Vibrational Spectroscopy 17 Ž1998. 1–30
The conformations of bromomethyl dimethyl silane and
bromomethyl dimethyl silane-d1 as studied by vibrational
spectroscopy and by ab initio calculations
Valdemaras Aleksa
a,1
, Peter Klaeboe a,) , Claus J. Nielsen a , Vinka Tanevska a ,
Gamil A. Guirgis b
a
b
Department of Chemistry, UniÕersity of Oslo, P.O. Box 1033, 0315 Oslo, Norway
Bayer, Bushy Park Plant, Research and DeÕelopment Department, Charleston, SC 29423-8088, USA
Received 12 December 1997; revised 7 April 1998; accepted 8 April 1998
Abstract
Bromomethyl dimethyl silane, CH 2 BrŽCH 3 . 2 SiH ŽBDS., and its deuterated analogue, CH 2 BrŽCH 3 . 2 SiD ŽBDSD., were
synthesised for the first time. Raman spectra of both liquids were recorded at various temperatures between 295 and 160 K
and spectra of the amorphous and crystalline solids were obtained. Infrared spectra were recorded in the vapour phase and in
the amorphous and crystalline states in the range 4000–60 cmy1. Additional infrared spectra of the compounds isolated in
argon and nitrogen matrices were recorded at 5 K before and after annealing to temperatures in the range 15–35 K. The
compounds exist in anti and gauche conformers due to restricted rotation of the CH 2 Br group. Raman temperature studies
0
Ž gauche–anti . of 0.6 " 0.3 kJ moly1 , anti being the low energy conformer in the liquid and in the
gave an average D Hconf
matrices. Two different crystals were observed for these molecules after careful annealing of the amorphous solid in the
range 110–150 K: one crystal ŽI. containing molecules in the gauche conformation and one crystal ŽII. with molecules in
the anti conformation. For the parent molecule, both of these crystals were repeatedly investigated in the infrared and in the
Raman spectra, while crystallisation was much more difficult to achieve for the deuterated compound. However, two
different crystals ŽI, III. both containing the gauche conformer, was observed in the Raman spectra and one with anti ŽII. in
the infrared spectrum of the deuterated compound. Ab initio calculations at the HFr6-311G) level gave vibrational
frequencies and Raman and infrared intensities for both conformers. Complete assignments were made for the anti and
gauche conformer spectra of both molecules. q 1998 Elsevier Science B.V. All rights reserved.
Keywords: Conformations; Infrared spectroscopy; Raman spectroscopy; Ab initio calculations; Matrix isolation; Crystal phases
1. Introduction
)
Corresponding author. Tel.: q47-22855678; Fax: q4722855441; E-mail: peter.klaboe@kjemi.uio.no
1
Permanent address. Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania.
Various silanes have been studied by Durig et al.
including vinyl silanes w1,2x and the halogenated
silanes: ethyl chlorosilane w3x, ethyldichlorosilane w4x
and ethyldifluorosilane w5x, all with conformational
0924-2031r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII: S 0 9 2 4 - 2 0 3 1 Ž 9 8 . 0 0 0 1 9 - 8
2
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
equilibria. The restricted rotation around the central
C–Si bond gives rise to anti and gauche conformers
in these molecules. A series of halomethyl dimethyl
halosilanes ŽCH 2 XŽCH 3 . 2 SiY; X s Cl, Br; Y s F,
Cl. is presently being investigated. They are analogous to the corresponding substituted ethanes and
should, due to restricted rotation of the –CH 2 X
group, exist as a mixture of an anti conformer with
C s symmetry and two spectroscopically equivalent
gauche conformers ŽFig. 1. with no symmetry ŽC 1 ..
It is our aim to investigate the infrared and Raman
spectra of these molecules and combine the results
with ab initio quantum chemical calculations to interpret the spectra and to evaluate their structure,
conformational energies and torsional barriers. It was
observed that in chloromethyl dimethyl fluorosilane
w6x and bromomethyl dimethyl fluorosilane w7x, the
gauche conformer had the lower energy in the liquid
and was also the one present in the crystals. In
chloromethyl dimethyl chlorosilane w8x and bromomethyl dimethyl chlorosilane w9x, anti was present in the crystals and was also the low energy
conformer. Thus, the conformational preferences in
these dihalogenated molecules is still an open issue.
Various saturated organic compounds containing
two silicon atoms w10x such as 1,2-dimethyl tetrachlorodisilane w11x and 1,1,2,2-tetrachlorodisilane
w12x or three silicon atoms w13x with conformational
equilibria have been investigated by Hassler et al.
The vibrational spectra of many molecules with Si–Si
bonds without conformers have also been studied by
McKean et al. w14,15x. It appears interesting to compare the vibrational spectra and the conformational
equilibria in halogenated ethanes having a C–C cen-
tral bond with the related molecules containing one
or two silicon atoms with Si–C or Si–Si central
bonds, respectively. It was therefore decided to extend this series to include the title compound, bromomethyl dimethyl silane, CH 2 BrŽCH 3 . 2 SiH Žto be
abbreviated BDS., and its deuterated analogue, bromomethyl dimethyl silane-d1 , CH 2 BrŽCH 3 . 2 SiD
ŽBDSD., containing only one halogen attached to
carbon.
It was observed that the dihalogenated molecules
of this series had very pronounced supercooling and
although their melting points were typically around
220 K, they could frequently be cooled to 160 K
without freezing. BDS had a very low melting point
below 145 K and two crystal forms were obtained by
annealing, one containing the gauche Žcrystal I., the
other the anti conformer Žcrystal II.. As is apparent
from the Raman and infrared spectra recorded, some
vibrational bands are present only in crystal I, others
in crystal II. In BDSD, the crystallisation was very
difficult to achieve, but after many attempts three
different crystals Žcrystals I, II and III. of which two
contained the gauche ŽI, III. and one contained the
anti conformer ŽII., were observed for this molecule.
An enthalpy difference of ca. 0.6 " 0.3 kJ moly1
was calculated as an average value from variable
temperature measurements of BDS and BDSD in the
liquid with anti being the low energy conformer. A
still lower enthalpy difference of ca. 0.2–0.4 kJ
moly1 was roughly estimated in the argon and nitrogen matrices since the equilibrium was shifted to the
anti conformer, but an appreciable concentration of
the gauche remained after annealing to ca. 35 K in
argon and 32 K in the nitrogen matrices.
Fig. 1. The anti and gauche conformers of bromomethyl dimethyl silane ŽBDS and BDSD..
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
3
2. Experimental
2.1. Sample preparation
The sample of BDS was prepared by reacting
bromomethyl dimethyl chlorosilane with lithium aluminumhydride in dry dibutylether at 273 K for 1 h.
For the deuterated species, BDSD, we used lithium
aluminium deuteride. The volatile material was collected using a vacuum system and purified with a
low pressure, low temperature distillation column.
The purity of the sample was checked by mass
spectrometry and no impurities were detected in any
of the compounds. Weak rotational bands of H–Br
could be detected in the far infrared vapour spectra.
2.2. Raman spectral measurements
Raman spectra of the liquid in two polarisation
directions were recorded at room temperature. Addi-
Fig. 2. Raman spectrum of bromomethyl dimethyl silane ŽBDS. as
a liquid at ambient temperature: curve ŽA., 3000–2000 cmy1 and
curve ŽB., 1500–100 cmy1 .
Fig. 3. Raman spectra of BDS Ž300–30 cmy1 . as Žfrom top to
bottom. a liquid at 295 K, and as amorphous, crystal I Žannealed
to 125 K. and crystal II Žannealed to 115 K. recorded at 80 K.
4
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
around 135 K and the spectra looked similar to those
of the amorphous phase containing bands from both
conformers. The two crystals of BDS were observed
many times in the infrared and Raman cryostats.
In BDSD, crystallisation was very difficult to
achieve and occurred only a few times. However, for
BDSD, a third crystal Žcrystal III. was identified in
the Raman spectra, containing the same conformer as
crystal I Ž gauche ., but having shifts and splittings
different from crystal I, particularly in the lattice
region below 100 cmy1 . The former two crystals
were by chance only observed in the Raman cryostat,
crystal II in the cryostat used in the mid-infrared
ŽMIR. region, whereas no crystals were ever formed
in the far-infrared ŽFIR. cryostat.
The Raman spectra were obtained using a Dilor
RTI-30 spectrometer Žsingle channel, triple monochromator. and recorded digitally. An argon ion laser
from Spectra Physics Žmodel 2000. was employed
using the 514.5 nm line with 908 excitation.
Fig. 4. Raman spectra of BDS Ž800–500 cmy1 . as a liquid,
amorphous, crystal I and crystal II Žfrom top to bottom.; same
annealing as for Fig. 3.
tional spectra were obtained at seven temperatures
between 298 and 163 K in a capillary tube of 2-mm
inner diameter surrounded by a Dewar, cooled by
gaseous nitrogen evaporated from a reservoir w16x.
0
From these spectra, the enthalpy difference, D Hconf
,
in the liquid between the conformers were calculated. Because of the low melting point of BDS and
BDSD, they did not crystallise using cold nitrogen
vapour as a refrigerant, reaching ca. 140 K w16x.
