Mechanistic study of the Me2SiH–Me3Si radical system

Abstract

Di- and trimethylsilyl radicals, generated by the reaction of H atoms with di- and trimethylsilane, react to produce three main products: 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane. These products are formed by both radical combination and radical disproportionation reactions. The disproportionation reactions form Me2Si which inserts into the Si–H bonds of the reactants. From a quantitative determination of the disilane products as a function of the reactant ratio, a value for the branching ratio of cross-disproportionation of di- and trimethylsilyl radicals relative to the branching ratio for the disproportionation of dimethylsilyl radicals can be extracted. Our results provide strong evidence that the ratio of the rate constants for hydrogen abstraction from di- and trimethylsilane by H atoms is larger than absolute rate measurements suggest. Analysis also shows that the geometric mean rule for cross-radical reaction is closely obeyed. Disproportionation reactions yielding silaethenes occur to a minor extent and are responsible for the formation of six trisilanes. Secondary reactions, mainly initiated by H-atom abstraction from tetra- and pentamethyldisilane by silyl radicals, also take place. The relative rate constants estimated for these reactions are in agreement with a previous determination.

Full text

Mechanistic study of the radical systemMe
2
SiH–Me
3
Si
I. Lein, C. Kerst,¤ N. L. Arthur” and P. Potzinger*
Max-Planck-Institut 10, 37073 Germanyfu

r Stro

mungsforschung, BunsenstraeGo

ttingen,
Di- and trimethylsilyl radicals, generated by the reaction of H atoms with di- and trimethylsilane, react to produce three main
products: 1,1,2,2-tetramethyldisilane, pentamethyldisilane and hexamethyldisilane. These products are formed by both radical
combination and radical disproportionation reactions. The disproportionation reactions form which inserts into the SiwHMe
2
Si
bonds of the reactants. From a quantitative determination of the disilane products as a function of the reactant ratio, a value for
the branching ratio of cross-disproportionation of di- and trimethylsilyl radicals relative to the branching ratio for the dispro-
portionation of dimethylsilyl radicals can be extracted. Our results provide strong evidence that the ratio of the rate constants for
hydrogen abstraction from di- and trimethylsilane by H atoms is larger than absolute rate measurements suggest. Analysis also
shows that the geometric mean rule for cross-radical reaction is closely obeyed. Disproportionation reactions yielding silaethenes
occur to a minor extent and are responsible for the formation of six trisilanes. Secondary reactions, mainly initiated by H-atom
abstraction from tetra- and pentamethyldisilane by silyl radicals, also take place. The relative rate constants estimated for these
reactions are in agreement with a previous determination.
Introduction
This paper presents another chapter in our continuing investi-
gation of the mechanistic pathways followed by silyl radicals.
So far we have studied the self-reaction of SiH
3
,1 Me
2
SiH,2
and radicals, and the cross-reactions ofMe
3
Si,3 Me
5
Si
2
4
and radicals.5 For the fully methylated silyl rad-Me
3
Si CH
3
icals the most important step is combination, while dispro-
portionation leading to silaethene plays only an inferior role:
for radicals the branching ratio for disproportionationMe
3
Si
is only 0.0632,3 and a similar value was found for rad-Me
5
Si
2
icals. For silyl radicals containing an SiwH bond a further
disproportionation pathway is opened: the formation of sily-
lene. Scavenging experiments with MeOH reveal2 that, in the
self-reaction of radicals, two thirds of the radicalsMe
2
SiH
choose this path, compared with 42% for radicals.1SiH
3
Here, we give an account of the cross-reactions of Me
2
SiH
and radicals. End-product analysis allows us to evalu-Me
3
Si
ate the ratio of rate constants for hydrogen abstraction by H
atoms from the reactants, and We are alsoMe
2
SiH
2
Me
3
SiH.
able to comment on the validity of the geometric mean rule as
it applies to the cross-reaction of these two radicals, and
to give a value for the branching ratio of cross-
disproportionation. Disproportionation reactions leading to
silaethenes, and rapid secondary reactions occurring even at
small conversions, are identiÐed. To conclude, a computer
simulation of the experimental results is described.
Experimental
Gas-handling, photolyses, actinometry and end-product
analyses were performed as described in the Hg-sensitized
photolyses of andMe
3
SiH3 Me
2
SiH
2
.2
Static photolyses were carried out in a 180 cm3 cylindrical
quartz cell with an optical path length of 10 cm. The cell was
attached to a conventional vacuum line and, at the same time,
reproducibly positioned in the optical light path. The light
source was a low-pressure mercury lamp (Gra ntzel Type 5)
operating in dc mode, thermostatically controlled and purged
¤ Present address: Institut fu r Physikalische Chemie, Christian-
Albrechts-Universita t, 24118 Kiel, Germany.
” On leave from the School of Chemistry, La Trobe University,
Melbourne, Victoria, Australia 3083.
by a continuous Ñow of nitrogen. The 185 nm line was
removed by a Vycor Ðlter, and the intensity of 254 nm light
transmitted through the photolysis cell was monitored by a
254 nm interference ÐlterÈphotomultiplier arrangement.
