Ultraviolet photolysis of 1,2-Dimethyldisilane in the gas phase

Article abstract of DOI:10.1016/j.jphotochem.2018.04.041

The formation of MeSiH is the primary process in the photolysis of 1,2-dimethyldisilane at 193 nm that are analogues of carbenes. Gas chromatographic technique was used with a flame ionization detector as an analysis tool to identify the products mixture. The photolysis light at 193 nm was provided by an Oxford KX2 pulsed laser operated with rare-gas halide (known as an excimer laser) as the gain medium to provide ultraviolet (UV) radiation. This work has confirmed that radical processes are not important in the photolysis of 1,2-dimethyldisilane. A method for the determination of rate constants for MeSiH reactions relative to the rate constants of 1,2-dimethyldisilane has been formulated. This has been used to determine some relative rate constants of MeSiH insertions with methylsilanes. The insertion reactions of MeSiH with SiH₄ and Methysilanes have shown to be fast and closer in reactivity to SiH₂ than to SiMe₂, whereas PhSiH looks to be slightly more reactive than MeSiH.

Full text of DOI:10.1016/j.jphotochem.2018.04.041

JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)145–151
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Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Ultraviolet photolysis of 1,2-Dimethyldisilane in the gas phase
Najem Al-Rubaiey
Petroleum Technology Department, University of Technology, Baghdad, Iraq
A R T I C L E I N F O
A B S T R A C T
Keywords:
The formation of MeSiH is the primary process in the photolysis of 1,2-dimethyldisilane at 193 nm that are
analogues of carbenes. Gas chromatographic technique was used with a flame ionization detector as an analysis
tool to identify the products mixture. The photolysis light at 193 nm was provided by an Oxford KX2 pulsed laser
operated with rare-gas halide (known as an excimer laser) as the gain medium to provide ultraviolet (UV)
radiation. This work has confirmed that radical processes are not important in the photolysis of 1,2-di-
methyldisilane. A method for the determination of rate constants for MeSiH reactions relative to the rate con-
stants of 1,2-dimethyldisilane has been formulated. This has been used to determine some relative rate constants
of MeSiH insertions with methylsilanes. The insertion reactions of MeSiH with SiH4 and Methysilanes have
shown to be fast and closer in reactivity to SiH2 than to SiMe2, whereas PhSiH looks to be slightly more reactive
than MeSiH.
Dimethyldisilane
Methylsilylene
Photolysis
Rate constants
UV
1. Introduction
2. Material and methods
The calculation of chemical reaction rate constants is of importance
to chemistry and biology. Related studies carried out by our group
concentrated on using time resolved spectroscopic studies [1–6]. In the
same time and for gaseous reactions, gas chromatography is one of the
most common and versatile methods of product analysis. This is be-
cause it can provide a separation and quantitative measurements of
complex gaseous mixtures.
Silylene can be considered as very reactive intermediates. They have
a very important role in the chemical vapor deposition of many thin
films that contain silicon compounds. These species have technological
importance in microelectronics industry besides the dry etching process
of silicon wafers [7,8].
Providing the reaction system is carefully selected, and the condi-
tions are well-defined, product yields can provide a mean of measuring
relative rate constants of many chemical intermediate species in the
presence of selected substrates.
This technique was employed in this research in order to:
(i) determine the primary processes and mechanism of the photo-
lysis of 1,2-dimethyldisilane (DMDS) using 193 nm excimer laser,
(ii) study the rate of trapping of MeSiH by SiH4, MeSiH3, and
Me2SiH2.
2.1. Apparatus
A Pyrex glass, static, high vacuum system has been used. The line
was pumped by a mercury diffusion pump backed by a single stage
rotary oil pump. A pre-calibrated MKS Baratron (type 170 M) was used
to measure the pressure. An Edward speedivac gauge model B5 in
conjunction with a Pirani gauge head model G6A was employed to
monitor the pressure in the line continuously.
