MOLSTR 10545
Journal of Molecular Structure 482–483 (1998) 391–396
Infrared and Raman spectra, conformations and ab initio
calculations of ethyl bromosilane and ethyl dibromosilane
D.L. Powell a,1, P. Klaeboe a,*, A. Gruodis a, c, C.J. Nielsen a, G.A. Guirgis b, J.R. Durig b,
V. Aleksa c
a
Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway
Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA
c
Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania
b
Received 24 August 1998
Abstract
Ethyl bromosilane (CH3CH2SiH2Br) and ethyl dibromosilane (CH3CH2SiHBr2) were synthesized and their infrared and
Raman spectra determined in vapour (IR), liquid (Raman), amorphous solid, and crystalline states. Additional infrared spectra
of the compounds isolated in argon and nitrogen matrices at 5 K were recorded before and after annealing to temperatures in the
range of 15–35 K. Raman spectra of the liquid were recorded at various temperatures between 295 and 153 K. Both the spectra
showed the existence of two conformers – anti and gauche – present in the fluid and amorphous phases. The crystals contained
the gauche conformer for bromide and anti conformer for the dibromide in accordance with the results for the chloro and iodo
homologues. The enthalpy differences in the liquids measured in Raman gave: DH (anti-gauche) ˆ 1.8 ^ 0.3 and DH (gaucheanti) ˆ 2.1 ^ 0.3 kJ mol 21, respectively.
Ab initio calculations were performed using the Gaussian 94 program with the HF/6-311G* basis set and gave optimized
geometries, infrared and Raman intensities and scaled vibrational frequencies for the anti and gauche conformers. The
conformational energies were calculated to be 0.02 kJ mol 21 for ethyl bromosilane and 0.3 kJ mol 21 for ethyl dibromosilane
with the anti and gauche conformers having the lower energy, respectively. q 1999 Elsevier Science B.V. All rights reserved.
Keywords: Conformations; Vibrational spectra; Bromosilanes; Ab initio calculations
1. Introduction
Ethyl bromosilane (CH3CH2SiH2Br) and ethyl
dibromosilane (CH3CH2SiHBr2), henceforth to be
called EBS and EDBS, were synthesized and their
vibrational spectra studied in various phases. The
* Corresponding author. Tel.: 47 22 85 56 78; fax: 47 22 85 54
41.
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe)
1
Permanent address: Department of Chemistry, The College of
Wooster, Wooster, OH 44691, USA.
chlorine-containing analogue ethyl chlorosilane [1]
was recently studied by infrared and Raman methods
revealing that the gauche conformer was more stable
in the liquid by 2.4 ^ 0.3 kJ mol 21 and was also
present in the crystal. In ethyl dichlorosilane [2], the
anti conformer is present in the crystal, and is the
more stable conformer with DH (gauche-anti) equal
to 1.7 ^ 0.1 in the liquid and equal to 0.7 ^
0.1 kJ mol 21 in liquid krypton. The two iodo analogues are presently being investigated [3] and their
conformations and spectra were found to be very
similar to those of EBS and EDBS.
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-286 0(98)00681-4
392
D.L. Powell et al. / Journal of Molecular Structure 482–483 (1998) 391–396
Fig. 1. MIR vapour spectra of EBS in the range 1500–400 cm 21, path 10 cm, at pressures 6 and 1.5 Torr.
In the present investigation the vapour, amorphous
and crystalline samples of the compounds were
recorded in the middle (MIR) and far infrared (FIR)
regions, and the infrared matrix isolation technique
employed.
Raman spectra of the liquid, including polarization
measurements, were obtained at ambient temperature
and with the sample cooled in a Dewar with nitrogen
gas [4]. Spectra of the amorphous and crystalline solids
were observed using different cooling techniques. Also,
the conformational energies, structure, force constants
and infrared and Raman intensities were calculated by
ab initio methods. Our preliminary data for EBS and
EDBS are given in the present communication.
