Journal of Molecular Structure 482–483 (1999) 563–569
Infrared and Raman spectra, conformations and ab initio
calculations of 2-chloroethyl trifluorosilane
V. Aleksa a,1, A. Gruodis a,1, P. Klaeboe b,*, C.J. Nielsen b, K. Herzog a, R. Salzer a,
G.A. Guirgis c, J.R. Durig c
a
Institute of Analytical Chemistry, University of Technology Dresden, D-01062 Dresden, Germany
b
Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway
c
Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA
Abstract
The infrared spectra of 2-chloroethyl trifluorosilane (ClCH2CH2SiF3) were recorded in the vapour, amorphous and crystalline
solid phases in the range 4000–50 cm 21. Spectra of the compound isolated in argon and nitrogen matrices at 10 K and variable
temperature spectra in liquified xenon were obtained. Raman spectra of the liquid were recorded at various temperatures
between 298 and 163 K, and the amorphous and crystalline solids studied. The spectra of 2-chloroethyl trifluorosilane showed
the existence of two conformers—anti and gauche—present in the vapour and in the liquid. Large variations in the infrared and
Raman spectra occurred during the annealing and an intermediate, probably metastable crystal appeared at ca. 125 K with
pronounced crystal splitting and apparently containing both conformers. More than 15 infrared and Raman bands present in the
fluid phases and in the 125 K crystal vanished in the stable crystal formed at ca. 160 K. From intensity variations with
temperature of five Raman band pairs, a DH8…anti-gauche† ˆ 0:8 ^ 0:3 kJ mol21 was obtained in the liquid, and in liquid
xenon under pressure a DH value of 20:7 ^ 0:1 kJ mol21 was obtained from IR spectra. Annealing experiments indicate that
the anti conformer has the lower energy in argon and nitrogen matrices as well as in xenon, and the barrier seemed to be ca.
8 kJ mol 21. Ab initio calculations were performed using the Gaussian 94 program with the HF/6-311G* basis set and gave
optimized geometries, infrared and Raman intensities and scaled vibrational frequencies for the anti and gauche conformers.
The conformational energy derived was 3.8 kJ mol 21 giving anti as the low energy conformer. q 1999 Elsevier Science B.V.
All rights reserved.
Keywords: Conformations; Vibrational spectra; Halosilanes; Ab initio calculations
1. Introduction
2-Chloroethyl trifluorosilane (ClCH2CH2SiF3),
later to be abbreviated CETFS, was synthesized for
the first time and it was decided to make an infrared
* Corresponding author. Tel.: 47 22855678; fax: 47 22855441.
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe)
1
Permanent address: Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania.
and Raman spectroscopic study of this compound.
Several related compounds have previously been
studied in these laboratories, and many of these
have conformational equilibria.
As is apparent from the structure CETFS can form
anti and gauche conformers owing to restricted rotation around the central C–C bond, and the silicon
moiety merely forms a substituent in the ethane.
This situation is different from a series of molecules
recently studied in these laboratories containing a
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-286 0(98)00676-0
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V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 563–569
Fig. 1. The anti and gauche conformers of 2 chloroethyl trifluorosilane (CETFS).
central C–Si linkage [1–4] with potential functions different from those of ethanes. Our preliminary results for CETFS are given in the
present communication and the two conformers
are illustrated in Fig. 1.
2. Experimental
The compound was prepared by the reaction of
freshly sublimed antimony trifluoride with 2-chloroethyl trichlorosilane at room temperature without
solvent for 1 h. Subsequently, the product was
purified in a low temperature, low pressure fractionation column and the purity was checked by mass
spectrometry.
The infrared spectra of CETFS were recorded in
various FT-IR spectrometers: Bruker models 66,
88 and 113v (vacuum bench) and Perkin–Elmer
model 2000. The vapour spectra were recorded
in cells of 10 cm (CsI windows) and 20 cm (PE
windows) path lengths whereas the amorphous and
crystalline solids were studied in cryostats with
CsI and Si windows cooled with liquid nitrogen.
Fig. 2. An MIR spectrum (1500–400 cm 21) of CETFS as a vapour, 10 cm path, 8 Torr.
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 563–569
565
Fig. 3. FIR spectra (650–50 cm 21) of CETFS as a vapour, 20 cm path, pressures 33 and 4 Torr.
