Temperature dependence of germylene reactions with acetylene, trimethylsilane, and phenylgermane

Article abstract of DOI:10.1016/S0009-2614(00)00189-5

Gas-phase reaction rate constants have been determined over the temperature range 295-436 K for the reactions of germylene, GeH₂, with acetylene (GeH₂ addition across a triple bond), trimethylsilane (GeH₂ insertion into a Si-H bond), and phenylgermane. The room-temperature rate constant for germylene reacting with benzene has been measured and is found to be a factor of ~300 smaller than that for phenylgermane, indicating that the latter reacts by GeH₂ insertion into the Ge-H bonds. A negative temperature dependence is observed in all cases. The activation energies, obtained from weighted linear fits to the data over the experimental temperature range, are -3.5±0.3, -11.0±0.4, and -3.6±0.3 kJ mol^-1 for acetylene, trimethylsilane, and phenylgermane, respectively, while the respective frequency factors, log(A/cm³ molecule^-1 s^-1), are -10.5±0.1, -11.8±0.1, -10.1±0.1.

Full text of DOI:10.1016/S0009-2614(00)00189-5

24 March 2000
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Chemical Physics Letters 319 2000 529–534
www.elsevier.nlrlocatercplett
Temperature dependence of germylene reactions with acetylene,
trimethylsilane, and phenylgermane
a
b
a,)
Ula N. Alexander , Keith D. King , Warren D. Lawrance
a
Department of Chemistry, Flinders UniÕersity, GPO Box 2100, Adelaide, S.A. 5001, Australia
Department of Chemical Engineering, Adelaide UniÕersity, Adelaide, S.A. 5005, Australia
b
Received 3 December 1999; in final form 2 February 2000
Abstract
Gas-phase reaction rate constants have been determined over the temperature range 295–436 K for the reactions of
Ž
.
Ž
.
germylene, GeH2, with acetylene GeH2 addition across a triple bond , trimethylsilane GeH2 insertion into a Si–H bond ,
and phenylgermane. The room-temperature rate constant for germylene reacting with benzene has been measured and is
found to be a factor of ;300 smaller than that for phenylgermane, indicating that the latter reacts by GeH2 insertion into
the Ge–H bonds. A negative temperature dependence is observed in all cases. The activation energies, obtained from
weighted linear fits to the data over the experimental temperature range, are y3.5"0.3, y11.0"0.4, and y3.6"0.3 kJ
moly for acetylene, trimethylsilane, and phenylgermane, respectively, while the respective frequency factors, log Arcm
1
3
Ž
moleculey sy1 , are y10.5"0.1, y11.8"0.1, y10.1"0.1. q 2000 Elsevier Science B.V. All rights reserved.
1
.
1. Introduction
We have recently become interested in the kinet-
ics of germylene as an extension of our work on the
other group-IV hydride radicals singlet methylene
The first direct kinetic measurements of germy-
lene, GeH2 , have recently been reported by Walsh’s
Ž1
.
Ž
.
CH2 and silylene SiH2 and have reported rate
constants for GeH2 with i-butylene, trimethylsilane,
w x
group 1 using the technique of laser flash photoly-
sis combined with time-resolved laser resonant ab-
sorption. This species is of considerable interest due
w x
acetylene, and phenylgermane 7 . Here we report
the results of studies of the temperature dependence
of GeH2 with acetylene, trimethylsilane, and phenyl-
germane. The reaction with acetylene provides in-
sight into the activation energy for GeH2 addition
across a triple bond; the reaction with trimethylsilane
gives information concerning GeH2 insertion into an
Si–H bond; and the reaction with phenylgermane
provides further information on GeH2 insertion into
Ge–H bonds. The activation energy for the latter
reaction may be compared with that obtained by
Becerra et al. for GeH2 insertion into the Ge–H
bonds in Et3GeH and GeH4.
w
x
to its relevance to chemical vapour deposition 2–4 .
w x
Becerra et al. 5 have also determined the tempera-
ture dependence of the insertion of GeH2 into the
Ž
.
Ge–H bond of triethylgermane Et3GeH , reporting
the first activation energy for a germylene reaction.
Most recently, Becerra et al. have completed a study
of the temperature dependence of the reaction of
Ž
. w x
GeH2 with germane GeH4 6 .
)
Corresponding author. Fax: q61-8-8201-3035; e-mail:
warren.lawrance@flinders.edu.au
0009-2614r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
Ž
.
PII: S0009-2614 00 00189-5