Two gas chromatographs were used for the analysis, both of which
were equipped with gas sampling valve. Most of the analysis of the runs
in this work was carried out using a Perkin-Elmer model 8310 gas
chromatograph with flame ionization detector (FID) connected to
Hewlett-Packard 3390 integrator. In addition a Perkin-Elmer model F33
gas-liquid chromatography with an FID was also used for some ana-
lyses. Several types of columns were employed for analytical purposes,
the packing materials and operating conditions of which were depen-
dent on the reaction under investigation. The main columns were of
15% PPG on 80–100 mesh chromosorb W (4.5m), 10% OV101 on
chromosorb W-HP 80/100 (4m), Porapak T (4m) and Porapak Q (3m).
Five main cylindrical reaction cells were used for the kinetic runs.
Two of these cells were constructed of spectrosil quartz tubing of length
E-mail address: 100108@uotechnology.edu.iq.
https://doi.org/10.1016/j.jphotochem.2018.04.041
Received 8 February 2018; Received in revised form 20 April 2018; Accepted 20 April 2018
Availableonline22April2018
1010-6030/©2018ElsevierB.V.Allrightsreserved.

N. Al-Rubaiey
JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)145–151
6.5 cm and internal diameter 2.8 cm to be used at high temperature.
The two other cells were built from Pyrex glass with silica windows
fixed by using black wax. These cells were suitable for possible cleaning
of the windows. These windows easily became covered with thin solid
films after a number of experiments. One cell was constructed of
spectrosil quartz tubing and wrapped (except for the window) in
“electro-thermal” heating tape and maintained at 343 K during the time
of experiments.
The furnace consisted of a horizontal silica tube (4 cm in diameter),
wound with nichrome resistance wire as a heater and contained in a
box packed with asbestos fiber. The furnace electrical supply was
controlled by using a Variac. Temperatures were measured using a
chrome-alumel thermocouple, located near the center of the furnace.
The photolysis light was provided by an Oxford KX2 pulsed laser
operated with rare-gas halide (known as an excimer laser) as the gain
medium to provide ultraviolet (UV) radiation. It was usually used with
Ar/F2 mixtures to provide a wavelength of 193 nm. Discharge effi-
ciencies were around 1–5% and emitted pulses have 0.1 to 1 J of energy
in the time range of 10–100 ns. Average powers are normally in the 10
to 100 MW range.
authentic sample. As will be seen, this photolysis provides a relatively
clean source of methylsilylene (MeSiH). MeSiH reacts with the pre-
cursor, DMDS to produce TMTS:
MeH2SiSiH2Me (DMDS) + hѵ → MeSiH + MeSiH3
MeSiH + DMDS → MeSiH2SiMeHSiMeH2 (TMTS)
To start with, the measurement of TMTS was
a problem.
Chromatographic peaks were irreproducible and had rather small yields
which made them difficult to analyze. However there were several
unrecorded, small peaks in the region where these molecules could be
expected. After heating the sample loop of the gas chromatograph the
peaks become reproducible. Because of the absence of TMTS authentic
sample, TMTS was identified by comparing its retention time with those
of other available trimethyltrisilane samples (ie. H3SiSiH2MeSiH2Me
and HSiMe2SiMeHSiH2Me) [9,10]. Its identity was supported by other
experiments [11]. The area for this peak was found to increase with the
number of shots and decrease in the presence of another substrate
under fixed photolysis conditions. TMTS was also observed as one of the
main products in the pyrolysis of DMDS by Ring et al. [10]. In the Ring
et al. work, the reaction was studied over the temperature range
295–573 K.
2.2. Source and purity of materials
Small traces of other compounds such as SiH4 and Me2SiH2 (in
addition to MeSiH3 and TMTS) have also been observed. The most
obvious suggestion for the formation of those compounds was due to
the presence of about 4% of the 1,1-dimethyldisilane impurity in
DMDS. This impurity was identified by comparing its retention time
with that of an authentic sample but under different GC conditions
[11]. At 193 nm the photodecomposition of this impurity may lead to
the formation of SiH4 and Me2SiH2 as follows:
1,2-Dimethyldisilane, DMDS, was kindly obtained from Professor E.