Fig. 4. The MIR spectra of CETFS at 80 K, unannealed (solid line); annealed to ca. 125 K (dashed) and annealed to 190 K (dotted) in the range
1100–700 cm 21.
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V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 563–569
Fig. 5. The FIR spectra of CETFS at 80 K, unannealed (solid curve); annealed to ca. 125 K (dashed) and annealed to 190 K (dotted) in the range
500–150 cm 21.
The sample of CETFS was mixed with argon or
nitrogen (1:1000) and slowly condensed on a CsI
window, cooled with a closed cycle system from
Leybold to ca. 10 K and annealed to temperatures
in the range 10–38 K.
Raman spectra were recorded with spectrometers
from Dilor (XY800 and TR 30), excited by argon
ion lasers using the 514.5 nm line. The sample was
enclosed in a capillary of 2 mm inner diameter and
low temperature measurements of the liquid and the
crystal were carried out in a Dewar cooled with
nitrogen gas [5]. Additional Raman spectra of the
amorphous and annealed crystalline phases were
measured of the sample deposited on a copper finger,
cooled with liquid nitrogen.
3. Results and Discussion
3.1. Infrared spectral results
Vapour spectra were recorded in the MIR (Fig. 2)
and FIR (Fig. 3) regions, and as is apparent many
bands had well resolved rotational A, B and C fine
structure. Large changes in the infrared spectra were
observed when the sample was deposited on a CsI
window at liquid nitrogen temperature and subsequently annealed to different temperatures. Among
the numerous curves recorded a few are shown in
the MIR (Fig. 4, 1100–700 cm 21) and FIR (Fig. 5,
500–150 cm 21) regions, revealing the amorphous
solid, a phase annealed to 125 K and one annealed
to 190 K. In the latter curve the following bands
disappeared or were reduced in intensities: 1450,
1418, 1318, 1198, 1131, 1026, 894, 888, 743, 695,
668, 426, 417, 353, 339, 267, 244 and 237 cm 21, interpreted as bands of one conformer which disappeared
in the crystal. The curve annealed to the intermediate
temperature (105 K) contained all of the bands disappearing in the solid annealed to 160 K, but the bands
were much sharper than those of the amorphous
phase. Although the bands of both conformers were
present at 105 K, the appearance of the infrared bands
suggests that it is a crystalline solid. The large differences between the anti and gauche spectra of CETFS
are in great contrast to the molecules with a central C–
Si linkage in which only a few bands from the conformers are separated in the spectra [1–4].
IR spectra of CETFS were recorded in argon and
nitrogen matrices (1:1000) at 10 K and the matrices
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 563–569
567
Fig. 6. Raman spectra (1500–150 cm 21) of CETFS in two directions of polarization.
were annealed to various temperatures between 20 K
and 38 K for argon and 32 K for the nitrogen matrices
before they were recooled to 10 K. As expected the
matrix spectra had sharper bands than those of the
vapour and the condensed phases. Certain changes
were observed in the band intensities after annealing,
indicating that the high energy conformer converted
to that of low energy. However, the intensity
Fig. 7. Raman spectra of CETFS as a liquid at ambient temperature and as a crystal at ca. 200 K.
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V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 563–569
Fig. 8. van’t Hoff plots of the band pairs: 1174/1197, 360/420, 666/734, 191/232 and 252/232 cm 21.
variations were in most cases quite small, indicating a
small energy difference between the conformers in the
matrices leading to an equilibrium at 20–38 K
between the conformers. Generally, the bands which
disappear in the crystal are reduced in intensities in
both matrix spectra after annealing while those
present in the crystal are enhanced. However, there
were a few exceptions to this rule making the conclusions somewhat uncertain. As the conformational
changes started around 30–35 K the conformational
barrier can be estimated to be 8–9 kJ mol 21 [6].
Infrared studies in xenon solution were carried out
in the temperature range 173–213 K studying two
band pairs and giving the value DH…anti-gauche† ˆ
1 2 0:7 ^ 0:1 kJ mol21 :
3.2. Raman spectral results
Raman spectra of CETFS in the region below
1500 cm 21 as a liquid at ambient temperature are
presented in Fig. 6 in two directions of polarization.
Additional spectra of the liquid and of the crystal were
recorded (Fig. 7). Below 175 K the sample crystallized spontaneously while the melting point was
close to 213 K. A substantial number of bands present
in spectra of the liquid vanished in the Raman
spectrum of the crystal and generally they were also
absent in the corresponding infrared spectrum (see
Figs. 4 and 5). The latter bands are attributed to the
second conformer which disappears in the crystal.