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)
530
U.N. Alexander et al.rChemical Physics Letters 319 2000 529–534
2. Experimental details
by several trap-to-trap distillations to remove the
water impurity. Infrared spectra of the resultant sam-
ple showed no evidence of impurity.
The experimental system is based on that used in
w
x
our extensive studies of methylene kinetics 8–10
and the features salient to the germylene situation
have been described in our earlier paper concerning
3. Results and discussion
w x
GeH2 kinetics 7 . Briefly, an excimer laser operat-
ing at 193 nm photolyses phenylgermane to produce
The experimental observable is the GeH2 relative
concentration as a function of time following the
photolysis laser pulse. A typical decay curve is
shown in Fig. 1. These decay curves are generally
analysed by fitting to a double-exponential function
to account for both the rise and fall behaviour. For
data with very rapid rise times a single exponential is
sufficient. The decay rate constant extracted from the
fits is the pseudo-first-order rate constant, k1, for the
reaction:
w x
GeH2. Our previous study 7 has shown that under
our experimental conditions phenylgermane is a suit-
able precursor for GeH2 kinetics, in contrast to
w x
earlier observations by Walsh’s group 1 . The GeH2
concentration is followed by time-resolved laser ab-
sorption using a single-mode cw ring dye laser tuned
to the 74GeH2 band at 17 111.31 cmy1. The experi-
ment is performed under conditions such that the
reactions are pseudo-first order. To minimise non-
germylene contributions to the signal, two sets of
decay traces are recorded, one with the laser tuned to
GeH2 qR ™ products.
Ž
.
a GeH2 absorption signal and one with the laser
Since the bimolecular rate constant is related to k1
Ž
.
detuned from GeH2 background . The background
is subtracted from the signal to provide the GeH2
decay signal. The precursor, reactant, and a buffer
gas flow continuously through the reaction cell.
The rate constants reported here were obtained
with a total cell pressure of 10 Torr. The PhGeH3
pressure was typically 10 mTorr. The reactant pres-
sure was typically varied in the range 0.02–1.0 Torr.
w x
w x
by k1 sk2 R , a plot of k1 vs. R yields k2 as the
slope.
Fig. 2 shows a plot of pseudo-first-order rate
constants vs. Me3SiH pressure over the range of
temperatures studied. This figure is typical of the
quality of the data obtained for all reactants. The
Ž
.
SF6 99.9%, BOC was used as the buffer gas as it is
an efficient collision partner that does not react with
GeH2. At 10 Torr of SF6, impurities at the 0.1%
level correspond to 10 mTorr, comparable to both
the lowest reactant pressure used and the precursor
pressure. BOC advise us that the impurities in the
SF6 are air and carbon tetrafluoride. Based on the
behaviour of methylene and silylene, and on a com-
parison of the relative rate constants for reaction by
these species and germylene, germylene is not ex-
pected to react with any of these impurities at a rate
w
x
that would be detectable in the experiments 11,12 .
Likewise, the reactants used will not be affected by
the presence of these impurities. We have established
previously that the reactions studied are in the high-
w x
pressure limit under these conditions 7 .
Ž
Fig. 1. A typical experimental decay trace in this case for
Phenylgermane was synthesised as described by
.
Me3SiH showing the variation in relative GeH2 concentration
w
x
Ž
.
Durig et al. 13 . Trimethylsilane Fluka, G97%
was used as supplied. Acetylene was synthesised by
the addition of water to calcium carbide and purified
with time. The points are the experimental data; the solid line
shows the least-squares fit to these data. The residuals from the fit
are shown below the decay trace.