Hengge (University of Graz/Austria), and found to be about 85% pure.
The purity was improved to better than 95%, by low temperature dis-
tillation using a dry-ice slush technique (DMDS has a vapor pressure of
0.3 mm at 195 K). Unfortunately amongst the remaining traces of other
compounds (MeSiH3, Me2SiH2, Me3SiH, SiH4, Me2SiHSiH3 and
MeSiH2SiHMeSiH2Me) were two of the main products viz, methylsilane
and 1,2,3-trimethyltrisilane. This means that correction had to be ap-
plied to the yields of these compounds in the photolysis runs.
Other compounds, such as SiH4 (> 99.9 purity, BOC Ltd. Electra-II
grade), MeSiH3 (99.6% purity, prepared in our lab.), Me2SiH2 (99.5%
purity, prepared in our lab.), and Me3SiH (99.8% purity, Fluorochem
Ltd.) were degassed by freezing down (77 K) in liquid nitrogen and
were used in this work. Electra-II grade Nitrogen (oxygen free, 99.9%
purity) was supplied by BOC Ltd.
Me2SiHSiH3 → Me2SiH2 + SiH2
Me2SiHSiH3 → SiH4 + SiMe2
Me2SiH2 and SiH4 were identified by comparison of their retention
times with those of authentic samples. Me2SiH2 was also detected by
gas chromatography in the work of Davidson et al. [9] on the pyrolysis
of DMDS. SiH2 and SiMe2 probably react with DMDS to give other
products (H3SiSiH2MeSiH2Me and HSiMe2SiMeHSiH2Me) but these will
have rather small yields which makes them difficult to analyze. How-
ever there were several unrecorded, small peaks in the region where
these molecules could be expected.
2.3. Experimental procedure
Samples for photolysis were prepared by putting the required
pressure of precursor into the reaction cell then adding the other re-
acting substrate (if required) and finally making the reaction mixture
up with nitrogen to the required pressure. Every precaution was made
to reduce the adsorption of the silicon compounds on the walls of the
gas line, the reaction cell and the gas sampling loop of the GC by
heating. After the photolysis, the mixture of gases in the cell was ana-
lyzed by gas chromatography. Kinetic experiments were carried out
with mixtures containing 50–100 mTorr precursor with partial pres-
sures of the reacting substrate in the range of 50–2000 mTorr and total
pressure ca, 200 Torr (made with nitrogen). An average of 2–10 laser
shots was used which was indicated by the amount of the product peaks
by GC. The number of shots was chosen as small as possible to keep the
conversion low. The procedure for a particular substrate gas was to vary
the ratio of precursor to the gas added and monitor the change in the
ratio of products produced by gas chromatography.
Preliminary experiments indicated the formation of an opaque solid
film which covered the walls of the cell, particularly when high laser
pulse energies were employed. The production of such films reduced
the transmission of laser radiation and also could lead to product
coming from direct photolysis of the adsorbed material. Usually those
compounds if produced were in small yields compared to those pro-
duced by the homogenous photolysis of DMDS in the gas phase. Many
efforts were made to reduce the effect of this film by using a low
pressure of DMDS and low conversion (a few numbers of shots). Regular
cleaning of the cell after five runs and leaving it overnight in an oven to
830 K prevented film build up.
DMDS photochemistry was explored by investigating the experi-
mental effects of the number of laser shots, DMDS pressure, total
pressure, added oxygen and temperature. In order to assess the effects,
The GC peaks ratio [MeSiH3]/[DMDS] was measured. This ratio gives
an indication of the photochemical conversion. TMTS was not identified
(and therefore not monitored) at the beginning of the work. The results
of these preliminary investigations are shown in Figs. 1–5. A number of
points can be extracted from this study:
3. Discussion
3.1. Results and discussion
1) The results in Fig. 1 show the linear dependence of the ratio with
the number of shots. This suggests a well behaved reaction with product
yields proportional to absorbed photons.