The absorbed light intensity was determined by Hg-
sensitized photolysis in the presence of butene,N
2
O U(N
2
) \
1.0.6 The Hg concentration in the photolysis cell was mea-
sured by an absorption experiment and kept constant in all
experiments.
Two capacitance manometers (MKS Baratron 122A 1000
mbar, 220BA 10 mbar), together with calibrated volumes
attached to the vacuum line, allowed the preparation of preci-
sely known reactant mixtures. All experiments were carried
out at room temperature.
Product analyses were carried out on a HP 5890 gas chro-
matograph equipped with a Ñame ionization detector and a 50
m ] 0.32 mm ]1.5 lm fused silica capillary column (OV1),
with He as the carrier gas (200 kPa). The split was adjusted to
1 : 23. The following temperature programme was used to
obtain a good separation of the product peaks: 50 ¡C (7 min)È
50 ¡C min~1È105 ¡C (0 min)È15 ¡C min~1È220 ¡C (8 min). Gas
chromatograms with the assigned peaks and retention times
are shown in Fig. 1.
and were of commercial origin.H
2
(6.0), Me
3
SiH Me
2
SiH
2
The gas-chromatographic purity of the latter two compounds
was better than 99.5%. Authentic samples of Me
4
Si
2
H
2
,
and were used in the calibration of the gasMe
5
Si
2
HMe
6
Si
2
chromatograph for analysis of the major products. For the
minor products, response factors were taken proportional to
the number of C atoms in the molecule.5
Results
Our main concern was the precise measurement of the disilane
products as a function of reactant composition. 1,1,2,2-Tetra-
methyldisilane pentamethyldisilane(Me
4
Si
2
H
2
), (Me
5
Si
2
H)
and hexamethyldisilane were by far the most impor-(Me
6
Si
2
)
tant products. From previous studies we know that disilanes
with an SiwH bond are susceptible to H atom2 and silyl
radical4 attack. To obtain reliable results, una†ected by sec-
ondary processes, it was considered necessary to use high
concentrations and high light intensities,Me
3
SiHÈMe
2
SiH
2
and to extrapolate the product ratios to zero photolysis time.
In Fig. 2(a)È(c) the concentrations of the three disilanes are
J. Chem. Soc., Faraday T rans., 1998, 94(16), 2315È2322 2315

Fig. 1 Typical gas chromatograms after photolysis of a 1 : 1 mixture of and (a) without MeOH, (b) with MeOH. 3.4Me
2
SiH
2
Me
3
SiH:
min 4 MeOH, 3.6 (impurity; \0.2%), 3.9 4.4 7.4 11.2min 4 MeSiH
3
min 4 Me
2
SiH
2
, min 4 Me
3
SiH, min 4 Me
3
SiOMe, min 4 Me
4
Si
2
H
2
,
13.4 15.5 17.2 23.0 23.5 min 4 silaethene product,min 4 Me
5
Si
2
H, min 4 Me
6
Si
2
, min 4 Me
2
HSiCH
2
SiMe
3
, min 4 Me
2
HSiSiMe
2
SiMe
2
H,
24.1 24.5 min 4 silaethene product, 24.7 25.1 25.5 min 4 silae-min 4 Me
2
HSiSiMe
2
SiMe
3
, min 4 Me
2
HSiSiMe
2
CH
2
SiMe
2
H, min 4 Me
8
Si
3
,
thene product, 25.8 min 4 silaethene product, 26.6 28.8 min, 29.7 min, 30.6 min 4 tertiary products. See Dis-min 4 Me
3
SiSiMe
2
CH
2
SiMe
3
,
cussion for assignment of silaethene products.
plotted as a function of photolysis time. Deviations from lin-
earity due to secondary reactions are not discernible from
such plots but can be seen if the product ratios,
and are plotted vs.Me
4
Si
2
H
2
/Me
6
Si
2
Me
5
Si
2
H/Me
6
Si
2
,
photolysis time [Fig. 2(d)]. does not possess an SiwHMe
6
Si
2
bond and is, therefore, not exposed to secondary attack. The
scatter of the experimental points is rather large and the
extrapolated intercept is loaded with a correspondingly sub-
stantial error. Using smaller concentra-Me
3
SiHÈMe
2
SiH
2
tions resulted in data of greater accuracy (Fig. 3). The
extrapolated intercept values and their errors (2p) are given in
Table 1 for the four reactant ratios studied.
Fig. 2 (a)È(c) Concentration of main products as a function of photolysis time. cm~3,cm~3,[Me
2
SiH
2
] \ 7.72 ] 1017 [Me
3
SiH] \ 7.76 ] 1017
cm~3. (d) Relative product concentration as a function of photolysis time.[H
2
] \ 2.21 ] 1019
Fig. 3 Relative product concentration as a function of photolysis time for di†erent reactant ratios. (a)(=) [Me
4
Si
2
H
2
], (…) [Me
5
Si
2
H].
