Infrared photoreaction of 2-chloroethyltrifluorosilane

Article abstract of DOI:10.1016/j.jphotochem.2011.05.004

Infrared laser dissociation of 2-chloroethyltrifluorosilane has been studied in light of silicon isotopes separation. The reaction occurs via beta-elimination mechanism to yield ethylene and chlorotrifluorosilane as products. The dissociation has a relatively low energy barrier what was confirmed by high-level quantum chemical calculations. High enrichment of ³⁰Si isotope was demonstrated.

Full text of DOI:10.1016/j.jphotochem.2011.05.004

Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 77–80
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Infrared photoreaction of 2-chloroethyltrifluorosilane
Petr S. Dementyeva,∗, Anton S. Nizovtseva,b, Evgenii N. Chesnokova
a Institute of Chemical Kinetics and Combustion, Institutskaya Str. 3, 630090 Novosibirsk, Russia
b Nikolaev Institute of Inorganic Chemistry, Lavrentiev Av. 3, 630090 Novosibirsk, Russia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 April 2011
Received in revised form 9 May 2011
Accepted 21 May 2011
Available online 27 May 2011
Infrared laser dissociation of 2-chloroethyltrifluorosilane has been studied in light of silicon isotopes
separation. The reaction occurs via beta-elimination mechanism to yield ethylene and chlorotrifluorosi-
lane as products. The dissociation has a relatively low energy barrier what was confirmed by high-level
quantum chemical calculations. High enrichment of 30Si isotope was demonstrated.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
2-chloroethyltrifluorosilane
Infrared photodissociation
Silicon isotopes separation
1. Introduction
occurs under mild laser fluence. High isotopic selectivity over rare
silicon isotopes was shown.
Recent progress in nanofabrication makes urgent the need for
isotopically pure materials [1–7]. Extremely important role of
silicon for prospective quantum computing devices requires its iso-
topes uppermost [8–12].Silicon isotopes (natural abundance: 28Si
92.23%, 29Si 4.67%, 30Si 3.10%) are currently available from the con-
ventional gas-centrifugal separation technology [13,14]. However,
the production of rare silicon isotopes 29Si and 30Si by this way is
quite expensive [15]. Therefore many efforts have been devoted to
development of alternative more economical separation methods,
especially laser ones [16–21]. Molecular laser isotopes separation
(MLIS) based on infrared multiphoton dissociation (IR MPD) is con-
sidered to be much attractive approach because it allows of high
enrichment due to significant isotopic shifts in vibrational spectra
of molecules. Although infrared laser enables to carry out a pri-
mary dissociative photoreaction with a great isotopic selectivity,
secondary radical reactions essentially degrade the isotopic com-
position of the end products. Consequently the manufacture of rare
isotopes by means of IR MPD is not so feasible.
It is not excluded that some other organotrifluorosilanes with
a halogen atom in the beta position of a side chain might behave
similarly:
To propose the most optimal substrate for the silicon MLIS
process we are continuing study of infrared photochemistry of
halogenoalkyltrifluorosilanes. A potential working substrate of
MLIS must satisfy certain conditions, in particular, it must be good
volatile and obtainable from the main semiproducts of organosili-
con industry. So the appropriate candidates for this role remain not
much, namely, it could be only mono- or di-halogenated ethyltriflu-
orosilanes. One of them is 1,2-dibromoethyltrifluorosilane (dBETFS)
analogous to dCETFS and being prepared from industrial reagents
through two stage. But experiments revealed that photodissocia-
tion of dBETFS proceeds with a radical detachment [23].
This paper presents the experimental study of a laser photore-
action of 2-chloroethyltrifluorosilane (CETFS), which is derivative
from one of the basic organosilicon monomers. We found that
CETFS well absorbs a radiation of the carbon dioxide laser and
readily undergoes a photoreaction. The reaction products were
identified and a mechanism was proposed. We also measured some
IR MPD parameters such as a dissociation yield and an isotopic
enrichment and compared it with those of dCETFS. Potential energy
profiles for CETFS and dCETFS were calculated by high-level ab initio
methods as well.
A promising silicon MLIS scheme utilizing a photoreaction of
1,2-dichloroethyltrifluorosilane (dCETFS) was lately suggested in
our group [22].Under infrared irradiation dCETFS undergoes the
beta-elimination to form vinylchloride and chlorotrifluorosilane
(CTFS). The products are stable and can be easily isolated in the
individual state. The reaction has a low dissociation threshold and
∗
Corresponding author. Tel.: +7 383 333 2944; fax: +7 383 330 7350.
E-mail address: dementyev@kinetics.nsc.ru (P.S. Dementyev).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.05.004

