Journal
of
MOLECULAR
STRUCTURE
ELSEVIER
Journal of Molecular
Structure 445 (1998) 161- 178
The conformers of chloromethyl dimethyl fluorosilane studied by
vibrational spectroscopy and ab initio methods’
Valdemaras
Aleksa2,“, Peter Klaeboe”‘*, Claus J. Nielsen”, Gamil A. Guirgisb
“Department ofChemistq
Universir): ofOslo, P.O. Box 1033, 0315 Oslo. Norway
‘Bayer Corporation, Bushy Park Plant, Research and Development Department, Charleston, SC 29423-8088,
Received 29 August 1997: accepted
7 October
USA
1997
Abstract
Chloromethyl
dimethyl
fluorosilane
(CH2CI-(CH3)2SiF) was synthesized and investigated by vibrational spectroscopy and
by ab initio quantum chemical methods. Raman spectra of the liquid were obtained at seven temperatures between 295 and
174 K, and spectra of the amorphous and crystalline solids were recorded. The infrared spectra of the vapour, of the amorphous
and crystalline states were obtained between 4000 and 50 cm-‘. The compound was mixed with argon and nitrogen and the
vapour mixture was deposited on a Csl window at 5 and at 15 K, the infrared spectra were recorded in the range 4000-400 cm-’
before and after annealing.
The spectra reveal that the compound exists as anti and gauche conformers in the vapour, liquid, in the unannealed matrices
and in the amorphous solid. Six cases were observed when infrared and Raman bands present in the fluid phases vanished after
crystallization. Raman temperature studies in the liquid gave A coniH = 0.2 + 0. I5 kJ mol-‘. The gauche conformer was the low
energy conformer and the only one present in the crystal. The IR bands vanishing in the argon and nitrogen matrix spectra after
annealing to ca. 28-34 K, suggested that the anti conformer had a lower energy than the gauche in both matrices. The
conformational barrier was estimated to be ca. 7 kJ mol-‘.
Ab initio calculations
at the HF/3-2 lG*, HF/6-3 1G*, HF/6-3 1 I G* and MP2/6-3 1G* levels of approximation gave optimized
geometries, IR and Raman intensities and vibrational frequencies for the anti and gauche conformers. After scaling, a reasonably good agreement between the experimental and calculated wavenumbers for the anti and gauche conformers was obtained.
0 1998 Elsevier Science B.V.
Keywords:
Ab initio
calculations;
Conformations;
Halosilanes;
1. Introduction
In our laboratories, a series of halomethyl dimethyl
halosilanes, CH2X-(CH3)2SiY
(X = Cl, Br; Y = H, F,
* Corresponding author.
’ In memory of Professor Otto Bastiansen; a good friend. an
excellent scientist, teacher and university rector.
’ Permanent address: Department of General Physics and
Spectroscopy, Vilnius University, Vilnius 2734, Lithuania.
0022-2860/98/$19.00
0 1998 Elsevier
PII SOO22-2860(97)00422-5
Science
Vibrational
spectra
Cl) are currently being investigated
by vibrational
methods.
We are interested
in
spectroscopic
comparing
the thermodynamic
and spectroscopic
results for this series of halosilanes and the differences
caused by the various halogen substituents. It is of
particular interest to elucidate the differences in bond’
mg between the carbon and silicon atoms by comparing the vibrational
spectra and the conformational
equilibria in halogenated molecules with C-C, Si-C
B.V. All rights reserved
162
V. Aleksa et al.Nournal c?fMolecular Structure 445 ( 1998) 161- I78
and Si-Si central bonds. Various saturated organic
compounds
containing
one [l-3],
two [4-61 or
three [7] silicon atoms with conformational equilibria
have previously been investigated in different laboratories and several molecules with Si-Si bonds without
conformers have been studied [8,9]. Two studies of
vinyl silanes with conformational equilibria have been
published
[ 10,l l] and we have recently reported
results from spectroscopic studies of four halomethyl
dimethyl halosilanes [12-151.
Chloromethyl
dimethyl
fluorosilane,
CH&l(CHj)#iF
(CDFS), was synthesized and the infrared
and Raman spectra were investigated. Raman spectra
of the liquid were recorded and polarization ratios
were obtained. Spectra of the liquid were recorded
at different temperatures,
some of them far below
the melting point since CDFS, like other related molecules, exhibit large super cooling. Raman spectra of
CDFS as a crystal were observed using different
cooling techniques.
The vapour, amorphous and crystalline samples of
CDFS were recorded in the middle and far infrared
regions. An infrared matrix isolation technique was
employed to obtain spectra of the compound trapped
in argon and nitrogen matrices. With this technique,
neighbouring
bands of different conformers, which
overlap in spectra of the vapour and liquid, can frequently be separated, due to the narrow band widths
observed in the matrix spectra. Also, by appropriate
annealing of the matrices in the temperature range
25-38 K, the conformational
barriers can often be
estimated in the matrices.
The conformational
energies, the structure, the
force constants and the Raman and infrared intensities
were calculated
by ab initio quantum
chemical
methods. Two staggered conformations of CDFS are
expected and they are illustrated in Fig. 1.
2. Experimental
2.1. Sample preparation
The sample of CDFS was prepared by reaction of
chloromethyl dimethyl chlorosilane [ 161 with freshly
sublimed antimony trifluoride at room temperature for
1 h. The compound was distilled in a low temperature,
low pressure fractionation
column and the purity
was checked by mass spectrometry.
No apparent
impurities were observed in the vibrational spectra.
2.2. Raman spectral measurements
The Raman spectra were obtained using a Dilor
RTI-30 spectrometer
(triple monochromator)
and
recorded digitally. An argon ion laser from Spectra
Physics (model 2000) was employed
using the
5 14.5 nm line with 90” excitation.
Spectra of the liquid, of the amorphous solid and of
the crystal were obtained at nine different temperatures between 295 and 174 K in a capillary tube of
2 mm inner diameter surrounded by a Dewar, cooled
by gaseous nitrogen evaporated from a reservoir [ 171.
From these spectra the enthalpy difference, A,,,rH, in
the liquid between the conformers was calculated. All
the halomethyl
dimethyl halosilanes
investigated,
including CDFS, had unusually large undercooling
and it was sometimes possible to study the liquid
50-60” below the freezing point. The crystallization
occurred
spontaneously
around
155 K and an
gauche
Fig. 1. The anti and gauche conformers
of chloromethyl
dimethyl fluorosilane
(CDFS).