Low temperature Raman spectra were independently
recorded of CETFS deposited on a copper finger of a
Raman cryostat, cooled with liquid nitrogen. The
amorphous solid first formed in the cryostat had
spectra similar to those of the liquid, while after
annealing to ca. 200 K and recooling to 80 K a crystal
was formed. This crystal had a spectrum quite similar
to that obtained from cooling the liquid [5].
Intensity variations of various Raman bands relative to the neighbouring bands were observed when
the liquid was cooled. It was observed that the bands
which disappeared in the crystal diminished in intensity at low temperatures, revealing a shift of the
conformational equilibrium. The following band
pairs (cm 21) were selected: 1174/1197, 360/420,
666/734, 191/232 and 252/232, in which the first
band of the pair represents the conformer absent in
the crystal while the second (generally a neighbouring
band) is tentatively attributed to the conformer present
in the crystal.
These five van’t Hoff plots based upon peak heights
are presented in Fig. 8, giving the values for DH: 0.8,
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 563–569
21
0.5, 1.0, 1.0 and 0.6 kJ mol . The reliability of these
values depends upon the intensity of the bands, if they
are well separated and based upon a flat background,
and that both bands are conformationally pure. The
average value DH…anti-gauche† ˆ 0:8 ^ 0:3 kJ mol21
in the liquid is different from DH ˆ 20:7 ^
0:1 kJ mol21 in liquid stabilized in the liquid. As
discussed later, the low energy conformer, which
also is present in the crystal, is undoubtedly the
gauche.
3.3. Quantum chemical calculations
The LCAO-MO-SCF calculations were performed
using the Gaussian-94 program with the basis function
HF/6-311G*. The conformational energy was
3.8 kJ mol 21 with anti being the low energy
conformer opposite from the experimental values.
3.4. Conformations
The force fields in Cartesian coordinates were
converted to internal coordinates in the usual manner
and the diagonal force constants scaled with a factor
of 0.9 for all wavenumbers above 400 cm 21 and no
scaling for those below 400 cm 21. The list of the
scaled calculated wave numbers correlated with the
observed infrared and Raman bands cannot be given
here for the sake of brevity. Among the calculated
fundamentals 11 had wave number differences larger
than 20 cm 21 between the anti and gauche conformers: n8 , n9 , n14 n 16,n 17 n 18, n 19, n 22, n 23, n 24 and
n 25. When the disappearing bands were assigned to
the anti and the remaining bands to the gauche
conformer all of these 11 fundamentals gave a qualitatively correct fit with the observed bands. Therefore,
there can be absolutely no doubt that the gauche
conformer is present in the stable crystal and is also
the low energy conformer in the liquid, while in liquid
xenon and possibily in the argon and nitrogen
matrices anti is the low energy conformer.
It is still quite uncertain if the phase obtained after
annealing to 110 K represents a truly crystalline solid
as both conformers were present. As is apparent from
Figs. 2 and 3 this phase gave infrared bands which are
shifted in frequencies from the amorphous phase and
are sharp with occasional crystal splitting. We have
recently studied three related silicon compounds:
dichlorodifluoro methyl silane [7], bromomethyl
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dimethyl silane [8] and chloromethyl methyldifluorosilane [9] which all have two crystalline phases each
containing a separate conformer. Therefore, a large
effort was made to anneal CETFS in very small
temperature intervals between 90 and 160 K, during
which the annealed solid was scanned both at the
annealing temperature and after being recooled to
80 K. However, we were never able to record spectra
in which the gauche conformer was absent or reduced
in intensity compared to anti.
In the related molecule 2-chloroethylsilane
(ClCH2CH2SiH3) [10] very recent results reveal that
the anti conformer is more stable than gauche in liquid
xenon solution by 2.2 kJ mol 21 and anti is also present
in the crystal. Substitution of three fluorines attached
to the Si atom therefore has a very pronounced effect
on the conformational equilibrium for this compound.
It should be noted that the gauche conformer of
CETFS has a much higher dipole moment than that
of 2-chloroethylsilane and should therefore be stabilized in the condensed state.
Acknowledgements
VA and AG are grateful to fellowships from
Konferenz
der
deutschen
Akademien
der
Wissenschaften (Volkswagenstiftung), Germany and
Det Norske Videnskaps-Akademi, Norway, respectively.
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