Instead, the compounds were condensed on a copper
finger at 80 K, forming an amorphous solid. A
number of annealing experiments were carried out,
giving two different crystals Žcrystals I and II. containing the gauche and anti conformers, respectively.
By annealing the amorphous solid formed at 80
K, crystal II containing the anti conformer first
appeared at ca. 115 K. At slightly higher temperatures, the gauche conformer was formed at ca. 125
K. After further annealing, the anti bands returned
Fig. 5. Raman spectra of BDS in the Si–H stretching range as
crystal I Žsolid line. and crystal II Ždashed line.; same annealing as
for Fig. 3.
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
Table 1
Infrared and Raman spectral data for bromomethyl dimethyl silane ŽBDS.
5
6
Table 1 Žcontinued.
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
7
Table 1 Žcontinued.
a
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; P, polarized; D, depolarized.
A, B and C denote vapor contours. Asterisks denote bands vanishing in the crystal spectra. Arrows pointing upwards and downwards signify
matrix bands which increase and decrease in intensities after annealing.
2.3. Infrared spectral measurements
The infrared spectra of BDS and BDSD were
recorded on a number of Fourier transform spectrometers: a Bruker spectrometer IFS-88 Ž4000–450
cmy1 ., a Nicolet model 800 Ž4000–450 cmy1 ., a
Perkin–Elmer model 2000 Ž4000–450 cmy1 . and on
Bruker model IFS-113v and Bomem model DA 3.002
vacuum spectrometers in the FIR Ž600–50 cmy1 ..
Except for the latter spectrometer which had a helium-cooled silicon bolometer, the instruments had
deuterated triglycine sulphate detectors ŽDTGS..
Beamsplitters of Ge substrate on KBr were em-
ployed in the MIR region, and beamsplitters with
3.5, 6.25 and 12 m thickness of Mylar and a metal
mesh beamsplitter were used in the FIR region.
The vapour was studied in cells with KBr Ž10 cm.
and polyethylene windows Ž20 cm and 1 m.. The
vapours were deposited on a CsI window in the MIR
region and on a wedge-shaped window of silicon in
the FIR region, both cryostats were cooled with
liquid nitrogen. As observed in the Raman cryostat,
two crystalline solids Žcrystals I and II. were observed in the infrared spectroscopy ŽIR. cryostat
after annealing, but at slightly different temperatures
at the CsI window as compared to the copper finger.
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
8
Fig. 6. Infrared spectrum Ž3950–400 cmy1 . of BDS as a vapour: pressure 19 Torr, 10 cm pathlength, resolution, 1 cmy1 .
In the case of BDSD, crystals I and III were only
obtained in the Raman cryostat, and crystal II in the
MIR cryostat, while the amorphous solid remained in
the FIR cryostat in spite of many attempts at crystallisation.
The sample of BDS was diluted with argon or
nitrogen Ž1:1000. whereas BDSD was observed in
argon only, due to the small amount of sample
available. It was deposited on a CsI window at either
5 or 15 K of a Displex cryostat from APD Žmodel
HS-4. with a three-stage cooling system. Cooling
was provided by two HC-4MK1 compressor modules, and the temperatures were controlled by a Lake
Shore model 330 controller using Si diode sensors.
The spectra of the unannealed matrices were first
recorded and subsequently annealed to temperatures
in the 30–39 K range for the argon, and 28–34 K for
the nitrogen matrices and recooled to 5 K before
recording.
finger of copper kept at 80 K in a Raman cryostat
cooled by liquid nitrogen. The solid deposit appeared
glassy and the Raman spectrum was quite similar to
3. Results for bromomethyl dimethyl silane (BDS)
3.1. Raman spectral results
A Raman spectra of BDS as a liquid at ambient
temperature is given in Fig. 2a for the region 3000–
1450 cmy1 and in Fig. 2b for the region below 1500
cmy1 . The vapour of BDS was condensed on a
Fig. 7. Infrared spectrum Ž2180–2110 cmy1 . of BDS as a vapour:
pressure 19 Torr, 10 cm pathlength, resolution, 1 cmy1 .
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
9
Fig. 8. Infrared spectrum of BDS Ž370–40 cmy1 . as a vapour: 25 Torr, 1 m pathlength, resolution, 0.1 cmy1 .
that of the liquid. This solid was gradually annealed
in order to facilitate crystallisation. Eventual changes
were also observed visually when the solid changed
from a glassy to a frosty look. Above 100 K, the
deposit changed appearance and the Raman spectrum
revealed large changes as compared to that of the
amorphous solid. It was immediately apparent that
the spectrum of the crystal formed Žcrystal II. was
different from that of the amorphous solid since a
number of the bands present in the amorphous phase
disappeared, revealing that only one conformer was
present. After further annealing a few degrees, large
changes occurred in the regions for the internal
modes and in the lattice modes below 120 cmy1 .
The spectrum of this solid Žcrystal I. was quite
different from that of crystal II. It was obvious that
not only a new crystal was formed, but this crystal
also contained a different conformer than crystal II.
The Raman spectra of the liquid at ambient temperature, of the amorphous solid at 80 K, of crystal
II annealed to ca. 115 K and crystal I annealed to ca.
125 K are given in the ranges 300–30 cmy1 and
800–500 cmy1 in Figs. 3 and 4, respectively. Additional Raman spectra comparing crystals I and II are
shown in the range 2170–2100 cmy1 , giving the
Si–H stretching modes for the anti and gauche
conformers ŽFig. 5..
Raman spectra were recorded of the liquid at
seven temperatures between 299 and 160 K and the
Fig. 9. Infrared spectra of BDS in the Si–H stretching region as
Žfrom bottom to top. an amorphous solid, crystal I Žannealed to
130 K. and crystal II Žannealed to 120 K. recorded at 80 K.
10
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
intensities of the band pairs 550r530 and 644r638
cmy1 were employed in van’t Hoff plots. They gave
0
Ž gauche–anti . values 0.4 " 0.2 and 1.0
the D Hconf
" 0.5 kJ moly1 , respectively, when peak heights
were used. Since the latter bands overlapped strongly,
the former value was considered to be the more
reliable. An independent determination of the deuterated molecule BDSD gave a value of 0.7 " 0.2 kJ
moly1 Žsee below., and an average value for all the
data was 0.6 " 0.3 kJ moly1 . The experimental values from the infrared and Raman spectra and the
vibrational assignments are collected in Table 1.
3.2. Infrared spectral results
An infrared vapour spectrum of BDS in the region
3950–400 cmy1 recorded at 19 Torr pressure in a
10-cm cell is presented in Fig. 6 and the Si–H
stretching bands for the gauche and anti conformers
under high dispersion with peaks at 2156 and 2127
Fig. 11. Infrared spectra of BDS Ž650–100 cmy1 . as an amorphous solid, crystal II Žannealed to 115 K. and crystal I Žannealed
to 130 K. Žbottom to top., recorded at 80 K.
Fig. 10. Infrared spectra of BDS Ž950–450 cmy1 . as an amorphous solid, crystal II and crystal I Žbottom to top.; same annealing as for Fig. 9.
cmy1 , respectively, are given in Fig. 7. An FIR
spectra from 370 to 40 cmy1 obtained in a 1-m cell
with 25 Torr pressure is presented in Fig. 8. A few
of the vapour bands had resolved rotational contours:
2972 and 2127 cmy1 Ž A contour. and 2951, 2946
and 2156 cmy1 Ž C contour.. However, most of the
vapour bands had indefinite contours; moreover, what
appeared to be rotational fine structure was in some
cases believed to be overlapping anti and gauche
conformer bands.
Infrared spectra of the amorphous phase and of
the two crystals I and II were obtained in agreement
with the recordings carried out in the Raman spectra.
The amorphous phase was formed when the vapour
was deposited on the CsI ŽMIR region. or the silicon
window ŽFIR region. at 80 K and the spectra recorded
in the two regions. The annealing temperatures necessary to obtain crystallisation were slightly higher
than in the Raman cryostat and were effected at ca.
120 K for crystal II and at ca. 130 K for crystal I. At
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
11
Fig. 12. Infrared spectra Ž825–600 cmy1 . of BDS in an argon matrix Ž1:1000. at 5 K, deposited at 15 K, unannealed, annealed to 28
and 35 K Žbottom to top..
still higher temperatures, the sample melted at ca.
140 K and the spectra were quite similar to the
amorphous phase. It was possible to rotate the
cryostats in a horizontal position to prevent the
liquid dropping from the window. After subsequent
cooling with liquid nitrogen, an amorphous solid was
again formed at 80 K and a new cycle of annealing
experiments giving crystals II and I could be
achieved. Thus, the phase changes could be observed
by annealing an amorphous solid formed simultaneously by condensing a gas or by freezing a liquid.