2. Experimental
The sample of EBS was prepared by the reaction of
ethylsilane with one equivalent of boron tribromide at
room temperature for one hour, while EDBS was
prepared from two equivalents of boron tribromide.
The compounds were purified in a low temperature,
low pressure fractionation column and the purities
checked by mass spectrometry.
Raman spectra were recorded with a spectrometer
from Dilor (TR 30), excited by a Spectra-Physics model
2000 argon ion laser using the 514.5 nm line. Capillaries
containing the samples were cooled with cold nitrogen
gas [4], and additional spectra of the amorphous and
annealed crystalline phases were measured on a
copper finger, cooled with liquid nitrogen.
The infrared spectra were recorded with the
following four FT-IR spectrometers: Bruker models
66, 88 and 113v (vacuum instrument) and PerkinElmer model 2000. The vapour spectra were recorded
in cells of 10 cm and 20 cm path lengths and the amorphous and crystalline solids were studied in cryostats
cooled with liquid nitrogen. The samples were studied
with argon or nitrogen (1 : 1000) at ca. 5 K, cooled with
a closed cycle system from APD and subsequently
annealed to temperatures in the range 20–35 K. Infrared
spectra were also recorded in liquified xenon under
pressure at various temperatures between 170 and
240 K in a copper cell with 4 cm path length.
3. Results and discussion
3.1. Infrared spectral results
MIR vapour spectra of EBS and EBDS in the region
D.L. Powell et al. / Journal of Molecular Structure 482–483 (1998) 391–396
393
Fig. 2. MIR vapour spectrum of EBDS in the range 1500–400 cm 21, path 10 cm at 3 Torr pressure.
1500–400 cm 21 are presented in Figs. 1 and 2, respectively, giving some well resolved contours. Low
temperature spectra were recorded in the MIR and
FIR cryostats at 80 K. As these compounds easily
formed glassy solids, the samples were annealed to
numerous temperatures to achieve crystallization,
and spectra were recorded both at the annealing
temperatures and after recooling to 80 K. Crystals
Fig. 3. MIR spectra (1500–400 cm 21) of EBDS in nitrogen matrices (1 : 1000) at 5 K, unannealed (solid line); annealed to 27 K (dashed).
394
D.L. Powell et al. / Journal of Molecular Structure 482–483 (1998) 391–396
Fig. 4. MIR spectra (1500–400 cm 21) of EBDS in argon matrices (1 : 1000) at 5 K, unannealed (solid line); annealed to 27 K (dashed).
were eventually formed in both molecules, as
apparent from the sharp peaks with crystal splitting.
Infrared bands of the amorphous phase at 694w, 684w
and 425w cm 21 in EBS and at 1010s and 462s cm 21 in
EBDS vanished and were attributed to a second
conformer.
The spectra of EBS in argon and nitrogen matrices
showed many sharp bands in the unannealed samples,
Fig. 5. MIR spectra (720–620 cm 21) of EBDS in argon matrices (1 : 1000) at 5 K, unannealed (solid line); annealed to 27 K (dashed).
D.L. Powell et al. / Journal of Molecular Structure 482–483 (1998) 391–396
395
Fig. 6. Raman spectra of EBDS as a liquid (295 K) (solid line) and crystal at 205 K (dashed) in the range 1500–50 cm 21.
but after annealing both spectra had a high background. In argon, matrix bands at 1095–1070, 906,
896, 732 and 450 cm 21 were reduced in intensities,
while those at 916 and 896 cm 21 were enhanced.
Significant changes occurred in the matrix spectra
of EBDS after annealing as is apparent from the
spectra given in Figs. 3–5 (nitrogen, 1500–
400 cm 21; argon, 1500–400 cm 21; and argon, 720–
620 cm 21). The bands at 2208, 2204 and 2194 cm 21
(Si–H stretches) and those at 1387, 1012, 979, 705,
701 and 469 cm 21 vanished after annealing to 27 K in
the nitrogen matrix. In the argon matrix the bands at
2205, 1355, 1352, 1321 and 479 cm 21 disappeared or
were highly reduced in intensity. Some of the changes
may be caused by matrix effects, as they were
different in the two matrices. The band at 469 cm 21
that disappeared in nitrogen also vanished in the
crystal, but it did not change the intensity in the
argon matrix until 36 K. Either the conformational
barrier was higher in the argon matrix than in a
nitrogen matrix or the enthalpy difference was
different.