V. Aleksa
Table
et al.Nournal
ofMoleculur
Structure
445 (I 998)
161-I
78
163
1
Infrared
and Raman
Vapour
spectral
Ar matrix
data a and assignments
N2 matrix
for chloromethyl
Solid
amorphous
2990 vvw
2992 vvw
2993 vvw
2982 w
2985 w
2985 w
2980 w
2982 m
2982 w
2916 m
2919 m
2978 m
dimethyl
fluorosilane
Liquid
ClyStal
(CDFS)
Solid
amorphous
crystal
85 K
85 K
2993~~
2992 w,br,D
2998 w
2915 m
2973 m,br,D
2975 m
2969 m
2967 m
Gauche
Anti
VlVZ
VIV2
V)
VI
Vl vs
VI v8
2957 VW
2950 w
2951~
2952 m
2945 w
2941 VVWJ
2948 w
2940 w
2942 VWL
2944 w
2938 w
2942 w
2935 m,P
2943 m
2934 VW
2934 vvw
2928 w
2933 ml
2929 VW
2909 w
2909 w
2914 s,P
2920 m,br
2810 w
2915 vvw
2915 VW
2868 vvw
2875 VW
2872 vvw
2798 VW
2789 VW
2800 vvw
1475 vvw
1475 VW
1440 VW
1445 “VW
1460 VW
1455 w
1445 w
1446 VW
1435 w
1434 w
1435
1431 w
1432
14lOm
1415 m
1416 m
1405 m
1407 m
1408 m
1407 m
1406 w
1393 w
1395 w
1391
1398 m
1391 m
1388 m
1377 w
1434 VW
1414 w
1302 VW
1412 w
1356 w
1360 VW
1306 VW
1308 VW
1305 w
1300 w
1294 VW
1295 VW
1286 VW
1264 vs
1257 vs
1405 m,D
1408 m
1415 w
1402 w
1401 m
1391 VW
1389 w
1360~
1359 VW
1260 w
1260 w
1251 w
1248 VW
1288 ww,D
1271 ml.
1268 vs,br
1263 vs
1259 vs
1183 m
1266 s
1265 w,P
1260 vs
1254 s
1184~
1193 w
1199ww,D?
1192~~
1186m
1180 w,D?
1183 w
1177m
1178mT
1181 m
1172m
1171 ml
1174 w
1184m
1185 w
V. A1ek.w et til./Journul of Molecular Structure 445 (1998) 161-178
164
Table
I (continued)
VapOUI
1099 m
AI matrix
N2 matrix
5K
5K
Solid
crystal
amorphous
crystal
85 K
85 K
85 K
85 K
1112ml
1108 w
1091 wl?
1102w
1092 VW
978 YW
IllOm
1109 w,D
1116
*
I100 vw,D
1lOOw
1112w
Anti
“17
1
“17
986 VW
968 VW
949 w,br
Gauche
Solid
amorphous
1097mT?
994 w,br
Liquid
941 m
941 m
924 w
932 w
903 sl
903 vsl
891 s
885 m
*
947 VW
912 w
900 s
896 VW
897 VW
895 VW
“I8
*
876 “s
??
888 VW
889 VW
852 s
854 m,D
852 yw
*
832 w,D
834 VW
812m
81Om
816m
“20
891 vs,br
886 “S
884 “S
893 s
878 ST
876 w
854 “s
854 “s
853 “S
834 “ST
835 “s
834 s
VI8
859 s
854 “s
863 VW
853 w !
“19
“19
847 “S
845 “s
837 “S
814 s
810~1
812 sl
812 “s
825 s
817 s 1
*
“20
779 WT
783 w,br
776 m
778 m
775 m
772 m
776 “VW
774 w
770 w
“21
“21
752 m
753 m
754 m
755 w
754 VW
755 s,P
753 s
756 s
“22
v22
748 “wl
145 wl
745 w
749 w
748 s,P
744 s
748 s
v23
v23
698 wl
700 m
700 w
702 w,D
702 m
702 m
v24
V24
687 s
681 s
681 s
683 m,D
682 m
681 m
v25
v25
738 vwl
692 s
695 wL
690 mT
686 sf
687 s
682 m
684 sl I
676 wl
V26
672 vwl
663 w
662 ml
662 m
660 m
660 m
654 VW
654 m
654 br
657 m,P
658 m
659 m
657 vwl
659 w,br
650 vwl
650 VW
630”“~
614 VW
*?
V26
V. Aleksa et al./Journal
Table
I
of Molecular
Structure
445 (1998)
161-178
165
(continued)
Interpretation
Raman
Vapour
Ar matrix
N2 matrix
Solid
Liquid
Solid
amorphous
crystal
5K
5K
85 K
85 K
607 W
609 wt
609 w
606m
*
607 vs,P
609 YS
602 w
601 vwL
600 vwl
598m
596 m
598 vs,P
598 vs
599 vvw
599 -NW
Gauche
amorphous
crystal
85 K
85 K
573 vvw
*
Anti
V27
596 vs
V27
572 vvw
560 w,br
*
532 w,br
397 VW
398 vw
398 VW
390 VW
389~~
385 VW
385 VW
376 VW
376 w
328 s
328 s,br
376 w
331 s
326 s1
330 m,P
329 m
330 m
325 w
??
Vlll
284 ww
280 vs,br
286 s
286 s
279 s
276m*?
285 w,D
281 m
283 m
*
267 w,P?
265 m
*
246 VW
250 w
237 vw,P
239 m
250 m
V30
213 m
213m
211 s,D
210 s,br
213 m
%I
%I
204 m,D?
203 w,sh
206 m
VI2
v32
v33 v34
h3
V35
V35
266 s
203 VW
115 W
202 W
207 VW
I77 w,br
184~~
133m
134m
v29
V28
v29
ho
182vw?
132 vw,P
13ow
135w
v34
127~~
94 m,br
105m
84 w
86 W
95 vw
78 w
70 W
64 w
56 m
80 w
61 m
a Abbreviations: s, strong; m, medium; w, weak; v, very: br, broad; P, polarized; D, depolarized;
and 1, signify bands which increase or decrease in intensities after annealing.