2) Fig. 2 demonstrates that the more DMDS in the cell, the less is the
[MeSiH3]/[DMDS] ratio. A possible explanation is that most of the
3.1.1. Experiments with DMDS alone
The first set of experiments indicated that the major products were
methylsilane (MS) and 1,2,3-trimethyltrisilane (TMTS). The MS peak
was identified by comparison of its retention time with that of an
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Fig. 1. Dependence of [MeSiH3]/[DMDS] on number of shots.
Fig. 4. Dependence of [MeSiH3]/[DMDS] on oxygen.
Fig. 2. Dependence of [MeSiH3]/[DMDS] on DMDS pressure.
Fig. 5. Dependence of [MeSiH3]/[DMDS] on temperature.
193 nm radiation light is absorbed by the DMDS at the front end of the
cell. Therefore, the more DMDS in the cell the more MeSiH3 formed is
diluted in unphotolysed DMDS. Another possible explanation is related
to the film build up which has similar effect by reducing the amount of
radiation going through the cell. To minimize difficulties of high light
absorption or film formation, the pressure of DMDS was kept as low as
possible. In practice the pressure range was 50–100 mTorr.
3) The results in Fig. 3 confirm that the ratio [MeSiH3]/[DMDS] did
not vary beyond experimental error, with change in total pressure. This
suggests that the reaction is pressure independent within the scatter.
4) The results in Fig. 4 show that adding oxygen does not affect the
ratio of [MeSiH3]/[DMDS] within experimental error which suggests
that the primary process is not radical process but rather molecular
extrusion process. No mechanistic information is available at present
concerning the reaction of MeSiH with O2, however Becerra et al. [12]
reported the time-resolved kinetic studies of the reaction of SiH2, with
O2 over the pressure range 1–100 Torr and temperature range
297–600 K. Becerra et al. reported that the reaction of SiH2 + O2 was
almost pressure independent over the temperature range studied. In
order to reduce the effect of such reaction, many efforts have been
made in this work to prevent the contamination of reaction system with
oxygen.
5) The effect of increasing the reaction temperature is shown in
Fig. 5.
6) In addition to MeSiH3, TMTS, Me2SiH2 and SiH4, no other pro-
ducts were detected at 298 K or higher temperatures.
3.1.2. Experiments with added gases
The second set of experiments was carried out with added SiH4,
Me2SiH2, and Me3SiH in order to confirm the production of MeSiH (by
scavenging it via. Si-H insertion processes). All the expected products
were detected. The stoichiometric equations are presented as follows:
SiH4 + MeSiH → H3SiSiMeH2
Me2SiH2 + MeSiH → Me2SiHSiMeH2
Me3SiH + MeSiH → Me3SiSi MeH2
The observation of these products provides strong evidence for the
intermediacy of methylsilylene, MeSiH. These products, H3SiSiMeH2,
Me2SiHSiMeH2 and Me3SSiMeH2 have been identified by comparing
their GC retention times with those from previous studies on the same
molecules [11]. It was also found that addition of these gases did not
deplete the observed amount of MS but it did reduce the amount of
TMTS relative to MeSiH3 which suggested that they were competing
with the DMDS for the methylsilylene.
It was subsequently found that by using a heated sample loop, the
peak area of TMTS increased and now became reproducible and formed
in fixed ratio to the quantity of MS. The ratio of peak areas [MeSiH3]/
Fig. 3. Dependence of [MeSiH3]/[DMDS] on total pressure.
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[TMTS] = 1.5
0.5 is constant for the initial DMDS in the range of
50–100 mTorr, total pressure 160–200 Torr and 1–10 shots/
80 mJ pulse−1. This suggests that the FID is more sensitive to MS than
to TMTS. However it is still possible that some TMTS is lost in the
system.
These experiments give an indication that relative rate studies can
be carried out to obtain rate constants of MeSiH removal with SiH4,
Me2SiH2 and Me3SiH at 298 K. These were carried out by monitoring
both the depletion of TMTS and by the increase of the insertion product
(H3SiSiMeH2, Me2SiHSiMeH2 and Me3SiSiMeH2) as substrate pressure
was varied.