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U.N. Alexander et al.rChemical Physics Letters 319 2000 529–534
531
quence of the reaction mechanism involving the
w
x
formation of an intermediate complex 15 .
The k2 values extracted from the second-order
plots are listed in Table 1. It is interesting at this
point to compare the reaction efficiencies for each of
the reactants at a particular temperature. We define
reaction efficiency as the ratio of the observed k2
value to the calculated Lennard-Jones collision rate
for the two species involved. A value of one indi-
cates reaction on every encounter. The values at 293
K are 0.20 for Me3SiH, 0.30 for C2 H2 , and 0.64 for
PhGeH3. This indicates broadly comparable efficien-
cies for GeH2 insertion into Si–H bonds and addi-
tion across the C–C triple bond but significantly
higher efficiency for insertion into Ge–H bonds. The
k2 value for the reaction of GeH2 with the alkene
Fig. 2. Plots of reactant pressure vs. pseudo-first-order rate con-
stant, k1, for GeH2 reaction with Me3SiH at various temperatures
in the range, 295–436 K. The slopes give the second-order rate
constant, k2. Note that the k2 values decrease with increasing
temperature.
w x
i-C4 H8 reported in Ref. 7 corresponds to a reaction
efficiency of 0.31. It appears that GeH2 reaction
with a C–C double bond is of comparable efficiency
to reaction with a C–C triple bond.
Arrhenius plots of the k2 values are shown in Fig.
3. It can be seen from the Arrhenius plots that the
data suggest a slight curvature compared to the linear
fit. Given the limited temperature range of the data
and the small changes in the magnitude of the rate
constants it is premature to conclude that this is
definitely the case. Nevertheless, we note that the
Arrhenius expression for the temperature dependence
of the reaction rate constants is an empirically deter-
mined relation that successfully describes the tem-
perature dependence of the rate constants of many
error bars in the figure correspond to one standard
deviation, determined by the uncertainty in the fits to
each decay trace and by the variation in k1 values
extracted from different decay traces. They do not
include other experimental uncertainties which, based
on the reproducibility of the rate constants from
different runs, are expected to be ;10%. A linear
least-squares fit to the second-order plot yields the
bimolecular rate constant, k2 , at each temperature. A
feature of the data shown in Fig. 2 is that the slopes
w x
w x
of the k1 vs. R plots decrease with increasing
simple reactions 16 . The negative temperature de-
pendence shows that these reactions are more com-
plex and so the Arrhenius equation is not necessarily
applicable to the description of their temperature
dependence. Systems comparable to those studied
temperature, in contrast with the usual Arrhenius
behaviour. This so-called negative temperature de-
pendence is, however, typical of the behaviour found
w
x
for similar systems 5,6,12,14 . It arises as a conse-
Table 1
3
Pressure-independent bimolecular rate constants, k2 cm moleculey1 sy1 measured for germylene reacting with acetylene, trimethylsi-
Ž
.
lane, and phenylgermane in SF6 bath gas at a total pressure of 10 Torr, from 295 to 436 K
y10
Ž
.
Ž
.
for reactant
Temperature K
k2 =10
Me3SiH
C2 H2
PhGeH3
295
337
373
417
436
1.03"0.05
0.85"0.02
0.57"0.014
0.35"0.006
0.29"0.028
1.38"0.04
1.34"0.06
1.14"0.02
0.92"0.02
0.77"0.05
3.00"0.10
3.06"0.09
2.76"0.04
2.30"0.07
2.05"0.04

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532
U.N. Alexander et al.rChemical Physics Letters 319 2000 529–534
fits where m is zero, the presence of an extra param-
eter leads to the fitting parameters being poorly
constrained. There is large uncertainty in the values
determined and they are of dubious worth. This
arises since the fits are of three parameters to only
five data points, each with an associated uncertainty,
which span a limited temperature range. The only
feature of note from these fits is that in all cases the
Ž
values of m must be large and negative in the range
.
y10 to y2, depending on the reactant in order to
match the trends in the data.
An Arrhenius treatment of each data set in princi-
ple indicates the frequency factor, A, from the inter-
cept and the activation energy for the reaction, Ea,
from the slope of the plot. The frequency factor
yields information on the entropy difference between
the reactants and the transition state. This is, of
course, only true when the Arrhenius equation is an
appropriate description of the behaviour. Fitting the
data to the Arrhenius equation using a weighted fit
yields the Arrhenius parameters given in Table 2.
This table includes values for other similar systems
for comparison. We have included in this table val-
ues for the GeH2 qtrimethylsilane reaction that Be-
cerra et al. have recently made available to us prior
Fig. 3. Arrhenius plots of the temperature dependence of the rate
Ž
.
constants for GeH2 reaction with C2 H2 open squares , Me3SiH
Ž
.
Ž
.
open circles , and PhGeH3 filled circles . The lines are weighted
least-squares linear fits to the data.
here also show curved Arrhenius plots. Examples
w
x
include the reaction of SiH2 with SiH4 17 , GeH2
w x
w x
with Et3GeH 5 , and SiH2 with Me3SiH 5 .
Beyond the Arrhenius equation there are a variety
of temperature-dependent forms for the rate constant
that can be used to fit the data. Laidler discusses
w
x
to publication 18 .
w
x
these various forms in detail 16 . The recommended
form when data do not conform to the Arrhenius
equation is
The Arrhenius parameters determined by Becerra
et al. for germylene reacting with trimethylsilane are
the only instance where there is another set of data
with which ours may be directly compared. A num-
ber of important points arise when comparing our
ksAT m exp yE rRT ,
1
Ž .
Ž
.
0
where E0 is the activation energy at 0 K, m is a
variable parameter, and the remaining parameters
have their usual meaning. We have fitted our data to
Ž .
results with those of Becerra et al.: 1 Becerra et al.
do not see a curvature when their data, which extend
Ž .
Eq. 1 but find that, in comparison to the Arrhenius
to higher temperature than ours, are plotted in the
Table 2
y1
3
Activation energy, Ea kJ mol , and pre-exponential factor, A cm moleculey1 sy1 , from Arrhenius fit to GeH2 qMe3SiH, C2 H2 and
Ž
.
Ž
.
PhGeH3 data and comparison with literature values
Ž
.
Reaction
log A
Ea
Ref.
GeH2 qMe3SiH
y11.8"0.1
y12.15"0.06
y10.5"0.1
y11.0"0.4
this work
w
18
x
y11.58"0.44
y3.5"0.3
GeH2 qC2 H2
GeH2 qPhGeH3
GeH2 qGeH4
GeH2 qEt3GeH
SiH2 qMe3SiH
SiH2 qC2 H2
this work
this work
y10.1"0.1
y3.6"0.3
y5.2"0.7
y10.64"1.11
y2.93"0.33
y3.3"0.19
w x
y11.17"0.10
y11.43"0.15
y10.11"0.05
y9.99"0.03
6
w x
5
w
x
x
12
19
w