3.2. Raman spectral results
Raman spectra of EDBS as a liquid at ambient
temperature and as a crystal cooled to 173 K in the
1500–50 cm 21 range are given in Fig. 6. A few bands
present in the amorphous, low temperature phase
vanished in the crystal: 1011m, 701m, 462m, 396s
and 267m cm 21. For EBS (figure not shown) bands
at 1009w, 686m, 430m and 276w cm 21 vanished on
crystallization. Both the compounds crystallized more
easily by cooling a liquid than by annealing an amorphous solid. It appears that the vanishing bands are
weak in EBS, but quite strong in EBDS, reflecting the
conformational equilibrium and the statistical weight
of 2 for the gauche conformer. The low temperature
Raman results support those obtained in the infrared
cryostats and are undoubtedly a result of bands
disappearing in the crystal. The number of vanishing
bands is quite small, revealing that most of the fundamentals of one conformer overlap with those of the
other.
Raman spectra of the two liquids were recorded
between 295 and 168 K (EBS) and between 295 and
153 K (EBDS). The enthalpy differences DH between
the conformers were calculated using the band pairs:
700*/650, 460*/440, 265*/233 cm 21 for EBS and
1026/1009*, 686*/736, 552/430* cm 21 (the bands
with asterisks vanished in the crystal). The van’t
Hoff plots gave the values DH (anti-gauche) ˆ 1.8 ^
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D.L. Powell et al. / Journal of Molecular Structure 482–483 (1998) 391–396
Table 1
Raman bands of ethylhalo and ethyldihalosilanes (670–200 cm 21)
Ethylhalosilanes
Chloro a
e
655w,p
562w,p
530m,p
507s,p
278m,p
242vw
Bromo b
Iodo c
Conf
Ethyldihalosilanes
Chloro d
Bromo b
Iodo c
Conf
647s,p
546w,p
430m,p
394s,p
276vw,dp
249w,p
639s,p
520w,p
379m,p
340s,p
267w
229m,p
g
g
a
g
a
g
661m,p
564m,p
492vs,p
380w
298s,p
260w,p
635w,p
358w,p
332m,p
292s,p
238m,p
210m,p
a
a
648m,p
443w,dp
394vs,p
341s,p
265ms,p
233m,dp
a
g
a
Ref. [1].
This work.
c
Ref. [3].
d
Ref. [2].
e
Abbreviations: s, strong; m, medium; w, weak; v, very; p, polarized; dp, depolarized; a, anti; g, gauche.
b
0.3 for EBS and DH (gauche-anti) ˆ 2.1 ^
0.3 kJ mol 21 for EBDS.
3.3. Quantum chemical calculations
Quantum chemical calculations using the Gaussian-94 programs [5] with basis function HF/6311G* was carried out giving the conformational
energy difference DH (gauche-anti) ˆ 0.02 kJ mol 21
for EBS and DH (anti-gauche) ˆ 0.3 kJ mol 21 for
EBDS. The ab initio calculated frequencies were
scaled with a factor of 0.9 for the fundamentals
above 400 cm 21. The spectral correlations showed
that the gauche and anti conformers were present in
the crystals of EBS and EBDS, respectively. These
results were in agreement with those reported for
the two chloro [1,2] and with the preliminary data
for the two iodo analogues [3]. The wave number
correlations in the range 670–200 cm 21 are presented
in Table 1.
Acknowledgements
AG and VA have received fellowships from Det
Norske Videnskaps-Akademi and the Research
Council of Norway, respectively.
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