
sured in the same temperature range as this work.
The reaction rates for carbon atoms are quite similar to
those obtained for silicon, and can be fitted by the relations:
k(T)ϭ(2.9Ϯ0.3)10Ϫ10(T/298)Ϫ(0.12Ϯ0.10) for acetylene,
and k(T)ϭ(3.0Ϯ0.4)10Ϫ10(T/298)Ϫ(0.11Ϯ0.07) for ethylene.
The measurements and their fits are represented in Figs. 9
and 10, together with the corresponding results for silicon.
The main difference is the slight decrease of the rate
constant that is observed below 50 K in the case of silicon.
This could be attributed to a dependence of the reactivity on
the spin–orbit state of the atom, as the relative spin–orbit
SiϩC H →c-Si͑CH ͒ 1A1͒ ⌬H5° ϭϪ216 kJ/mol,
͑5͒
͑6͒
͑
2 2
2
4
→c-Si͑CH ͒ 3B1͒ ⌬H°6ϭϪ72 kJ/mol.
͑
2 2
This adduct could be collisionally stabilized, or could un-
dergo isomerization. Direct rearrangement of silylacetylene
or vinylsilylene would need four chemical bonds to be bro-
ken or reformed, and thus is highly improbable. However,
c-Si͑CH2͒2 is very close in energy to the adduct formed by
the reaction of SiH2 with acetylene, and the isomerization of
silacyclopropylidene to silacyclopropene requires the 1–2
shift of two hydrogen atoms. Silacyclopropene could then
rearrange further to form more stable products. As far as we
know, no studies have been made on the rearrangements of
silacyclopropylidene, and the fate of the adduct depends on
the height of its isomerization barriers, among other param-
eters.
Channels ͑2͒, ͑4͒, and ͑4Ј͒ correspond to the dissociation
of the adduct, forming molecular hydrogen and a reaction
product. The energy excess removal is then ensured by hy-
drogen ejection. Reactions ͑2͒ and ͑4͒ are spin-forbidden,
and the intermediate adduct has to be sufficiently long lived
to allow time for intersystem crossing to a singlet surface.
The termolecular channels cannot be eliminated on the
basis of our measurements. The only indication we have that
FIG. 9. Rate constants for the reaction of Si(3PJ) with acetylene ͑᭹, this
work͒ and C(3PJ) with acetylene ͑Ref. 59͒ ͑ᮀ͒, as a function of temperature
plotted on a log–log scale. The continuous line is a fit to the data
for Si(3PJ)ϩC2H2, yielding kϭ(2.6Ϯ0.6)ϫ10Ϫ10 (T/300)Ϫ(0.71Ϯ0.24)
ϫexp(Ϫ(29Ϯ10)/T) cm3 moleculeϪ1 sϪ1. The dashed line is a fit to the data
for C(3PJ)ϩC2H4, yielding kϭ(2.9Ϯ0.3)ϫ10Ϫ10(T/298)Ϫ(0.12Ϯ0.10)
cm3moleculesϪ1
.
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J. Chem. Phys., Vol. 115, No. 14, 8 October 2001
Canosa et al.
as the products of the reaction of silicon atoms with the same
hydrocarbons. The reaction of C with C2H4 leads mainly to
C3H3ϩH ͑Ref. 75͒ and the corresponding channel is endot-
hermic for silicon. H ejection is also observed in the
CϩC2H2 reaction,76,77 leading to C3H, but in a recent experi-
mental study, Loison et al.78 have shown that the ratio of H
atom detected per C atom consumed is only about 50%.
They suggest that the other products for this reaction are
C3ϩH2. The corresponding channel for silicon is the forma-
tion of silicon carbide SiC2. The structures of the products
are, however, not the same, as C3 is linear and SiC2 cyclic.
