Infrared and Raman spectra, conformational stability, barriers to internal rotation, normal-coordinate calculations and vibrational assignments for vinyl silyl bromide

Article abstract of DOI:10.1016/S1386-1425(99)00204-8

The infrared (3200-30 cm^-1) spectra of gaseous and solid, the Raman spectra (3200-30 cm^-1) of the liquid and solid vinyl silyl bromide, CH₂CHSiH₂Br, have been recorded. Additionally, quantitative depolarization values have been obtained. Both the gauche and cis conformers have been identified in the fluid phases but only the gauche conformer remains in the solid. Variable temperature studies from 0 to -87 °C of the Raman spectrum of the liquid was carried out. From these data, the enthalpy difference has been determined to be 22±6 cm^-1 (0.26±0.08 kJ/mol), with the gauche conformer being the more stable form. The predictions from the ab initio calculations up to MP2/6-311++G(2d,2p) basis set favor the gauche as the more stable form. A complete vibrational assignment is proposed for both the gauche and cis conformers based on infrared band contours, relative intensities, depolarization values and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing the force constants from ab initio MP2/6-31G(d) calculations and the potential energy terms for the conformer interconversion have been obtained from the same calculations. Complete equilibrium geometries have been determined for both rotamers by ab initio calculations employing a variety of basis sets up to 6-311++G(2d,2p) at levels of restricted Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The results are discussed and compared to those obtained for some similar molecules.

Full text of DOI:10.1016/S1386-1425(99)00204-8

Spectrochimica Acta Part A 56 (2000) 1065–1078
www.elsevier.nl/locate/saa
Infrared and Raman spectra, conformational stability,
barriers to internal rotation, normal-coordinate calculations
and vibrational assignments for vinyl silyl bromide
Gamil A. Guirgis 1, Yasser E. Nashed 2, James R. Durig *
Department of Chemistry, Uni6ersity of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA
Received 27 July 1999; accepted 1 August 1999
Abstract
The infrared (3200–30 cm−1 spectra of gaseous and solid, the Raman spectra (3200–30 cm−1) of the liquid and
solid vinyl silyl bromide, CH2CHSiH2Br, have been recorded. Additionally, quantitative depolarization values have
been obtained. Both the gauche and cis conformers have been identified in the fluid phases but only the gauche
conformer remains in the solid. Variable temperature studies from 0 to −87°C of the Raman spectrum of the liquid
was carried out. From these data, the enthalpy difference has been determined to be 2296 cm−1 (0.2690.08
kJ/mol), with the gauche conformer being the more stable form. The predictions from the ab initio calculations up to
MP2/6-311+ +G(2d,2p) basis set favor the gauche as the more stable form. A complete vibrational assignment is
proposed for both the gauche and cis conformers based on infrared band contours, relative intensities, depolarization
values and group frequencies. The vibrational assignments are supported by normal coordinate calculations utilizing
the force constants from ab initio MP2/6-31G(d) calculations and the potential energy terms for the conformer
interconversion have been obtained from the same calculations. Complete equilibrium geometries have been
determined for both rotamers by ab initio calculations employing a variety of basis sets up to 6-311+ +G(2d,2p) at
levels of restricted Hartree-Fock (RHF) and/or Moller-Plesset (MP) to second order. The results are discussed and
compared to those obtained for some similar molecules. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Conformational stability; FT-IR spectra; ab initio calculations; Vinyl silyl bromide
1. Introduction
* Corresponding author. Tel.: +1-816-2351136; fax: +1-
816-2355191.
The conformational stability of the allyl halides
(3-halopropenes) CH2CHCH2X where X=F, Cl,
Br and I have been the subject of several studies
in the past decade [1–8]. From these studies it has
been concluded that these molecules exist as a
mixture of cis and gauche conformers in the fluid
E-mail address: durigj@umkc.edu (J.R. Durig)
1 Permanent address: Analytical Research and Development
Department, Bayer Corp., Charleston, SC 29411, USA.
2 Taken in part from the dissertation of Y.E. Nashed which
will be submitted to the Department of Chemistry in partial
fulfillment of the Ph.D. degree.
1386-1425/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(99)00204-8