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)
U.N. Alexander et al.rChemical Physics Letters 319 2000 529–534
533
Ž .
Arrhenius form; 2 the Arrhenius parameters deter-
mined by the two groups are very similar. We find
Ea sy11.0"0.4 kJ moly1 while Becerra et al.
different, with the germylene value being y11.8
compared with a silylene value of y10.1. Becerra et
al. have discussed the significance of such changes
in the context of the GeH2 qtriethylgermane reac-
obtain a value of y11.58"0.44 kJ moly1 ; and 3
Ž .
3
Ž
w x
while the frequency factors are similar, log Arcm
tion 5 . Essentially, the probable mechanism is via a
moleculey1 sy1 sy11.8"0.1 us vs. y12.15"
.
Ž .
H-bonded intermediate Me3SiH PPP XH2 , leading to
the product Me3SiXH3, XsSi, Ge. If we denote the
rate constant for formation of the complex by k1,
that for its decomposition to reactants by ky1, and
that for its conversion to products by k2 , the larger
A value for silylene indicates that the ratio k2rky1
is larger for silylene than for germylene. There are
no data available for the temperature dependence of
the reaction rate for silylene with phenylgermane
with which to compare.
Ž
.
0.06 Becerra et al. , the difference arises from a
systematic difference in the magnitudes of the rate
constants determined by the two groups. We consis-
tently measure rate constants slightly larger than
those found by Becerra et al. While this was noted
w x
previously 7 , we are unable to account for this
trend.
The activation energies, listed in Table 2, show
that the trimethylsilane reaction rate is the most
strongly dependent on temperature while the phenyl-
germane reaction rate is the least temperature depen-
dent. The frequency factors suggest that the reactions
GeH2 qphenylgermane and GeH2 qacetylene have
a smaller change in geometry between the reactants
and the transition state than does the GeH2 q
trimethylsilane reaction. Both the activation energies
for these reactions, which are small and negative,
and the frequency factors are of the order seen for
Acknowledgements
This work was supported by the Australian Re-
search Council. U.N.A. acknowledges the financial
support provided by an Australian Postgraduate
Award. The help of the Mechanical and Engineering
Workshop staff at Flinders University is gratefully
acknowledged. We thank Professor Robin Walsh for
fruitful discussions and correspondence, and for pro-
viding preprints of his work prior to publication. We
also thank Professor John Barker for helpful com-
ments on the manuscript.
Ž
.
analogous silylene reactions see Table 2 .
The magnitude of the activation energy found
here for GeH2 qPhGeH3 is similar to the value
w x
reported by Becerra et al. for GeH2 qEt3GeH 5
and is a factor of ;2 greater than that measured for
w x
GeH2 qGeH4 6 . We have measured the rate con-
stant for GeH2 with benzene and find it to be
y12
Ž
.
1"1 =10
cm3 moleculey1 sy1, a factor of
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Et3GeH and GeH2 qGeH4.
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