Journal
of
MOLECULAR
STRUCTURE
ELSEVIER
Journal of Molecular Structure 410-411 (1997) 489-495
The conformers of bromomethyl dimethyl fluorosilane studied by
vibrational spectroscopy and ab initio methods
H.M. Jensena, P. Klaeboea’*, C.J. Nielsena, V. Aleksa”, Gamil A. Guirgisb,
J.R. DurigC
“Department of Chemistry, University of Oslo, P.O. Box 1033, 03I5 Oslo, Norway
bBayer Corporation, Bushy Park Plant, Research and Development Department, Charleston, SC 29208, USA
‘Department of Chemistry, University of Missouri - Kansas City, Kansas City, MO ~54110-2499, USA
Received 26 August 1996; accepted 6 September 1996
Abstract
Bromomethyl dimethyl fluorosilane (CH2Br-(CH3)2SiF) was synthesized for the first time. Raman spectra of the liquid were
obtained at various temperatures between 295 and 163 K, and spectra of the amorphous and crystalline solids were recorded.
The infrared spectra were recorded of the vapour, and of the amorphous and crystalline states at liquid nitrogen temperature.
Additional IR spectra were obtained at 5 and 15 K, isolated in argon and nitrogen matrices.
The compound exists as anti and gauche conformers. Five IR bands and eight Raman bands present in the fluid phases
vanished upon crystallization, suggesting a large overlap of the vibrational bands between the conformers. Raman temperature
studies in the liquid gave a AW(anti -gauche) value of 0.4 ? 0.3 kJ mol-‘, gauche being the low energy conformer which was
also present in the crystal. The IR bands vanishing in the argon and nitrogen matrix spectra after annealing to about 25-28 K
suggested that the anti conformer had a lower energy than the gauche one in both matrices, and the conformational barrier was
6-7 kJ mol-‘.
Ab initio calculations with the basis sets HF/3-21G’ and HF/6-3lG*, HF/6-31 IG’ and MP2/6-3lG* gave optimized geometries, IR and Raman intensities, and scaled vibrational frequencies for the anti and gauche conformers. Reasonably good
agreement was obtained between the experimental and calculated wavenumbers for the anti and gauche conformers. 0 1997
Elsevier Science B.V.
Keywords:
Ab initio calculations;
Conformations;
Halosilanes;
1. Introduction
Bromomethyl
dimethyl
fluorosilane,
CH2Br(CH&SiF,
abbreviated
to BDFS, was synthesized
for the first time, and the vibrational spectra were
investigated as part of a series of halomethyl dimethyl
halosilanes, CH2X-(CH&SiY
(X = Cl, Br; Y = H, F,
Vibrational
spectra
Cl). Several related compounds
have previously
been
studied in these laboratories, and many of these have
conformational equilibria [l-3]. In the present investigation, the vapour, amorphous and crystalline samples were recorded in the mid- and far-infrared
regions, and the infrared matrix isolation technique
was employed to obtain spectra of the compound
trapped in argon and nitrogen matrices.
* Corresponding author.
0022-2860/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved.
PI1 SOO22-2860(96)09460-4
Raman
spectra
of the liquid,
including
polarization
H.M. Jensen et al./Journd of Molecular Structure 410-411
490
(1997) 489-495
communication
and the two conformations
strated in Fig. 1.
are illu-
2. Experimental
2.1. Sample preparation
anti,cs
gauche,Cl
Fig. 1. The anti and gauche conformers
tluorosilane (BDFS).
of bromomethyl
dimethyl
measurements,
were obtained and spectra of the
liquid were recorded at 13 temperatures
made
possible by the excessive super cooling of these
molecules. Raman spectra of BDFS as a crystal
were observed using different cooling techniques.
Moreover, the conformational energies, the structure,
the force constants, and infrared and Raman intensities were calculated by ab initio methods. Our
preliminary data for BDFS are given in the present
The sample of BDFS was prepared for the first time
by reaction of bromomethyl dimethyl chlorosilane [4]
with freshly sublimed antimony trifluoride at room
temperature for 1 h. The compound was purified in
a low temperature, low pressure fractionation column
and the purity was checked by mass spectrometry
(mp = 208 K).
