Journal of Molecular Structure 554 (2000) 251±269
www.elsevier.nl/locate/molstruc
Conformational equilibrium in dimethyl vinyl ¯uorosilane studied
by infrared and Raman spectroscopy
A. Horn a, P. Klaeboe a, V. Aleksa a,b, A. Gruodis a,b, C.J. Nielsen a, Y.E. Nashed c,
G.A. Guirgis c,1, J.R. Durig c,*
a
Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway
Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania
c
Department of Chemistry, University of Missouri-Kansas City, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA
b
Received 12 April 2000; accepted 30 May 2000
Abstract
The Raman spectra (3500±20 cm 21) of liquid with depolarization values and solid, as well as the infrared spectra of the gas,
the sample isolated in argon and nitrogen matrices at ca. 5 K and solid dimethyl vinyl ¯uorosilane, CH2yCHSi(CH3)2F, have
been recorded. Both gauche and syn rotamers have been identi®ed in the ¯uid phases but only the syn conformer remains in the
solid. Variable temperature (255 to 21508C) studies of the infrared spectra (4000 and 400 cm 21) of dimethyl vinyl ¯uorosilane
dissolved in liquid xenon and krypton have been recorded. From the xenon and krypton data, the enthalpy differences have been
determined to be 53 ^ 9 cm 21 (0.64 ^ 0.10 kJ/mol) and 44 ^ 7 cm 21 (0.53 ^ 0.09 kJ/mol), respectively, with the gauche
conformer being the more stable form. The intensity variations with temperature of the Raman spectrum of the liquid gave
an enthalpy difference of 25 ^ 15 cm 21 (0.30 ^ 0.18 kJ/mol) also with the gauche conformer being the more stable form.
Vibrational assignments are provided for both conformers. Complete equilibrium geometries have been determined for both
rotamers using ab initio calculations employing the 6-31G(d), 6-3111G(d,p) and 6-3111G(2d,2p) basis sets at the levels of
restricted Hartree±Fock (RHF) and/or with full electron correlation by the perturbation method, Moller±Plesset (MP), to
second order. The syn conformer is predicted to be the more stable conformer from all ab initio calculations except those of
MP2/6-31(d) which predict the gauche form being the more stable conformer by 54 cm 21 (0.65 kJ/mol) although the values
favoring the syn form are all very small. These results are compared to the corresponding quantities of some similar molecules.
q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Conformational stability; FT-IR spectra; Ab initio calculations; Dimethyl vinyl ¯uorosilane
1. Introduction
A number of silanes in which the silicon atom is
* Corresponding author. Tel.: 11-816-235-1136; fax: 11-816235-5191.
E-mail address: durigj@umkc.edu (J.R. Durig).
1
Permanent address: Analytical Research and Development
Department, Bayer Corp., P.O. Box 118088, Charleston, SC
29423, USA.
attached to a sp 2 hybridized carbon atom have been
investigated by infrared and Raman spectroscopy.
When the silicon atom has different substituents
attached, these molecules will have possibilities for
conformational equilibria. The vinyl silanes
CH2yCHSiX2Y in which X and Y are different groups
will, like the corresponding propenes, exist in a syn
conformer with a plane of symmetry and in two
equivalent gauche conformers. Thus, from the
0022-2860/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.
PII: S 0022-286 0(00)00677-3
252
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Fig. 1. Raman spectra of dimethyl vinyl ¯uorosilane in two polarization directions, 1700±800 cm 21 with ordinate scale 0±3000 counts, 700±
100 cm 21 with 0±30,000 counts.
vibrational spectral data [1,2] and microwave investigations [3] it was reported that vinylsilylchloride is
present in syn and gauche conformers in the vapor and
liquid states, whereas the gauche conformer is the
stable form in the crystal. However, for the vapor
state these investigators ®rst reported [1] the gauche
as the more stable form but from a later investigation
[2], Si-d2 isotopomer, the syn rotamer was reported as
the more stable conformer. Although the gauche
rotamer was calculated to be the more stable
conformer from ab initio RHF/6-31G(d) calculations,
the syn conformer was determined to be more stable in
the liquid by variable temperature Raman studies
[4,5]. However, it should be noted that the conformer
that is the most stable form in the liquid, may not be
the most stable rotamer in the gas. Therefore, we [6]
carried out a variable temperature FT-IR investigation
of rare gas solutions of vinylsilyl chloride. These
studies indicated that the gauche was more stable by
78 ^ 11 cm 21 (0.93 ^ 0.13 kJ/mol).