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P.S. Dementyev et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 77–80
2. Materials and methods
2.1. Chemicals
CETFS was prepared through
a
fluorination of 2-
chloroethyltrichlorosilane [24]. The latter was synthesized by
the free-radical chlorination from commercially purchased ethyl-
trichlorosilane (Fluka, 97%) [25]. The final compound was purified
by trap-to-trap distillation in a high-vacuum line and its purity
was checked by infrared and mass spectrometry.
2.2. Experimental
Infrared photodissociation setup was previously described in
detail elsewhere [21,22]. Briefly, we used a tunable pulse carbon
dioxide laser for vibrational excitation and a magnetic mass-
spectrometer for product analysis. The laser beam was not focused
in all experiments. Infrared spectrum was obtained with a Fourier-
transform spectrometer of 1 cm−1 in resolution.
Fig. 1. (A) IR spectrum of CETFS, recorded at pressure of about
1 torr(1 torr = 133.3 Pa). Arrows show some of the CO2 laser lines used. (B)
MPA of CETFS measured at the R30 line, sample pressure was 100 mtorr. Dash line
corresponds to the linear extrapolation of absorption cross-section from the IR
spectrum.
2.3. Computational details
Geometric parameters of species under consideration were fully
optimized by hybrid meta density functional M05-2X [26] with all-
electron Dunning’s correlation-consistent basis set of triple-zeta
quality, cc-pVTZ [27], on the all atoms (next the basis set is abbre-
viated as VTZ). All the following single-point calculations were
carried out with M05-2X/VTZ geometries. Ultrafine grid was used
in all density functional theory calculations.
In order to calculate correlation of valence electrons the
coupled-cluster method [CCSD(T)] [28] with VTZ basis set was
applied within frozen core approximation (ECCSD(T)/VTZ).
The correlation of the core electrons (Ecore) was calculated at
the MP2/cc-pwCVTZ level [29,30]. These effects were determined
as the difference between the energy values obtained without (all-
electron correlation) and with frozen core approximation.
Scalar relativistic corrections (ESR) were computed as the differ-
ence between MP2(DKH)/cc-pVTZ-DK and MP2/cc-pVTZ energies
[31].
The absorption spectrum of CETFS in the region 850–1350 cm−1 is
shown in Fig. 1A, since a working space of the carbon dioxide laser is
in fact interesting for us. Very intense band (centered at 981 cm−1
)
belonging to the Si–F stretchings is located in the laser range.
We calculated that the frequency difference between conform-
ers for these specific vibrations is small as against the bandwidth,
therefore the conformers are indistinguishable for the laser. The
substance can thus absorb infrared radiation, and under powerful
laser field this can give a strong vibrational excitation. Furthermore,
the molecules being vibrationally excited, the difference between
conformers disappears at all.
We measured
a multiphoton absorption (MPA) of CETFS,
expressed as the number of quanta absorbed per a molecule
depending on laser fluence (Fig. 1B). It is seen that CETFS absorbs
many photons even in mild fluence conditions. This allows a rapid
intramolecular heating and thus provides enough energy to over-
come an activation barrier.
To anticipate a little, the infrared spectrum of CETFS is better
for MLIS than that of dCETFS. Firstly, the band involved is narrower
than the corresponding band of dCETFS, which is splitted. Secondly,
for CETFS the band is shifted to a long-wave range as compared with
dCETFS (centered at 990 cm−1). So in case of CETFS the laser gap
gets to the place, where the isotopic selectivity is not great, while
for dCETFS the gap lies right in the spectral range of the maximum
isotopic selectivity over rare isotopes.
Zero-point vibrational energies (EZPE) were calculated from the
M05-2X/VTZ harmonic vibrational frequencies without scaling fac-
tors. The thermal correction to the enthalpy (ETC) was calculated
within rigid-rotor-harmonic-oscillator approximation and used for
obtaining reaction enthalpies at 298 K.
Isotopic shifts for the 29Si and 30Si isotopomers of CETFS were
also calculated at M05-2X/VTZ theoretical level.
All of the stationary points were characterized by their har-
monic vibrational frequencies as minima or saddle points (number
of imaginary frequencies was equal to 0 and 1 for local minima and
transition states, respectively). Connections of the transition state
structures between designated minima were confirmed by intrinsic
reaction coordinate (IRC) calculations [32].
The calculations were performed with the use of Gaussian 03
program package [33]. Some basis sets were taken from the EMSL
database [34].
3.2. Laser-induced reaction
CETFS readily reacts under laser action according to the
nhv
equation:F3SiCH2CH2Cl−→ F3SiCl + C2H4
Fig. 2 illustrates characteristic evaporation curves of a sample
recorded before and after irradiation.
Full information about results of quantum chemical calculations
is placed in Supplementary material.
We cooled down a sample in a small cold finger by liquid nitro-
gen and then slowly heated it. Vapor pressure in a closed volume
was registered as a function of evaporator temperature. This is actu-
ally the analogue of a usual distillation curve. One can see two
substances are formed in equimolar quantities during reaction. And
the amount equals to that of consumed CETFS. The products have
quite different physical properties and thereby they can be eas-
ily separated from each other and from initial substance. In such
a manner the products were simply distilled and analysed with
a mass-spectrometer. These are ethylene and CTFS, it implies the
3. Results and discussion
3.1. Infrared spectrum and multiphoton absorption
Infrared and Raman spectra of CETFS were described earlier
by Aleksa and coworkers [24]. They also showed that free CETFS
molecules are the equilibrium mixture of two conformers – anti
and gauche, which have slightly different vibrational frequencies.