cm~3,cm~3,cm~3. (b) cm~3,[Me
2
SiH
2
] \ 1.63 ] 1017 [Me
3
SiH] \ 8.1 ] 1016 [H
2
] \ 2.2 ]1019 [Me
2
SiH
2
] \ 1.21 ] 1017 [Me
3
SiH] \ 1.21
] 1017 cm~3,cm~3. (c) cm~3,cm~3,cm~3. (d)[H
2
] \ 2.2 ] 1019 [Me
2
SiH
2
] \ 7.0 ]1016 [Me
3
SiH] \ 1.76 ] 1017 [H
2
] \ 2.2 ] 1019
cm~3,cm~3,cm~3.[Me
2
SiH
2
] \ 5.3 ] 1016 [Me
3
SiH] \ 1.86 ] 1017 [H
2
] \ 2.2 ]1019
2316 J. Chem. Soc., Faraday T rans., 1998, Vol.94

Table 1 Relative product concentration as a function of reactant ratio
Me
2
SiH
2
/Me
3
SiH
2.02 : 1 1 : 1 0.40 : 1 0.29 : 1
Me
4
H
2
Si
2
/Me
6
Si
2
2.04 ^ 0.14
8.236 ^ 0.076 2.064 ^ 0.032 0.355 ^ 0.008 0.171 ^ 0.008
Me
5
Si
2
H/Me
6
Si
2
2.87 ^ 0.18
5.537 ^ 0.032 2.798 ^ 0.026 1.148 ^ 0.014 0.784 ^ 0.006
Besides these three main products there exists a number of
lesser products. Three of these products have already been
characterized in previous studies: Me
2
HSiCH
2
SiMe
3
,
andMe
2
HSiSi(MeH)CH
2
SiMe
2
HMe
3
SiSiMe
2
CH
2
SiMe
3
.
The remaining four primary product peaks in the gas chro-
matogram form two doublets, with retention times which are
typical of trisilanes. These trisilanes are characterized by the
ease with which they are scavenged by MeOH [Fig. 1(b)]. It is
likely that they all possess the SiCSi structural unit, and are
formed by radical addition to a species containing the SixC
bond.
This analysis is supported by the following observations.
The retention time di†erence between Me
3
SiSiMe
2
CH
2
SiMe
3
and is the same as the retention time di†erence for theMe
8
Si
3
Ðrst doublet and and the same as forMe
2
HSiSiMe
2
SiHMe
2
,
the second doublet and We, therefore,Me
2
HSiSiMe
2
SiMe
3
.
suggest that two out of the three substances
andMe
2
HSiSi(HMe)CH
2
SiMe
3
,Me
3
SiSi(HMe)CH
2
SiHMe
2
correspond to the Ðrst doublet,Me
2
HSiSiMe
2
CH
2
SiHMe
2
and two out of the three substances
andMe
2
HSiSiMe
2
CH
2
SiMe
3
,Me
3
SiSi(HMe)CH
2
SiMe
3
to the second doublet. The intensityMe
3
SiSiMe
2
CH
2
SiHMe
2
of these peaks was so small, (only a few per cent of the disilane
products) that the integrator did not evaluate them most of
the time. Quantitative results were therefore difficult to extract
but, under favourable experimental conditions, it can be
shown that all these products are of primary origin (Fig. 4).
Secondary products are rapidly formed in our system. After
a conversion of only 0.3% a triplet of peaks is observed in the
gas chromatogram. The size of the peaks grows quadratically
with photolysis time (Fig. 5). These three peaks are identiÐed
as andMe
2
HSiSiMe
2
SiHMe
2
,Me
2
HSiSiMe
2
SiMe
3
Me
8
Si
3
.
At a conversion of ca. 1.5% the generation of tertiary products
is already discernible. Again a triplet of peaks appears, and
the retention times are consistent with the products being
tetrasilanes.
Discussion
Basic mechanism of the reaction H + Me
2
SiH
2
–Me
3
SiH
Mercury-sensitized photolysis of in the presence of aH
2
mixture results almost exclusively in theMe
2
SiH
2
ÈMe
3
SiH
Fig. 4 Concentration of minor products as a function of photolysis
time
formation of and radicals:Me
2
SiH Me
3
Si
H ] Me
2
SiH
2
] Me
2
SiH ] H
2
(1)
H ] Me
3
SiH ] Me
3
Si ] H
2
(2)
Only for is it known3 that attack on the methylMe
3
SiH
group occurs, leading to the formation of Me
2
HSiCH
2
:
H ] Me
3
SiH ] Me
2
HSiCH
2
] H
2
(3)
The branching ratio k(3)/[k(2) ] k(3)] is very small, ca. 0.003,3
and reaction (3) will, therefore, be neglected in further dis-
cussion. The minor product probablyMe
2
HSiCH
2
SiMe
3
results from the occurrence of this reaction. No abstraction
Fig. 5 Relative secondary product concentration as a function of photolysis time for di†erent reactant ratios. ()) [Me
3
SiSiMe
2
SiMe
3
], (K)
reactant concentration as in Fig. 3.[Me
2
HSiSiMe
2
SiMe
3
], (L) [Me
2
HSiSiMe
2
SiHMe
2
],
J. Chem. Soc., Faraday T rans., 1998, Vol.94 2317

from the CwH bond has been found for the system
H ] Me
2
SiH
2
.2
The and radicals produced in our systemMe
2
SiH Me
3
Si
combine and disproportionate according to the following
mechanism:
2Me
2
SiH ] Me
4
Si
2
H
2
(4)
2Me
2
SiH ] Me
2
SiH
2
] Me
2
Si (5)
Me
2
SiH ] Me
3
Si ] Me
5
Si
2
H (6)
Me
2
SiH ] Me
3
Si ] Me
3
SiH ] Me
2
Si (7)
2Me
3
Si ] Me
6
Si
2
(8)
The dimethylsilylene produced in reactions (5) and (7) inserts
into the SiwH bond of andMe
3
SiH Me
2
SiH
2
:7,8
Me
2
Si ] Me
2
SiH
2
] Me
4
Si
2
H
2
(9)
Me
2
Si ] Me
3
SiH ] Me
5
Si
2
H (10)
Analysis of the mechanism
In the mechanism described so far, the disproportionation
reactions leading to silaethene formation, and ultimately to
minor trisilane products, have been omitted. Neglecting these
reactions in the Ðrst instance allows us to carry out an analyti-
cal treatment of the mechanism within the framework of the
familiar steady-state assumption.