105w
88 VW
lattice
74 VW
lattice
64 m
V36
v3b
43 m
lattice
28 m
lanice
*, denotes vanishing bands; t
V. Akksa
166
et ol.Nournal
of Molecular
anisotropic crystal containing only one conformer was
obtained. Independently,
the vapour of CDFS was
condensed on a copper finger at 80 K. An amorphous
phase was first formed and the spectra were quite
similar to those of the liquid. After annealing to ca.
145 155 K and retooling to 80 K, a crystalline phase
was obtained having much sharper Raman bands than
those of the amorphous solid.
2.3. Infrared spectral measurements
The spectra in the middle infrared region (MIR)
were recorded on the following Fourier transform
spectrometers:
a Bruker model IFS-88 (4000-450
cm-‘), a Nicolet model 800 (4000-450 cm-‘) and a
Perkin-Elmer
model 2000 (4000-450 cm-‘), while
the far infrared region (FIR) was covered by a Bruker
IFS- 113~ vacuum
spectrometer
(600-50
cm-‘).
Beamsplitters of Ge substrate on KBr were employed
in the MIR, and beamsplitters with 3.5 and 12 p thickness of Mylar and of the metal mesh type were used in
the FIR. The vapour was studied in cells with KBr
(10 cm) and polyethylene
windows (20 cm). The
amorphous and crystalline solids were deposited on
a CsI window in the MIR region and on a wedge
Structure
445 (1998)
161-l
78
shaped window of silicon in the FIR region, both
cryostats were cooled with liquid nitrogen.
The sample was diluted with argon or nitrogen (1:500
and 1: 1000) and deposited on a CsI window at either 5 or
15 K of a Displex cryostat from APD (model HS-4)
with a three stage cooling system. The spectra of the
unannealed matrices were first recorded. Subsequently,
the matrices were annealed to temperatures between 15
and 20 K to remove site effects. They were then
annealed in steps of 3-5 K to a maximum of 34 K in
argon and 32 K in nitrogen, in periods from 10 min to
1 h. Higher temperature measurements are not feasible
since the inert gases then have too high a pressure in the
cryostat and the matrices turn ‘soft’, followed by diffusion of the solute. After each annealing the window
was retooled to 5 K and the spectra were recorded
3. Results and discussion
3.1. Raman spectral results
The experimental
results for CDFS from Raman
and infrared spectra in different phases are collected
in Table 1.
Tr
I
I
I
6000
x
.z
2
2
E
4000
600
Wavenumber
Fig. 2. Raman spectra (900-50
I cm-’
cm-‘) of CDFS as a liquid at 298 K and as a crystalline
solid at 80 K.
V. Aleksa
et d./Journal
of Molecular
Structure
445 (1998)
161-I
167
78
very small intensity variations with temperature of
certain bands relative to neighbouring
bands were
observed, and were interpreted as a displacement of
the conformational
equilibrium.
The Raman bands
which vanished upon crystallization
belong to one
conformer, and they were paired with other bands
(often neighbours)
which remained in the crystal.
However, it is a priori quite uncertain whether these
remaining bands are characteristic of only one conformer or whether they belong to overlapping bands of
both conformers. In some cases the results of the force
constant calculations strongly suggest that the remaining bands may be caused by one conformer only.
To determine the conformational enthalpy difference
from the variable temperature spectra the bands of the
two possible conformers of CDFS should both have
reasonably high intensities, they should also be situated
on a flat background and not overlap other bands in the
spectra. Moreover, they must be ‘pure’, meaning that
their intensities should be due to one conformer only,
with no contribution, neither from fundamentals nor
from combination bands of the other conformer. For
this reason, we have measured more than one pair of
bands and carried out independent
calculations
of
A,,,@ for each pair. Considerable
discrepancies
Raman spectra below 1500 cm-’ of the liquid and
of the annealed crystalline solid at 80 K are compared
in Fig. 2. Certain bands present in Raman spectra of
the liquid and of the amorphous solid disappear in the
spectrum of the crystal. These bands are equipped
with asterisks in Table 1 and attributed to the second
conformer which is absent in the crystal. The same
bands also disappear in the infrared spectra of the
crystalline CDFS. (By comparison with the calculated
wavenumbers obtained from the ab initio calculations
after appropriate scaling, it will be shown below that
the vanishing bands pertain to the anti conformer.
Accordingly, the bands present in the crystal should
belong to the gauche rotamer.) The small number of
vanishing Raman and infrared bands reveal that most
of the fundamentals of one conformer overlap those of
the other. This conclusion
agrees with the earlier
results obtained
for saturated
[l-3]
and vinyl
[ 10,l l] silanes with conformational
equilibria. It is
probably due to a low conformational
barrier, a relatively weak interaction between the end groups and
the long Si-C distance in these molecules compared
to the C-C distance in ethanes.
Raman spectra of the liquid were recorded at nine
different temperatures between 295 and 174 K. Only
1.5
8
-I
1.0
Ei
.g
2
0.5
0.0
-1
3200
3100
3000
Wavenumber
Fig. 3. Infrared vapour spectrum of CDFS in the C-H stretching
2900
2800
I cm-’
region. Pressure, 45 Torr; path length, IO cm.
V. Aleksa et al.Nournal
168
1600
of Molecular
Structure
1200
1400
1000
Wavenumber
Fig. 4. Infrared vapour spectrum (1600-500
I
800
600
/cm-’
cm-‘) of CDFS as a vapour. Path length, 10 cm; 9 Torr pressure.
might occur between the A,,,,H values obtained from
different pairs of bands, indicating either poor signal
to noise ratio, overlapping
bands or an ill-defined
background. Eventually,
one or both bands of the
I
445 (1998) 161-178
I
pair may be contaminated
by the other conformer,
meaning a superposition between a fundamental
of
one conformer and a fundamental, combination band
or overtone belonging to the other conformer.
I
I
I
1.0
8
El
.i
E
OS
0.0
I
I
I
I
I
I
600
500
400
300
200
100
Wavenumber
I cm-’
Fig. 5. Far infrared spectum (630-50 cm-‘) of CDFS as a vapour. Path length, 20 cm; 45 Torr pressure. Curve A: beamsplitter,
resolution. Curve B: beamsplitter, 12 p’; resolution, 4 cm-‘.
