Rate constants for H+(CH3)4-nSiHn, n=1-4

Article abstract of DOI:10.1016/S0009-2614(97)01263-3

Rate constants for the reactions of H atoms with SiH₄ and the methylsilanes, (CH₃)_4-nSiH_n, n=1-4, have been measured at 298 K in pulsed photolysis experiments with H atoms produced by the mercury-sensitised phot

Full text of DOI:10.1016/S0009-2614(97)01263-3

9
January 1998
Chemical Physics Letters 282 Ž1998. 192–196
ž
/
4yn
Rate constants for Hq CH
SiH , ns1–4
3
n
Neville L. Arthur, Luke A. Miles
School of Chemistry, La Trobe UniÕersity, Melbourne, Victoria, Australia 3083
Received 17 September 1997; in final form 23 October 1997
Abstract
Rate constants for the reactions of H atoms with SiH4 and the methylsilanes, ŽCH3.4ynSiH , ns1–4, have been
n
measured at 298 K in pulsed photolysis experiments with H atoms produced by the mercury-sensitised photolysis of H2 and
y
13
monitored by Lyman-a absorption. The values obtained, for ns1–4, respectively, are: k sŽ2.70"0.20.=10
,
1
y1
y
13
y
13
y
13
3
k sŽ3.94"0.29.=10 , k sŽ3.88"0.21.=10 , and k sŽ3.38"0.16.=10
cm s . These results show that
methyl substitution enhances the reactivity of the Si–H bond towards H atom attack, and comparisons are made with the
2
3
4
corresponding reactions of Cl, Br and O atoms. q 1998 Elsevier Science B.V.
1
. Introduction
tion has no discernible effect on the strength of the
Si–H bond w4x.
The reactions of H atoms offer a less complicated
yardstick of reactivity than attack by more complex
species, and therefore occupy a central position in
this discussion. Unfortunately, although many inves-
Whether methyl substitution has an effect on the
strength and reactivity of the Si–H bond in SiH is a
4
contentious topic w1x. The results of Ding and Mar-
shall w1,2x indicate that for Cl and Br attack the room
temperature rate constant corrected for reaction path
tigations of the reactions of H atoms with SiH and
4
degeneracy, krn, is higher for ŽCH . SiH than SiH
the methysilanes have been reported, the values of
the rate constants obtained cover a wide range, mak-
ing it difficult to establish reactivity patterns. Further
uncertainty has been introduced recently by Marshall
and coworkers w5x, who have called into question the
3
3
4
by a factor of about 4. Although in each case this
was accompanied by an increase in A-factor, an
increase in activation energy was also observed,
from which they deduced that the dissociation en-
y1
ergy of the Si–H bond in ŽCH . SiH is 14 kJ mol
results of Potzinger and coworkers w6x for SiH ,
3
3
4
higher than in SiH . By contrast, for O atom attack,
results which until then had been regarded as the
most reliable available. This in turn casts doubt on
their w6x data for the methylsilane reactions, and in
view of the topicality of the effect of methyl substi-
tution on the reactivity of the Si–H bond and the
thermochemistry of the corresponding silyl radicals
w1,4,7x, the kinetics of H atom attack on the methylsi-
4
Horie et al. w3x found that krn was greater for
ŽCH . SiH than SiH by a factor of 35. According
3
3
4
to unpublished results referred to by Ding and Mar-
shall w1x, this is because the activation energy of the
y1
ŽCH . SiH reaction is about 5 kJ mol
less than
3
3
for SiH , a result which could be interpreted as an
4
indication that methyl substitution reduces the Si–H
dissociation energy. Both of these conclusions are
contrary to the hitherto accepted view that methyla-
lanes are now likely to become the subject of re-
newed interest. We have therefore undertaken a fresh
investigation of the reactions of H atoms with SiH4
0
009-2614r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S0009-2614Ž97.01263-3

N.L. Arthur, L.A. MilesrChemical Physics Letters 282 (1998) 192–196
193
and the methylsilanes. The values of the rate con-
3. Results
stants obtained are compared with those found in
previous studies, and with results reported for attack
by Cl, Br and O atoms.