To obtain somewhat more manageable expressions, we
deÐne the terms: k(1)[Me
2
SiH
2
]/k(2)[Me
3
SiH] \ n,
k(4) \ (1 [ a) k
D
, k(5) \ ak
D
, k(6) \ (1 [ b)k
DT
, k(7) \ bk
DT
,
and is givenk(8) \ k
T
k(9)[Me
2
SiH
2
]/k(10)[Me
3
SiH] \ q. k
DT
by the expression where f takes into accountk
DT
\ 2fJk
D
k
T
possible deviation from the geometric-mean rule.
When the steady-state assumption is made for the interme-
diates and we arrive at the followingMe
2
HSi, Me
3
Si SiMe
2
,
expressions:
[Me
2
SiH]
ss
[Me
3
Si]
ss
\
f
2
m
S
k
T
k
D
(I)
[Me
2
Si]
ss
\
fJk
D
k
T
G
ma
2
] 2b
H
k
10
[Me
3
SiH](q ] 1)
[Me
2
SiH]
ss
[Me
3
Si]
ss
(II)
where It is then only am \ (n [ 1) ] J(n [ 1)2]4n/f 2.
small step to derive expressions for the relative product con-
centrations:
[Me
4
Si
2
H
2
]
[Me
6
Si
2
]
\ (1 [ a)m2
f 2
4
]
q
q ] 1
G
am2
f 2
4
] bmf 2
H
(III)
[Me
5
Si
2
H]
[Me
6
Si
2
]
\ (1 [ b)mf 2]
1
q ] 1
G
am2
f 2
4
] bmf 2
H
(IV)
In the well known case when only combination occurs, a \ 0,
b \ 0 and f \ 1, the expressions (III) and (IV) reduce to
[Me
4
Si
2
H
2
]
[Me
6
Si
2
]
\ n2 (V)
[Me
5
Si
2
H]
[Me
6
Si
2
]
\ 2n (VI)
The other extreme case arises when the radicals undergo
only disproportionation reactions, corresponding to a \ 1,
b \ 1, and f \ 1. This leads to
[Me
4
Si
2
H
2
]
[Me
6
Si
2
]
\
q
q ] 1
Mn2]2nN (VII)
[Me
5
Si
2
H]
[Me
6
Si
2
]
\
1
q ] 1
Mn2]2nN (VIII)
Of the Ðve unknowns n, q, f, a and b, three have been the
subject of independent studies. For n to be known, the ratio
k(1)/k(2) must be known. Both rate constants have been mea-
sured by means of di†erent methods by several groups. The
results are summarised in Table 2 as given in ref. 9. The
results for k(1) and k(2) show appreciable scatter, but the ratio
of the rate constants measured by the same group show much
better agreement. For a 99% conÐdence interval we get
1.01 O k(1)/k(2) O 1.25.
The factor q can be evaluated if we know the rate constants
k(9) and k(10). Both rate constants have been measured by
Walsh and co-workers7,8 and, from their values, k(9)/
k(10) \ (1.22 ^ 0.18).
Simple collision and activated complex theories suggest that
the cross-combination rate constant should not be exactly
twice the geometric mean of the self-combination rate con-
stants.14 Furthermore, Arthur and Christie15 have shown that
the factor f introduced in our analysis should always be larger
than one. Experimental support is scarce, however, and for
radicals with similar masses and similar reaction cross-
sections, deviations from f \ 1 are small and well buried in the
uncertainties of the experimental values.
The branching ratio a \ k(5)/[k(4) ] k(5)] has been deter-
mined by a scavenging experiment with MeOH,2 and a value
of a \ (0.64 ^ 0.10) was found.