Infrared and Raman studies combined with ab initio
calculations have also been carried out for dimethyl
vinyl chlorosilane [7] and for methyl vinyl dichlorosilane [8]. Since the methyl group and the chlorine
have approximately the same size, the conformational
preference is by no means obvious for these molecules. In the former molecule the gauche conformer
was more stable and was present in the crystal [7]
whereas in the latter the syn conformer had lower
energy and was the sole conformer present in the
crystal [8]. These results indicate a preference of the
methyl group eclipsing the double bond over
the chlorine atom.
When these studies are extended to ¯uorine substituents it was observed that in methyl vinyl di¯uorosilane [9,10] the gauche conformer has a lower energy
in the gas and liquid whereas the syn form is present in
the crystal. In order to obtain more information on the
relative stability of the conformers of dimethyl vinyl
halosilanes we have recorded the infrared and Raman
spectra to determine the conformational stability of
dimethyl vinyl ¯uorosilane, CH2yCHSi(CH3)2F,
(DVFS), and the results of this study will be described
in the present paper.
2. Experimental
The sample was prepared by the reaction between
the chloro derivative CH2yCHSi(CH3)2Cl, and freshly
sublimed antimony tri¯uoride at room temperature
without solvent for 1 h. The sample was puri®ed by
a low temperature, low pressure fractionation column
and the purity was checked by mass spectrometry.
The Raman spectra of the liquid, amorphous solid
and crystal were obtained at different temperatures in
a capillary tube of 2 mm inner diameter, surrounded
by a Dewar, cooled by gaseous nitrogen evaporated
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
253
Fig. 2. Raman spectra (1050±50 cm 21) of amorphous (solid line) and annealed (dotted line), crystalline solids of dimethyl vinyl ¯uorosilane.
from a reservoir [11]. These spectra were employed
for calculating the enthalpy difference DH between
the conformers in the liquid. DVFS has a pronounced
hysteresis (undercooling) and it was possible to study
the liquid far below the freezing point. The crystallization often occurred spontaneously at ca. 21238C
and the anisotropic crystal containing only one
conformer was obtained. Independently, the vapor
of DVFS was condensed on a copper ®nger at
21968C, and the Raman spectrum of the amorphous
phase was recorded. Subsequently, the amorphous
solid was annealed to temperatures slightly below
the melting point, the sample turned crystalline from
Fig. 3. Infrared spectrum of a gas of dimethyl vinyl ¯uorosilane.
visual inspection and was recooled to 21968C before
the spectrum was obtained. The Raman spectra were
recorded digitally using a Dilor RTI-30 spectrometer
(triple monochromator, with a Peltier cooled detector)
coupled to a PC. An argon ion laser from Spectra
Fig. 4. Far infrared spectra of dimethyl vinyl ¯uorosilane: (A) gas;
(B) amorphous (solid line) and annealed solid (dotted line).
254
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Fig. 5. Infrared spectra (1100±400 cm 21) of amorphous (solid line) and crystalline (dotted line) solids of dimethyl vinyl ¯uorosilane.
Physics (model 2000) was employed with perpendicular illumination using the 514.5 nm line for excitation. The Raman spectra are shown in Figs. 1 and 2.
The infrared spectra (Figs. 3±8) were recorded with
Fig. 6. Infrared spectra (1350±800 and 860±500 cm 21) of dimethyl
vinyl ¯uorosilane in an argon matrix unannealed (solid line) and
annealed (dashed line).
various Fourier transform spectrometers: Bruker
models IFS-88 and IFS-66 (4000±450 cm 21), a
Nicolet model 800 (4000±450 cm 21), a Perkin±
Elmer model 2000 (4000±450 cm 21) and on
two vacuum benches: Bruker IFS-113v spectrometer (600±50 cm 21) and Bomem model DA
3.002 (600±50 cm 21). The latter instrument had
Fig. 7. Infrared spectra (1025±950 cm 21) of dimethyl vinyl ¯uorosilane in a nitrogen matrix unannealed (solid line) and annealed.