2.2. Spectral measurements
The experimental techniques employed and the various infrared and Raman spectrometers used in this
investigation
have been described for the related
molecule bromomethyl dimethyl chlorosilane [4].
I
!m
I
I
800
700
wavemnnber I cm-t
Fig. 2. Raman spectra of BDFS as an amorphous
(solid line) and crystalline
solid (dotted line).
I
600
49
H.M. Jensen et al./Journal of Molecular Structure 410-41 I (I 997) 489-495
I
amorphous
I
580
I
570
wavenumber I cm-’
I
I
I
560
550
540
Fig. 3. Raman (upper) and IR (lower) spectra of the 576/557 cm-’ bands as liquid and crystal, and as amorphous
and crystalline
solids.
492
H.M. Jensen et a/./Journal
of Molecular
3. Results and discussion
3.1. Raman spectral results
In Fig. 2, Raman spectra of the liquid at ambient
temperature and of the annealed crystalline solid at
80 K are shown in the range 1200-500 cm-‘. Certain
bands, present in the spectrum of the liquid (marked
by asterisks), vanish in the Raman spectrum of the
crystal and are also absent in the corresponding
infrared spectrum (see Fig. 5). These bands are
assigned to the second conformer which is absent in
the crystal. The number of vanishing bands is small
compared to those of the corresponding
ethanes,
revealing that many of the fundamentals of one conformer overlap those of the other. This conclusion
agrees with the earlier results obtained for silanes
[l-3] and is undoubtedly due to the weak interaction
Structure 410-411
(1997) 489-495
of the end groups and the long Si-C distance in these
molecules.
Raman spectra of the liquid were recorded between
295 and 163 K (a super cooled liquid), and the
enthalpy difference AH between the conformers was
calculated. Small intensity variations with temperature of certain bands relative to neighbouring bands
were observed, revealing a displacement of the conformational
equilibrium.
Seven band pairs (cm-‘)
were employed
in estimating
the conformational
enthalpy change. Both peak heights and integrated
band areas were employed.
The 197/191 cm-’ pair showed a large deviation
from the other band pairs, while the pair at 576/
557 cm-’ of nearly equal intensities was very strong
in the Raman and seemed well suited to quantitative
calculations (Fig. 3). However, this band pair gave a
smaller value (AH = 0.13 kJ mol-‘) than the average.
wavenumber
/ cm-’
Fig. 4. Mid-IR vapour spectrum of BDFS.
H.M. Jensen et al./Journal of Molecular Structure 410-411
493
(1997) 489-495
1.c
I
I
I
I
I
750
700
650
600
550
(a)
Fig. 5. (a) Mid-IR spectra of amorphous (solid line) and crystalline
crystalline (dotted Ike) solids at 80 K.
wavenumber
/ cm-’
(dotted line) solids at 80 K. (b) Far-IR spectra of amorphous
The mean value of AZf(anti - gauche) was 0.4 t- 0.3
kJ mol-‘, gauche being the low energy conformer (see
below), compared to anti being the low energy conformer for bromomethyl
dimethyl chlorosilane
[4]
and chloromethyl
dimethyl chlorosilane
[5]. The
van’t Hoff plots compiled from integrated areas of
curve resolved bands instead of peak intensities
gave a larger scatter of points, probably because of
uncertain background corrections.
3.2. Infrared spectral results
The infrared vapour spectrum of BDFS in the
1500-500 cm-’ range is presented in Fig. 4, and spectra of the amorphous and crystalline solids are shown
in Fig. 5(a) (800-500cm-‘)
and Fig. 5(b) (60050 cm-‘). The bands vanishing in the crystal spectra
are indicated by asterisks and are the same as those
(solid line) and
disappearing
in the Raman crystal spectra, corresponding to those diminishing in intensity at lower
temperatures.