The infrared spectra of the amorphous phase and
of crystals I and II are presented in Fig. 9, giving the
Si–H region around 2140 cmy1 , in Fig. 10 giving
Fig. 13. Infrared spectra Ž725–500 cmy1 . of BDS in a nitrogen matrix Ž1:1000. at 5 K, deposited at 15 K, unannealed, annealed to 28 and
31 K Žbottom to top..
12
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
recorded in the MIR region immediately after the
vapour mixture was deposited on the CsI window.
Subsequently, the matrices were annealed in small
steps of 2–58, recooled to 5 K and the spectra were
again recorded.
If the barrier height to conformational equilibrium
is high enough to prevent conversion, 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. Fairly small spectral changes
occurred in the argon or nitrogen matrices when the
samples were annealed to temperatures below 20 K,
indicating negligible site effects for BDS isolated in
the matrices. At higher annealing temperatures of ca.
31 K in nitrogen and 35 K in argon matrices, larger
spectral changes were observed which were correlated with conformational changes in the molecule.
The spectral ranges 825–600 cmy1 for BDS in
argon and 725–500 cmy1 in nitrogen matrices are
presented in Figs. 12 and 13.
Fig. 14. Infrared spectra of BDS Ž2200–2100 cmy1 . in argon
matrix Ž1:1000. at 5 K, unannealed Žbottom. and annealed Žtop. to
34 K.
the region 950–450 cmy1 and in Fig. 11, showing
the FIR region 650–100 cmy1 . These spectra supplement those obtained in the Raman effect and
confirm the presence of two conformers in the amorphous phase and one conformer in each of crystals I
and II. The crystals obtained in the infrared cryostats
were not as ‘pure’ as those observed in the Raman
experiments and it appears from the spectra that each
of the crystalline samples was slightly contaminated
with the amorphous solid or with the other crystals.
The infrared bands of BDS in matrices at 5 K are
usually very sharp and frequently separate rotamer
bands can be detected which overlap in the vapour,
amorphous and liquid phases. The sample was mixed
with argon or nitrogen and the gas mixtures Ž1:1000.
were deposited at 5 or at 15 K. The quality of the
matrix spectra is often superior when deposited at 15
K since with 5 K deposition, a strong scattering of
the matrix may lead to an intense background. First,
the infrared spectra of matrix-isolated BDS were
Fig. 15. Infrared spectra of BDS Ž2200–2100 cmy1 . in nitrogen
matrix Ž1:1000. at 5 K, unannealed Žbottom. and annealed Žtop. to
31 K.
A
X
A
X
A
X
A
Y
A
X
A
Y
A
X
A
Y
A
X
A
X
A
Y
A
X
A
Y
A
X
A
X
A
X
A
Y
A
X
A
Y
A
X
A
Y
A
X
A
X
A
X
A
Y
A
Y
A
X
A
X
A
Y
A
X
A
Y
A
Y
A
X
A
X
A
Y
A
Y
Species
597
495
307
220
215
161
154
118
68
727
689
677
651
610
2976
2923
2921
2919
2908
2907
2853
2852
2058
1441
1439
1432
1429
1414
1300
1294
1165
1069
894
887
851
799
a
ncalc
scaled
0.8
12
12
3
0.3
0.0002
1
1
1
44
0.8
11
1
10
4
26
20
5
35
10
6
15
216
6
10
1
0.2
1.7
20
43
14
6
159
245
45
39
I IR
intensity b
16
29
1
2
1
0.07
0.03
0.6
1
6
10
6
3
2
72
105
98
26
140
42
235
3
178
0.8
12
0.1
0.7
0.3
3
0.1
2
4
7
4
1
0.5
IR
intensity c
0.36
0.13
0.60
0.72
0.75
0.26
0.75
0.62
0.75
0.74
0.36
0.75
0.75
0.75
0.75
0.13
0.73
0.75
0.74
0.75
0.01
0.75
0.2
0.6
0.74
0.75
0.75
0.70
0.003
0.75
0.74
0.75
0.75
0.73
0.6
0.75
Polarization
602
533
289
212
212
134
121 f
108
73
742
718
666
647
625
2972
2972
2946
2946
2914
2914
2851
2851
2127
1428
1428
1410
1410
1393
1263
1263
1134
1056
892
885
843
802
d
nobs
vw
w
m
vw
vw
w
w
w
w
w
w
vw
m
vw
vs
vs
m
m
m
m
w
w
vs
w
w
w
w
m
vw
s
m
m
vs
vs
s
m
I IR
vw
vs
m
m
s
vw
w
m
vw
w
s
vw
s
vw
m
m
s
s
vs
vs
vw
vw
vs
vw
vw
w
w
w
vw
m
w
w
w
w
vw
vw
IR
P
P
D
D
P
P
P
D
D
P
D
P
P
D
D
D
P
P
P
Polarization
b
Calculated at the HFr6-311G) level in Gaussian-94, and scaled with factors of 0.9 for bands above 400 cm y1 , and 1.0 for fundamentals below 400 cmy1 .
Calculated infrared intensities Žkm moly1 ..
c
˚ 4 moly1 ..
Calculated Raman cross-sections ŽA
d
From infrared spectra of the vapor, except when noted.
e
Contributions of less than 10% are omitted.
f
Raman liquid.
a
n 28
n 29
n 30
n 31
n 32
n 33
n 34
n 35
n 36
n 23
n 24
n 25
n 26
n 27
n1
n2
n3
n4
n5
n6
n7
n8
n9
n 10
n 11
n 12
n 13
n 14
n 15
n 16
n 17
n 18
n 19
n 20
n 21
n 22
Vibration
number
Table 2
Calculated and observed fundamentals of the anti conformer of bromomethyl dimethyl silane ŽBDS .
S22 Ž100 .
S5 Ž97 .
S8 Ž68 ., S7 Ž28 .
S25 Ž64 ., S24 Ž35 .
S7 Ž70 ., S8 Ž30 .
S24 Ž65 ., S25 Ž35 .
S6 Ž99 .
S23 Ž99 .
S3 Ž100 .
S19 Ž92 .
S18 Ž92 .
S33 Ž92 .
S34 Ž92 .
S13 Ž92 .
S15 Ž95 .
S30 Ž95 .
S14 Ž91 .
S28 Ž95 .
S26 Ž57 ., S31 Ž12., S32Ž11.
S17 Ž41 ., S10 Ž40.
S16 Ž79 .
S32 Ž39 ., S29 Ž29 ., S1 Ž11 .,
S31 Ž11 .
S10 Ž40 ., S17 Ž29 ., S2 Ž13 .
S2 Ž51 ., S4 Ž24 ., S10 Ž14 .
S21 Ž71 ., S31 Ž21 .
S29 Ž41 ., S31 Ž32 .
S26 Ž36 ., S32 Ž32 ., S29 Ž12 .,
S31 Ž11 .
S1 Ž74 ., S4 Ž11 .
S4 Ž43 ., S12 Ž20 ., S1 Ž11 .
S9 Ž43 ., S12 Ž19 ., S17 Ž14 .
S11 Ž64 ., S16 Ž22 .
S27 Ž72 .
S20 Ž93 .
S36 Ž97 .
S12 Ž55 ., S13 Ž25 ., S9 Ž10 .
S35 Ž88 ., S36 Ž10 .
PED e
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
13
a
ncalc
2973
2925
2920
2908
2905
2903
2851
2847
2091
1442
1437
1432
1430
1415
1300
1293
1165
1071
891
882
849
799
753
689
670
623
607
597
523
253
241
197
166
154
124
69
Vibration
number
n1
n2
n3
n4
n5
n6
n7
n8
n9
n 10
n 11
n 12
n 13
n 14
n 15
n 16
n 17
n 18
n 19
n 20
n 21
n 22
n 23
n 24
n 25
n 26
n 27
n 28
n 29
n 30
n 31
n 32
n 33
n 34
n 35
n 36
1
0.1
0.03
1
1
33
4
4
4
14
10
12
11
24
5
16
19
24
46
1
11
13
156
11
4
3
0.2
1.5
22
39
12
3
140
222
64
31
I IR
intensity b
3
0.1
0.07
0.2
0.9
28
2
1
12
13
2
7
3
4
68
69
98
100
153
18
151
92
118
0.5
17
0.8
17
10
3
0.3
3
6
5
3
1
2
IR
intensity c
0.54
0.34
0.65
0.68
0.75
0.17
0.36
0.63
0.41
0.67
0.38
0.20
0.27
0.46
0.74
0.74
0.13
0.73
0.71
0.73
0.01
0.001
0.18
0.70
0.74
0.74
0.74
0.67
0.007
0.33
0.72
0.74
0.75
0.64
0.55
0.72
Polarization
197
134
121 f
108
73
558
250
231
718
651
643
613
609
769
2972
2972
2946
2946
2914
2914
2851
2851
2156
1428
1428
1410
1410
1393
1263
1263
1134
1056
892
885
843
802
d
nobs
Table 3
Calculated and observed fundamentals of the gauche conformer of bromomethyl dimethyl silane ŽBDS .
vw
w
w
w
w
s
w
m
w
m
m
w
w
w
vs
vs
m
m
m
m
w
w
vs
w
w
w
w
m
m
s
m
m
vs
vs
s
m
I IR
s
w
m
w
vw
vs
m
m
s
s
s
vw
vvw
m
m
w
w
w
w
vw
vw
m
m
s
s
vs
vs
vw
vw
vs
vw
vw
w
w
w
IR
P
P
P
P
P
P
P
P
P
D
D
P
D
P
P
D
D
D
P
P
D
Polarization
S22 Ž100 .