If only combination reactions take place, corresponding to
a \ 0, b \ 0 and f \ 1, n may be calculated from eqn. (V), (VI)
or (IX):
[Me
4
Si
2
H
2
]
[Me
6
Si
2
]
]
[Me
5
Si
2
H]
[Me
6
Si
2
]
\ n2]2n (IX)
The agreement between experimental and calculated relative
product concentrations is astonishingly good. The values of n
obtained from the di†erent expressions do not di†er very
much, and the values for k(1)/k(2) given in Table 3 (entries
1È4) have been calculated from expression (IX) for di†erent
values of the reactant ratio, k(1)/k(2)Me
2
SiH
2
/Me
3
SiH.
changes very little with the experimental conditions, in any
case not in a systematic manner, and the mean value is given
by k(1)/k(2) \ (1.41 ^ 0.03).
If one also allows f to vary, we have
[Me
4
Si
2
H
2
]
[Me
6
Si
2
]
]
[Me
5
Si
2
H]
[Me
6
Si
2
]
\ f 2m
A
1 ]
m
4
B
(X)
[Me
4
Si
2
H
2
]
[Me
5
Si
2
H]
\
m
4
(XI)
for the two unknowns n and f, and the experimental results
will be reproduced exactly, independent of the true nature of
the mechanism. One expects, however, that a change in the
reactant ratio would lead to a change in the calculated k(1)/
k(2) and f values if the true and assumed mechanism were not
in agreement with each other. Entries 5È8, Table 3, show that
only a small increase in the k(1)/k(2) values is observed,
“compensatedÏ for by an f value smaller than one.
Table 2 Literature values of the rate constants for hydrogen
abstraction from and by H atomsMe
2
SiH
2
Me
3
SiH
k(1)/10~13 cm3 s~1 k(2)/10~13 cm3 s~1 k(1)/k(2) ref.
11 ^ 310^ 3 1.10 ^ 0.45 10
4.1 ^ 0.9 3.7 ^ 1.0 1.11 ^ 0.39 11
3.4 ^ 0.4 3.0 ^ 0.3 1.13 ^ 0.17 12
2.2 ^ 0.2a 1.4 ^ 0.2a 1.57 ^ 0.27a 13
3.1 ^ 0.1 2.6 ^ 0.1 1.19 ^ 0.06 9
a Rate constants for hydrogen abstraction by D atoms.
2318 J. Chem. Soc., Faraday T rans., 1998, Vol.94

Table 3 Relative rate constants for di†erent mechanistic models as a funtion of reactant ratio
no. Me
2
SiH
2
/Me
3
SiH k(1)/k(2) abfk(9)/k(10) Me
4
Si
2
H
2
/Me
6
Si
2
Me
5
Si
2
H/Me
6
Si
2
[1.01, \1.25a 0.64a 1.22 ^ 0.18a
1 2.02 1.41 0 0 1 È 8.086 5.687
2 1.00 1.42 0 0 1 È 2.020 2.842
3 0.40 1.46 0 0 1 È 0.339 1.164
4 0.29 1.37 0 0 1 È 0.159 0.796
5 2.02 1.45 0 0 0.96 È
6 1.00 1.42 0 0 1.00 È
7 0.40 1.48 0 0 0.96 È
8 0.29 1.39 0 0 0.95 È
9 2.02 1.41 1 1 1 0.74
10 1.00 1.42 1 1 1 0.74
11 0.40 1.46 1 1 1 0.77
12 0.29 1.37 1 1 1 0.75
13 2.02 1.41 0.64 0.41 1 1.22
14 1.00 1.42 0.64 0.40 1 1.22
15 0.40 1.46 0.64 0.42 1 1.22
16 0.29 1.37 0.64 0.42 1 1.22
17 2.02 1.13 0.64 0.55 1.35 1.22
18 1.00 1.13 0.64 0.57 1.50 1.22
19 0.40 1.13 0.64 0.68 3.08 1.22
20 0.29 1.13 0.64 0.67 4.71 1.22
21 2.02 1.25 0.64 0.49 1.17 1.22
22 1.00 1.25 0.64 0.50 1.25 1.22
23 0.40 1.25 0.64 0.59 1.64 1.22
24 0.29 1.25 0.64 0.55 1.49 1.22
25 2.02 1.35 0.64 0.44 1.06 1.22
26 1.00 1.35 0.64 0.44 1.09 1.22
27 0.40 1.35 0.64 0.51 1.24 1.22
28 0.29 1.35 0.64 0.45 1.06 1.22
a Experimental results.
The results may also be explained by pure dispro-
portionation reactions, for which a \ 1, b \ 1 and f \ 1. The
sum of the two relative product concentrations is again given
by eqn. (IX), while the product ratio is given by:
[Me
4
Si
2
H
2
]
[Me
5
Si
2
H]
\ q (XII)
The results are documented in Table 3, entries 9È12. k(9)/k(10)
is much smaller than the experimental value but changes very
little with a change in the reactant ratio.
This analysis shows that, for a mechanism involving only
combination (a \ 0, b \ 0, f \ 1), and for one involving only
disproportionation (a \ 1, b \ 1, f \ 1), we deduce values for
k(1)/k(2), f and k(9)/k(10) which vary very little with a change
in the reactant ratio. Both mechanisms cannot be right and,
therefore, we have to conclude that the independence of the
reactant ratio shown by the evaluated parameters is not a suf-
Ðcient condition for the correctness of the assumed mecha-
nism. In both cases we derive values for k(1)/k(2) and k(9)/k(10)
which deviate greatly from those obtained in more direct
experimental determinations, calling both mechanisms into
doubt.