Spectra of BDFS were recorded in argon (1:500)
and nitrogen (1: 1000) matrices at 5 K, and gave
sharper peaks than the vapour, liquid, amorphous
and crystal spectra. Annealing in the 15-20 K range
led to small spectral changes due to relaxation of
BDFS in the matrix lattice. At higher annealing temperatures (25-35 K), it was observed that bands vanishing in the matrices were present in the IR and
Raman spectra of the crystals. For example, the
bands at 724 cm-’ in the matrices vanished after
annealing, belonged to the low energy conformer in
the liquid and amorphous phases, and remained in the
crystal spectra. The corresponding bands at 736 cm-’
were still present in the matrix spectra after annealing,
but disappeared
in the crystal spectra. A further
494
H.M. Jensen et al./Journal
of Molecular Structure 410-411 (1997) 489-495
0.6
0.3’
*
I.
I
I
I
I
I
500
400
300
200
100
wavenumber / cm-’
Fig. 5. Continued.
example is the band pair at 580 and 572 cm-‘, the
latter band vanishing in the matrices (Fig. 6) and the
former in the crystal after annealing. A conformational barrier equal to 6-7 kJ mol-’ was estimated
[6] from the annealing temperatures.
Consequently, argon and nitrogen apparently both
stabilize the anti conformer in the matrices, while the
gauche conformer is of lower energy in the liquid and
amorphous phases, and is also present in the crystal
(see below). Quite similar observations were made for
the related molecule chloromethyl dimethyl fluorosilane [7]: the gauche conformer had the lower energy
in the liquid and amorphous phases, and was present
in the crystal, while the anti conformer was of lower
energy in the two matrices.
3.3. Quantum chemical calculations
Quantum chemical calculations
were performed
using the GAUSSIAN 94 programs with different basis
functions: HF/3-21G*, HF/6-31G*, HF/6-31 lG* and
MP2/6-3 lG*. The conformational
energies derived
were 3.7, 4.7, 4.8 and 5.4 kJ mol-’ for these basis
sets, respectively. The calculations all favour anti as
the low energy conformer, while the experimental
values in connection with the assignments based on
the force constant calculations
for BDFS favour
gauche. Moreover, the calculated energy differences
are an order of magnitude higher than the experimental value of 0.4 kJ mol-’ (see above) obtained in the
liquid. Thus, in BDFS, chloromethyl dimethyl chlorosilane [5] and bromomethyl dimethyl chlorosilane
[4] the energy of the anti conformer was calculated
to be 4-7 kJ mol-’ lower than that of the gauche form.
However, based on the Raman spectra of the liquids,
only chloromethyl dimethyl chlorosilane [5] seemed
to have anti as the low energy conformer, with a value
of 0.7 kJ mol-‘.
The Si-C bond length was 1.89 A for both anti and
gauche. Undoubtedly, this long bond contributes to a
H.M. Jensen et d/Journal
of Molecular
Structure 410-411
(1997) 489-495
495
weak interaction between the two parts of the molecule compared to the corresponding ethane derivative,
resulting in a small enthalpy difference. A dihedral
angle of 66” for the Br-Si-C-F
angle of the gauche
conformer was obtained, compared to 69” for chloromethyl dimethyl chlorosilane [5].
The ab initio force fields in Cartesian coordinates
were converted to internal coordinates, and the frequencies were scaled with a factor of 0.9 for the fundamentals above 400 cm-’ and unscaled for those
below. Correlation between the observed and calculated wavenumbers of the anti and gauche conformers
(see Table 1 of Ref. [5]) suggests that gauche is the
low energy conformer in the liquid for BDFS, but anti
has a lower energy in the matrices.
References
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[2]
582.5
580.0
577.5
575.0
572.5
[3]
WIVatUnbeKCttt’
Fig. 6. The argon matrix spectra of the hand pair at 580 and 573
cm-’ before and after annealing to 24 and 28 K.
[4]
[5]
[6]
[7]
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