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G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
phases but only the fluoride molecule has the cis
conformer the more stable form.
electron correlation by the perturbation method
to second order (MP2) with larger basis sets than
previously utilized [10]. From the MP2/6-311+
G(d,p) calculation the gauche conformer is pre-
dicted to be more stable by 123 cm−1 (1.47
kJ/mol) and even at the level of MP2/6-311+
G(2d,2p) the gauche conformer is still predicted to
be more stable by 21 cm−1 (0.25 kJ/mol). Thus,
the ab initio calculations seem to favor the gauche
conformer for vinyl silyl chloride but maybe not
by quite as large amount as for the corresponding
carbon compounds.
As a continuation of our studies on silicon
analogues of the 3-halopropenes, we have
recorded the infrared and Raman spectra of the
fluid and solid phases of vinyl silyl bromide along
with variable temperature studies of the Raman
spectra of the liquid to determine the conforma-
tional stability. We have also carried out ab initio
calculations employing a variety of basis sets up
to 6-311+ +G(2d,2p) at levels of restricted
Hartree-Fock (RHF) and with full electron corre-
lation by the Moller-Plesset (MP) perturbation
method to second order to obtain complete equi-
librium geometries. The force constants, vibra-
tional frequencies, infrared and Raman
intensities, and conformational stabilities have
also been obtained from the ab initio calculations.
The results of these vibrational spectroscopic and
theoretical studies are reported herein.
Ab initio calculations have been carried out for
both the allyl fluoride and allyl chloride (gauche
more stable form). For the chloride, the correct
conformer stability is predicted at all level of the
calculations up to MP2/6-311+ +G(d,p) but the
energy difference of 457 cm−1 (5.47 kJ/mol) is
much larger than the experimentally determined
enthalpy difference of 9695 cm−1 (1. 1590.06
kJ/mol) from a variable temperature of a krypton
solution and 147920 cm−1 (1.76924 kJ/mol)
for the xenon solution. For the fluoride molecule
the MP2/6-311+ +G(d,p) ab initio predict the
gauche conformer to be the more stable form by
117 cm−1 (1.40 kJ/mol) whereas the experimental
enthalpy value was determined to be 6098 cm−1
(0.7290.10 kJ/mol) from a xenon solution and
8191 cm−1 (0.97+0.001 kJ/mol) from a kryp-
ton solution. Therefore the ab initio predictions
appear to favor the gauche conformer by at least
200 cm−1 (2.39 kJ/mol) for these allyl halides. In
order to determine whether a similar problem
exists for the corresponding vinyl silyl halides,
CH2CHSiH2X where X=F, Cl, Br, we have ini-
tiated a study of the conformational stabilities of
these molecules, both experimental and from ab
initio calculations.
The first of these silyl compounds we investi-
gated was vinyl silyl chloride [9,10]. Prior to our
recent conformational study [11] of this molecule,
the enthalpy difference was determined from a
variable temperature study of the Raman spec-
trum of the liquid [10] where an enthalpy value of
98911 cm−1 (1.1890.13 kJ/mol) was reported
with the cis conformer the more stable form. This
result was at variance with the conformational
stability prediction from the RHF/6-31G(d) calcu-
lations [10] However, it is well known that the
conformer that is the most stable form in the
liquid may not be the most stable rotamer in the
gas. Therefore, we carried [11] out variable tem-
perature infrared studies for both xenon and
krypton solutions and determined enthalpy differ-
ences of 112924 cm−1 (1.3490.29 kJ/mol) and
8198 cm−1 (0.9790.10 kJ/mol), respectively
with the cis conformer the more stable form. We
[11] also carried out ab initio calculations with full
2. Experimental
Vinyl silyl bromide was prepared in two steps.
First vinyl silyl trichloride was reduced with
lithium aluminum hydride in dry dibutyl ether to
obtain vinyl silane. The later compound was
purified first then reacted with tin(IV) bromide at
50°C for 40 h. Vinyl silyl bromide was purified
using a low pressure, low temperature fractiona-
tion column and the purity of the sample was
checked with infrared spectroscopy and mass
spectrometry.
The Raman spectra of vinyl silyl bromide from
3200 to 30 cm−1 (Fig. 1) were recorded on a
SPEX model 1403 spectrophotometer equipped
with a Spectra-Physics model 164 argon ion laser

G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
1067
,
operating on the 5145A line. Laser power at the
sample ranged from 0.4 to 1 W depending on the
Fig. 1. Raman spectra of vinyl silyl bromide in the (A) liquid;
and (B) annealed solid.
Fig. 3. Mid-infrared spectra of vinyl silyl bromide (A) gas; (B)
calculated spectrum of the mixture of both conformers (DH=
22 cm−1); (C) calculated spectrum of the cis conformer; (D)
calculated spectrum of the gauche conformer.
physical state of the sample. The spectrum of the
liquid was obtained from the sample sealed in a
glass capillary. The spectrum of the solid was
obtained by condensing the liquid in a Miller–
Harney cell [12] cooled by boiling liquid nitrogen.
Multiple annealings were carried out to obtain
good polycrystalline material, which was indi-
cated when no further changes in the spectrum
were noted. The reported frequencies are expected
to be accurate to 92 cm−1
.
The mid-infrared spectra of gaseous and solid
vinyl silyl bromide (Figs. 2 and 3A) were recorded
on a Perkin-Elmer model 2000 Fourier transform
interferometer equipped with a ceramic source,
Ge/CsI beamsplitter and a DTGS detector. A 10
cm cell with CsI windows was used to obtain the
spectrum of the gas. The spectrum of the solid
was obtained by condensing the sample on a CsI
substrate held at −196°C by boiling liquid nitro-
Fig. 2. Mid-infrared spectra of vinyl silyl bromide in the (A)
gas; (B) annealed solid.

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G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
Fourier transform interferometer equipped with a
vacuum bench, a Globar source, and a liquid He
cooled silicon bolometer with a wedged sapphire
filter and polyethylene windows. The gaseous
sample was contained in a 1 m cell equipped with
polyethylene windows and recorded at a spectral
resolution of 0.2 cm−1 with a 6.25 mm Mylar
beamsplitter. The far infrared spectrum of the
solid (Fig. 4B and C) was recorded on the previ-
ously described Perkin-Elmer model 2000 interfer-
ometer. A grid beamsplitter and a cryostat cell
with polyethylene windows were used and the
sample was deposited on a silicon substrate
cooled by boiling liquid nitrogen. All observed
infrared and Raman bands with significant inten-
sities are listed in Table 1.
gen. The sample was repeatedly annealed until no
further changes were observed in the spectra.
The far infrared spectrum of the gas (Fig. 4A)
was recorded on a Bomem model DA3.002
3. Ab initio calculations
The LCAO-MO-SCF restricted Hartree-Fock
calculations were performed with the Gaussian-94
program [13] using Gaussian-type basis functions.
The energy minima with respect to nuclear coor-
dinates were obtained by the simultaneous relax-
ation of all of the geometric parameters using the
gradient method of Pulay [14]. The structural
optimizations for both the gauche and the cis
conformers were carried out with initial parame-
ters taken from those of vinyl silyl chloride [11].
The 6-31G(d) and 6-311+G(d,p) basis sets were
employed at the level of restricted Hartree-Fock
(RHF) and Moller-Plesset (MP2) to second order
and the determined structural parameters are
listed in Table 2.
Fig. 4. Far infrared spectra of vinyl silyl bromide in (A) gas,
(B) unannealed solid, and (C) annealed solid. The bands
labeled with an asterisk are due to the rotational transitions of
HBr.
The ab initio NIP2/6-31G(d) calculations were
utilized to obtain force constants for both the
gauche and the cis conformers of vinyl silyl bro-
mide, and the resulting wavenumbers were used
for comparison with the experimental results. To
obtain the approximate descriptions of the normal
modes the ab initio force fields of Cartesian co-
ordinates were transformed into force fields in
internal coordinates. The corresponding transfor-
mation matrices, B, were generated using the
Cartesian coordinates obtained from the opti-
mized geometries and the complete set of 24
internal coordinates are shown in Fig. 5. This
Fig. 5. Internal coordinates of vinyl silyl bromide.