Taking disproportionation into account, and assuming the
values for k(1)/k(2), k(9)/k(10) and f to be known, one would be
tempted to conclude that a and b can be calculated from eqn.
(III) and (IV). The two relations can be transformed to
qb [
am
4
\
q ] 1
mf 2
G
[Me
4
Si
2
H
2
]
[Me
6
Si
2
]
[
m2f 2
4
H
(IIIa)
qb [
am
4
\
q ] 1
mf 2
G
mf 2[
[Me
5
Si
2
H]
[Me
6
Si
2
]
H
(IVa)
It can be clearly seen that only one relation remains for the
two unknowns a and b and, taking f \ 1, there exists an equa-
tion for n which we have already met in eqn. (IX). With
a \ 0.64 we calculate the b values given in Table 3, entries
13È16. We expect a b value which is smaller than the a value
and this is indeed observed. The disadvantage of this model is
again the large discrepancy between the extracted and the
measured k(1)/k(2) value.
This problem can be avoided if we allow f to be varied,
assuming n to be known. From the two relations (IIIa) and
(IVa) an equation for f can be extracted which is identical to
eqn. (X). b can then be determined by either eqn. (III) or (IV).
In eqn. (X) and (III) we Ðrst introduced the mean value k(1)/
k(2) \ 1.13. As can be seen in Table 3, entries 17È20, the value
of f changes systematically with the reactant ratio, not to
mention that its absolute value is much too high. A k(1)/k(2)
value of 1.13 is, therefore, not able to account for our experi-
mental results. Increasing k(1)/k(2) to the upper conÐdence
limit improves the situation somewhat, but a systematic
change in the values of f is still visible, and the b values show
large scatter (Table 3, 21È24). Only if k(1)/k(2) is increased to
1.35 is a possible trend in the f values masked by the scatter of
the experimental values (Table 3, 25È28).
We must conclude, then, that within the framework of the
proposed mechanism we are not able to reproduce the experi-
mental results if we choose a k(1)/k(2) value within the con-
Ðdence limit, i.e. 1.01 O k(1)/k(2) O 1.25. Our analysis suggests
that k(1)/k(2) \ (1.40 ^ 0.05) with an f value very close to one.
As the Ðnishing touches were being put to this paper, we
learned from Arthur and Miles16 that they have recently rein-
vestigated H atom attack on and obtainingMe
2
SiH
2
Me
3
SiH,
k(1) \ (3.95 ^ 0.29) ] 10~13 cm3 s~1 and k(2) \ (2.78 ^ 0.19)
] 10~13 cm3 s~1. These give k(1)/k(2) \ (1.42 ^ 0.14), thus
conÐrming the value deduced in this work, and reinforcing our
analysis of the mechanistic pathway open to andMe
2
SiH
radicals.Me
3
Si
Although a mechanism involving only combination reac-
tions explains our results satisfactorily, we know from a pre-
vious study2 that the disproportionation reaction (5) does
occur, i.e. If this is so, our analysis demands that reac-a D0.
tion (7) must also occur, and we obtain b/a \ (0.6 ^ 0.1),
where the standard deviation quoted is a rough estimate,
taking into account the errors in n, q, and the product concen-
trations.
J. Chem. Soc., Faraday T rans., 1998, Vol.94 2319

One question remains to be answered: how is it that a
model which takes only combination reactions into account,
despite the known occurrence of disproportionation reactions,
explains the data so well? The answer can be seen in relations
(IIIb) and (IVb), which follow straightforwardly from (IIIa)
and (IVa). Taking f \ 1
[Me
4
Si
2
H
2
]
[Me
6
Si
2
]
\ n2]
n
q ] 1
M2qb [ anN (IIIb)
[Me
5
Si
2
H]
[Me
6
Si
2
]
\ 2n [
n
q ] 1
M2qb [ anN (IVb)
If the value of term (2qb [ an) on the right-hand side of eqn.
(IIIb) and (IVb) is close to zero, then the same result is
obtained as in eqn. (V) and (VI), where only combination reac-
tions are considered. The second term is zero if b/a \ n/2q
and, from the deÐnitions given at the beginning of this section,
b
a
\
n
2q
\
k(1)
k(2)
2
k(9)
k(10)
\ 0.58
The value of b/a has to be close to 0.58 so that a mechanism
involving only combination accounts for the experimental
results.
Computer simulation studies: Expansion of the mechanism
Our analysis so far is incomplete as we have not taken into
account the disproportionation reactions of the radicals
involved in the formation of the silaethenes. The following
reactions have to be considered:
2Me
2
SiH ] Me
2
SiH
2
] MeHSixCH
2
(11)
Me
2
SiH ] Me
3
Si ] Me
2
SiH
2
] Me
2
SixCH
2
(12)
Me
2
SiH ] Me
3
Si ] Me
3
SiH ] MeHSixCH
2
(13)
2Me
3
Si ] Me
3
SiH ] Me
2
SixCH
2
(14)
The branching ratio relative to combination for reaction (14)
is well known;2,3 for reaction (11) we know that it is very
small, k(11)/[k(4) ] k(5) ] k(11)] P 0.007,2 and nothing is
known about the branching ratios of reactions (12) and (13).