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Fig. 8. Infrared spectra of dimethyl vinyl ¯uorosilane: (A) experimental spectrum of the liquid Krypton (21258C); (B) calculated
spectrum of the mixture (DH ˆ 44 cm 21); (C) calculated spectrum
of the syn conformer; (D) calculated spectrum of the gauche
conformer.
a helium cooled Bolometer as detector, the other
instruments had detectors of DTGS. Beamsplitters
of Ge substrate on KBr were used in the midinfrared regions (MIR) whereas beamsplitters of
Mylar of thickness 3.5 and 12 m as well as one
of a metal mesh were employed in the far infrared
(FIR) region. The spectrum of the vapor was
recorded with the sample contained in cells with
KBr windows and path length 10 cm in MIR and
in cells of 20 cm and 1 m path lengths with polyethylene windows in the FIR region. The spectra
of the amorphous and crystalline solids were
obtained by depositing the vapor on a CsI window
and on a wedge shaped window of silicon, cooled
with boiling liquid nitrogen, for the MIR and FIR
regions, respectively.
The sample was diluted with argon and nitrogen
(1:500 and 1:1000) and deposited on a CsI window
of a three stage Displex cryostat from APD (model
HS-4) at either 2268 or 22588C. The matrices were
255
subsequently annealed to various temperatures from
2253 to 22368C (argon) and from 2253 to 22398C
(nitrogen) in periods from 10 min to 1 h and the
window was recooled to 22688C and the spectra
recorded (Figs. 6 and 7).
The mid-infrared spectra (Fig. 8) of the samples
dissolved in lique®ed xenon (255 to 21008C) and
krypton (2105 to 21508C) as a function of temperature were recorded on a Bruker model IFS 66 Fourier
transform interferometer equipped with a Globar
source, a Ge/KBr beamsplitter and a TGS detector.
In all cases, 100 interferograms were collected at
1.0 cm 21 resolution, averaged and transformed with
a boxcar truncation function. For these studies, a
specially designed cryostat cell was used. It consisted
of a copper cell with a path length of 4 cm with
wedged silicon windows sealed to the cell with
indium gaskets. It was cooled by boiling liquid
nitrogen to 21968C. The temperature was monitored
with two Pt thermoresistors. The complete cell was
connected to a pressure manifold, allowing the ®lling
and evacuation of the cell. After the cell had cooled to
the desired temperature, a small amount of the
compound was condensed into the cell. Next the pressure manifold and the cell were pressurized with the
noble gas, which immediately started to condense in
the cell, allowing the compounds to dissolve. All
observed infrared and Raman bands with signi®cant
intensities are listed in Table 1.
3. Ab initio calculations
The
LCAO±MO±SCF
calculations
were
performed with the gaussian-94 program [12] with
Gaussian-type basis functions. The energy minima
with respect to the nuclear coordinates were obtained
by the simultaneous relaxation of all of the geometric
parameters, except for the symmetry restrictions for
the gauche and cis conformers, using the gradient
method of Pulay [13]. The structural parameters
were determined from RHF/6-31G(d) (restricted
Hartree±Fock), MP2/6-31G(d) (full electron correlation by the perturbation method to the second order),
MP2/6-3111G(d,p)
and
MP2/6-3111G(2d,2p)
calculations and the results are given in Table 2.