In order to ensure that pseudo first-order condi-
tions prevailed in our work, the concentration of H
atoms produced by the photolytic pulse was deter-
mined in a separate set of experiments by the method
described in detail in previous papers w8,9x. The
2
. Experimental
resultant calibration plot relating ln ŽI rI. to wHx,
0
where I rI is the Lyman-a transmittance, showed
0
The pulsed photolysis system used in these exper-
iments has been described previously w8x. Briefly, H
atoms are produced by mercury-sensitized photolysis
that Beer–Lambert behaviour was followed up to wHx
1
2
y3
f3.0=10 cm , corresponding to a photolysis
pulse length of 3.8 ms for the methylsilanes, and 6.0
Ž253.7 nm. of H2 by flowing a dilute substrate-H2
ms for SiH , where a mesh was inserted between the
4
mixture over a drop of mercury in a thermostatted
reservoir, and then through a quartz reaction cell,
where it is illuminated with light from a pulsed
low-pressure mercury lamp. The concentration of H
atoms is followed after the photolysis pulse by moni-
toring the radiation transmitted through the cell from
a Lyman-a lamp situated perpendicular to the pho-
tolysis lamp. The Lyman-a line Ž121.6 nm. is se-
lected by a vacuum-ultraviolet monochromator and
detected by a solar-blind photomultiplier, the output
from which is amplified, digitised and then counted
on a multichannel scaler card installed in a PC. The
PC also controls the variables that define the experi-
mental reaction conditions Žlength of the photolysis
photolysis lamp and the reaction cell. All rate con-
stant measurements were therefore carried out with
maximum pulse lengths less than these values, and
the resultant
H atom concentrations led to
wsilanexrwHx ratios in the range 10–500.
Under these conditions, the variation of wHx with
time immediately after the photolysis pulse is given
1
1
by wHxswHx expŽyk t., where k is the pseudo
0
first-order rate constant. Substitution of the Beer–
L a m b e r t e x p r e s s io n
y ie ld s
I s
1
I0 exp
Ä
yP expŽyk t.
4
, where PsQwHx , and Q is
0
1
the slope of the calibration plot. The value of k was
obtained by fitting this double exponential function
to the experimental decay data by a non-linear least-
pulse, period between pulses, flow rates, temperature
and data collection time. by means of a file created
squares procedure ŽWavemetrics Igor Pro. with I ,
P and k as adjustable parameters.
0
1
for each series of runs. In these experiments, good
quality data were obtained between 15 and 60 min.
As we have pointed out previously w9x, the value
1
of k determined in this manner does not rely on
Mixtures of silane Ž0.80–1.10 Torr. and H Žf
absolute values of wHx being known, and hence does
not depend on the values of the intensity of the
absorbed photolysis light and the rate constants of
the reactions leading to the production of H atoms in
2
9
00 Torr. were prepared in a 20 l bulb, allowed to
stand overnight to allow complete mixing, and used
within 48 h to ensure constancy of composition. To
achieve the desired concentration, the silane–H
the Hg-sensitized photolysis of H . The only condi-
2
2
3
y1
mixture Žflow rate 1–10 standard cm min . was
diluted by combining it with a stream of hydrogen in
a mixing chamber, the total flow rate being main-
tions that need to be fulfilled are that pseudo first-
order concentration ratios be established, and H atom
concentrations be restricted to the Beer–Lambert
region.
3
y1
tained at 500 standard cm min . The pressure in
the reaction cell was held constant at 500 Torr.
1
The values of k obtained increased strongly with
H ŽLinde, 99.999%. was passed through an Oxy-
photolysis pulse length. As has been shown previ-
ously w6,8x, this behaviour can be accounted for by
2
trap and an Indicating Oxytrap ŽAlltech. to absorb
any residual traces of oxygen and water. SiH4
ŽMatheson, 99.99%., and the methylsilanes ŽPeninsu-
lar ChemResearch, 99%. were further purified by
the occurrence of secondary reactions between the H
atom and the silyl radical formed by H-abstraction
from the silane. As before, to obtain the true value of
1
low-temperature trap-to-trap fractionation, and thor-
k , several experiments were carried out at different
oughly degassed.
pulse lengths, and the results extrapolated to zero.