3.5 p; 2 cm-’
V. Aleksa
et al.Nournal
of Molecular
Structure
700
750
cm-‘) of amorphous
1
(-)
I
I
I
600
500
400
600
/cm-’
and crystalline
.) solids of CDFS at 80 K
(.
I
cm-‘) of amorphous
200
300
Wavenumber
Fig. 7. Far infrared spectra (630-50
169
I78
independent of temperature. In spite of careful spectral deconvolution and measuring the band areas with
advanced computer programs, the calculations based
upon band areas were invariably erratic and showed
The intensities of each band pair were fitted to the
equation In K = AconfHIRT + constant, where K is the
ratio in peak heights or integrated areas between anti
and gauche bands, and it is assumed that A,,,,H is
0.0
16/-
650
Wavenumber
Fig. 6. Infrared spectra (800-500
445 (I 998)
(-)
I
100
/cm-’
and crystalline
(.
.) solids of CDFS at 80 K
V. Aleksa
170
et al.Nourtd
of’Molecular
results
An infrared vapour spectrum of CDFS in the C-H
stretching region (3150-2750
cm-‘) is presented in
Fig. 3 while a spectrum in the 1600-500 cm-’ region
is given in Fig. 4. A vapour spectrum in the FIR region
650-50 cm-’ at the full pressure of 45 Tot-r appears in
Fig. 5. As is apparent from these figures, practically
no rotational fine structure was observed except for
I
445 (I 998)
161-
178
the bands at 2946, 1177 and 687 cm-’ which all
seemed to have ill defined C-type contours.
The infrared
spectra of the amorphous
and
crystalline solids at 80 K are presented in the regions
800-500 cm-’ (Fig. 6) and 600-50 cm-’ (Fig. 7). A
few infrared
bands present
in spectra of the
amorphous solid vanished in the spectra of the crystal
after annealing: 1092, 876, 834,606,266
and possibly
276 cm-‘. They are generally the same bands as those
disappearing in the Raman spectra of the crystal (see
above). These bands are enhanced in the Raman
spectra with higher temperatures and therefore belong
to the high energy conformer of the liquid.
In the infrared spectra of CDFS in matrices at 5 K
the bands are sharp and contain much information.
Spectra were recorded of argon and in nitrogen
matrices with solute to matrix ratios of 1:500 and
1: 1000 and deposited both at 5 and 15 K. The quality
of the infrared spectra of CDFS isolated in matrices is
generally better when deposited at 15 K; however, in
the case of very low conformational
barriers, the 5 K
deposition temperature is advantageous
to prevent
instantaneous
conformational
conversion.
Infrared
spectra of CDFS in unannealed
matrices and in
matrices annealed to 32 K in nitrogen deposited at
large deviations in the van’t Hoff plots. Therefore,
only peak height measurements
were employed in
the final quantitative calculations.
All together six band pairs were included in the
van’t Hoff plots 288/264*,
606*/598, 883*/812,
884*/854 and 1109/l lOO* cm-‘, in which the band
equipped with asterisks vanished in the crystal. The
bands at 606*/598 cm-’ were both very intense in
Raman and assigned to the C-Cl stretching modes of
the two conformers. The A,,,‘H(anti - gauche) values
calculated from the various band pairs gave remarkably
constant
values,
0.16,
0.18,
0.22,
0.24
and
0.18 kJ mall’, respectively, leading to a average value
of A,,,‘H = 0.2 2 0.1 kJ mall’ with gauche apparently
being the low energy conformer (see below).
3.2. Infrared spectral
Structure
I
I
800
Wavenumber
600
I cm-’
Fig. 8. Infrared spectra (I 350-600 cm-‘) of CDFS in a nitrogen matrix (I : 1000) deposited and recorded at 5 K: unannealed (-)
(- -) to 33 K.
and annealed
V. Aleksa
et al./Journul
ofMolecular
Structure
I
I
1200
1000
800
Fig. 9. Infrared spectra (1350-600cm-‘) of CDFS in an argon matrix
annealed to 35 K (. .).
cm-‘). Corre5 K are given in Fig. 8 (1000-600
sponding spectra in argon matrices, deposited at
15 K and annealed to 34 K are shown in Fig. 9
(1300-600 cm-‘).
Supposedly, the conformational
equilibrium of the
vapour phase is maintained when the gas mixture is
quickly frozen on the CsI window at 5 or 15 K,
provided
that
the
barrier
to conformational
equilibrium
is high enough to prevent conversion.
Very small spectral changes occurred both in the
argon and nitrogen matrices when the samples were
X
s
Hj
T
Y
Fig. IO. Internal coordinates
16 I- I78
I
Wavenumber
HZ
445 (1998)
of CDFS.
171
600
I cm-’
( I : 1000) deposited at 15 K and recorded at 5 K: unannealed (---_)
and
annealed to temperatures below 20 K, indicating negligible site effects. This is in contradiction
to the
related molecule bromomethyl dimethyl fluorosilane
[ 12,131, in which large site effects were observed. At
higher annealing
temperatures
(20-34 K) changes
were observed both in the argon and nitrogen matrix
spectra which were correlated with conformational
changes in CDFS.
It is highly significant that the bands vanishing in
the matrix spectra after annealing did not disappear on
crystallization,
but were present in the infrared and
Raman spectra of the crystals. These bands remained
in the crystal and belonged to the low energy conformer
in the
liquid.
Accordingly,
opposite
conformers were more stable in the liquid and in the
matrices,
exactly as observed
for bromomethyl
dimethyl fluorosilane [ 131.
It can be clearly seen in both matrices that the bands
at 1112, 903, 812 and 600 cm-’ (nitrogen matrix)
vanish or are reduced in intensity after annealing.
As is apparent from Fig. 9, the band at 1 112 cm-’
was reduced in intensity
compared
to that at
1102 cm-’ in nitrogen matrices after annealing. The
bands around 903 and 812 cm-’ nearly vanished in
both matrices after annealing compared to those at
172
V. Aleksa et al./fournal
of‘Molecular
884 and 835 cm-’ (Figs. 8 and 9). Another example is
the band at 600 cm-’ diminishing
in intensity compared to that at 609 cm-‘. Therefore, it can be coneluded that the high energy conformer in the liquid
(anti) is the low energy conformer in both matrices.