The energy difference that resulted from these various
calculations ranged from 46 cm 21 (0.55 kJ/mol) from
Infrared
256
Table 1
Infrared and Raman spectral data (abbreviations used: s, strong, m, moderate; w, weak; v, very; bd, broad; sh, shoulder; p, polarized; d, depolarized. A, B and C denote vapor
contours; asterisks denote band vanishing in the crystal spectra; arrows pointing upwards and downwards signify matrix bands which increase and decrease in intensities after
annealing; and P, Q, and R refer to the rotational±vibrational branches) for dimethyl vinyl ¯uorosilane (CH2yCHSi(CH3)2F)
Raman
Solid
Vapor
m
3025
w
2986 R
2973 Q,C
2961 P
2915 max
1937 R
1928 Q,A
1919 P
1608 R
1601 Q,A
1595 P
m
w
Amorphous (80 K)
Crystalline (80 K) Liquid
3074 w
3063 m
3027 w
3018W
3071 w
3062 m
3026 w
3019 w
3059 m
3059 m
3064 m,br,P
n10
n1
3018 w/m
3022 w,D?
n2
3000 w
3000 w
2988 vw
3024 w
3018 w
2993 w,sh
2986 w/m,sh
2974 m/w,sh
2963 m
2954 m
2976 m
2970 w
2957 w
2916 vw
1941 vw w
m
1929 vw
1923 vw
1605 w
1603 m
1598 w o
"
1446 w
1442 w
1423 w
1421 vw "
1420 Q
m
1414 Q
1411 Q
1405 sh
m
m
m
1413 m
1408 m
1297 sh
w
1272 R
1266 Q m
1263 Q m
1254 sh
1398 m #
1293 w
1277 w
m
m
m
o
1262 vs
1257 vs "
2982 vw
2975 m
2963 w
2963 s
2957 w
2916 vw
2888 w
2954 s
2907 w
2876 vw
1939 w
1930 w
1601 m #
1599 m "
1597 m/s
1596 w/m
1596 w/m
1447 m
1446 w
2907 w
2882 vw
1930w
1917
Amorphous (80 K)
Crystalline (80 K) Interpretation
n3
2981 vs,P
2960 m,sh,D
n4
2911 vs,P
n 5,n 6
n7
n 8,n 9
o
1601 vs,P
1601 vs,P
1597 m
1597 m
1601 m
1595 m
n11, n12
1425 vw
1421 w #
n13
n14
1415 w
1413 s
1409 m
1403 m "
1401 m #
1294 vw
1277 w #
1263 s
1258 vs
1253 s "
1250 s
n10
n10 0
o
1409 s
1412 m
1410 m
1398 m
1299 w
1397 m
1305 w
1275 w
1271 w
1279vs,P
1272m
1254 vs,br
1257 m
1252 m
1260 vw
1258 vw
1416 vs,P
1412 s
1415 m
1413 m
1402 w,sh
1405 w,sh
1274 m
1270 m
1263 w
1258 w
n15
n15 0
n 16
n 17
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
3071 R
3065 Q,C
3058 P
Ar matrix (5 K) N2 matrix (5 K)
Table 1 (continued)
Infrared
Raman
Solid
Vapor
Ar matrix (5 K) N2 matrix (5 K)
1013 max
m
1007 max
m
904 R
895 Q
886 sh
855 R
850 Q,C
848 Q,C
841 P
806 R
799 Q
789 P
772 Q
m
1244 w
1018 w "
1015
m # 1011 m
Crystalline (80 K) Liquid
Amorphous (80 K)
Crystalline (80 K) Interpretation
1244 m
1252 vw
1250 vw
n 18
1008 w
1009 w
n 19
1004w
1002 w
n 20 n 20 0
n 21 0
n 21
1017 vw
1012 s
1012 s
1008 s #
1006 w
1005 s
1002 s
969 vs "
970 s
959 s
967 w,D
971 w
957 w
967 w,D
963 w
p
883 w,D
874 vw
967 m "
966 s,sh "
964 s #
963 s
961 vs
958 m
890w
966 s #
p
890 m "
893 w,sh
vs
vs
866 vs
884 vs
876 w #
883 vs #
879 vs #
873 m #
876 s
865 s
862 s,sh
vs
vs
847 vs "
845 vs #