S7 Ž49 ., S24 Ž48 .
S5 Ž99 .
S24 Ž47 ., S7 Ž45 .
S8 Ž93 .
S25 Ž98 .
S6 Ž63 ., S23 Ž34 .
S23 Ž64 ., S6 Ž35 .
S3 Ž100 .
S19 Ž94 .
S18 Ž93 .
S33 Ž83 .
S34 Ž87 .
S13 Ž91 .
S15 Ž94 .
S30 Ž93 .
S14 Ž91 .
S28 Ž95 .
S26ŽŽ53., S31Ž11., S32Ž11.
S17Ž34., S10Ž36., S26Ž8 .
S16Ž76 .
S29Ž34., S10Ž19., S17Ž13.,
S32 Ž12 .
S32Ž29., S31Ž18., S10Ž13.,
S21 Ž12 .
S2 Ž55 ., S4 ŽŽ 19 .
S21 Ž68 ., S31Ž12., S2 Ž10 .
S31 Ž32 ., S26 Ž30 ., S32 Ž13 .
S1 Ž57 ., S4 Ž14 ., S2 Ž10 .
S29Ž33., S10Ž15., S32Ž10.
S0?
S4 Ž40 ., S12 Ž18 ., S1 Ž10 .
S9 Ž52 ., S11 Ž11 ., S16 Ž11 .
S27 Ž22 ., S12Ž16., S9 Ž16 .,
S11 Ž15 .
S11Ž54., S27Ž15., S16 Ž12 .
S20 Ž83 .
S36 Ž94 .
S12Ž49., S13Ž22., S27Ž22.
S35 Ž92 .
PED e
14
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
It can be seen that the infrared bands at 770, 653,
643 and 555 cmy1 were reduced in argon matrices
as compared to those at 737, 667, 645, 620 and 533
cmy1 after annealing to 35 K. Similar intensity
changes occurred in the nitrogen matrices after annealing to 31 K. As compared with the infrared and
Raman spectra of the crystals, it can immediately be
concluded that the bands being reduced in intensities
in the matrices after annealing were present in crystal I while the enhanced bands are present in crystal
II. In both matrices, the gauche conformer apparently has a higher energy than anti in the matrices at
5 or 15 K and the gauche conformer partly converted to anti after annealing. In chloromethyl
dimethyl fluorosilane w6x, bromomethyl dimethyl fluorosilane w8x and dichloromethyl methyldifluoro
silane w17x, the gauche bands vanished after annealing, but in BDS, they were merely reduced in intensity. This suggests that the energy difference between the anti and gauche conformers is very low
for BDS and an equilibrium is maintained in both
matrices at a temperature as low as 35 and 31 K.
It cannot be ruled out that the intensity changes
after annealing might be caused by a redistribution
of the band intensities due to site splitting, rather
than a conformational change. However, Figs. 14
and 15 for BDS and Fig. 21 for BDSD suggest a
surprising shift of the S–H stretch in argon and
nitrogen matrices.
0
values in the matrices are not known,
The D Hconf
since the present matrix spectra were not recorded
with a heatable nozzle which can reveal the enthalpy
difference by plotting intensities of gaucher anti
0
band pairs in van’t Hoff plots. The D Hconf
should be
still lower than the value of 0.6 kJ moly1 obtained
from the liquid. At an appropriate annealing temperature, the gauche conformer has sufficient energy to
pass the barrier and convert to anti. The lowest
15
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. This temperature was ca. 35
K for the argon and 31 K for the nitrogen matrix.
From the curves correlating annealing temperature
and activation energyrbarrier height given by Barnes
w18x, the conformational barrier was estimated to be
ca. 8–9 kJ moly1 . In Table 1, the infrared and
Raman bands vanishing in crystals I and II are
equipped with asterisks, and the infrared bands observed in argon and nitrogen matrices are fitted with
arrows pointing upwards or downwards if the bands
increase or decrease, respectively, in intensity after
annealing.
3.3. Quantum chemical calculations
Hartree Fock quantum chemical calculations at
the HFr6-311G) level of approximation were performed using the Gaussian-94 program w19x. The
minima on the potential surface were found by relaxing the geometry, and the bond distances and angles
for both the anti and gauche conformers were calculated. The geometrical parameters calculated with the
basis set 6-311G) are very similar to those of
bromomethyl dimethyl fluorosilane w7x and they have
not been presented for the sake of brevity.
The calculations for BDS favour anti as the low
energy conformer in agreement with the results w20x
for the other CH 2 XŽCH 3 . 2 SiY molecules studied.
The conformational energy difference from the basis
set 6-311G), employed for comparing the series of
molecules CH 2 X-ŽCH 3 . 2 SiY w20x, was 1.3 kJ moly1 .
The difference between the calculated thermal corrections to the enthalpy for the gauche and anti
conformations was negligible.
Notes to Table 3:
a
Calculated at the HFr6-311G) level in Gaussian-94, and scaled with factors of 0.9 for bands above 400 cmy1 , and 1.0 for fundamentals
below 400 cmy1 .
b
Calculated infrared intensities Žkm moly1 ..
c
˚ 4 moly1 ..
Calculated Raman cross-sections ŽA
d
From infrared spectra of the vapor, except when noted.
e
Contributions of less than 10% are omitted.
f
Raman liquid.
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
16
3.4. Normal coordinate calculations
Analytical HF force constants were derived for
each of the two conformers in BDS using the 6311G) basis set. The calculated ab initio force
constants were 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.
Reasonably good agreement between the experimen-
tal 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 cmy1 . The infrared intensities, Raman scattering
cross-sections and Raman polarisation ratios were
calculated and these data are listed in Tables 2 and 3
for the anti and gauche conformers, respectively.
The potential energy distribution ŽPED. given in
Tables 2 and 3 is expressed in terms of the symmetry
coordinates. The normalised symmetry coordinates
ŽTable 4. have been constructed from a set of valence coordinates ŽFig. 16.. Only PED terms larger
than 10% have been included in Tables 2 and 3. The
Table 4
Symmetry coordinates for bromomethyl dimethyl silane ŽBDS and BDSD.
X
A
Y
A
a
1
2
3
4
5
6
7
8
9
10
11
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
Si–C symmetric stretch
Si–C antisymmetric stretch
Si–H ŽD. stretch
C–Br stretch
CH 2 symmetric stretch
CH 3 symmetric stretch
CH 3 antisymmetric stretch
CH 3 antisymmetric stretch
Symmetric C–Si–C bend
Antisymmetric C–Si–C bend
Antisymmetric C–Si–C bend
Si–C–Br bend
CH 2 scissor
CH 2 wag
CH 3 symmetric deformation
CH 3 antisymmetric deformation
CH 3 antisymmetric deformation
CH 3 antisymmetric deformation
CH 3 antisymmetric deformation
CH 3 torsion
Si–C antisymmetric stretch
CH 2 antisymmetric stretch
CH 3 symmetric stretch
CH 3 antisymmetric stretch
CH 3 antisymmetric stretch
C–Si–H ŽD. deformation
C–Si–C deformation
CH 2 twist
CH 2 rock
CH 3 symmetric deformation
CH 3 antisymmetric deformation
CH 3 antisymmetric deformation
CH 3 antisymmetric deformation
CH 3 antisymmetric deformation
CH 2 Br torsion
CH 3 torsion
For meaning of valence coordinates, see Fig. 16.
S1 s 3y1r2 Ž R1 q R 2 q R 3 . a
S2 s 6y1r2 Ž2 R1 y R 2 y R 3 .
S3 s S
S4 s T
S5 s 2y1r2 Ž S7 q S8 .
S6 s 6y1r2 Ž d1 q d 2 q d 3 q d 4 q d 5 q d 6 .
S7 s 12y1r2 Ž2 d1 y d 2 y d 3 q 2 d 4 y d 5 y d 6 .
S8 s Ž1r2.Ž d 2 y d 3 q d 5 y d 6 .
S9 s 6y1r2 Ž J 1 q J 2 q J 3 y Q 1 y Q 2 y Q 3 .
S10 s 6y1r2 Ž J 1 y J 2 y J 3 .
S11 s 6y1r2 ŽQ 1 y Q 2 y Q 3 .
S12 s V
S13 s Ž1r2.Žg 1 q g 2 q u 1 q u 2 .
S14 s Ž1r2.Žg 1 q g 2 y u 1 y u 2 .
S15 s 12y1r2 Ž b 1 q b 2 q b 3 y a 1 y a 2 y a 3 q b4 q b5 q b6 y a 4 y a 5 y a 6 .
S16 s 12y1r2 Ž2 b 1 y b 2 y b 3 q 2 b4 y b5 y b6 .
S17 s Ž1r2.Ž b 2 y b 3 q b5 y b6 .