1
94
N.L. Arthur, L.A. MilesrChemical Physics Letters 282 (1998) 192–196
Inspection of Table 1 shows that our value of the
rate constant is a factor of 1.7 higher than that of
Potzinger and coworkers w6x. This difference in re-
sults is unexpected, as our experimental system is
based on an earlier version constructed by Potzinger,
Reimann and coworkers w6,10x. The reason for the
disparity is not obvious, but may be connected with
the H atom concentrations employed in our work,
which are at least a factor of 10 less than those in the
experiments of Potzinger and coworkers. This allows
the experimental Lyman-a decay curve to be fitted
1
to a double exponential function to evaluate k ,
rather than the triple exponential function used in the
1
earlier work, and consequently extrapolation of k
Fig. 1. Plot of k1 versus wsilanex for the evaluation of k: Ž`.
SiH ; Žv. ŽCH . SiH .
values to zero pulse-length is now more straightfor-
ward. In addition, the automation of some aspects of
the experiment, described previously w8x, has led to
4
3
2
2
much more data being collected for a particular
reaction system than formerly, and this may also
contribute to more reliable and consistent results
being obtained.
The bimolecular rate constant for H atom attack
1
1
on the silane, k, is related to k by k skwsilanexq
k , where k is the rate constant for the diffusion of
w
w
H atoms out of the Lyman-a observation zone.
Values of k were determined at several silane
The most recent study of HqSiH is that of
4
1
Marshall and coworkers w5x. Our only reservation
concerning their work is that H atom concentrations
were not independently determined, and although it
is unlikely that pseudo first-order conditions did not
apply in their work, it was not verified. Their rate
constant is 18% lower than ours, but having regard
concentrations, and k was then evaluated from the
1
slope of a plot of k versus wsilanex. Typical plots,
for SiH and ŽCH . SiH , are shown in Fig. 1, and
4
3
2
2
the values of k obtained for ŽCH . SiH , ns1–4,
3
4yn
n
y13
are: k sŽ2.70"0.20.=10 , k sŽ3.94"0.29.
1
2
y13
y13
=
"
10 , k sŽ3.88"0.21.=10
and k sŽ3.38
3
4
y13
3
y1
0.16.=10
cm
s
, where the error limits
correspond to one standard deviation.
Table 1
Rate constants for H q SiH4
T ŽK.
k Ž10y13 cm sy1.
3
Ref.
4
. Discussion
2
98
)2.2
21"7
26"3
85"34
4.6"0.3
w11x
w12x
w13x
w14x
a
room
room
3
3
305
room
4
.1. HqSiH4
00
00
The rate constants determined in this work are
15
w x
b
compared with previously reported values in Table 1
Žalso see Refs. w21,22x.. Setting aside the anoma-
lously high values, we see that the remaining nine
rate constants fall within the range 2.0–4.6=10
cm s . It is interesting to note that the average of
these is Ž3.4"0.5.=10
that found in this work, and close to the best value
4.4"1.0
w16x
w17x
w6x
w18x
w19x
4.4"0.7
2.0"0.1
2.2"0.2
4.0
2.5"0.5
2.8"0.3
2
2
94
93
y13
room
room
298
3
y1
w
20
w5x
this work
x
y13
3
y1
c
cm s , the same as
2
98
3.4"0.3
suggested by Arthur and Bell w23x in their review of
a
b
c
y13
y12
cm3
cm
y1
y1
Relative to kŽHqC H .sŽ3.78"0.30.=10
s
s
w21x.
w22x.
y13
2
2
data published prior to 1978, Ž3.5"0.7.=10
3
Relative to kŽHqC H .sŽ1.24"0.22.=10
2
4
3
y1
cm s
.
Evaluated at 298 K from quoted Arrhenius parameters.

N.L. Arthur, L.A. MilesrChemical Physics Letters 282 (1998) 192–196
195
Table 2
Rate constants for HqŽCH3.