Table 2
Calculated
and observed fundamentals
a
“<al‘
Vib.
no.
(cm-‘) for the anti conformer
h
Y,,lc
scaled
IIR
int.
c
IR
int.
d
Structure 445 (1998) 161-I 78
Accordingly,
the conformational
stabilities in the
liquid state and in the argon and nitrogen matrices
are opposite, as observed for bromomethyl dimethyl
fluorosilane [ 131. As discussed below, the conformer
present in the crystal, which is also the low energy
of chloromethyl
dimethyl Ruorosilane
(CDFS)
Dep.
Y,,b\.
fIK
IR
PED ’
m
m
m
w
w
w
w
w
VW
m,D
m.D
_
S22( 100)
SO(49), S8(30), S7(20)
SS(SO), S8(37), S7( 12)
S25(55), S24(45)
S7(67), S8(32)
S24(55), S25(45)
S6(99)
S23(99)
Sl9(78)
Sl8(81), Sl9(12)
S33(89)
S34(89)
Sl3(86)
S15(95)
S30(94)
S 14(92)
S28(94)
Sl6(38), S3(34), S17(10)
S17(43), Sl6(22), S2(10)
S31(25), S29(34), S21(22)
S3(5l), S16(18), S17(16)
S32(74)
S2(37), SS4(35), S l7( 13)
S21(52), S31(43)
S29(47), S28( 14), S31( 14)
S32(13)
S4(44), S2(23)
Sl(69), Sl2(11), S3(10)
S9(32), Sl2(27), Sl7(16),
S13(12)
S26(62), S27(12). S31(lO)
SlO(48). Sl l(20)
S 11(40), S 10( 18) S9( 16).
Sl6(15)
S27(58), S26( 19)
S20(81)
S36(92)
Sl2(40), S13(20), Sll(l1)
S35(87), S36( IO)
A”
A’
A’
A”
A’
A”
A’
A”
A’
A’
A”
A”
A’
A’
A”
A’
A”
A’
A’
A”
A’
A”
A’
A”
A”
3304
3248
3246
3245
3239
3237
3174
3173
1600
1595
1588
1586
1581
1452
1447
I345
1235
967
948
918
852
842
812
755
734
2974
2923
2921
2920
2915
2914
2857
2856
1440
1436
1429
1427
1423
1307
1302
1210
1111
870
853
826
767
758
730
680
661
6
41
2
5
29
7
3
8
9
8
2
0. I
17
28
56
9
2
151
I60
140
41
17
3
8
0.1
78
125
86
31
136
37
232
3
1
10
2
15
15
2
0.2
2
6
0.5
0.7
0.04
1
2
11
3
2
0.75
0.3 1
0.38
0.75
0.74
0.75
0.01
0.75
0.61
0.75
0.75
0.75
0.75
0.03
0.75
0.3 1
0.75
0.58
0.30
0.75
0.65
0.75
0.54
0.75
0.75
2973
2973
2969
2935
2935
2935
2914
2914
1446
1434
1408
1405
1391
1265
1251
1174
1100
888
854
832
776
755
748
702
683
S
A’
A’
A’
720
628
300
648
565
300
69
4
II
9
22
0.7
0.7 I
0.01
0.74
657
607
285
m
w
s
w,D
A”
A’
A’
291
275
211
291
275
211
20
20
0.06
1
I .4
1.3
0.75
0.59
0.58
285
267
211
s
s
m
w,D
W,P
s,D
A”
A’
A”
A’
A”
205
153
143
128
68
205
153
143
128
68
0.9
0.5
0.1
5
4
0.4
0.06
0.03
0.5
2
0.75
0.41
0.75
0.73
0.75
204
132
132
95
80
w
w
w
m
w
m,D
vw,P
VW,P
VW,P
m,P
m,P
m,P
s,P
SF
w
m
m
w
m,D
“S
w.P
“S
m
VW
vw,D
“S
VW
“S
m,D
VS
W,D
m
m
w
m
s,P
s,P
’ Calculated at the HF level employing the 6-31 I G* basis set.
h Scaled ab initio values with factors of 0.9 for stretches and bends and 1.O for frequencies
’ Calculated infrared intensities (km mol-‘)
d Calculated Raman cross sections (A” mot-‘).
e Contributions of less than 10% are omitted.
VW
w,D
m,D
m.P
V&P
W
below 300 cm
_I
V. Aleksu et al./Joumul
conformer in the liquid and in the amorphous solid, is
believed to be gauche. Therefore the anti conformer
has the lower energy in the two matrices.
The lowest annealing temperature
at which the
equilibrium of the solute molecule is displaced from
the high to the low energy conformation was used for
estimating
the conformational
barrier. For both
matrices this temperature was ca. 28 K. From the
curves given by Barnes [ 181 the conformational
barrier was therefore be estimated to be 7-8 kJ mol-‘.
Table 3
Calculated
Vib.
no.
and observed fundamentals
173
of Molecular Structure 445 (199X) 161-178
(cm-‘) for the gauche conformer
Secondary effects due to matrix viscosity or matrixsolute interactions might influence the barrier height,
but the estimated value is supposedly valid for the
isolated molecules in the vapour phase. In Table 1
the infrared and Raman bands vanishing in the crystalline solids are signified by asterisks, but the infrared
bands in argon and nitrogen matrices are fitted with
arrows pointing upwards or downwards, respectively,
if the bands increase or decrease in intensity after
annealing.
of chloromethyl
dimethyl
fluorosilane
(CDFS)
’
Y,,lc
U’crlc
scaled
IIR
int.
IR
int.
Dep.
Y”tl\.