850 vs "
848 vs #
845 vvs
846 vs
m
1014 w,D
p
839 m "
805 m
796 m
793 s
788 m/w
769 m
765 m
741 vw
p
n 22
n 22 0
864 w
855 vw
850 w
848 w
n 23
n 23 0
799 m,D
806 w
795 w
n 24
770 m
767 w
805 w
800 w
792w
769 vw
V25
743 vw
742 vw
n 26
801 s "
796 vs #
799 vs
797 vs
772 m #
766 w "
736 vw #
766 s
739w
711 s "
711 s
p
712 m,p
714 m
p
n 27
m
770 M #
768 M "
733 W #
714 W "
712 vs "
710 m "
692 m #
694 m #
695 m
696 m,P
695 m
695 m
n 28,n 27 0
616 R
607 Q
vw
607 w
606 w #
601 w #
607 m
696 m
692 w
613 vs,P
621 m,sh
613 vs
n 29 0
521 max
m
611 vs
530 w
m
527 m "
525 m #
522 w #
526 w,D
515 max
528 s "
526 s #
523 s #
716 R
710 Q,C
703 P
694 sh
vs
m
m
524 s
519 w
n 29
n 30
523 vw
n 30 0
257
532 s
p
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
974 R
969 Q,C
966 P
966 R
964 Q,C
955
1249 w #
1016 m "
1014 m #
1011 m #
1008 m
1007 s
1005 w,sh
Amorphous (80 K)
258
Table 1 (continued)
Raman
Solid
Vapor
400 R
394 Q, A/C
388 P
365 max
273 max
m
m
s
267
sh
218
vw,sh
180
175
170
164
w
vw
w
vw
Ar matrix (5 K) N2 matrix (5 K)
Amorphous (80 K)
Crystalline (80 K) Liquid
520 m "
519 s
519 m
394 s
394 s
396 m,P
397 m
p
275 s
371 m,P
275 m,D
261 s
265 m,P
368
286 w
277 w
263 m
p
225 w,sh
209 w,sh
195 s,D
187 w,sh
178 w
230 w
214 m
201 m
190 w,sh
178 w
101 w
95 vw
104 m,br
515 m "
366 m
275 s
272 s
263 s
228 w
208 w
201 w
179 vw
151 vw
209 w
178 vw
178 vw
151 vw
Amorphous (80 K)
Crystalline (80 K) Interpretation
517 vw
395 m
n 31 0
p
n 31
n 32,n 33 0
285 w
264 m
258 m
n 33,n 32 0
p
n 34
209 m
187 w
175 w
n 35,n 34 0
n 36,n 35 0
n 37
n 36 0
n 39
110 m
102m
64 m
47 m
lattice
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Infrared
Table 2
Ê , bond angles in (8), rotational constants in MHz, and dipole moments in Debye), rotational constants, dipole moments and energy for
Structural parameters (bond distances in A
dimethyl vinyl ¯uorosilane
RHF/6-31G(d)
MP2/6-31G(d)
MP2/6-3111G(d,p)
MP2/6-3111G(2d,2p)
syn
gauche
syn
gauche
syn
gauche
syn
rC1 ±C2
rSi±C2
rF±Si3
rC1 ±H5
rC1 ±H6
rC2 ±H7
rSi±C8
rSi±C9
rC8 ±H10
rC8 ±H11
rC8 ±H12
rC9 ±H13
rC9 ±H14
rC9 ±H15
/SiC2C1
/FSiC2
/H5ClC2
/H6ClC2
/H7C2C1
/C8SiF
/C9SiF
/H10C8Si
/H11C8Si
/H12C8Si
/H13C9Si
/H14C9Si
/H15C9Si
t FSiC2C1
t H5ClC2Si
t H6C1C2H5
t H7C2C1Si
t C8SiFC2
t C9SiFC2
t H10C8SiF
t H11C8SiH10
t H12C8SiH10
t H13C9SiF
t H14C9SiH13
1.325
1.869
1.609
1.078
1.077
1.081
1.876
1.876
1.087
1.086
1.087
1.087
1.087
1.087
124.5
107.4
122.3
122.4
117.4
107.5
106.7
111.7
111.8
110.3
111.2
111.3
111.3
117.9
180.0
180.0
180.0
120.9
2119.0
178.7
121.1
2119.4
2179.3
2119.9
1.325
1.868
1.608
1.078
1.076
1.082
1.876
1.876
1.088
1.086
1.087
1.088
1.086
1.087
123.3
106.4
122.3
121.9
117.8
107.9