In order to get reliable results it was important to select the lowest
conversion but at the same time suitable experimental conditions in
which the product peaks could be observed and monitored.
When DMDS is photolysed in the presence of added gas then the
latter will compete with the unphotolysed DMDS to trap the methylsi-
lylene formed. A depletion in the amount of TMTS will be observed, and
thus will be an increase in the ratio of MS to TMTS.
For the experiments in the presence of SiH4, MeSiH would react as
follows:
MeSiH + SiH4 → MeSiH2SiH3 (MDS)
By following the procedure explained in Ref. [13], the following
expressions have been obtained:
(1) [MeSiH3]/[TMTS] = 1+ (kSiH4/kDMDS) ([SiH4]/[DMDS])
(2) [MDS]/[TMTS] = (kSiH4/kDMDS) ([SiH4]/[DMDS])
(3) [MeSiH3]/[MDS] = 1+ (kDMDS/kSiH4) ([DMDS]/[SiH4])
For reactions of MeSiH with Me2SiH2 and Me3SiH, similar expres-
sions are obtained. Gaseous mixtures involving the additives SiH4,
Me2SiH2, and Me3SiH were made up and studied over a suitable range
of starting pressures for each added gas. These gases were picked out as
examples to illustrate the insertion reaction of methylsilylene.
Fig. 6. Dependence of product on added substrate: DMDS + SiH4.
MeSiH + Me2SiH2 → MeSiH2SiMe2H (TMDS)
MeSiH + Me3SiH → MeSiH2SiMe3 (TTMS)
In this study (to improve analysis) different columns were used from
the preliminary study. The experiments with silane, dimethylsilane and
trimethylsilane (which do not absorb significantly at 193 nm) were
carried out at room temperature. In these, partial pressures in the range
of 50–100 mTorr of DMDS were photolysed in the presence of varying
partial pressures of the added reactant gases at 0.05–2 Torr (in the
presence of nitrogen to a total pressure of 200 Torr), 1–5 shots
(60–80 mJ per pulse energy of the laser) were used. These conditions
were chosen to keep conversions low (≈10%) and to minimize film
formation.
The results of these experiments are shown in Figs. 6–8. It is worth
mentioning here that only equations (1) and (3) have been used to plot
the data in Figs. 6–8. For example, in Fig. 6, it has been shown that plot
6a represents equation (1) and plot 6b represents equation 3. Thus both
plots (6a and 6b) should give the same relative rate value (kSiH4/kDMDS).
Substrate to DMDS ratios in all cases show that these results confirm to
the relationship determined here. Applying the relative rate procedure
described previously, the relative rate constants can now be calculated
with two values for each reaction.
Two methods of calculating the relative rate constants of SiH4 and
methylsilanes offered a good check on the data. Table 1 shows these
data at 298 K and about 200 Torr of total pressure. The agreement be-
tween the two sets of values for SiH4 is quite good but for Me2SiH2 and
Me3SiH is less good.
Fig. 7. Dependence of product on added substrate: DMDS + Me2SiH2.
precursor, DMDS was obtained by fitting unpublished values from time-
resolved studies by Becerra et al. [14] at different temperatures.
3.2. Comparison with other data
log (k/cm3molecule−1 s−1) = −10.74
2.303 RT
0.1 + 1.93
0.1 kcal mol−1
/
There are no other published, room temperature, relative rate stu-
dies for methylsilylene to compare with. However there are absolute
rate data [6]. An Arrhenius equation for the MeSiH reaction with the
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Table 3
Absolute rate constant at 298 K, k/10−11 cm3 molecule−1 s−1
.
Substrate
SiH2[17]
SiMe2[18]
MeSiH
PhSiH[19]
SiH4
42
37
33
25
0.02
0.19
0.55
0.45
3.97
–
23.18
6.62
5.1
39
21
39
MeSiH3
Me2SiH2
Me3SiH
Becerra et al. and that described here are big and therefore close
agreement cannot be expected. This does suggest, however, that more
work is needed to improve these values at 298 K.