Table 1
Observed infrared and Raman wavenumbers (cm−1) for vinyl silyl bromidea
Infrared
Gas
Raman
Liquid
Assignment
Rel. Int.
Solid
3058
Rel. Int.
w
Rel. Int. and Depol.
w 0.47
Solid
3063
3019
Rel. Int.
wib
Approximate description
CH2 antisymmetric stretch
3078 R
3072 Q, A
3066 P
w
3067
3025
2990
w
w1
3032 max
2995 max
vw
vw
3021
3014
vw
w
sh, vw
vw
2989
vw
m 0.06
2994
2978
w
vw
w2, w%
CH2 symmetric stretch
3
/
2967 max
2202 R
2194 Q, C
vw
vs
2949
2212
2202
w
2956
2197
2183
vw 0.11
sh,s 0.07
vs. 0.07
2953
2220
2205
w
w3, w%
CH stretch
2
m
vs
m
vs
w4
w5
SiH2 antisymmetric stretch
SiH2 symmetric stretch
2187 Q, A
2181 P
vs
1940 R
1937 min, A
1931 P
vw
w
Overtone
6
1604 R
(
1598 QA
1593 P
1589
1588
m
m
1596
m 0.01
1591
m
w6
CC stretch
1
1421 Q, A
1430
vw
1
1409 R
1403 Q, A
1398 P
1405
1382
m
vw
1409
1402
w 0.27
w 0.27
1409
1385
w
vw
w7
CH2 deformation
CH in-plane bend
1274 R
1270 Q,A
1264 P
1295
1266
vw
1263
1259
vw
vw
m 0.06
1260
1015
m
w
w8
1008 max
w
w
1014
1012
1003
999
Vw
vw
s
1013
bd,w 0.66
w9
CH2 wag
CH2 twist
1001 Q
997 Q
w10
m

Table 1 (Continued)
Infrared
Raman
Liquid
Assignment
Gas
Rel. Int.
Solid
972
Rel. Int.
s
Rel. Int. and Depol.
sh,vw 0.75
Solid
976
Rel. Int.
vw
wib
Approximate description
971 Q,C
967 QC
m
m
970
944
w%
11
CH2 rock
960 R
955 min, B
948 P
m
m
bd,w 0.75
w%
12
948 R
943 min, B
939 P
927
912
s
vw
944
840
734
695
666
bd,w 0.75
w 0.60
924
m
w12
SiH2 deformation
855 R
850 Q, A
844 Q, A
844
837
820
sh,m
s
s
vs
vs
848
830
w
vw
w%
13
w13
SiH2 wag
SiH2 twist
740 R
735 Q,C
727 Q,C
/
w
w
737
691
w
w 0.33
747
m
w18
w%
18
696 R
693 min, B
685 P
703
698
sh,m
m
w
m
m 0.13
w15
S-C stretch
669 R
664 min, B
657 P
vw
w 0.18
vw
w%
15
609 Q, C
592 Q, C
m
m
610
590
w%
16
5
586
m
vw
592
602
vw
522
515
vw
w16
w17
w%
SiH2 rock
(
512
505
sh,w
m
m
w
5
500 max
w
498
w 0.33
CH bend (out-of-plane)
469 sh
434 max
vw
vw
455
m
460
430
404
sh,vw
m 0.05
s 0.06
459
vw
7
393
382
w
m
396
387
sh,m
s
w1188, w%
SiBr stretch
17
352
280
168
140
w 0.27
m 0.09
sh,m 0.75
sh,m
w%
19
w19
w20
w%
20
w21
280
190
vw
vw
281
205
m
m
SiCC bend
CSiBr bend
190 max
w
137
Asymmetric torsion
Lattice modes
122
98
80
48
34
73
50
42
a Abbreviations used: s, strong; m, moderate; w, weak; v, very; bd, broad; sh, shoulder; A, B, and C refer to infrared band envelopes; P, Q, and R refer to the
rotational-vibrational branches.
b w and w% refer to the assignments made for the gauche and cis conformers, respectively.