IIR
IR
PED
3291
3255
3239
2962
2930
2915
7
II
24
71
73
128
0.72
0.74
0.69
2973
2973
2969
m
m
m
m,D
3236
3234
323 I
3173
3168
1601
1593
1589
1586
1577
1451
1446
1353
I249
983
951
883
855
835
822
758
750
694
624
348
289
241
215
203
157
146
126
63
2913
2910
2908
2856
285 I
1441
1434
1430
1427
1419
1306
1301
1218
II24
885
856
795
770
751
740
682
675
625
561
314
289
241
215
203
157
146
126
56
21
31
3
7
6
I5
3
2
2
I3
30
52
I2
0.9
I85
141
122
27
II
2
23
6
33
5
39
23
I
I
0.2
0.1
0.07
0.9
3
77
121
23
160
70
2
I4
7
I2
8
3
0.2
0.9
7
0.2
I
0.4
1
2
I9
I
6
3
22
3
I
1
2
0.4
0.03
0.04
0.2
0.8
0.18
0.53
0.65
0.01
0.00
0.73
0.75
0.75
0.75
0.72
0.04
0.35
0.5 I
0.75
0.55
0.47
0.7 I
0.75
0.7 I
0.42
0.75
0.72
0.48
0.05
0.50
0.70
0.36
0.65
0.72
0.55
0.74
0.75
0.74
2935
2935
2935
2914
2914
1446
1434
1408
I405
1391
1265
1251
1180
1109
896
854
812
776
755
748
702
683
657
598
330
285
237
211
204
132
132
95
80
w
w
w
w
w
VW
m,P
S22(99)
S7(49), S24(48)
S24(33). S7(31), SE(l8).
S5(14)
S5(74), S7( I I ). S24( I I)
SE(77). S5( IO)
S25(89)
S6(70). S23(27)
S23(7 1). S6(28)
S19(85)
Sl8(82), S19(10)
S34(49), S33(43)
S33(41), S34(46)
S13(84). S33(1 I)
s I S(94)
S30(93)
S l4(92)
S28(95)
S 17(22), S3(42), S l6( 12)
Sl6(51), Sl7(20). S2(14)
S31(35),S21(32)
S3(53), Sl7(25), Sl6( 14)
S32(80)
S4(50). S2(23)
S29(52), S28( 12)
S21(44), S31(43)
S2(37), S4(21), Sl(l6)
Sl(48). Sl2(16). S4(13)
SO(23). S 10(20), S26( IO)?
S26(39), S I O(23)
S9(67), Sl7(14)
S27(21), S26(25), Sl2(15)
Sll(52), SlO(21)
S20(82)
S36(90)
S I2(42), S27(24), S I3( 19)
S35(83)
a See footnotes to Table 2.
m
m
w
“S
“S
m
m
s
“S
s
m
m
w
m
s
m
w
s
s
VW
m
w
w
w
m
w
m.D
m.P
m,P
S,P
S,P
VW
m,D
W,P
w,D
w,D
VW
m,D
m
VW
LP
S,P
W.D
m.D
m.P
VS,P
m,P
w,D
VW.P
s,D
m,D
vw,P
vw,P
vw,P
VW
174
V. Aleksa et al./Journul
C$ Molecular
3.3. Quantum chemical calculations
Quantum chemical calculations
were performed
using the GAUSSIAN-94 program [19]. Several levels
of approximations
were employed:
HF/3-2 1G*,
HF/6-3 1G*, HF/6-3 1 lG*, MP2/6-3 I G* and MP2/
6-31 lG*. The minima on the potential surface were
found by relaxing the geometry. The geometrical
parameters calculated at the HF/6-3 1 lG* level are
very similar to those of bromomethyl
dimethyl
fluorosilane [ 131 and they have not been presented
for the sake of brevity.
The calculations of conformational energies of anti
and gauche CDFS all favour anti as the low energy
Table 4
Symmetry
A’
A”
coordinates
1
2
3
4
5
6
7
8
9
IO
II
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
for chloromethyl
dimethyl fluorosilane
Si-C symmetric stretch
Si-C antisymmetric stretch
Si-F stretch
C-Cl stretch
CH: symmetric stretch
CH 1symmetric stretch
CH, antisymmetric stretch
CH1 antisymmetric stretch
Symmetric C-Si-C bend
Antisymmetric C-Si-C bend
Antisymmetric C-Si-C bend
Si-C-F bend
CH2 scissor
CH2 rock
CH2 symmetric deformation
CH2 antisymmetric deformation
CH2 antisymmetric deformation
CH, antisymmetric deformation
CH? antisymmetric deformation
CH, torsion
Si-C antisymmetric stretch
CHZ antisymmetric stretch
CH? symmetric stretch
CH? antisymmetric stretch
CH? antisymmetric stretch
C-Si-F deformation
C-Si-C deformation
CH2 twist
CH 2 rock
CH 1symmetric deformation
CH i antisymmetric deformation
CHj antisymmetric deformation
CH, antisymmetric deformation
CH2 antisymmetric deformation
CH #Zl torsion
CHx torsion
Structurr
445 (I 998) 16/L I78
conformer in agreement with the results [20] for the
other CH2X-(CH3)$5iY
molecules
studied. The
conformational
energy difference predicted by the
HF/6-3 11 G* calculations,
employed for comparing
the CH2X-(CH3)$iY
series of molecules [20], was
4.6 kJ mol-‘. This energy difference did not change
substantially when using second order Moller-Plesset
perturbation.
3.4. Normal coordinate
calculations
Analytical HF force constants were derived for each
of the two conformers in CDFS, using the 6-31 lG*
basis set. The calculated ab initio force constants were
(CDFS)
S, =3-“?(R,
&=6-“‘(2R,
+Rz+R,)
-R?-R3)
s, =s
S, = T
Ss=2m”z(.s,+s8)
S,=6m’l”(d,
+d2+d3+dz,+d5+dG)
S,=12+(2d,-d,-d,+2d,-d,-de)
S,=$(d>-d?+ds-de)
S,=6-“2(X,+177+Xj-~,-~2-~j)
S,,=6-“‘(2Z,
--Ez-E3)
S,, =6-“‘(2+,
-@y$)
.S,,=Q
~,,=~(Yl+Y?+4+~2)
.~,4=;(Y,+Y?-e,-ez
Sls=l2~‘!?(P,+P2+P3--01,--012--013+Pq+P5+Ph-Q14-015-01h)
S,,=W’iZ(2P,
-Pz-P3+2P4-Ps-P6)
&,=f(fi,-&+0,-L&)
s,8=12~‘~~(201,-01~-oIj+201~-01s-01~)
Sw=~(olz-%+~S-%)
szo=2~“*(7*+Q)
S,, =2m”2(Rz-R3)
S,,=2-“*(s,-s,)
&=6-“*(d,
+dz+d,-d,-d,-d,)
S>1=12-‘i2(2d,
-d2-d3-2d4+ds+dh)
S>s=t(d2-d-(-ds+de)
SZh=2-“*(E2-E3)
S,,=6-“+-@3)
Szx=f(~l-~*-~,+~2)
&9=f(Yl-Y*+h-b)
s,“=l2-“*(p,+P2+P3-a,-~~-a~--P~-P5-P6+(Y~+(Ys+01~)
S,,=12-‘12(2P,--P?-P3-2P4+Ps+Pb)
&*=f(P*-P3-Ps+&)
s,,=12~“~(201,--01*--01~-2~~+01s+01~)
s,,=~(oI?--(y~--oIs+oL~)
&5 =71
s36=2-“*(72-73)
V. Aleksa et al./Journal
of Molecular
transformed from Cartesian to symmetry coordinates,
derived from a set of valence coordinates. The ab
initio calculated wavenumbers
are invariably larger
than the experimental values. In order to make a complete assignment of the observed infrared and Raman
bands, a normal coordinate analysis with scaled force
constants was carried out.