107.9
111.1
111.2
111.4
111.1
111.2
111.4
0.0
180.0
180.0
180.0
119.9
2119.9
181.8
119.8
2119.9
2181.8
2119.8
1.344
1.859
1.634
1.088
1.087
1.091
1.866
1.866
1.094
1.093
1.094
1.094
1.093
1.094
123.4
107.7
122.6
121.9
117.5
107.7
107.2
111.3
111.8
110.2
110.7
111.3
111.3
115.0
180.0
180.0
180.0
120.5
2119.2
178.8
121.0
2119.4
2179.3
2119.8
1.344
1.859
1.632
1.087
1.086
1.091
1.867
1.867
1.094
1.093
1.094
1.094
1.093
1.094
122.5
106.2
122.6
121.3
117.8
108.1
108.1
110.8
111.0
111.4
110.8
111.0
111.4
0.0
180.0
180.0
180.0
119.6
2119.6
182.1
119.7
2119.9
2182.1
2119.7
1.347
1.859
1.637
1.087
1.088
1.091
1.861
1.861
1.094
1.093
1.094
1.094
1.094
1.094
123.4
107.3
122.1
121.7
117.2
107.4
106.7
111.4
111.4
110.1
110.7
111.0
111.1
120.7
180.0
180.0
180.0
119.9
2119.4
178.4
121.0
2119.5
2180.6
2119.7
1.347
1.858
1.636
1.087
1.087
1.091
1.861
1.861
1.095
1.093
1.094
1.095
1.093
1.094
122.9
106.2
122.0
121.3
117.4
107.6
107.6
110.6
110.8
111.2
110.6
110.8
111.2
0.0
180.0
180.0
180.0
119.7
2119.7
181.7
119.6
2119.8
2181.7
2119.6
1.340
1.857
1.621
1.080
1.081
1.084
1.859
1.859
1.087
1.086
1.087
1.087
1.086
1.087
123.2
107.5
122.1
121.5
117.0
107.8
106.8
111.2
111.4
110.1
110.6
110.9
111.1
121.6
180.0
180.0
180.0
119.8
2119.4
178.2
120.9
2119.4
2179.7
2119.8
1.340
1.856
1.621
1.081
1.080
1.084
1.859
1.859
1.087
1.086
1.087
1.087
1.086
1.087
122.7
106.3
122.0
121.1
117.3
107.8
107.8
110.6
110.8
111.1
110.6
110.8
111.1
0.0
180.0
180.0
180.0
119.7
2119.7
181.9
119.7
2119.9
2181.9
2119.7
259
gauche
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Parameter
119.9
3407.7
2069.9
1997.3
0.152
0.000
1.832
1.838
1.450232
120.2
3352.0
2064.1
1973.3
1.071
0.283
1.845
2.152
1.450188
10
119.8
3381.3
2055.2
1985.9
0.128
0.000
2.030
2.034
1.348714
120.2
3327.1
2048.2
958.4
1.205
0.278
2.062
2.404
1.348615
22
t H15C9SiH13
A
B
C
um a u
u m bu
um c u
um t u
2(E 1 545)
DE (cm 21)
120.0
3314.7
2048.5
1959.5
1.114
0.209
1.833
2.155
0.943956
119.9
3389.8
2055.0
1971.9
0.004
0.000
1.698
1.698
0.142580
120.1
3320.0
2052.6
1945.2
1.033
0.213
1.716
2.014
0.142371
46
119.9
3371.9
2054.9
1987.6
0.041
0.000
1.796
1.796
0.943711
54
gauche
gauche
syn
gauche
Parameter
MP2/6-31G(d)
RHF/6-31G(d)
Table 2 (continued)
syn
MP2/6-3111G(d,p)
syn
gauche
syn
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
MP2/6-3111G(2d,2p)
260
RHF/6-31G(d), 254 cm 21 (20.64 kJ/mol) from
MP2/6-31G(d) and 22 cm 21 (0.26 kJ/mol) from
MP2/6-3111G(d,p) and 10 cm 21 (0.12 kJ/mol)
from MP2/6-3111G(2d,2p) calculations with the
syn rotamer the more stable conformer from each
calculation except from the MP2/6-31G(d) calculation which predicted the gauche rotamer being more
stable.