Table 2
Structural parametersa, rotational constants, dipole moments and energy for vinyl silyl bromide
Parameters
RHF/6-31G(d)
MP2/6-31G(d)
MP2/6-311+G(d,p)
MP2/6-311+G(2d,2p)
MP2/6-311++G(2d,2p)
gauche
cis
gauche
cis
gauche
cis
gauche
cis
gauche
cis
SiBr
C3Si
C4C3
SiH5
SiH6
C3H7
C4H8
C4H9
C3SiBr
C4C3Si
H5SiBr
H6SiBr
H7C3Si
H8C4C3
H9C4C3
C4C3SiBr
H5SiBrC3
H6SiBrC3
H7C3SiC4
H8C4C3Si
H9C4C3H8
A
2.231
1.858
1.325
1.469
1.469
1.079
1.077
1.077
110.8
122.1
107.3
106.2
119.3
122.2
122.2
114.7
119.8
2.228
1.864
1.324
1.470
1.470
1.081
1.077
1.076
2.229
1.850
1.344
1.480
1.480
1.089
1.087
1.087
110.3
121.1
107.6
106.6
120.2
122.3
121.8
113.0
118.8
2.226
1.857
1.342
1.481
1.481
1.091
1.087
1.086
2.234
1.851
1.347
1.472
1.471
1.089
1.087
1.088
109.8
121.7
108.2
106.4
120.0
121.9
121.6
117.0
118.9
−121.5
179.3
179.3
2.230
1.857
1.345
1.472
1.472
1.091
1.087
1.086
2.253
1.857
1.340
1.468
1.467
1.082
1.080
1.081
109.5
121.4
107.8
105.9
120.4
121.9
121.5
117.1
118.9
2.247
1.853
1.338
1.468
1.468
1.084
1.080
1.080
2.253
1.847
1.340
1.468
1.467
1.082
1.080
1.081
109.5
121.4
107.8
105.9
120.4
121.9
121.4
117.0
118.9
2.247
1.8.3
1.338
1.468
1.468
1.084
1.080
1.080
/
110.6
109.3
109.9
125.2
109.4
109.4
125.5
107.1
107.1
116.3
121.9
122.2
0.0
121.6
−121.5
180.0
180.0
180.0
6859.2
1938.1
1537.7
124.3
107.4
107.4
117.4
122.0
121.6
0.0
121.3
−121.3
180.0
180.0
180.0
6689.0
1994.9
1564.6
124.9
107.3
107.3
117.5
121.6
121.3
0.0
121.0
−121.0
180.0
180.0
180.0
6710.2
1965.1
1547.0
124.9
107.3
107.3
117.5
121.6
121.3
0.0
121.0
−121.0
180.0
180.0
180.0
6710.8
1965.4
1547.2
107.5
107.5
117.1
121.5
121.7
0.0
121.0
−121.0
180.0
180.0
180.0
6739.0
1962.6
1547.1
5
−121.6
179.6
179.3
−122.0
180.3
179.8
−121.6
179.6
179.5
−121.6
179.6
179.4
(
179.9
180.0
179.7
179.8
179.8
1
10 335.4
1474.1
1368.4
2.504
10 086.0
1490.2
1381.2
2.483
10 290.0
1475.7
1369.0
10 238.6
1471.0
1363.3
10 231.8
1471.4
1363.7
B
C
1
ꢀvaꢀ
2.135
0.188
0.000
2.143
2.102
0.145
0.000
2.107
2.512
0.352
0.016
2.537
2.105
0.291
0.000
2.125
2.467
0.258
0.021
2.480
2.057
0.219
0.000
2.069
2.464
0.258
0.022
2.478
2.055
0.220
0.000
2.067
ꢀvbꢀ
0.225
0.040
2.515
0.487251
0.193
0.043
2.491
0.975143
ꢀvcꢀ
ꢀvtꢀ
−(E+2937)
0.486491
0.974977
4.066079
4.065415
4.126099
4.126056
4.126468
4.126383
DE (cm−1
)
167
36
146
10
19
a
,
Bond distances in A, bond angles in degrees, rotational constants in MHz, and dipole moments in Debye.