T ŽK.
w
x
w x
al. 17 , Potzinger and coworkers 6 , and in this
work, the early values being much higher than the
later ones.
SiH , ns1–3
4yn
n
k Ž10y13 cm sy1.
3
Ref.
CH SiH
12.6"3.8
11.5"2.0
The work of W o¨ rsdorfer et al. and Potzinger and
coworkers is from the same laboratory, the former
employing the discharge-flow technique, the latter,
pulsed photolysis-resonance absorption. While there
is reasonable agreement between their rate constants
for CH SiH and ŽCH . SiH , the value obtained
3
3
a
room
room
w12x
w13x
b
3
05
6.3"1.2
w16x
room
3.8"0.2
4.0"0.1
3.9"0.2
w17x
2
2
91
98
w6x
this work
3
3
3
2
2
ŽCH . SiH
3
2
2
for SiH4 in the discharge-flow experiments is more
than twice that found by pulsed photolysis. Potzinger
and coworkers have suggested that surface effects
may have influenced the results of the earlier work,
but as has been pointed out above, their value for
SiH4 now appears to be low compared with our
value and that of Marshall and coworkers 5 .
a
room
room
17.6"4.8
4.1"0.9
6.8"1.3
2.8"0.2
3.1"0.1
3.9"0.3
w12x
w13x
b
3
05
w16x
room
w17x
2
2
92
98
w6x
this work
ŽCH . SiH
3
3
w x
a
room
room
15.9"4.0
3.7"1.0
5.8"1.2
2.6"0.1
2.7"0.2
w12x
Comparison of our results for the methylsilanes
with those of Potzinger and coworkers shows that
there is agreement only for CH SiH . For both
w13x
b
3
2
2
05
90
98
w16x
w6x
3
3
this work
Ž
.
Ž
.
CH3 SiH2 and CH 3SiH our values are higher,
2
3
the difference in the case of ŽCH . SiH being
a
y13
cm3
cm
y1
y1
Relative to kŽHqC H .sŽ3.78"0.30.=10
s
s
w21x.
w22x.
3
2
2
2
2
b
y12
3
particularly marked.
.3. ReactiÕity trends
The effect of increasing methyl substitution on the
Relative to kŽHqC H .sŽ1.24"0.22.=10
2
4
4
to the range of values reported for this reaction, this
constitutes an acceptable degree of agreement.
reactivity of the Si–H bond can be seen by evaluat-
ing krn, the rate constant corrected for reaction-path
degeneracy, where n is the number of Si–H bonds.
4
.2. Hq(CH3 )4y n SiH , ns1–3
n
Before ascribing the measured rate constants to
attack on the Si–H bonds in the methylsilanes, the
possibility must be considered that attack also occurs
on their methyl groups. Arrhenius parameters for the
reaction of H atoms with ŽCH . Si have been deter-
3
4
mined by Potzinger and coworkers w24x, and the rate
y16
3
constant evaluated at 298 K is 1.15=10
s
cm
y1
. Taking into account the number of methyl
groups in the various methylsilanes, the resultant rate
constants are 0.03, 0.02 and 0.01% of the experimen-
tal values for ns1–3, respectively. The rate con-
stants obtained in this work can therefore be confi-
dently attributed to attack solely on the Si–H bonds
of the methylsilanes.
Rate constants for H atom attack on the methylsi-
lanes from this and previous studies are listed in
Table 2. There is a clear separation between the
values of Hong w12x, Cowfer et al. w13x, and Austin
and Lampe w16x, and those reported by W o¨ rsdorfer et
Fig. 2. Effect of methyl substitution on Si–H bond reactivity: Ž`.
this work; ŽI. Ref. w6x; ŽB. Ref. w17x.