For the normal coordinate analysis, the force ®eld
in Cartesian coordinates was obtained with the gaussian-94 program [12] from the MP2/6-31G(d) calculations. Internal coordinates were de®ned as shown in
Fig. 9, which were used to form the symmetry coordinates listed in Table 3. The Cartesian coordinates
obtained from the optimized geometry were used to
calculate the B-matrix elements with the G matrix
program of Schachtschneider [14]. These B-matrix
elements were used to convert the ab initio force
®eld in Cartesian coordinates to a force ®eld in the
desired internal coordinates. The resulting force ®elds
for the gauche and cis conformers are available from
the authors. These force ®elds were used in a massweighted Cartesian coordinate calculation to reproduce the ab initio vibrational frequencies and to determine the potential energy distribution (PED) which is
given in Table 4 for the two conformers. All the
elements of the force ®eld in internal coordinates
from the MP2/6-31G(d) calculation were then
assigned scaling factors of 0.9 for the stretches and
bends and 1.0 for the torsions and the calculation
repeated to obtain the ®xed scaled force ®eld and
scaled vibrational frequencies.
To aid in the vibrational assignment for the
CH2CHSi(CH3)2F molecule, the infrared and Raman
spectra were calculated using frequencies, Raman
scattering activities (RHF/6-31G(d)), and infrared
intensities (MP2/6-31G(d)) determined from the ab
initio calculations. The evaluation of the Raman
activity by using the analytical gradient method has
been developed [15,16]. The activity Sj can be
expressed as:
Sj ˆ gj …45a2j 1 7b2j †
where gj is the degeneracy of the vibrational mode j,
a j the derivative of the isotropic polarizability and b j
the derivative of the anisotropic polarizability. The
Raman scattering cross sections, 2s j =2V; which are
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
261
Fig. 9. Internal coordinates of dimethyl vinyl ¯uorosilane.
proportional to the Raman intensities, can be calculated from the scattering activities and the predicted
wavenumbers for each normal mode using the relationship [17,18]:
2s j
ˆ
2V
0
!
2 4 p4 B
B
B
45 @
1
!
C
n0 2 nj †4
h
C
Sj
C
A
2hcnj
8p2 cnj
1 2 exp
kT
where n 0 is the exciting frequency, n j the vibrational
frequency of the jth normal mode and Sj the corresponding Raman scattering activity.
To obtain the polarized Raman scattering crosssections, the polarizabilities are incorporated into Sj
by Sj‰…12…j†=…11…j†Š where r j is the depolarization ration
of the jth normal mode. The Raman scattering crosssections and calculated frequencies were used
together with a Lorentzian function to obtain the
calculated spectrum. The experimental and predicted
Raman spectra of dimethyl vinyl ¯uorosilane are
shown in Fig. 10. The predicted spectra are compared
to the experimental Raman spectrum of the liquid,
which is shown in Fig. 10A. These spectra were
very useful for making the vibrational assignments
to the correct bands for the two conformers.
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 MP2/6-31G(d) and
transformed to normal coordinates by
!
X
2m u
2mu
Lij
ˆ
2Qi
2Xj
j
where Qi is the ith normal coordinate, Xj the jth Cartesian displacement coordinate and Lji the transformation matrix between the Cartesian displacement
coordinates and normal coordinates. The infrared
intensities were then calculated by
"
#
2my 2
Np
2mx 2
2m z 2
1
1
Ii ˆ 2
2Qi
2Qi
2Qi
3c
In Fig. 8, the predicted infrared spectra are shown for
the pure gauche (Fig. 8D), pure syn (Fig. 8C) and the
mixture (Fig. 8B). The experimental infrared spectrum of the normal species dissolved in liquid krypton
at 21258C is also shown for comparison in Fig. 8A.