In order to examine these results in more detail the rate constants
obtained here have been used together with those of earlier studies to
make a comparison of reactivity of MeSiH with those of SiH2[15–17],
SiMe2[18], and PhSiH [19]. Table 3 shows these comparison at 298 K in
which there are some interesting points worth noting:
(i) MeSiH is much closer in reactivity to SiH2 than SiMe2. The value
of the ratio, k(SiH2)/k(MeSiH), lies in the range ca. 1–10 whereas the
ratio k(MeSiH)/k(SiMe2), has values in the range ca. −14-200.
(ii) PhSiH looks to be slightly more reactive than MeSiH, although
not by much for Me2SiH2.
3.3. Mechanism of the reaction
In order to explain the type of reaction between MeSiH and the
methylsilanes, we need to come back to the theoretical ideas of the
nature of insertion process [20] originating with Hoffman [21] (see
Scheme 1). This work suggests that the reaction will happen in two
stages, the first “electrophilic” stage in which the electrons from the Si-
H bond feeds into the empty p orbital on the silylene, then followed by
the second “nucleophilic” stage which correspond to donation of the
lone-pair on the silylene to make a new Si–Si bond.
Fig. 8. Dependence of product on added substrate: DMDS + Me3SiH.
Table 1
Relative rate constants of methylsilylene removal by silane, dimethylsilane and
trimethylsilane at 298 K (from two different processes).
Substrate
(kx/kDMDS
)
(kx/kDMDS)
2
1
The results obtained here suggest that MeSiH has a closer reactivity
to SiH2 than to SiMe2. Previous results also suggest an increase in re-
activity with methyl-substitution on the silane substrate. The results
here are not clear cut on this point. The sequence of increasing re-
activity is SiH4, Me3SiH, Me2SiH2. It seems probable that this is caused
by experimental error and at the same trend is likely as with SiH2 and
SiMe2. These trends have been explained as follows.
There are three possibilities in which silylenes interact with silanes:
(i) That both “electrophilic” and “nucleophilic” occur simulta-
neously and at the same time.
SiH4
Me2SiH2
Me3SiH
0.08
0.66
0.19
0.01
0.47
0.04
0.08
0.49
0.09
0.01
0.09
0.04
Table 2
Absolute rate constants comparison for thee reaction of MeSiH with mono-
silanes at 298 K and 200 Torr total pressure.
Substance
k/10−11 cm3 molecule−1 s−1
(ii) A process led by the “electrophilic” interaction as shown in
Scheme 1(a).
Becerra et al. [6]
This work
(iii) a process led by “nucleophilic” interaction with a different in-
termediate complex (see Scheme 1(b)). If the process (ii) was func-
tioning in the Si-H insertion process, then the “electrophilic” stage is
unlikely to be rate determining step. Thus the process should not be
affected by the methyl substituent.
D (1)
D (2)
SiH4
8.1
12.9
19.5
25.5
16.3
3.97
–
23.18
6.62
MeSiH3
Me2SiH2
Me3SiH
25.5
16.9
27.8
Reported ab initio calculations by Becerra et al. [16,22] (quantum
mechanical procedures to calculate the potential energy surface) of
SiH2 insertion into SiH4 provide support for process (ii). The inter-
mediate complex for this reaction has been illustrated in Scheme 2
where the geometry of the structure has been explained. The bond
lengths in this scheme suggest that H transfer from SiH4 to SiH2 is
considerably more advance than Si–Si bond formation. It may be un-
certain whether there is any Si–Si interaction at all. This is clear in the
observation of the high entropy requirement (structure looseness) of the
complex intermediate. This means that the “electrophilic” interaction
may proceed the “nucleophilic” to some degree, although they are
evidently coupled.