Silaethenes combine and add radicals rapidly, with rate con-
stants similar to those for radical combination reactions.5 In
our case, the steady-state concentrations of radicals are much
higher than those of silaethenes and, therefore, only radical
addition will be considered.
R ] SixC ] RSiC (15)
RSiC ] R ] RSiCR (16)
The products originating from a silaethene precursor are char-
acterized by an SiSiCSi skeleton. With R 4 Me
3
Si, Me
2
SiH
and we expect eightSixC 4 Me
2
SixCH
2
, MeHSixCH
2
products: two previously characterized compounds,
andMe
2
HSiSi(MeH)CH
2
SiMe
2
HMe
3
SiSiMe
2
CH
2
SiMe
3
,
and two triplets of isomers with, respectively, six and seven
methyl groups attached to the SiSiCSi skeleton.
The expected products are listed in Table 4 . Experimen-
tally, only two doublets were observed [Fig. 1(a)]. The assign-
ment of these four product peaks was made solely on the basis
of computer simulation of the product yields and must, there-
fore, be regarded with caution. The following rate constant
ratios were used as input data: k(11)/[k(4) ] k(5) ] k(11)] \
7 ] 10~3, k(12)/[k(6) ] k(7) ] k(12) ] k(13)] \ 5 ] 10~3,
k(13)/[k(6) ] k(7) ] k(12) ] k(13)] \ 1.6 ]10~3, k(14)/
[k(8) ] k(14)] \ 6 ]10~2.2,3 The rate constant for reaction
(15) was taken as equal to and for reactionk(15) \ k
D
\ k
T
,
(16), k(16) \ k
DT
.
The experimental and calculated values (Ðrst entry in
Table 4) of the two characterized substances,
andMe
2
HSiSi(MeH)CH
2
SiMe
2
HMe
3
SiSiMe
2
CH
2
SiMe
3
,
agree well with each other; in particular the di†erent behav-
iour of the yields of the two substances with respect to the
reactant ratio is correctly described.
The two peaks of the Ðrst doublet are characterized by an
intensity ratio of ca. 3 : 1, which is independent of the reactant
ratio. The intensity, relative to decreases by more thanMe
6
Si
2
a factor of 10 as the ratio is varied fromMe
2
SiH
2
/Me
3
SiH
2.02 to 0.29. The computer simulations yield a triplet with two
peaks of equal intensity. Our mechanism implies that the
yield of equals that of EitherR
1
SiCR
2
R
2
SiCR
1
.
orMe
2
HSiSi(MeH)CH
2
SiMe
3
Me
3
SiSi(MeH)CH
2
SiMe
2
H
can be correlated with the low-intensity peak, while the calcu-
lated intensity of agrees with theMe
2
HSiSiMe
2
CH
2
SiMe
2
H
intensity of the larger peak of the doublet. The dependence on
the reactant ratio is again fairly well described by our simula-
tions.
Table 4 Experimental and calculated product concentrations relative to for di†erent reactant ratios[Me
6
Si
2
]
Me
2
SiH
2
/Me
3
SiH
2.02 1.0 0.4 0.29
exp calca exp calca exp calca exp calca
Me
2
HSiSi(HMe)CH
2
SiMe
2
H 0.039 0.036 0.008 0.006 0.0004 0.0006 È 0.0002
0.034 0.006 0.0005 0.0002
Me
2
HSiSi(HMe)CH
2
SiMe
3
0.017 0.017 0.007 0.005 0.0014 0.001 0.001 0.0005
0.018 0.006 0.0012 0.0007
Me
3
SiSi(HMe)CH
2
SiMe
2
H 0.017 0.005 0.001 0.0005
0.004 0.002 0.0004 0.0002
Me
2
HSiSiMe
2
CH
2
SiMe
2
H 0.059 0.051 0.024 0.027 0.007 0.009 0.004 0.006
0.034 0.018 0.006 0.004
Me
2
HSiSiMe
2
CH
2
SiSiMe
3
0.029 0.018 0.024 0.019 0.017 0.016 0.013 0.014
0.017 0.019 0.016 0.014
Me
3
SiSi(HMe)CH
2
SiMe
3
0.004 0.003 0.002 0.001
0.006 0.004 0.002 0.002
Me
3
SiSiMe
2
CH
2
SiMe
2
H 0.023 0.018 0.018 0.019 0.012 0.016 0.008 0.014
0.012 0.012 0.011 0.009
Me
3
SiSiMe
2
CH
2
SiMe
3
0.01 0.006 0.016 0.013 0.027 0.028 0.03 0.033
0.006 0.013 0.028 0.033
a See text for the method of calculation.
2320 J. Chem. Soc., Faraday T rans., 1998, Vol.94

Fig. 6 Calculated relative product concentrations as a function of
photolysis time (ÈÈÈÈ). cm~3,[Me
2
SiH
2
] \ 1.21 ] 1017
cm~3,cm~3. Experimen-[Me
3
SiH] \ 1.21 ] 1017 [H
2
] \ 2.2 ] 1019
tal results are depicted as symbols.