S18 s 12y1r2 Ž2 a 1 y a 2 y a 3 q 2 a 4 y a 5 y a 6 .
S19 s Ž1r2.Ž a 2 y a 3 q a 5 y a 6 .
S20 s 2y1r2 Žt 2 q t 3 .
S21 s 2y1r2 Ž R 2 q R 3 .
S22 s 2y1r2 Ž S7 y S8 .
S23 s 6y1r2 Ž d1 q d 2 q d 3 y d 4 y d 5 y d 6 .
S24 s 12y1r2 Ž2 d1 y d 2 y d 3 y 2 d 4 q d 5 q d 6 .
S25 s Ž1r2.Ž d 2 y d 3 y d 5 q d 6 .
S26 s 2y1r2 Ž J 2 y J 3 .
S27 s 6y1r2 ŽQ 2 y Q 3 .
S28 s Ž1r2.Žg 1 y g 2 y u 1 q u 2 .
S29 s Ž1r2.Žg 1 y g 2 q u 1 y u 2 .
S30 s 12y1r2 Ž b 1 q b 2 q b 3 y a 1 y a 2 y a 3 y b4 y b5 y b6 q a 4 q a 5 q a 6 .
S31 s 12y1r2 Ž2 b 1 y b 2 y b 3 y 2 b4 q b5 q b6 .
S32 s Ž1r2.Ž b 2 y b 3 y b5 q b6 .
S33 s 12y1r2 Ž2 a 1 y a 2 y a 3 y 2 a 4 q a 5 q a 6 .
S34 s Ž1r2.Ž a 2 y a 3 y a 5 q a 6 .
S35 s t 1
S36 s 2y1r2 Žt 2 y t 3 .
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
17
4. Results for bromomethyl dimethyl silane-d 1
(BDSD)
4.1. Raman spectral results
Fig. 16. Valence coordinates of BDS and BDSD.
C–H, Si–H, C–Br and Si–F stretching modes are
reasonably well-localised, but the CH 3 rock, C–C
stretches and the skeletal deformations are highly
mixed. Obviously, the vibrational modes for the anti
conformer, separated into symmetry species AX and
AY are more localised than those of the gauche
conformer in which all the modes belong to the same
species.
For BDSD, the calculated fundamental modes of
vibration are collected and correlated with the experimental wavenumbers in Tables 6 and 7 for the anti
and gauche conformers, respectively.
The sample of BDSD was synthesised and the
spectra recorded in order to support the results for
the parent molecule ŽBDS.. A fairly small sample of
BDSD was available, restricting the number of independent experiments which could be carried out. The
isotopic purity was very good since the Si–H stretching region for the two conformers of BDS at 2160–
2120 cmy1 had no bands in the spectra of BDSD.
As predicted from the normal coordinate calculations, the calculated wavenumbers were nearly identical between BDS and BDSD in the regions 3000–
2100 cmy1 , 1500–1100 cmy1 and 400–60 cmy1 .
The observed infrared and Raman spectra of the
isotopomers were also nearly identical. Clear-cut
changes occur in the Si–H ŽD. stretching region
which is shifted from ca. 2150 cmy1 in BDS to 1550
cmy1 in BDSD. Significant wavenumber shifts between the corresponding modes of BDS and BDSD
were also observed Žand calculated. in the range
950–450 cmy1 since many modes in this region
involve Si–H ŽD. bending. This is demonstrated by
the Raman spectra of BDS and BDSD as liquids in
the range 750–450 cmy1 ŽFig. 17..
Fig. 17. Raman spectra Ž750–100 cmy1 . of BDS Žsolid line. and BDSD Ždashed line. as liquids at ambient temperature.
18
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
As mentioned above, it was more difficult to
crystallise BDSD than BDS. In the Raman cryostat,
crystal I was obtained containing the gauche conformer, but no crystal II containing the anti conformer was ever observed in spite of many attempts.
The close similarities in the lattice region below 120
cmy1 between the Raman spectra of BDS and BDSD
reveal without doubt that these crystals are the same
Žcrystal I.. However, still another crystal was observed in the Raman cryostat of BDSD not seen in
the spectra of BDS. As is apparent from Fig. 18, the
two bottom Raman curves of BDSD are both spectra
of crystals containing one conformer Ž gauche . since
the anti bands at 729, 606 and 478 cmy1 Žliquid. are
absent. It is clearly seen that in crystal I, there is a
doublet at 571 and 564 cmy1 while in crystal III,
there is a doublet at 518 and 511 cmy1 . Various
frequency shifts between the Raman spectra of BDSD
in crystals I and III were observed ŽTable 5., but
there is no doubt that both crystals contain the
gauche conformer. This crystal III of BDSD was not
detected in any of the infrared cryostats with CsI or
silicon windows.
The enthalpy difference between the conformers
were calculated from the band pair 606r573 cmy1
recorded at five temperatures between 298 and 163
K, representing an anti r gauche pair since the former band disappears in the Raman spectra of crystal
0
I ŽTable 5.. From the van’t Hoff plot, a D Hconf
y1
Ž gauche–anti . equal to 0.7 " 0.2 kJ mol
was
obtained in reasonable agreement with the results for
BDS.
4.2. Infrared spectral results
Infrared spectra of BDSD as a vapour were
recorded in the MIR and FIR regions 4000–50 cmy1 .
Apart from the Si–H ŽD. stretch, the significant
changes from the IR vapour spectra of BDS occurred
in the range 950–450 cmy1 . A vapour spectrum of
BDSD covering the region 1600–430 cmy1 is given
in Fig. 19.
Only one crystalline phase of BDSD was obtained
in spite of attempts to record both crystals I and II.
Spectra of the amorphous and one crystal are presented in Fig. 20 and the crystal formed is definitely
different from that investigated in the Raman cryostat and should therefore be crystal II. This is clearly
demonstrated by comparing the spectra of BDSD
with those of BDS in the regions where the spectra
are nearly identical. The bands which vanish after
crystallisation belong to the gauche conformer.
Attempts were made to crystallise BDSD on the
silicon window of the FIR cryostat without success.
No crystalline solids Žcrystals I, II or III. were
formed and the spectra recorded in this region were
Fig. 18. Raman spectra of BDSD Ž750–450 cmy1 . as an amorphous solid, as crystal I Žannealed to 124 K. and crystal III Žannealed to
134 K. Žtop to bottom., recorded at 80 K.
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
Table 5
Infrared and Raman spectral data for bromomethyl dimethyl silane-d1 ŽBDSD.
19
20
Table 5 Žcontinued.
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
21
Table 5 Žcontinued.
a
Abbreviations: s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; P, polarized; D, depolarized.
A, B and C denote vapor contours. Asterisks denote bands vanishing in the crystal spectra. Arrows pointing upwards and downwards signify
matrix bands which increase and decrease in intensities after annealing.
all from the amorphous solid. Obviously, the experimental data for this compound were not as complete
as for the parent compound BDS. Therefore, the
spectra of the deuterated molecule did not support
the interpretation of the parent molecule as much as
expected.
Matrix spectra of BDSD were restricted to argon
and no nitrogen matrix spectra were recorded. As
observed in the spectra of BDS, a number of bands
in the argon matrix spectra of BDSD changed intensities after annealing to ca. 35 K. Thus, the bands at
1257, 1137, 846, 833, 803, 775, 610, 573, 508 and
22
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
Fig. 19. Infrared spectrum of bromomethyl dimethyl silane-d1 ŽBDSD. Ž1600–450 cmy1 . as a vapour: pressure 25 Torr, 10 cm path,
respectively, 1 cmy1 .
478 cmy1 were reduced in intensities as compared
to the bands at 1253, 1134, 848, 828, 819, 781, 738,
604 and 482 cmy1 . In most cases, the diminishing
bands were the same as those found in crystal I
attributed to the gauche, while the enhanced bands
were observed in crystal II and belonged to the anti
conformer. The conformer bands at 1255, 1135 and
847 cmy1 coincided in the condensed states, but
they appeared as separate anti and gauche bands in
the matrix spectra. The matrix bands at 1556 and
1541 cmy1 assigned to the Si–D stretching modes
behaved exactly as those in the argon matrix spectra
Fig. 20. Infrared spectra of BDSD as an amorphous solid Žsolid line. and as crystal II Žannealed to 120 K. Ždashed line..
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
Fig. 21. Infrared spectra of BDSD in an argon matrix at 5 K in the
Si–D stretching region, unannealed Žsolid line. and annealed to
34 K Ždashed line..
of BDS. We therefore ascribe the 1564r1556 cmy1
doublet to the anti and the 1551r1541r1538 cmy1
triplet to the gauche conformer ŽFig. 21. for spectral
consistency with BDS ŽTable 5..
The anti bands were enhanced and the gauche
bands reduced in intensities after annealing, suggesting that an equilibrium was achieved at the annealing
temperature of ca. 34 K. From a simple calculation
assuming a concentration ratio of anti r gauche of
0
Ž gauche–anti . was esti2–3 at 35 K, the D Hconf
y1
mated to be 0.4 kJ mol in the matrices, a slightly
lower value than what was found in the liquid from
the van’t Hoff plots.