1
96
N.L. Arthur, L.A. MilesrChemical Physics Letters 282 (1998) 192–196
A plot of krn versus n comparing our results with
those of Potzinger and coworkers and W o¨ rsdorfer et
al. is shown in Fig. 2. Our values show that methyl
substitution leads to a progressive increase in Si–H
bond reactivity. The same trend is followed by the
values of Potzinger and coworkers, but as a result of
their low rate constants for ŽCH . SiH and SiH4,
their values are rather scattered. The data of
W o¨ rsdorfer et al. show little variation for the three
silanes studied.
an oddly low value for ŽCH . SiH, Potzinger and
3 3
coworkers came to the conclusion that the activation
energy was unaffected by methylation, and that the
main contribution to the change in krn along the
series was a parallel change in A-factor.
The conflicting behaviour shown by the different
attacking species points to the need for further stud-
ies of the temperature dependence of these reactions,
particularly those involving H and O atom attack.
3
2
2
A comparison of the reactivity of ŽCH . SiH and
SiH4 towards attack by various atoms is shown in
3
3
References
Table 3. The ratios of the krn values for ŽCH . SiH
3
3
w1x L. Ding, P. Marshall, J. Am. Chem. Soc. 114 Ž1992. 5754.
and SiH are 3.2, 2.8, 4.6 and 35, for H, Cl, Br and
4
2
w x
L. Ding, P. Marshall, J. Phys. Chem. 96 1992 2197.
Ž .
O attack, respectively. Thus the introduction of
methyl groups enhances the reactivity of the Si–H
bond to a similar extent for the H, Cl and Br
reactions, but for O atom attack the effect is much
more striking. For both the Cl and Br atom reactions,
the Arrhenius parameters of Ding and Marshall w1,2x
w3x O. Horie, R. Taege, B. Reimann, N.L. Arthur, P. Potzinger,
J. Phys. Chem. 95 Ž1991. 4393.
w4x R. Walsh, in: S. Patai, Z. Rappoport ŽEds.., The Chemistry
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to ŽCH . SiH is the result of an increase in the value
3
3
of Arn, offset to some extent by an increase in
activation energy. In the case of the O atom reac-
tions, however, the unpublished results of Ding and
Marshall w1x indicate that the activation energy for
ŽCH . SiH is substantially less than for SiH , and
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Ž1995. 3367.
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3
3
4
2
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served for these reactions. For H atom attack, despite
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3
3
4
Ž1975. 1752.
krn Ž10y13 cm sy1.
3
Silane
Ref.
w15x D. Mihelcic, V. Schubert, R.N. Schindler, P. Potzinger, J.
Phys. Chem. 81 Ž1977. 1543.
H
y13
w
16x
17x
Ž
.
ŽCH . SiH
SiH4
2.7=10
0.85=10
Cl
1.56=10
0.554=10
this work
this work
E.R. Austin, F.W. Lampe, J. Phys. Chem. 81 1977 1134.
K. W o¨ rsdorfer, B. Reimann, P. Potzinger, Z. Naturforsch. A
3
3
y13
w
3
8 Ž1983. 896.
y10a
w18 M. Koshi, F. Tamura, H. Matsui, Chem. Phys. Lett. 173
x
ŽCH . SiH
SiH4
w1x
w2x
3
3
y10a
Ž
.
1990 235.
w19x N.M. Johnson, J. Walker, K.S. Stevens, J. Appl. Phys. 69
Br
y12a
1991 2631.
Ž .
ŽCH . SiH
5.23=10
1.14=10
O
3.06=10
0.88=10
w1x
w2x
3
3
y12a
w
x
Ž
.
SiH4
20 S.K. Loh, J.M. Jasinski, J. Chem. Phys. 95 1991 4914.
w21x K. Sugawara, K. Okazaki, S. Sato, Bull. Chem. Soc. Jpn. 77
y12b
y13b
1955 334.
Ž .
ŽCH . SiH
w3x
w3x
3
3
w
22 P.D. Lightfoot, M.J. Pilling, J. Phys. Chem. 91 1987 3373.
x Ž .
SiH4
w23x N.L. Arthur, T.N. Bell, Rev. Chem. Intermed. 2 Ž1978. 37.
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Chem. Soc. Faraday Trans. 86 Ž1990. 1407.
a
Evaluated at 298 K from Arrhenius parameters.
Room temperature.
b