Journal of
MOLECULAR
STRUCTURE
ELSEVIER
Journal of Molecular Structure 4 1O-4 I 1 ( 1997) 477-48 1
The vibrational spectra, including matrix isolation, conformations and
ab initio calculations of bromomethyl dimethyl chlorosilane
Gamil A. Guirgisa, A. Nilsenb, P. Klaeboebq*, V. Aleksab, C.J. Nielsenb,
J.R. DurigC
“Bayer Corporation, Bushy Park Plant, Research and Development Department, Charleston, SC 29208, USA
‘Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway
‘Department of Chemistry, University of Missouri at Kansas City, Kansas City, MO 64110-2499, USA
Received 26 August 1996; accepted 6 September 1996
Abstract
A vibrational spectroscopic study of bromomethyl dimethyl chlorosilane (CH2Br-(CH3)$XI)
was carried out. Infrared
spectra of the vapour, the amorphous and crystalline solids at liquid nitrogen temperature, and spectra of argon and nitrogen
matrices (I: 1000) at about 5 K were recorded. Raman spectra of the liquid were obtained at five temperatures between 295 and
190 K, and spectra of the crystalline solid were recorded.
Owing to restricted rotation around the C-Si bond, the compound apparently exists as anti and gauche conformers. Approximately five IR and Raman bands present in the fluid phases vanished upon crystallization. From intensity variations with
temperature in the Raman spectra of the liquid, a AH” value of 1.O ? 0.4 kJ mol-’ was obtained. The high energy conformer
bands did not vanish in the matrix spectra after annealing to approximately 39 K in the argon matrix, suggesting a barrier higher
than IO kJ mol-‘.
Ab initio calculations were carried out with the GAUSSIAN 94 program using the basis sets HF/3-21G* and HF/6-31 IG’;
optimized geometries, IR and Raman intensities, and the vibrational frequencies for the anti and gauche conformers were
calculated. After appropriate scaling, reasonably good agreement was obtained between the experimental and calculated
wavenumbers for the anti and gauche conformers, suggesting the anti conformer to be the more stable and present in the
crystal. 0 1997 Elsevier Science B.V.
Keywords:
Ab initio calculations;
Conformations;
Halosilanes;
1. Introduction
Bromomethyl
dimethyl
chlorosilane
(CH2Br(CH3)2SiCl), henceforth abbreviated to BDCS, was
first synthesized in 1951 [l], but to our knowledge
this compound has not previously been investigated
by spectroscopic
methods. Earlier studies in our
* Corresponding author.
Vibrational
spectra
laboratories involved vinyl silanes [2,3] and halogenated silanes: ethyl chlorosilane [4], ethyl dichlorosilane [5] and ethyl difluorosilane
[6], all with
conformational
equilibria. We are presently investigating a series of halomethyl dimethyl halosilanes,
CH*X-(CH3)*SiY
(X = Cl, Br; Y = H, F, Cl). These
are analogous
to the corresponding
substituted
ethanes and should, owing to restricted rotation
around the Si-C central bond, exist as a mixture of
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.
PI/ SOO22-2860(96)09458-6
478
G.A. Guirgis et al./Journal
Fig. 1. The conformers of bromomethyl
(BDCS): anti (left) and gauche (right).
dimethyl
of Molecular
chlorosilane
an anti conformer with C, symmetry and two spectroscopically equivalent gauche conformers (Fig. 1) with
no symmetry (C,). It is our aim to determine the infrared and Raman spectra of these molecules and combine the results with ab initio quantum chemical
calculations in order to interpret the spectra and to
evaluate the structure, conformational
energies and
torsional barriers for these molecules. In the present
communication,
we shall present our preliminary
results for BDCS.
2. Experimental
2.1. Sample preparation
The sample of BDCS was prepared by the reaction
of chlorotrimethylsilane
with bromine, as reported by
Speier [I], subsequently purified in a low temperature,
low pressure fractionation column, and the purity was
controlled by mass spectrometry.