P.S. Dementyev et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 77–80
79
Fig. 2. Typical evaporation curves of a sample before and after reaction. The heating
rate was about 0.15 K/s.
Fig. 4. Isotopic composition of product CTFS, measured mass-spectrometrically by
m/z 85-87 lines of SiF3 ions. CETFS pressure was 300 mtorr. Relative consump-
+
tion of CETFS was kept within limits of 0.5–1.3%. Laser fluence was varied in range
beta-elimination mechanism of reaction. Such molecular mecha-
nism prevents the mass loss and could provide an effective MLIS
process.
0.45–0.7 J/cm2. Arrows on the top show the location of some CO2 laser lines used.
3.4. Isotopes separation
3.3. Dissociation yield and energy profile calculation
Very high enrichment of rare silicon isotopes was achieved in
reaction product CTFS by tuning laser lines. In the Fig. 4 the silicon
isotopic composition of CTFS depending on laser frequency used is
shown. We accurately distilled the product and observed its mass-
spectrum. Break between 957 and 968 cm−1 corresponds to the gap
between P- and R-branches of a CO2 laser.
The reaction has a rather low activation barrier. We measured
a dissociation yield as an average probability of one molecule to
dissociate for a single laser pulse
ꢀN
N0
Virr
V0
1
p
Y =
×
×
The strong band at 981 cm−1in the infrared spectrum of CETFS
contains the spectra of three silicon isotopomers. Calculated mass-
dependent red shifts for these vibrations are 7 cm−1 between 28Si
and 29Si and 7 cm−1 between 29Si and 30Si isotopomers respec-
tively. We can expect that an isotopically selective laser-induced
reaction is possible.
where N0 is the starting concentration, ꢀN is the small concen-
tration change during irradiation, Virr is the irradiated volume of
reactor, V0is the full reactor volume, p is the number of introduced
pulses. The dissociation yield describes indirectly a reaction rate
constant in pulse experiments. In Fig. 3 the dissociation yield of
CETFS depending on laser fluence is represented in comparison
with the same function of dCETFS. CETFS reacts almost as easily as
dCETFS. We also calculated the potential energy profiles of reaction
assuming the four-membered transition states (insert in Fig. 3). The
activation barriers are 45.9 and 45.3 kcal/mol for CETFS and dCETFS
respectively.
Of course, an enrichment factor of MLIS process strongly
depends on many conditions such as isotopic shifts, sample pres-
sure and degree of conversion. In order to demonstrate the silicon
isotopes separation we chose an initial sample pressure so that,
on the one hand, to minimize the pressure-dependent decrease
in selectivity, but on the other hand, to reliably detect the form-
ing product. Laser energy was varied so as to make a dissociation
yield moderate. When laser is tuned at the R-branch (wavenum-
ber > 970 cm−1) close to the band centre no significant isotopic
enrichment is observed. In case when we move to the long-wave
range of the P-branch (wavenumber < 950 cm−1), the formation of
29Si and 30Si CTFS’s isotopomers becomes preferential, and the con-
tent of 29Si passes over an extremum. Near 935 cm−1 the content of
28Si is less than 10%, but the dissociation yield becomes very low.
As mentioned above CETFS is more effective in isotopes sepa-
ration than dCETFS. The maximum isotopic enrichment achieved
with CETFS is 86% of 30Si and 31% of 29Si while in case of dCETFS
the maximum contents of 30Si and 29Si which could be obtained
are 75% and 16% respectively. We can also use a ratio of reaction
rate constants (so-called isotopic selectivity) as a universal com-
parative feature. The maximum isotopic selectivity reached on30Si
is about 1650 for CETFS as against 240 for dCETFS. Regarding 29Si
the corresponding values are 200 for CETFS versus 35 for dCETFS.
In sum, by controlling conditions one can produce CTFS enriched
by 30Si up to 90% and simultaneously enhance concentration of 28Si
in CETFS. However, the 29Si isotope was found as being difficultly
extracted, though the unprecedented 29Si content of about 30% was
achieved in our experiments. The spectral overlaps of isotopomers
in initial substance seem to be too strong to permit the interjacent
Fig. 3. Laser fluence dependence of dissociation yield of CETFS, measured at the R30
laser line. Sample pressure was 100 mtorr, relative consumption of CETFS was less
10%. The same function of dCETFS is shown for comparison (adopted from [22]).
Insert: schematic energy profiles of reaction, calculated for CETFS and dCETFS. The
scale is ꢁ(E + ZPE) in kcal/mol.

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P.S. Dementyev et al. / Journal of Photochemistry and Photobiology A: Chemistry 222 (2011) 77–80
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CETFS readily dissociates under infrared irradiation. The reac-
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Acknowledgments
We thank the Siberian Branch of the Russian Academy of Sci-
ences for financial support (Grant No. 52/2009). A.S.N. is also
grateful to the program of the Russian Government “Research
and educational personnel of innovative Russia” (Contract No.
14.740.11.0722). The computing time on NKS-160 cluster has been
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