Various scaling factors to the different types of
motions
were tested in an iteration
procedure.
However, reasonably good agreement between the
experimental
and calculated
wavenumbers
was
achieved by using scaling factors of 0.9 for the
stretching and bending modes above, and 1.0 for the
modes below 400 cm-‘. The infrared intensities,
Raman scattering cross-sections and Raman polarization ratios were calculated and these data are listed in
Tables 2 and 3.
The potential energy distributions (PEDs) given in
Tables 2 and 3 are expressed in terms of the symmetry
coordinates. The normalized symmetry coordinates
have been constructed from a set of valence coordinates
which are given in Table 4. Only PED terms greater
than 10% have been included in Tables 2 and 3. The
C-H, C-Cl and Si-F stretching modes are reasonably
well localized, but the CH3 rock, C-C stretches and
the skeletal
deformations
are highly
mixed.
Obviously, the vibrational modes for the anti conformer, separated into symmetry species A’ and A”,
are more localized than those of the gauche conformer
in which all the modes belong to the same species.
4. Discussion
4. I. Conformations
It can be seen from Table 1 that the vibrational
bands at 1100, 888, 834, 609 and 265 cm-’ (Raman
bands of the liquid) vanish during crystallization both
in the infrared and Raman spectra. In addition, the
Raman bands at 947, 560 and 325 cm-’ with no IR
counterparts seem to disappear and the IR band at
279 cm-’ possibly vanishes in the crystal. These
bands not only disappear in the crystal, they are also
enhanced in the IR spectra of the argon and nitrogen
matrices after annealing.
Finally, they increase in
intensity with temperature as observed in the Raman
spectra of the liquid.
Structure
445 (1998)
161-l
78
175
As discussed for bromomethyl
dimethyl fluorosilane [13], neither the infrared vapour contours,
Raman polarization data nor the ab initio calculated
energies could answer the following question: do the
bands vanishing in the crystal spectra belong to the
anti or the gauche conformer? While the calculated
energies for the anti and gauche rotamers, derived
from the ab initio calculations have large uncertainties, the force constants and the wavenumbers
after
appropriate scaling usually give a good agreement
with those of the observed fundamentals. Within the
group frequency regions for the CH3 and CH2 stretching and deformation vibrations, the calculated wavenumbers for the anti and gauche fundamentals overlap
completely (Tables 2 and 3). Below 1200 cm-’ there
are eight instances in which the anti and gauche
fundamentals
are separated
more than 10 cm-‘.
Frequently,
the observed anti/gauche
band pairs
were situated approximately at these frequencies.
The following observed band pairs from Raman
spectra of the liquid, 1109/1100*, 896/888*, 832*/
812, 607’1598 and 267*/237 cm-’ (the bands with
asterisks vanished in the infrared and/or Raman
spectra), were correlated with the scaled, calculated
wavenumbers
of the anti and gauche conformers,
respectively (Tables 2 and 3). It was found merely
from the direction of the shifts that in all instances
the bands with asterisks were preferably fitted with
anti and the remaining bands with gauche. The experimental and calculated shifts were also in reasonable
agreement for these band pairs. An exception is the
band pair 285/279*, tentatively assigned to Vet, in
which 285 cm-’ supposedly coincides with v2x of the
anti conformer, and 279 cm-’ was observed only in
the infrared spectra. For this uncertain band pair, the
279” cm-’ band was correlated with the anti conformer, and the 285 cm-’ band with the gauche. However, the calculated wavenumbers
were separated
only by 2 cm-’ making this band pair very uncertain
as a conformational
probe.
Thus, we feel that there are compelling reasons to
assign the vanishing bands to the anti conformer, meaning that the gauche conformer remains in the crystal
and is also the low energy conformer in the liquid.
Some additional arguments support this conclusion.
1. In bromomethyl
dimethyl fluorosilane
[ 131, in
which the vibrational spectra and conformations
176
V. Aleksa
et d./Jnurnd
of Molecuhr
are strikingly similar to those of CDFS, the experimentally determined band pairs and the results of
the calculations also revealed that the gauche conformer was present in the crystals similar to the
conclusion for CDFS.
2. We expect the anti conformer to be the more stable
conformer not only in the vapour but probably in
the matrices as well, considering the low enthalpy
difference between the conformers in the liquid
(0.2 kJ mall’). The bond moments of C-Cl and
Si-F are much larger than those of the other
bonds in CDFS. It is therefore expected that the
anti conformer
with opposite C-Cl and Si-F
bonds should have a much smaller dipole moment
than gauche in which these bonds make approximately a tetrahedral angle. In numerous conformational
systems
a stabilization
of the polar
conformer in polar solvents and in the liquid compared to the vapour have been reported [2 11. This
observation also agrees with the dielectric theory
first proposed by Onsager [22].
Therefore, while anti is the low energy conformer
in the matrices and, presumably, in the vapour, the
stabilization
of the polar gauche molecules in the
liquid leads to gauche being the low energy conformer
in the liquid. Again, the findings in bromomethyl
dimethyl fluorosilane [ 133 are in complete agreement
with the present results. In the related molecules in
this series, however,
i.e. chloromethyl
dimethyl
chlorosilane [ 141 and bromomethyl dimethyl chlorosilane [ 1.51,the low energy conformer in the liquid and
in the matrices was anti in both cases. Thus, the size,
electronegativity
and polarization
of the halogens
apparently play an important role for the relative conformational stability.