Structure 410-411
(1997) 477-481
cooled via a three stage closed cycle system (APD
model HS-4) to about 5 K and annealed to temperatures in the range lo-38 K.
Raman spectra were recorded with a triple monochromator, monochannel spectrometer, model RT 30
from Dilor, digitally controlled by a PC and excited
with an argon ion laser model 2000 from SpectraPhysics using the 514.5 nm line. The sample was
drawn into 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 cold
nitrogen gas [7]. Additional Raman spectra of the
amorphous
and annealed crystalline
phases were
measured when the sample had been deposited on a
copper finger, cooled with liquid nitrogen.
3. Results and discussion
3.1. Raman spectral results
Raman spectra of the liquid and amorphous phases
are nearly identical, while the bands at 823, 746 and
225 cm-l vanish in the spectrum of the crystal. They
are also absent in the corresponding
infrared spectrum. These bands are attributed to a second conformer which is absent in the crystal. Since the number of
vanishing bands is small, most of the fundamentals of
one conformer overlap those of the other.
Raman spectra of the liquid, including polarization
measurements, were recorded at ambient temperature
and were also convoluted with the R(n) function [8]
(see below). Moreover, a series of spectra was
2.2. Spectral measurements
__.___._....._
Liquid,
The infrared spectra of BDCS were recorded in
various FT-IR spectrometers: Nicolet model 800, Bruker models 88 and 113~ (vacuum bench), PerkinElmer model 2000 and Bomem model DA 3.002
(vacuum bench). The vapour spectra were recorded
in cells of 10 cm (CsJ windows), 20 cm (PE windows)
and 1 m (PE windows) path lengths, whereas the
amorphous and crystalline solids were studied in cryostats with inner and outer windows of CsI (mid-IR
region) or an inner window of wedge shaped silicon
and outer windows of PE (far IR region). The sample
of BDCS was mixed with argon or nitrogen (1:lOOO
and 1:500) and slowly condensed on a CsI window,
-
T
=
173K
Liquid, T = 295K ,+,
750
Wavenumberlcm-t
Fig. 2. Raman spectra of BDCS at 295 and 173 K in the 7807 10 cm-’ range.
GA. Guirgis et al./Journal
of Molecular
l/r
Fig. 3. Van? Hoff plots of the band pairs 823/802 and 746/728 cm-‘.
recorded between 295 and 163 K (the latter temperature represented a strongly super cooled liquid
since the melting point is around 237 K). Slight
intensity
variations
with
temperature
were
observed (Fig. 2) which are interpreted as due to a
displacement
of the conformational
equilibrium.
Various band pairs were attempted, but only two
Structure 410-411
419
(1997) 477-481
were selected
for the quantitative
calculations:
8231802 and 746/728 cm-‘. The first band of the
pair represents the conformer absent in the crystal.
The van’t Hoff plots for the 823/802 and 746/728
cm-’ pairs, based upon peak heights at five temperatures, are presented in Fig. 3, giving the values
0.95 and 1.06 kJ mol-’ with an average AZf(gauche anti) = 1.0 + 0.4 kJ mol-‘. Corresponding plots from
integrated band areas, which in principle are preferable for evaluating band intensities, gave a larger
uncertainty.
The low energy conformer, which is
also present in the crystal, is probably anti in agreement with the results for bromomethyl dimethyl fluorosilane
[9],
whereas
chloromethyl
dimethyl
chlorosilane [lo] and chloromethyl dimethyl fluorosilane [ 1 l] have gauche as the low energy conformer.
These conclusions were drawn from the correspondence between the observed spectra and the results
of the normal coordinate analyses (see Table 1,
Ref. [lo]).
m
Wavenumber /cm-*
Fig. 4. Far-IR vapour spectrum of BDCS at 1 m path length, 7 torr pressure (0.1 cm-’ resolution)
superposed
by HCI rotational
lines.