1072
G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
Table 3
4. Conformational stability
Symmetry coordinates for vinyl silyl bromide
There are several fundamentals, which show
conformer doublets in both the infrared and Ra-
man spectra of the fluid phases. The two bands at
693 and 664 cm−1 in the Raman spectrum of
liquid are assigned to the Si–C stretching modes,
but in the spectrum of the solid only the higher
frequency band remains. Similarly, the two Q-
branches at 609 and 592 cm−1 in the infrared
spectrum of the gas are assigned as SiH2 rocks,
but this time the lower frequency band remains in
the spectrum of the solid. Other bands which are
observed in the spectra of the fluid phases and
amorphous solid but not present in the spectrum
of the annealed solid are observed at 971, 727, 434
Species
A
Description
Symmetry
Coordinatea
CH2 antisymmetric
stretch
CH stretch
CH2 symmetric stretch
SiH2 antisymmetric
stretch
SiH2 symmetric stretch
CC stretch
CH2 deformation
CH in-plane bend
CH2 wag
CH2 twist
CH2 rock
S1=r1+r2
S2=r3
S3=r1+r
S4=r4−r5
S5=r4+r5
S6=R2
S7=2a−b1−b2
S8=c1−c2
S9=b1−b2
S
S
S
10=m
11=j1
SiH2 deformation
12=4o−u1−u2−g1
and 348 cm−1
.
−g2
These data clearly show that there are two
conformers present in the fluid phases at ambient
temperatures but only one rotamer remains in the
polycrystalline solid. The band at 352 cm−1 is the
only one predicted by the ab initio calculations in
the 300 cm−1 region and is definitely assigned to
the SiꢀCꢁC bend of the cis conformer. Since this
band disappears from the spectrum of the solid
(Fig. 4), it can be concluded that only the gauche
conformer remains in the annealed solid. Further
support for this conclusion is found from the
assignments for the Si–C stretch and SiH2 rock
which are assigned at 664 and 609 cm−1 for the
cis form in the gas phase and at 693 and 592
cm−1, respectively, for the gauche rotamer. Only
the latter gauche bands remain in the spectrum of
the polycrystalline solid. Similarly, the other listed
bands which disappear upon solidification and
annealing are all assigned as arising from the cis
conformer, and they will be discussed later.
Therefore, all the spectral data indicate that the
gauche form is the stable conformer in the an-
nealed solid.
SiH2 wag
SiH2 twist
S
S
S
S
S
S
S
S
13=g1+g2−u1−u2
14=u1−u2−g1+g2
15=R1
16=u1−u2+g1−g2
17=j2
18=T
Si–C stretch
SiH2 rock
CH bend (out-of-plane)
SiBr stretch
SiCC bend
19=2p−c1−c2
20=5v−o−u1−u2
CSiBr bend
−g1−g2
S21=t
Asymmetric torsion
a Symmetry coordinates S4, S11, S13, S14, S16, S17 and S21 are
A%% modes for the cis conformer.
complete set of internal coordinates used to form
the symmetry coordinates are listed in Table 3.
The resulting force fields in internal coordinates
for the gauche and cis conformers can be obtained
from the authors. These force constants were
input along with the unsymmetrized G-matrices
and scaling factors, into the perturbation program
written by Schachtschneider [15]. Initially, all scal-
ing factors were kept fixed at a value of 1.0 to
produce the pure ab initio calculated vibrational
frequencies. Scaling factors of 0.88 for carbon–
hydrogen stretches, 0.90 for silicon–hydrogen and
heavy atom stretches, carbon–hydrogen bends
and 1.0 for heavy atom bends and asymmetric
torsion were used to obtain the ‘fixed scaled’ force
fields and vibrational wavenumbers along with the
potential energy distributions (P.E.D.). All of
these data are listed in Table 4 for the gauche and
cis conformers.
In order to gain information about enthalpy
difference between the two conformers variable
temperature studies in liquified xenon and kryp-
ton were attempted. Unfortunately, the com-
pound decomposed to siloxanes when dissolved in
both rare gas solutions. The decomposition was
believed to be due to a small amount of water in
the noble gases. In another attempt to determine

Table 4
Observed and calculated frequencies for gauche and cis vinyl silyl bromide
Description
gauche
cis
ab initioa Fixed
scaleb
IR.
Raman
act.d
dp.
Obs.e
3072
P.E.D.
99S1
ab initioa Fixed
scaleb
IR.
Raman
act.d
dp.
Obs.e
3072
P.E.D.
99S1
Int.c
Ratiod
Int.c
Ratiod
A
w1
CH2 antisymmetric
stretch
3286
3083
9.5
81.6
0.64
3297
3092
6.8
58.9
0.69
w2
w3
w4
CH stretch
3216
3097
2348
3017
2999
2228
2.0
7.9
134.8
148.5
41.3
64.8
0.20
0.56
0.74
2995
2967
2194
91S2
92S3
100S4
3188
3208
2340
2990
3006
2219
3.9
7.2
142.1
97.0
142.6
83.3
0.50
0.16
0.75
2967
2995
2194
87S2, 12S3
87S3, 12S2
100S4
CH2 symmetric stretch
SiH2 antisymmetric
stretch
/
w5
w6
w7
w8
w9
w10
SiH2 symmetric stretch
CC stretch
CH2 deformation
CH (in-plane-bend)
CH2 wag
2333
1675
1485
1319
1058
1053
2213
1589
1409
1253
1035
1004
108.5
11.0
19.1
0.6
11.7
17.8
157.4
30.9
33.0
15.1
0.7
0.05
0.16
0.38
0.29
0.74
0.75
2187
1598
1403
1270
1008
1001
100S5
2332
1680
1483
1320
1053
1052
2212
1594
1404
1258
1033
999
130.6
3.9
18.1
3.4
41.6
31.6
196.3
22.0
24.4
10.9
4.6
0.06
0.14
0.46
0.27
0.68
0.75
2187
1598
1403
1270
1008
1001
100S5
61S6, 33S7
66S7, 26S6
60s8, 26S9
61S9, 29S8
63S10, 29S17
63S6, 30S7
70S7, 24S6
62S8, 26S9
61S9, 28S8
62S10, 15S9,
258S17
CH2 twist
1.1
1.0
w11
w12
w13
w14
w15
w16
CH2 rock
SiH2 deformation
SiH2 wag
SiH2 twist
SiC stretch
SiH2 rock
997
986
904
780
729
606
946
935
857
742
693
577
72.3
102.4
366.1
6.5
22.1
43.0
5.4
0.64
0.75
0.71
0.75
0.33
0.54
967
943
850
735
693
592
88S11
89S12
96S13
73S14, 12S15
63S15, 17S14
73S16
1009
1001
890
762
690
957
952
844
728
654
617
148.0
17.4
306.5
29.9
8.1
24.2
0.2
13.3
5.1
7.0
7.9
0.71
0.75
0.70
0.75
0.34
0.75
971
955
844
727
664
609
99S11
96S12
86S13
78S14
81S15
65S16, 18S15
10S16
47S17, 28S16
20S10
66S18, 17S19
56S19, 32S18
79S20, 19S19
98S21
15.1
12.0
10.7
6.0
5
3.0
647
46.5
,
,
)
w17
CH bend (out-of-plane)
523
505
20.0
10.5
0.73
500
36S17, 16S18,
10S20, 19S10
77S18, 10S17
78S19, 14S21
76S20
420
399
0.2
12.7
0.75
393
w18
w19
w20
w21
SiBr stretch
SiCC bend
CSiBr bond
Asymmetric torsion
426
278
168
74
405
271
166
74
24.3
0.7
2.0
0.1
7.6
3.7
5.7
9.2
0.21
0.32
0.73
0.74
393
460
369
148
116
440
360
147
116
36.4
8.6
0.7
0.1
8.0
4.2
2.0
5.0
0.13
0.64
0.64
0.75
434
352*
140*
–
1
280*
168*
(137)
7
77S21, 11S20
a Calculated with the MP2/6-31G(d) basis set.
b Scaling factors of 0.88 for carbon-hydrogen stretches, 0.90 for silicon-hydrogen and heavy atom stretches, carbon–hydrogen bends and 1.0 for heavy atom bends and asymmetric torsion.
c Calculated infrared intensities in km/mol.
d
4
,
Calculated Raman activities in A /amu, and dp ratio using RHF/6-31G(d) basis set.
e Frequencies are taken from the infrared spectrum of the gas, except the one in parentheses, which is taken from the Raman spectrum of the solid and the ones with asterisks are taken from the Raman spectrum of
the liquid.
f For a description of the symmetric coordinates see Table 3.