5. Discussion
5.1. Conformers
The essential question remains: do the infrared
and Raman bands present in crystal I and II belong
23
to the gauche or the anti conformer, respectively, or
vice versa? Neither the infrared vapour contours,
Raman polarisation data nor the ab initio calculated
energies can answer this question unambiguously.
The infrared vapour bands had poorly defined contours and those present could be due to neighbouring
conformer bands. The Raman spectra of the liquid
frequently consisted of overlapping rotamer bands,
making the polarisation ratios of little use. The energies for the anti and gauche conformers derived
0
from the ab initio calculations revealed a D Hconf
y1
Ž gauche–anti . equal to 1.3 kJ mol . However,
these calculations have large uncertainties and cannot decide which conformer has the lower energy in
the vapour and liquid states, particularly when the
calculated enthalpy difference was low.
A much more reliable clue to determine the conformers is derived from the calculated wavenumbers
of the anti and gauche fundamentals. The
wavenumbers obtained after appropriate scaling of
the ab initio calculations usually give a good agreement with those of the observed fundamentals. Below 1200 cmy1 , there are several instances where
the anti and gauche fundamentals are clearly separated for BDS and for BDSD from the band positions in crystals I and II and from the matrix spectra.
When the 10 band pairs for BDS ŽTable 1.: n 9 , n 23 ,
n 25 , n 26 , n 27 , n 28 , n 29 , n 30 , n 31 and n 32 are compared with the calculated frequencies for the anti
and gauche conformers, assuming crystal I to contain the gauche and crystal II the anti conformer,
there are nine cases where the experimental and
calculated shifts agree qualitatively and one case
Ž n 28 . which is arbitrary.
Among the 11 band pairs in BDSD: n 9 , n 20 , n 21 ,
n 23 , n 24 , n 27 , n 28 , n 29 , n 30 , n 31 and n 32 , nine cases
were in agreement, and two cases Ž n 23 , n 24 . were
opposite from the calculated shifts ŽTables 5 and 6..
These data reveal with a large degree of certainty
that the anti and gauche conformers should be
applied to the bands present in crystals II and I,
respectively, in both molecules. These correlations
were qualitative, meaning that in the ‘correct’ instances, the wavenumber shifts between the experimentally determined band pairs and those obtained
from the force constant calculations were both positive or both negative. A correlation between the size
of the observed and calculated shifts was in most
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
24
Table 6
Calculated and observed fundamentals of the anti conformer of bromomethyl dimethyl silane-d1 ŽBDSD.
Vibration number
n1
n2
n3
n4
n5
n6
n7
n8
n9
n 10
n 11
n 12
n 13
n 14
n 15
n 16
n 17
n 18
n 19
n 20
n 21
n 22
n 23
n 24
n 25
n 26
n 27
n 28
n 29
n 30
n 31
n 32
n 33
n 34
n 35
n 36
Species
Y
A
X
A
X
A
Y
A
X
A
Y
A
X
A
Y
A
X
A
X
A
X
A
Y
A
Y
A
X
A
X
A
Y
A
X
A
Y
A
X
A
X
A
Y
A
Y
A
X
A
Y
A
Y
A
X
A
X
A
Y
A
X
A
X
A
X
A
Y
A
X
A
Y
A
X
A
Y
A
a
ncalc
b
nobs
I IR
IR
Polarization
PED c
2976
2923
2921
2919
2908
2907
2853
2852
1480
1441
1438
1432
1429
1414
1300
1294
1164
1068
852
839
812
788
702
676
647
616
571
499
455
304
219
213
161
154
118
68
2972
2972
2948
2948
2914
2914
2864
2864
1546
1426
1426
1418
1418
1393
1263
1263
1133
1054
843
831
823
779
736
708
647
620d
608
514
482
286
227
221e
124
124
109
66e
vs
s
m
m
m
m
w
w
vs
s
s
w
w
m
vs
vs
w
m
vs
vs
vs
m
m
vw
s
w
s
vs
m
m
m
m
vw
vw
w
vw
m
m
vs
vs
vs
vs
m
m
vs
m
m
D
D
P
P
P
P
P
P
P
S22Ž100.
S5Ž97.
S8Ž68., S7Ž28.
S25Ž64., S24Ž35.
S7Ž70., S8Ž30.
S24Ž65., S25Ž35.
S6Ž99.
S23Ž99.
S3Ž98.
S19Ž92.
S18Ž91.
S33Ž92.
S34Ž92.
S13Ž92.
S15Ž95.
S30Ž95.
S14Ž91.
S28Ž96.
S16Ž76.
S17Ž75.
S31Ž34., S29Ž23., S21Ž19., S26Ž12.
S32Ž76.
S2Ž69.
S21Ž61., S31Ž33.
S29Ž51., S31Ž15., S21Ž12.
S4Ž36., S1Ž30., S10Ž20.
S1Ž48., S10Ž28.
S26Ž64., S32Ž14.
S10Ž28., S4Ž28., S12Ž13.
S9Ž43., S12Ž19., S17Ž14.
S11Ž63., S16Ž21.
S27Ž72.
S20Ž93.
S36Ž97.
S12Ž55., S13Ž25., S9Ž10.
S35Ž88.
m
m
m
m
m
m
vw
w
w
m
vw
vs
vs
s
vs
m
m
m
m
m
vw
vw
P
P
P
P
P
a
Calculated at the HFr6-311G) level of Gaussian-94, and scaled with factors of 0.9 for bands above 400 cmy1 , and 1.0 for fundamentals
below 400 cmy1 .
b
From infrared spectra of the vapor, except when noted.
c
Contributions of less than 10% are omitted.
d
Infrared amorphous.
e
Raman liquid.
cases also quite good as can be seen from the data in
Tables 2, 3, 6 and 7. In agreement with the results of
the ab initio calculations, the anti conformer should
have the lower energy in the liquids according to the
van’t Hoff plots, and also slightly lower in the
matrices from the annealing experiments, and probably in the vapour as well.
The bond moment of the Si–H ŽD. bond is negligible as compared to that of the C–Br bond in BDS
and BDSD and we therefore expect nearly the same
enthalpy difference between the anti and gauche
conformers in the vapour and liquid states, supported
by the experiments. In the series of halomethyl
dimethyl halosilanes, however, the polar gauche
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
25
Table 7
Calculated and observed fundamentals of the gauche conformer of bromomethyl dimethyl silane-d1 ŽBDSD.
Vibration number
a
ncalc
b
nobs
I IR
IR
Polarization
PED c
n1
n2
n3
n4
n5
n6
n7
n8
n9
n 10
n 11
n 12
n 13
n 14
n 15
n 16
n 17
n 18
n 19
n 20
n 21
n 22
n 23
n 24
n 25
n 26
n 27
n 28
n 29
n 30
n 31
n 32
n 33
n 34
n 35
n 36
2973
2925
2920
2908
2905
2903
2851
2847
1504
1442
1437
1432
1430
1415
1299
1293
1164
1069
850
842
800
774
707
685
669
604
547
498
492
252
238
196
166
154
122
68
2972
2972
2948
2948
2914
2914
2864
2864
1566
1426
1426
1418
1418
1393
1263
1263
1133
1054
843
833d
807
775d
721
708
647
610d
576
514
506
244
239e
207
124
124
109
70
vs
s
m
m
m
m
w
w
s
s
s
w
w
m
vs
vs
w
m
vs
vs
s
m
s
vw
s
w
w
s
m
m
m
m
vw
vw
w
w
m
m
vs
vs
vs
vs
m
m
s
m
m
D
D
P
P
P
P
P
P
P
S22Ž100.
S7Ž49., S24Ž48.
S5Ž99.
S24Ž47., S7Ž45.
S8Ž93.
S25Ž98.
S6Ž63., S23Ž34.
S23Ž64., S6Ž35.
S3Ž99.
S19Ž94.
S18Ž93.
S33Ž83.
S34Ž87.
S13Ž91.
S15Ž94.
S30Ž93.
S14Ž91.
S28Ž96.
S16ŽŽ76.
S17Ž70.
S31Ž36., S26Ž19., S21Ž16., S32Ž12.
S32Ž58., S29Ž16.
S29Ž37., S31Ž14., S10Ž14.
S2Ž48., S4Ž17.
S21Ž60., S31Ž22.
S1Ž68., S4Ž14., S2Ž11.
S4Ž28., S12Ž16., S10Ž12.
S26Ž57., S32Ž13.
S10Ž42., S4Ž14., S17Ž12.
S9Ž55., S16Ž11.
S27Ž23., S11Ž16., S12Ž16., S9Ž13.
S11Ž54., S27Ž15., S16Ž11.
S20Ž83.
S36Ž94.
S12Ž49., S13Ž23., S27Ž21.