In the two related
molecules,
bromomethyl
dimethyl silane (CH2Br-(CH&SiH)
and dichloromethyl
methyl
difluorosilane
(CHC12-CH$iF&
interesting phase transitions were observed in the
crystals. Still unpublished
results from our laboratories reveal that for both molecules two different
crystals were formed after annealing, each crystal
contained a separate conformer for these compounds,
making it possible to obtain very complete spectra of
each conformer.
Surprisingly,
the results for the
deuterated molecule CH2Br-(CH&SiD
was not as
clear cut as for the parent compound, possibly because
Structure
445 (1998)
161-178
of small impurities of the latter, lowering the melting
and transition points.
4.2. Spectral assignments
The assignments of the infrared and Raman spectra
of CDFS to the anti and gauche conformers appear in
Tables l-3. For the sake of similarity, the fundamentals of both the anti and gauche conformers
have been numbered consecutively,
instead of the
conventional
numbering
of the modes in the anti
conformer belonging to species A’ before those of
A”‘.
We expect eight C-H stretching fundamentals,
VI-v& of each conformer due to eight hydrogens
belonging to the CH3 and CHZ groups in CDFS.
Some of these are accidentally degenerate and, moreover, the fundamentals
of the anti conformer completely overlap those of gauche as expected for
these group frequencies.
The vI-vx modes were
assigned to infrared and Raman bands between 2973
and 2914 cm-’ (wavenumbers from Raman spectra of
the liquid when available) while the weak or very
weak bands above or below this region are supposedly
combination bands or overtones.
Similar to the C-H stretching region the ab initio
calculations suggest that the CH3 deformation and the
CH2 scissoring and wagging modes should overlap
considerably. Thus, the scaled wavenumbers suggest
(Tables 2 and 3) that five fundamentals (v~---v,~) for
both the anti and gauche conformers
should be
situated between 1440 and 1400 cm-‘, while four
fundamentals (Y IJ-~ ,,) were expected between 1350
and 1100 cm-‘. We have assigned v~--v~~ for the two
conformers to bands present both in the infrared and
Raman spectra at 1446, 1434, 1408, 1405 and 1391,
1265 and 1251 cm-’ (R aman values), respectively.
Most of these bands were medium or weak in intensity
both in the infrared and Raman spectra, an exception
being the very strong infrared bands at 1263 and
1259 cm-’ in the argon matrix, assigned to ~14 and
vls. It appears from the infrared matrix spectra that
vi6 might be separated into a gauche and an anti
fundamental at 1178 and 117 1 cm-‘, respectively.
The v i6 mode consisted of an anti and a gauche pair
at 1100 and 1109 cm-‘, concluded from the vanishing
bands during crystallization
and supported by the
calculations.
The band pair at 896 and 888 cm-’
V. Aieksa et al.Noun~al
of Molecular
gave rise to very weak bands in the Raman spectra,
but intense infrared bands in the amorphous state at
903 and 884 cm-‘. They were attributed to the anti and
gauche fundamentals Y,a in excellent agreement with
the calculations.
The infrared and Raman bands
around 854 cm-’ are attributed to overlapping
anti
and gauche bands of vt9, calculated to be a few
wavenumbers apart.
A band pair at 832 and 8 12 cm-’ are assigned to v20,
supported by spectra of the crystal and by the varit
Hoff plots. All the modes between v21 and vZ6 were
assigned
as coinciding
anti and gauche bands
although for uZi and u25 the calculations indicated a
separation of 10 and 14 cm-‘, respectively, between
the two conformers. It is quite uncertain if the anti and
gauche bands of u26 coincide at 657 cm-’ or if an anti
band is situated at 682 cm-’ in the infrared vapour
spectrum, as suggested by the calculations.
The two intense Raman bands at 607 and 598 cm-’
and their infrared counterparts form the band pair for
anti and gauche conformers of the uz7 mode, mainly
associated with CC1 stretch. In bromomethyl dimethyl
fluorosilane [ 131 the corresponding
C-Br stretches
were situated at 576 and 561 cm-‘, and for both compounds they represented
the most intense Raman
bands in the spectra.
For CDFS and for bromomethyl
dimethyl fluorosilane [ 131 the ab initio calculations suggested that vzg
of the gauche and anti conformers should be separate
at 48 and 38 cm-‘, respectively. In the spectra of both
compounds, however, no obvious interpretation of ~28
could be found, probably due to overlap of yZ9. In
CDFS v2x was tentatively assigned to the bands at
330 (gauche) and 285 cm-’ (anti), the latter possibly
overlapping ~29 (gauche) and with a very uncertain ~29
(anti) at 279 cm-‘. More certain are the anti and
gauche modes of vjl) at 267 and 237 cm-‘, respectively, well supported by the calculations,
whereas
for bromomethyl
dimethyl
fluorosilane
[13] the
tentative assignment of vjo was contradicted by the
calculations.
The vibrational
modes ~3~--u35 were tentatively
attributed
to overlapping
anti and gauche bands
between 220 and 100 cm-’ observed in the infrared
and Raman spectra. Particularly
uncertain are the
methyl torsions, yX3 and yX4, tentatively attributed to
the weak, broad band around 132 cm-‘. Finally, the
CH2Cl torsional mode of both conformers,
vj6, is
Structure
445 (1998)
161-178
177
attributed to the infrared band around 70 cm-‘. The
corresponding Raman band in the liquid at 80 cm-’ is
clearly evident in the R(v) representation
[23].
While the assignments of the anti and gauche conformers of CDFS are mainly based upon the changes
in the infrared and Raman spectra on crystallization
and the results of the force constant calculations, the
intensity variations in the matrix spectra after annealing, including argon and nitrogen, support the conclusions. As is apparent from the arrows pointing
upwards and downwards in Table 1, in a number of
cases the anti bands increased and the gauche bands
decreased in intensities after annealing. However, in
other instances the intensity variations among the
matrix bands were probably caused by site effects
and could not be correlated with conformational
equilibria.
Acknowledgements
The authors are grateful to Mrs. Anne Horn for
valuable assistance. VS acknowledges a Norwegian
Government Scholarship under the Cultural Exchange
Programs.
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