1074
G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
DH, a variable-temperature study of the Raman
spectra of the liquid was performed (Fig. 6). The
relative intensities of the Raman lines at 690, 663,
428 and 397 cm−1 were measured. By utilizing the
equation −lnK=(DH/RT)−(DS/R), one can
determine DH from a plot of −lnK versus the
inverse temperature where DH/R is the slope of
the line. In this equation, K is the ratio of Ic/Ig,
where I is the intensity of the Raman lines, and it
Five sets of spectral data were obtained at
various temperatures ranging from 0 to −87°C
(Table 5). A value of 24913 cm−1 was obtained
from the 690/663 cm−1 pair and 1993 cm−1
from the 397/428 cm−1 pair. The average value
from these two determinations is 2296 cm−1
(0.2690.08 kJ/mol) with the gauche conformer
the more stable rotamer.
is assumed that DH is not
temperature.
a function of
5. Vibrational assignments
The conformational analysis of vinyl silyl bro-
mide shows that the molecule exists in two stable
conformations in the gas and liquid states. The cis
conformer has a plane of symmetry which in-
cludes the a and b axis (Cs) and the 21 normal
modes span 14A% and 7A%% species. The 14 A%
modes should produce polarized Raman bands
and A/B-type infrared band contours, whereas the
7 A%% modes should be depolarized in the Raman
spectrum and give rise to C-type infrared band
envelopes. The gauche conformer has only the
trivial C1 symmetry and all 21 vibrations should
yield polarized Raman bands and A/B/C-type
infrared band contours. Guided by these consider-
ations and also by the calculated spectral intensi-
ties predicted wavenumbers from the ab initio
calculations, we proposed the vibrational assign-
ments in Table 1.
The assignments of the carbon–hydrogen
modes have been previously reported [10,11] for
vinyl silyl chloride, and with only minor
wavenumber shifts, they remain essentially the
same for vinyl silyl bromide. In fact, the spectra
of these two compounds look remarkably similar
down to about 1000 cm−1. The SiH2 stretches
give rise to a very strong and polarized band in
the Raman spectrum of the liquid, which splits
into two bands, one at 2194 cm−1 for the anti-
symmetric stretch and another at 2187 cm−1 for
the symmetric stretch in the spectrum of the solid.
The corresponding SiH2 wag is well resolved and
appears as Q-branches at 850 cm−1 for the
gauche form and 844 cm−1 for the cis conformer
in the infrared spectrum of the gas. The C-type
Q-branches located at 735 and 727 cm−1 in the
infrared spectrum are assigned to the SiH2 twists
Fig. 6. Temperature dependent Raman spectra of vinyl silyl
bromide.
Table 5
Temperature and intensity ratios for the conformational study
of liquid vinyl silyl bromide
T (°C)
1000/T (K)
I
690/I663
I
397/I428
0
−24
−42
−64
−87
DHa
3.66
4.02
4.33
4.78
5.38
1.9037
1.8482
1.7610
1.8218
1.7714
2.5810
2.5819
2.5249
2.5072
2.4737
24913
1993
a Average value of 2296 cm−1 (0.2690.08 kJ/mol) with
the gauche conformer the more stable form.