480
G.A. Guirgis et al./Journal
of Molecular
3.2. Infrared spectral results
Complete vapour spectra with negligible rotational
fine structure were recorded in the mid- and far-IR
regions. A high resolution far-IR vapour spectrum is
given in Fig. 4, revealing about nine bands below
350 cm-‘. The bands with centres at 96 and 67 cm-’
are probably due to the torsional transitions of the anti
and gauche conformers, respectively.
These bands
may be the same as those observed in the Raman
spectrum of the liquid at 110 and 76 cm-‘, employing
the R(v) function [8] which “removes” the Rayleigh
wing. Spectra of the amorphous and crystalline solids
in the mid- and far-IR regions are shown in Fig. 5. The
IR bands which disappeared on crystallization generally agree with the corresponding bands in the Raman
spectra. In the far-IR region, the band at 275 cm-’ and
probably that at 73 cm-’ of the amorphous phase
vanished in the crystal spectra.
BDCS was deposited
in argon and nitrogen
matrices at both 5 and 15 K, and the IR spectra
were recorded before annealing and after subsequent
annealing for 15 min at every 3 K. The argon matrices
were heated to a limiting temperature of 39 K and the
nitrogen matrices to 34 K.
Supposedly, the conformational equilibrium of the
vapour phase is maintained when the gas mixture hits
the CsI window at 5 K. The high energy conformer
might convert to the low energy one when the temperature is raised, passing the potential barrier. However, from the large number of annealing experiments
carried out both in argon and nitrogen matrices, no
significant changes in band intensities were observed.
The high energy conformer supposedly remained in
the matrices, as was also observed for chloromethyl
dimethyl chlorosilane [lo]. From the plots given by
Barnes [ 121 neglecting the effects of matrix viscosity,
the barrier height must therefore be larger than 10 kJ
mall’.
Structure 410-411
(1997) 477-481
the low energy conformer for BDCS and for the
related bromomethyl dimethyl fluorosilane [9], chloromethyl dimethyl chlorosilane [9] and chloromethyl
dimethyl fluorosilane [ 111, the value of AH varying
between 4.6 and 7.4 kJ mall’ with the largest basis
sets. They are all much higher (an order of magnitude)
than the experimental
AH values for these compounds, found to be between 1.0 and 0.2 kJ mall’ in
the
liquid.
Moreover,
chloromethyl
dimethyl
fluorosilane [ 111 and bromomethyl dimethyl fluorosilane [9] apparently have gauche and anti as the
low energy conformer
in the liquid and in the
matrices, respectively.
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. No complete table containing the observed and
calculated
wavenumbers
of the anti and gauche
900
I
I
I
1
850
800
750
I
I
700 650 600
Wavenumber,cm-r
I
1
I
550
500
450
3.3. Quantum chemical calculations
The LCAO-MO-SCF
calculations were performed
using the GAUSSIAN
94 program with a variety of basis
functions: STO-3G, HF/3-21G* and HF/6-31 lG*. The
conformational energies derived from these basis sets
were 10.8,8.3 and 7.4 kJ mol-‘. Thus, the calculations
based upon various basis sets invariably give anti as
1
r
275 250 225 200 175 150 125 100 75
Waveatimber
cm”
Fig. 5. IR curves (90%450 cm-‘, upper, and 290-60 cm-‘, lower) of
amorphous (stronger) and crystalline (weaker bands) BDCS at 80 K.
G.A. Guirgis et al/Journal of Molecular Structure 410-41 I (1997) 477-481
conformers is given here for the sake of brevity. A
correlation between the observed and scaled ab initio
frequencies for the anti and gauche conformers suggests that the bands vanishing in the crystal spectra
and in the annealed matrix spectra belong to the
gauche conformer. The low temperature crystal is
accordingly anti.
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(1991) 239.
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