V. CONCLUSION
We present the first kinetics measurements of gas-phase
reactions of silicon atoms with ethylene and acetylene below
300 K. These reactions have been found to be fast, proceed-
ing at close to the collisional limit. From an energetic point
of view, SiϩC2Hn can form SiC2Hn and/or SiC2HnϪ2ϩH2.
Our experiment does not allow us to determine the nature of
the products, nor to distinguish between a termolecular satu-
rated reaction and a true bimolecular reaction. These results,
however, and especially the results concerning acetylene,
will have consequences for astrochemical models of IRC
ϩ10 216, where the abundance and the spatial distribution of
SiC2 remain to be explained.
FIG. 10. Rate constants for the reaction of Si(3PJ) with ethylene ͑᭹, this
work͒ and C(3PJ) with ethylene ͑Ref. 59͒ ͑ᮀ͒, as a function of temperature
plotted on a log–log scale. The continuous line is a fit to the data for
Si(3PJ)ϩC2H4,
yielding
kϭ(3.7Ϯ0.3)ϫ10Ϫ10(T/300)Ϫ(0.34Ϯ0.10)
ϫexp(Ϫ(16Ϯ4)/T) cm3 moleculeϪ1. The dashed line is a fit to the data for
C(3PJ)ϩC2H4,
yielding
kϭ(3.0Ϯ0.4)ϫ10Ϫ10(T/298)Ϫ(0.11Ϯ0.07)
cm3 moleculeϪ1 sϪ1
.
populations change dramatically between 295 K ͑where
N
Jϭ0 :NJϭ1 :NJϭ2ϭ1.0:2.7:1.72͒ and 15
K
͑where
NJϭ0 :NJϭ1 :NJϭ2ϭ1.0:0.001:Ͻ10Ϫ4͒.
The analogy between reactions of carbon and silicon at-
oms has been discussed. The main conclusion that can be
drawn is that reaction rates ͑at least their order of magnitude͒
may be guessed for silicon reactions, from their analogs for
carbon. This is true namely when the reactions are driven by
long-range forces as is the case here, where the first step is
the formation of an adduct between the silicon and the ␲
orbitals of the unsaturated hydrocarbon. The analogy fails,
however, with the products of the reaction, which are very
different according to the nature of the reactants.
2. The products
Although Si and C belong to the group IVA͑14͒ of the
periodic classification, their chemical behavior is very
different.1,71 The chemistry of carbon is characterized by the
ability of this element to form single, double, or triple bonds
with itself and with other atoms. On the other hand, silicon
leads to form multidirectional single bonds.72 The best illus-
tration of the difference existing between the carbon and sili-
con ‘‘analogs’’ is provided by CO2 and SiO2: carbon dioxide
is a gas, properly written OvCvO, whereas SiO2 is a net-
work substance, with each Si atom singly bonded to four
oxygen atoms. In fact, while stoichiometric similarities exist
between carbon and silicon compounds, there is no structural
or chemical similarity between them. This difference is not
limited to the multiplicity of chemical bonds in neutral mol-
ecules, but also concerns the maximum coordination
number71 and the structure of the ions.72–74
With regard to silicon–carbon bonds, before a stable
compound containing a doubly bonded silicon atom was syn-
thesized less than 30 years ago, it was thought that silicon
͓and the remainder of the Group IVA͑14͔͒ could not form
stable multiple bonds with either carbon or another atom.1 A
variety of stable compounds containing silicon–carbon
double bonds have now been synthesized, but the first
silicon–carbon triple bond has been reported only recently in
an excited state of silicon methylidine ͑SiCH͒.36 These mul-
tiple bonds are weaker, however, than multiple bonds involv-
ing only carbon atoms,71 so that when an alternative structure
is possible ͑see the isomers of formula SiC2H2͒, silicon is
rather involved in single bonds.65
ACKNOWLEDGMENTS
The authors wish to thank the Program National PCMI
͑Physique et Chimie du Milieu Interstellaire͒ for its financial
support and their colleagues from the University of Birming-
ham for the kind loan of the 15 K nozzle.
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