S35Ž92.
m
m
m
m
m
m
w
w
w
s
vw
vs
vs
s
m
s
m
m
w
w
P
P
P
P
a
Calculated at the HFr6-311G) level of Gaussian-94, and scaled with factors of 0.9 for bands above 400 cmy1 , and 1.0 for fundamentals
below 400 cmy1 .
b
From infrared spectra of the vapor, except when noted.
c
Contributions of less than 10% are omitted.
d
Infrared argon matrix.
e
Raman crystal I.
molecule is stabilised in the liquid as compared to
the anti conformer as observed for chloromethyl
dimethyl chlorosilane w8x, bromomethyl dimethyl fluorosilane w7x, chloromethyl dimethyl fluorosilane w6x
and bromomethyl dimethyl chlorosilane w9x. In this
series of two halogen substituents, the gauche invariably had larger dipole moments than the anti
conformers, and the polar gauche rotamer was sta-
bilised in the liquid state as compared to the matrices
w20x.
5.2. Phase transitions
More than 40 years ago, Kagarise w21x reported
that by careful cooling of 1,1,2,2-tetrabromoethane,
the liquid gave different crystal phases, one contain-
26
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
ing the anti the other the gauche conformer. A
number of instances have been reported by various
authors describing two crystals containing molecules
of different conformers. Most often, these phases are
obtained by condensing the vapour on a cold window Žor finger. followed by very careful annealing,
first giving one crystalline phase while annealing to
higher temperatures gives another phase. Although
such cases are rare, various instances have been
found in our laboratories, most recently in
dichloromethyl methyldifluorosilane w17x. Some of
the molecules known to us with two or more crystals
each containing a different conformer were listed in
the latter paper w17x. It is believed that in all these
examples, the phase transitions are irreversible and
while one crystal is stable, one or more additional
crystals are metastable. This means that a thermodynamically unstable crystal can be formed from a
liquid or from an amorphous solid, but when a stable
crystal is formed, it will not convert to a metastable
crystal.
Another phase transition between at least three
solid phases connected with conformational changes
was observed for bicyclohexyl ŽC 6 H 11 . 2 w22,23x.
The high temperature phase Ž274–277 K. contained
a mixture of diequatorial Ž ee . anti and gauche
conformers, but in one crystalline phase Ž256.5–274
K. only the anti conformer, below 256.5 K only the
gauche conformer, was present. Unlike the other
examples encountered, these phase changes were all
reversible in terms of both temperature and pressure
changes w22x.
0
, between the
The enthalpy difference, D Hconf
conformers were invariably small Žtypically below 2
kJ moly1 . in the examples where two specific conformers can be isolated in different crystals. With
0
, the crystal energies cannot overcome
larger D Hconf
the enthalpy differences, and the more stable conformer will invariably be present in all the crystal
phases. The existence of two or more crystal phases
containing separate conformers are often detected
accidentally during annealing, and their presence can
easily be overlooked.
As reported above, two or three different crystals
were observed in the spectra of BDS and BDSD;
crystals I and II were observed for both molecules,
while crystal III was detected only in the Raman
spectrum of BDSD. Because of the very low melting
point, these phases could not be reached using cold
nitrogen vapour w16x as the refrigerant. The cryostats
employed for the infrared and Raman spectra are
fitted with thermocouples, but they cannot provide
reliable temperature control over an extended time
period since the refrigerant is added manually.
It was apparent from the similar lattice modes
detected in the Raman spectra below 100 cmy1
ŽTables 1 and 5. that crystal I for BDS and BDSD
was isomorphous and the same was apparently the
case with crystal II in the two compounds. These
crystals contained the gauche ŽI. and anti ŽII. conformers, respectively, while crystal III also contained
the gauche conformer. Crystals I and II were repeatedly observed in the MIR, FIR and Raman cryostats
for BDS, but crystal III was not seen for this
molecule. In BDSD, these crystals were obtained
only after many annealings, crystals I and III were
observed in the Raman and crystal II in the infrared
spectra only.
The phase transitions in BDS and the deuterated
BDSD were much more difficult to understand that
those recently observed for dichloromethyl methyldifluoro silane w17x. In the latter molecule, the crystals
were invariably formed from the amorphous solid
after annealing to 202 and 208 K. Also, none of the
molecules studied earlier with separate conformers in
different crystals Žsee Ref. w17x. gave such confusing
results as in BDS and BDSD. No complete understanding of the phase transitions or stabilities of the
present crystals can be presented, partly because of
the low temperatures involved and the apparent small
regions of stability of each phase.
As reported for the deposits in the Raman cryostat
of BDS, crystal II was first formed at ca. 115 K and
crystal I formed after subsequent annealing to ca.
125 K. In the FIR cryostat, however, a number of IR
curves were investigated ŽFig. 11.; in this case,
crystal II was first formed at 115 K, then the sample
appeared amorphous at 125 K and crystal I was
formed at 130 K before the sample melted at ca. 140
K. In the Raman spectra of BDSD, crystal I was
formed at 124 K and crystal III was subsequently
formed at 134 K without crystal II being detected. It
appears as if the heating rate and the surface: copper
finger ŽRaman., CsI window ŽMIR. or silicon window ŽFIR. may play a role for the phases formed.
Obviously, none of the temperatures reported pre-
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
sents thermodynamic equilibria and there are undoubtedly irreversible transitions and metastable
phases involved. It is not even clear which of the
crystals I, II or III represents the thermodynamically
stable phase. Although the anti conformer appears to
have the lower energy and is present in crystal II,
this crystal is not necessarily stable. Calorimetric
measurements and very careful annealing experiments under accurately controlled temperatures are
needed to establish the phase transitions in BDS and
BDSD.
Obviously, when two different conformers can be
isolated in different crystal phases, the vibrational
spectra can be interpreted with a high certainty. In
the common case of one conformer Žamong two.
being present in the crystalŽs., it can only be concluded that the bands vanishing upon crystallisation
belong to one conformer while those being present
belong either to the other conformer or they are
common to both.
5.3. Spectral assignments of BDS and BDSD
The assignments of the infrared and Raman spectra of BDS to the anti and gauche conformers
appear in Tables 1–3 and those of BDSD in Tables
5–7. The fundamentals have been numbered consecutively of both the anti and gauche conformers to
make a comparison more convenient, rather than the
conventional method where the AX modes are listed
before those of AY in the anti conformer.
It is significant that the anti and gauche fundamentals n 1 – n 22 , covering the region 3000–800 cmy1
in BDS and n 1 – n 19 for BDSD overlap in the vapour,
liquid and crystalline states although some of them
may appear as separate bands in the matrix spectra.
This is also in agreement with the results of the
calculations ŽTables 2, 3, 6 and 7.. The only exception being n 9 , the Si–H stretch is situated around
2150 cmy1 for BDS ŽFigs. 5, 7 and 9. and the Si–D
stretch around 1550 cmy1 for BDSD ŽFigs. 19 and
21., in which the conformer bands are well-separated.
We expect eight C–H stretching fundamentals
n 1 – n 8 of each conformer connected with two hydrogens in the CH 2 Br moiety and six hydrogens belonging to the two methyl groups in each of the compounds. The fundamentals in BDS and BDSD are
27
calculated to lie at the same wavenumbers in both
compounds and those of the anti and gauche conformers overlap. As is apparent from Tables 1 and 5,
a number of infrared and Raman bands were observed in the range 3000 to 2800 cmy1 , many of
them weak and observed only in the matrix spectra.
In both spectra, prominent bands were observed at
2972, 2946 and 2914 cmy1 , attributed to the modes
n 1 – n 6 for both conformers. The remaining two CH 3
symmetric stretches are attributed to the weak bands
at 2851 cmy1 in BDS and 2864 cmy1 in BDSD.
The weak or very weak bands above or below this
region are supposedly combination bands or overtones.
The two bands at 2156 and 2127 cmy1 are attributed to the Si–H ŽD. stretches Ž n 9 . of the gauche
and anti conformers in BDS, while they were shifted
to 1566 and 1546 cmy1 in BDSD. These assignments are in agreement with the fact that the high
and low frequency components are present in crystals I and II, respectively, both in the infrared and
Raman spectra. Although these bands were all situated at higher wavenumbers than the scaled calculated values, the shifts between the gauche and anti
rotamers in the vapour phase were observed to be 29
and 20 cmy1 for BDS and BDSD, respectively, in
good agreement with the calculations.
Very surprising results can be seen from Figs. 14
and 15, giving the infrared spectra in argon and
nitrogen matrices, respectively, of the conformer
doublet in the Si–H stretching region of BDS. As is
apparent, the two low wavenumber bands in argon
and the series of four high wavenumber bands in the
nitrogen matrices are reduced in intensity after annealing. It has been concluded both from the infrared
and Raman spectra, as well as from the force constant calculations, that the high wavenumber band of
the doublet is due to the gauche and the low
wavenumber band to the anti conformer. This observation alone would have suggested the surprising
result that one conformer is stabilised in argon the
other in the nitrogen matrix as formerly observed for
1,1,2-trichloro-2,3,3-trifluorocyclobutane w24x. However, since a number of bands forming anti r gauche
band pairs change intensities alike in both matrices
ŽFigs. 12 and 13., this assumption is not feasible.