G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
1075
with the gauche conformer having the higher fre-
quency, whereas the other C-type Q-branches lo-
cated at 609 and 592 cm−1 are assigned to the
SiH2 rock with the lower frequency band assigned
to the gauche rotamer.
the gauche conformer compared to this parameter
in the cis rotamer, again for all basis sets. Addi-
tionally, a comparison of the structural parame-
ters obtained for vinyl silyl bromide with the
corresponding ones previously reported [11] for
vinyl silyl chloride reveals many interesting simi-
larities. Comparing structural parameters ob-
tained from the MP2/6-311+G(d,p) calculations
for the bond distances and bond angles between
these two molecules shows that the bond distances
The Si–C stretching conformer pair is observed
in the Raman spectrum of the liquid at 695 and
666 cm−1 for the gauche and cis conformers,
respectively, where only the former band is ob-
served in the spectrum of the crystalline solid. The
polarized Raman bands at 430 and 404 cm−1 are
assigned to Si–Br stretch for the gauche and the
cis conformers, respectively. The SiCꢁC bend vi-
bration at 348 cm−1 of the cis form is evident in
the far-infrared spectrum of the amorphous solid
but disappears upon annealing (Fig. 4). The ab
initio calculations predict very weak infrared in-
tensities for the CSiBr bend and asymmetric tor-
sions for both conformers. They are expected to
be about 160 and 140 cm−1 for the former mode
and about 74 and 116 cm−1 for the latter mode
for the cis and gauche conformers, respectively.
These modes are not observed in the far infrared
spectrum of the gas, but the CSiBr bend of the
gauche form is assigned to the band at 190 cm−1
observed the infrared spectrum of the solid. The
two Raman lines at 168 and 140 cm−1 in the
spectrum of the liquid are assigned to the CSiBr
bends of the gauche and cis rotamers, respectively.
,
are within 0.007 A and the angles are within 2°
with the exception of the Si–Cl and Si–Br dis-
tances and angles.
The frequencies for the Si–H stretching modes
in vinyl silyl bromide are exactly the same as
those found for the corresponding modes in vinyl
silyl chloride. Since we made the isotopomer
CH2CHSiHDCl, we utilized the isolated CH
stretching vibrational frequency to obtain the SiH
,
bond distance. We obtained a value of 1.480 AA
for this distance for both the cis and gauche
conformers. Therefore, we expect the same values
for the SiH bond distance in the bromide. As will
be seen in Table 2, the predicted SiH distance is
1.472 A from the MP2/6-311+G(d,p) calcula-
tions which is essentially the same values pre-
dicted at this level of calculation for the chloride.
Therefore, the SiH distance is estimated to be too
,
short by approximately 0.008 A from this level of
The asymmetric torsion is assigned at 137 cm−1
a
calculation; however, the MP2/6-31G(d) calcula-
tions predict the correct values for the SiH dis-
tances. Many of the other parameters from the
MP2/6-31G(d) calculations are similar to those
predicted with the larger basis set, MP2/6-311+
G(d,p), which indicates that the predicted
parameters from the smaller basis set, i.e. MP2/6-
31G(d), are reasonably accurate if one uses full
electron correlation. Thus, such calculations can
be utilized to predict reasonably well structural
parameters for these types of molecules.
Raman band in the spectrum of the annealed
solid whereas no band was observed to be as-
signed for the same mode for the cis rotamer.
6. Discussion
The structural parameters listed in Table 2 are
fairly consistent among the three ab initio calcula-
tions with full electron correlation with the major
exceptions occurring for the Si–Br and Si–H
distances. The Si–Br distance becomes longer and
the Si–H distance shorter with the larger basis
sets with polarization functions. The C–Si bond
We have measured the depolarization values of
most of the pronounced Raman lines (Table 1)
and the values have been obtained from the RHF/
6-31G(d) calculations. For many of them the
agreement is satisfactory, particularly for the ones
that have depolarization ratios near 0.75. How-
ever, there are a significant number of them that
have depolarization ratios lower than those pre-
,
distance is approximately 0.006 A shorter for the
gauche conformer compared to the corresponding
parameter for the cis form with all basis sets. The
CCSi bond angle is approximately 3.5° smaller for

1076
G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
Table 6
quencies, scattering activities and intensities deter-
mined from the MP2/6-31G(d) calculations. The
Gaussian-94 program [13] with the option of cal-
culating the polarizability derivatives was used.
The evaluation of Raman activity by using the
analytical gradient method has been developed
[16,17]. The activity Sj can be expressed as:
Potential energy parameters (cm−1) for the confortnational
interchange of vinyl silyl bromide
Parameter
mp2/6-31G(d)
V1
V2
V3
V4
37
−153
671
90
Sj=gj(45h2j +7ij2)
V5
3
V6
DE
−49
36
where gj is the degeneracy of the vibrational mode
j, hj is the derivative of the isotropic polarizability,
and ij is that of the anisotropic polarizability.
The Raman scattering cross sections, #|j/#V,
which are proportional to the Raman intensities,
can be calculated from the scattenng activities and
the predicted frequencies for each normal mode
using the relationship [18,19]:
dihedral angle
cis/gauche barrier
gauche/gauche barrier
gauche/cis barrier
113.0
634
747
670
Fig. 7. Potential function of the asymmetric torsion of vinyl
silyl bromide from MP2/6-31G(d) calculations. A dihedral
angle of 0° corresponds to the cis conformer.
dicted from the ab initio calculations. Neverthe-
less, there is some useful information obtained
from the predicted depolarization ratios for mak-
ing the vibrational assignment.
The potential function parameters obtained
from the data of the MP2/6-31G(d) calculations
are listed in Table 6. The barriers of the cis to
gauche, gauche to gauche and gauche to cis wells
are 634, 747 and 670 cm−1, respectively with an
energy difference of 36 cm−1. Therefore, the po-
tential function is nearly threefold, with the
gauche and cis conformers having almost the same
barriers for interconversion (Fig. 7).
Fig. 8. Raman spectra of vinyl silyl bromide: (A) experimental
spectrum of the liquid; (B) calculated spectrum of the mixture
(DH=22 cm−1); (C) calculated spectrum of the cis con-
former; (D) calculated spectrum of the.gauche conformer.
Raman and infrared spectra (Figs. 3 and 8) for
vinyl silyl bromide were calculated using the fre-