Using this equation at 298 K, the absolute rate constant for the re-
action of MeSiH + DMDS can now be calculated, to be
k = 4.73 × 10−10 cm3molecule−1s−1. This enables the relative rate
constants reported (Table 1, average) to be converted to absolute rate
constants (see Table 2, column 3). Table 2 shows a comparison of these
rate constants between this work and that of Becerra et al. [6]. Two sets
of activation of energies were calculated by Becerra et al. First, the
Arrhenius parameters were obtained from the directly measured rate
constants at each temperature and second by fitting the mid-range rate
constants with an assumed A-factor (per Si-H bond) of
10−12.4 cm3 molecule−1 s−1 (average value). The latter values (D1 in
Table 2) were preferred because they provide a more constant variation
than the apparent best fit values. The uncertainties in the work of
For mechanism (a) (Scheme 1), where “electrophilic” interaction
occurs initially, the effect of an X atom or group which is an electron
withdrawing substituent on the silicon center will affect the orbitals.
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Scheme 1. Nature of the silylene insertion process.
substituent effect in the silane becomes important.
4. Conclusion
The following points can be concluded:
(i) The primary process for the DMDS decomposition at 193 nm is
proposed as follows: MeH2SiSiH2Me + hѵ → MeSiH + MeSiH3
In the presence of a large amount of DMDS, MeSiH can react to give
TMTS:
MeSiH + DMDS → MeSiH2SiMeHSiMeH2 (TMTS)
(ii) The photolysis of DMDS provides a relatively clean source of
methylsilylene, MeSiH.
(iii) The data presented here shows that MeSiH is much closer in
reactivity to SiH2 than to SiMe2 whereas PhSiH looks to be slightly
more reactive than MeSiH.
(IV) This work also suggests that the reaction will happen in two
stages, the first “electrophilic” stage in which the electrons from the Si-
H bond feeds into the empty p orbital on the silylene, this follows by the
second “nucleophilic” stage which corresponds to donation of the lone-
pair on the silylene to make new Si–Si bond.
V) Further work is required on varying the laser energy pulse on the
photolysis of 1,2 DMDS. This will determine between one-photon and
multiple-photon excitation and may lead to differentiate between
photolytic paths.
VI) Investigation of the formation, structure, composition and de-
composition of the film formed on the windows during photolysis of
DMDS.
Scheme 2. Silylene insertion: intermediate complex[22].
They draw off negative charges. However, they resist formation of
positive charges at the silicon center. This might appear that they are in
disagreement with our study. However, the silylenes are so “electro-
philic”, it may be assumed that they may have little difficulty in ac-
cepting an electron pair whatever the substituent (X = H or Me). The
methylsilanes donor character (Y = Me) is also probable not changed
much by Me groups. The bond dissociation energies [23–25] gave
support for this effect. The effect of methyl substituent on the Si-H bond
strength in methylsilanes seems to be very little, whereas the case of the
simple alkanes, the effect of methyl substituent on CeH bond strength is
to weaken them.
VII) Further search for a clean photolysis source of MeSiH.
However, it is in the second, “nucleophilic” stage where Me groups
that may exert their control. If Me groups are found in the silylene
(X = Me) then it will be more disinclined to donate its lone pair. Thus it
destabilizes the intermediate and second transition state. On the other
hand, if Me groups are found in the silanes (Y = Me) then they will help
withdraw charge and make possible the acceptance of the core pair
(stabilizing the intermediate). Consequently, if the second stage turns
out to be the rate determining step (as with the SiMe2 insertion) or at
least rate-influencing (as with the MeSiH insertion), the methyl
Acknowledgments
The author would like to express his gratitude to the Department of
Chemistry, University of Reading, UK for providing the possibility of
completing this work. The author would like also to express his sincere
appreciation and thanks to Professor Robin Walsh (Reading University)
for his support and advice. A special thanks is given to Professor E.
Hengge (University of Graz/Austria) for supplying some chemicals.
150

N. Al-Rubaiey
JournalofPhotochemistry&PhotobiologyA:Chemistry360(2018)145–151
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