In the vapour, liquid and crystalline states, the
high wavenumber band is correlated with the gauche
28
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
and the low wavenumber band with the anti conformer. While this is also true of the nitrogen matrix,
it cannot be the case for the argon matrix. The
intensity changes after annealing reveal that the bands
at 2121 and 2116 cmy1 in argon ŽFig. 14. correspond to the bands at 2182, 2178, 2173 and 2162
cmy1 in the nitrogen matrix Ž gauche conformer..
Correspondingly, the bands at 2159, 2152 and 2146
cmy1 in argon and at 2143, 2140 and 2132 cmy1 in
nitrogen ŽFig. 15. must belong to the anti conformer. As compared with the vapour spectra, the
gauche conformer bands are at widely different
wavenumbers in the two matrices, whereas the anti
bands are less influenced. The gauche component of
the Si–H stretch is ca. 60 cmy1 higher in the
nitrogen as compared to argon spectra while the anti
component is approximately 15 cmy1 higher in the
argon spectra.
The matrix bands at 1564 and 1556 cmy1 in
argon are enhanced and those at 1551, 1541 and
1538 cmy1 assigned to the Si–D stretching modes in
BDSD are reduced in intensities after annealing. We
therefore ascribe the 1564 and 1556 cmy1 doublet to
the anti and the 1551r1541r1538 cmy1 triplet to
the gauche conformer ŽFig. 21.. Qualitatively, the
spectra of BDSD agree with those of BDS since the
gauche bands are shifted to lower, the anti bands to
higher wavenumbers in the argon matrix as compared to the vapour values.
Specific interactions between the Si–H Žand Si–D.
moiety of the anti and the gauche conformers in the
argon and nitrogen matrices are difficult to explain.
Further studies of BDS in additional matrices such as
krypton and xenon are desirable to establish eventual
specific interactions of the Si–H groups in the matrices.
Among the four antisymmetric CH 3 deformation
modes, n 10 and n 11 were observed as coinciding
vapour bands at 1428 cmy1 , and n 12 and n 13 were
observed at 1410 cmy1 , with the anti and gauche
conformers overlapping. In BDSD, these fundamentals were found at nearly the same wavenumbers as
in the spectra of BDS, and they were all predicted to
be of low intensities in the IR and Raman spectra as
observed. The fundamental n 14 connected with CH 2
scissor was found at 1393 cmy1 for both BDS and
BDSD common to both rotamers. No good candidates for n 15 were found in the spectra of BDS and
BDSD and the two CH 3 deformation modes Ž n 15
and n 16 . were therefore considered to coincide at
1134 cmy1 . This assumption was supported by the
existence of close-lying bands in the matrices and in
the Raman spectra of the crystals for both compounds.
The infrared vapour bands around 1134 and 1056
cmy1 in both spectra were interpreted as the CH 2
wagging and twisting modes, n 17 and n 18 . In BDS,
the fundamentals n 19 and n 20 are attributed to the
vapour bands at 892 and 885 cmy1 , respectively. In
agreement with the calculations ŽTables 2 and 3.,
these infrared bands are the most intense in the
spectrum besides n 9 and involve C–Si–H out-ofplane deformation and methyl antisymmetric deformation, respectively. The n 21 and n 22 modes were
found at 843 and 803 cmy1 , in good agreement with
the scaled calculated values. The number of strong
peaks in the argon and nitrogen matrix spectra,
changing intensities on annealing, suggests separate
close-lying conformer bands which could not be
detected in the crystal spectra.
In BDSD, the anti and gauche conformers n 19
coincide at 843 cmy1 . Unlike the parent compound
BDS, the n 20 , n 21 and n 22 modes in BDSD, mainly
involving methyl deformations, all seem to have
separate bands for the two rotamers as predicted by
the calculations. As is apparent from Table 5, the n 20
and n 21 modes fit with the bands of crystals I and II,
whereas the gauche conformer of n 22 is based upon
the matrix doublet at 777 and 774 cmy1 which is
reduced in intensities after annealing
The series of fundamental modes n 23 to n 32 in
BDS appeared as separate anti and gauche bands
ŽTable 1. in excellent agreement with their presence
in crystals I and II. Their separations were well-predicted by the calculations, and the assignments in
Table 1 need no further comments. An exception
was the anti and gauche fundamentals of n 24 which
coincided at 718 cmy1 in agreement with the results
of the calculations. Most of the fundamental n 23 to
n 32 are mixed between different symmetry coordinates as is apparent from Tables 2 and 3, but in most
of them, the largest contribution to PED stems from
Si–C stretch, C–Si–H bend and C–Si–C bend.
However, n 29 had the largest contribution from C–Br
stretch, the gauche component was observed at 25
cmy1 higher wavenumber than anti in the vapour as
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
compared with 28 cmy1 in the calculations. Both
were intense in the Raman spectra and employed for
the van’t Hoff plots. The gauche component was
much stronger than anti in the infrared spectra of
n 29 , in agreement with the calculated intensities of
33 for the gauche and 12 km moly1 for the anti
rotamer ŽTables 2 and 3.. The assignments of n 31
and n 32 were somewhat uncertain since the anti
components of both were considered to coincide
Žaccidental degeneracy. at 212 cmy1 . However, they
belong to different symmetry species ŽAX and AY ., a
doublet at 226 and 223 cmy1 were observed in the
Raman spectra of crystal II and the observed and
calculated shifts between the anti and gauche components were in excellent agreement.
The remaining fundamentals n 33 to n 36 gave rise
to weak or very weak bands both in the infrared and
Raman spectra in agreement with the calculations
ŽTables 2 and 3.. The anti and gauche bands appeared to overlap for each of these modes which
were observed at 134, 124, 108 and 73 cmy1 , respectively. Whereas n 33 and n 34 involved the two
methyl torsions, n 35 was mostly Si–C–Br bend
while the lowest fundamental n 36 involved torsion
of the central Si–C bond.
The infrared and Raman bands of BDSD were
considerably displaced as compared to those of BDS,
since the Si–H ŽD. bond through stretching and
bending contributed to many of the fundamentals in
this range Žsee Fig. 17.. Moreover, with the present
numbering, many fundamentals of the anti conformers of BDS and BDSD Ž n 24 , n 26 , n 27 and n 28 . had
opposite symmetry species, revealing totally different vibrations in the two molecules ŽTables 2 and 6..
Also, the PED terms in Tables 6 and 7 reveal that the
anti and gauche components of the same fundamentals in BDSD often had quite different contributions
of the symmetry coordinates.
The anti and gauche fundamentals n 23 were
assigned to the bands at 736 and 721 cmy1 , although
the gauche fundamentals were calculated to be at a
higher wavenumber than anti. The fundamentals n 24
and n 25 were both attributed to coinciding anti and
gauche modes at 708 and 646 cmy1 , respectively.
None of these assignments agreed with the calculations since the gauche fundamentals should be at
higher wavenumbers than the anti for both modes.
Except for n 28 which was attributed to coinciding
29
conformer bands at 514 cmy1 , the fundamentals n 26
to n 32 were all attributed to separate anti and gauche
components as seen from Tables 5–7, and the observed and calculated shifts were in qualitative
agreement. Some of these bands were uncertain since
the anti and gauche bands of n 26 were observed
only in the infrared spectra and the gauche component of n 29 did not have any Raman counterpart.
The PED indicates that n 26 for the anti and n 27
for the gauche conformer had the largest contribution from C–Br stretch. Unlike the spectra of BDS
where the band pair 552r529 cmy1 Ž n 29 . was the
most intense in the Raman spectra and localised to
C–Br stretch, Fig. 17 reveals that a number of
Raman bands of BDSD between 650 and 450 cmy1
had similar intensities. With more fundamentals in
this region, the C–Br stretch in BDSD is more
delocalised. However, the large intensities in the
Raman spectra would indicate that n 27 in both conformers might be predominantly C–Br stretch. This
band pair 606r573 cmy1 was employed in the van’t
Hoff plots of BDSD. Other possibilities are the
bands at 514 or 482 cmy1 assigned to n 28 and n 29 .
The gauche conformer of n 31 was tentatively attributed to the Raman band at 239 cmy1 observed in
the crystal only. Since the sample of BDSD did not
crystallise in the FIR cryostat, but remained amorphous after annealing, the positions of the conformer
bands in low wavenumber region below 450 cmy1
were uncertain.
The conformers of the n 33 and n 34 modes were
assumed to overlap at 124 cmy1 while the n 35
fundamentals were supposedly situated at 109 cmy1 .
Finally, the n 36 fundamentals involving torsion
around the Si–C central bond were attributed to the
Raman bands at 72 and 66 cmy1 in the liquid state.
The Raman spectra of the liquids in the low frequency region were also investigated employing the
RŽ n . representation w25x and all the Raman bands
below 200 cmy1 were verified.
Acknowledgements
The authors are grateful to Anne Horn for very
valuable assistance. Helpful comments from the referee are highly appreciated. V. Aleksa acknowledges
a grant from the Norwegian Research Council re-
30
V. Aleksa et al.r Vibrational Spectroscopy 17 (1998) 1–30
served for the Baltic Countries and Northwest Russia
while V. Tanevska had a student grant for The
Macedonian Republic through IAESTE ŽNorway..
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