G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
1077
(|j
(V
24y4
45
:
(w0−wj)4
;
h
ꢁ ꢂ
ꢁ ꢂ
where the Qi is the ith normal coordinates, Xj is
the jth Cartesian displacement coordinates, Lji is
the transformation matrix between the Cartesian
displacement coordinates and normal coordinates.
The infrared intensities were then calculated by:
=
Sj
−hc6
kT
8y2cwj
ꢃ n
j
1−exp
where w0 is the exciting frequency, wj is the vibra-
tional frequency of the jth normal mode, h, c and
k are universal constants, and Sj is the corre-
sponding Raman scattering activity. To obtain
the polarized Raman scattering cross section, the
polarizabilities are incorporated into Sj by Sj[(1−
rj)/(1+rj)] where rj is the depolarization ratio of
the jth normal mode. The Raman scattering cross
sections and calculated frequencies are used to-
gether with a Lorentzian line shape function to
obtain the calculated spectrum. Since the calcu-
lated frequencies are :10% higher than those
observed, the frequency axis of the theoretical
spectrum was compressed by a factor of 0.9. The
predicted Raman spectra of the cis and gauche
pure conformers are shown in Fig. 8C and D,
respectively. The predicted Raman spectrum of
the mixture of the two conformers with a deter-
mined DH of 22 cm−1 is shown in Fig. 8B. This
spectrum should be compared to the experimental
spectrum of the liquid (Fig. 8A). The calculated
Raman spectrum is quite similar to the experi-
mental spectrum with the exception of the intensi-
ties of the CH2 deformation and CH (in-and
out-of-plane bends) fundamentals which are pre-
dicted too weak. The same problem was encoun-
tered with the calculated spectrum of vinyl silyl
chloride [11] where the same type of modes were
also predicted to be weak compared to the exper-
imental intensities. Actually, the calculated dipole
moment of the gauche conformer is larger than
that of the cis (Table 2), which should stabilize the
gauche form in the condensed phases. This factor,
coupled with other packing interactions, may be
the reason why the gauche conformer is more
stable in the solid.
2
2
Nv (v
(v
(v
ꢃꢁ ꢂ ꢁ ꢂ ꢁ ꢂn
x
y
z
2
Ii=
+
+
3c2 (Qi
(Qi
(Qi
In Fig. 3C and D, the predicted infrared spectra
of the two conformers are shown. The combina-
tion of the spectra of the two conformers with a
DE of 36 cm−1 predicted from MP2/6-31G(d)
calculation is shown in Fig. 3B. The experimental
infrared spectrum of the gas is also shown for
comparison. Excluding the overtones, and combi-
nation bands that are present in the spectrum of
the gas, the agreement between the observed and
calculated spectra is satisfactory and it provided
support for the vibrational assignments.
The vibrational modes of CH2CHSiH2Br are
relatively pure as can be seen from the P.E.D.
given in Table 4. The exception is in the CH bend
(out-of-plane) mode, which is extensively mixed
with the CH2 twist, Si-Br stretch and CSiBr bend
for the gauche conformer and the CH2 twist and
SiH2 rock for the cis form.
Using scaling factors of 0.88 for the carbon–
hydrogen stretches, 0.90 for all the other vibra-
tions except 1.0 for the heavy atom bends and
asymmetric torsion, the wavenumbers for the fun-
damentals are predicted from the MP2/6-31G(d)
calculation to be within 1.2% for the gauche con-
former and 1.15% for the cis conformer. There-
fore, the frequency predictions from the
MP2/6-31G(d) calculations are quite good and
very helpful in the vibrational assignment. These
results show that two scaling factors are sufficient
to adequately predict the frequency for the ob-
served fundamentals for this type of molecule.
All the basis sets favor the gauche conformer as
the more stable form. The DE varies from 36
cm−1 from the MP2/6-31G(d) calculations to 10
cm−1 from the MP2/6-311+G(2d,2p) calcula-
tions with full electron correlation which is a
reasonable trend. However, the DE has a value of
146 cm−1 from the MP2/6-311+G(d,p) calcula-
tion. The failure of the variable temperature study
in the noble gases to determine the enthalpy dif-
Infrared intensities were also calculated based
on the dipole moment derivatives with respect to
the Cartesian coordinates. The derivatives were
taken from the ab initio calculations and trans-
formed to normal coordinates by:
(v
(v
ꢁ ꢂ ꢁ ꢂ
v
v
=%
Lji
(Qi
(Xi
j

1078
G.A. Guirgis et al. / Spectrochimica Acta Part A 56 (2000) 1065–1078
[6] B.J. van der Veken, W.A. Herrebout, D.T. Durig, W.
Zhao, J.R. Durig, J. Phys. Chem. A 103 (1999) 1976.
[7] J.R. Durig, D.T. Durig, M.R. Jalilian, M. Zhen, T.S.
Little, J. Mol. Struct. 194 (1989) 259.
[8] J.R. Durig, D.T. Durig, B.J. van der Veken, W.A. Herre-
bout, J. Phys. Chem. 104 (1999) in press.
[9] J.F. Sullivan, M.A. Qtaitat, J.R. Durig, J. Mol. Struct.
Theochem. 202 (1989) 159.
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Phys. Chem. 91 (1991) 1563.
ference between the two conformers does not give
the opportunity to check the ab initio predictions.
Since the ab initio calculation predict higher
dipole moment for the gauche form rather than
the cis form, it is expected that gauche form will
be slightly more stable in condensed phases rela-
tive to its stability in the gas phase. Therefore, the
DH of 2296 cm−1 with the gauche conformer
the more stable form is expected to be the higher
limit for the DH in the gas.
[11] J.R. Durig, Y.E. Nashed, M.A. Qtaitat, G.A. Guirgis, J.
Mol. Struct. 400 (1999) in press.
[12] F.A. Miller, B.M. Harney, Appl. Spectrosc. 24 (1970)
291.
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Acknowledgements
JRD acknowledges the University of Kansas
City Trustees for a Faculty Fellowship award for
partial financial support of this research.
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