The second doublet consists of two peaks of almost equal
intensity which change little with variation in the reactant
ratio. Computationally, we obtain the same relative yield for
and while, forMe
2
HSiSiMe
2
CH
2
SiMe
3
Me
3
SiSiCH
2
SiMe
2
H
an appreciably smaller yield is cal-Me
3
SiSi(MeH)CH
2
SiMe
3
,
culated. The small change in the product yields with the
change of the reactant ratio is also approximately reproduced.
In this case one could argue that the small intensity product
is hidden under the barely separat-Me
3
SiSi(MeH)CH
2
SiMe
3
ed two high-intensity peaks and, therefore, only a doublet is
observed.
Further simulations of the products associated with the Ðrst
doublet show that when the mechanism is expanded, and
small adjustments are made to the values of the rate constants
of the reactions involved, two main products and a minor
product are predicted. Just as for the second doublet, we
could then argue that the small peak associated with the
minor product is hidden by its larger companions.
The mechanism was expanded in the light of our previous
result, that radicals add preferentially to the silicon side of the
SixC double bond5,17 and, also, that the substituted methyl
radical may undergo disproportionation as well as com-
bination (16). For the reaction with a trimethylsilylradical we
therefore have to include
R
3
SiCH
2
] Me
3
Si ] R
3
SiCH
3
] Me
2
SixCH
2
(17)
while, for a dimethylsilylradical, two reactions are possible:
R
3
SiCH
2
] Me
2
SiH ] R
3
SiCH
3
] Me
2
Si (18)
R
3
SiCH
2
] Me
2
SiH ] R
3
SiCH
3
] MeHSixCH
2
(19)
The branching ratio k(18)/[k(16) ] k(18) ] k(19)] is, presum-
ably, much greater than k(17)/[k(16) ] k(17)]. In extending the
mechanism we took only reaction (18) into account, with
k(18)/[k(16) ] k(18)] \ 0.3, and the value for k(11)/
[k(4) ] k(5) ] k(11)] was increased to 1 ]10~2. The calcu-
lated values for the relative product yields are given as the
second entry in Table 4. No attempt was made to achieve the
best possible agreement; the calculations are only intended to
show that the mechanism leading to these minor products is
by-and-large understood. Beyond that, the calculations
conÐrm that the branching ratio for reaction (11) is much
smaller than for reaction (14).
Secondary reactions
The observation of rapid secondary reactions in our system is
not unexpected. In the Hg-sensitized photolysis of Me
2
SiH
2
2
similar behaviour was observed. There, the early appearance
of secondary products was attributed to the availability of an
easily abstractable H atom in the disilane product. H atoms,
as well as silyl radicals, are potential reaction partners in this
process, forming disilanyl radicals:
R ] Me
4
Si
2
H
2
] RH ] Me
4
Si
2
H (20)
R ] Me
5
Si
2
H ] RH ] Me
5
Si
2
(21)
where R 4 H, The disilanyl radicals will reactMe
2
SiH, Me
3
Si.
predominantly with and the radicals presentMe
3
Si Me
2
SiH,
in the highest stationary concentration:
Me
2
SiH ] Me
4
Si
2
H ] Me
6
Si
3
H
2
(22)
Me
3
Si ] Me
4
Si
2
H ] Me
7
Si
3
H (23)
Me
2
SiH ] Me
5
Si
2
] Me
7
Si
3
H (24)
Me
3
Si ] Me
5
Si
2
] Me
8
Si
3
(25)
The reaction sequence (20)È(25) explains the formation of the
secondary trisilane products (Fig. 5). We have also endeav-
oured to simulate the formation of secondary products by
computer calculations. Rate constants for reaction (21) have
been reported for R 4 H18 as well as for R 4 Me
2
SiH,
The rate constants for reaction (20) were taken to beMe
3
Si.19
a factor of 1.5 higher. With this set of rate constants, second-
ary product formation was found to be too large, and the rate
constants k(20) and k(21) for had to beR 4 Me
3
Si, Me
2
SiH
reduced by a factor of three to achieve agreement with the
experimental results. A similar situation was found when we
attempted to simulate the product distribution in our earlier
work on the direct photolysis of It appears thatMe
5
Si
2
H.20
the relative rate constant k(21)/[k(8)]1@2 \ 8 ]10~11 cm3@2
s~1@2 reported in a previous publication19 is too large, and a
value of k(21)/[k(8)]1@2 \ (3 ^ 1) ] 10~11 cm3@2 s~1@2 is more
appropriate. The calculated relative di- and trisilane concen-
trations as a function of photolysis time are plotted in Fig.
6(a) and (b), respectively. Good agreement with the experimen-
tal results is obtained, and this is also true for other reactant
ratios. Further reactions of the trisilanes have not been taken
into account and, therefore, the levelling o† of the relative
product concentrations is not reproduced.
thank Prof. K. Hassler, Technische Universita t Graz, forWe
supplying us with a sample of 1,1,2,2-tetramethyldisilane.
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Paper 8/02360C; Received 26th March, 1998
2322 J. Chem. Soc., Faraday T rans., 1998, Vol.94