Excluding the overtones or combination bands, the
calculated spectra have some differences from the
experimental one especially in comparing the relative
intensities of the bands in the 1300 cm 21 region.
Nevertheless, they provide support for the assignments of the observed bands to the indicated fundamentals for each conformer.
4. Conformational stability
There are a few fundamentals which show
conformer doublets in the infrared and Raman spectra
262
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Table 3
Symmetry coordinates (not normalized) for dimethyl vinyl ¯uorosilane
Description
CH2
CH3
CH3
CH3
antisymmetric
antisymmetric
antisymmetric
antisymmetric
Internal coordinate
stretch
stretch
stretch
stretch
CH2 symmetric stretch
CH3 antisymmetric stretch
CH stretch
CH3 symmetric stretch
CH3 symmetric stretch
C ˆ C stretch
CH2 deformation
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH3 antisymmetric deformation
CH3 symmetric deformation
CH3 symmetric deformation
CH in-plane bend
CH2 twist
CH2 wag
CH2 rock
SiF stretch
CH3 rock
CH3 rock
CH3 rock
CH3 rock
SiC stretch
SiC2 antisymmetric stretch
SiC2 symmetric stretch
C±H out-of-plane bend
CCSi bend
SiC2 wag
SiC2 rock
SiC2 deformation
methyl torsion
CSiF bend
SiC2 twist
methyl torsion
Asymmetric torsion
S1 ˆ r1 2 r2
S2 ˆ r5 2 r6 2 r8 1 r9
S3 ˆ r1 1 r2
S4 ˆ 2r4 2 r5 2 r6 1
2r7 2 r8 2 r9
S5 ˆ r1 1 r2
S6 ˆ 2r4 2 r5 2 r6 2
2r7 1 r8 1 r9
S7 ˆ r3
S8 ˆ r4 1 r5 1 r6 1 r7 1
r8 1 r9
S9 ˆ r4 1 r5 1 r6 2 r7 2
r8 2 r9
S10 ˆ Q
S11 ˆ 2a 2 b1 2 b2
S12 ˆ 2d1 2 d2 2 d3 2
2d4 2 d5 2 d6
S13 ˆ d1 2 d3 2 d4 1 d6
S14 ˆ 2d1 2 d2 2 d3 2
2d4 1 d5 1 d6
S15 ˆ d1 2 d3 1 d4 2 d6
S16 ˆ d1 1 d2 1 d3 2
f1 2 f2 2 f3 1 d4 1
d5 1 d6 2 f4 2 f5 2 f6
S17 ˆ d1 1 d2 1 d3 2
f1 2 f2 2 f3 2 d4 2
d5 2 d6 1 f4 1 f5 1 f6
S18 ˆ y1 2 y2
S19 ˆ g
S20 ˆ b1 2 b2
S21 ˆ e
S22 ˆ S
S23 ˆ f2 2 f3 1 f5 2 f6
S24 ˆ f2 2 f3 2 f5 1 f6
S25 ˆ 2f1 2 f2 2 f3 1
2f4 2 f5 2 f6
S26 ˆ 2f1 2 f2 2 f3 2
2f4 1 f5 1 f6
S27 ˆ R
S28 ˆ X1 2 X2
S29 ˆ X1 1 X2
S30 ˆ g 0
S31 ˆ 2p 2 y1 2 y2
S32 ˆ v1 1 v2 2 u1 2 u2
S33 ˆ up
2 u2 1 v1 2 v2
1 
Sp
34ˆ … 6 1 2†S2
6 2 2†v 2 u 1 2 u2 2
v1 2 v2
S35 ˆ tp
2 t3
2 
Sp
36ˆ … 6 2 2†S2
6 1 2†v 1 u 1 1 u2 1
v1 1 v2
S37 ˆ u1 2 u2 2 v1 1 v2
S38 ˆ t2 1 t3
S39 ˆ t1
Fig. 10. Raman spectra of dimethyl vinyl ¯uorosilane: (A) experimental spectrum of the liquid; (B) calculated spectrum of the
mixture (DH ˆ 44 cm 21); (C) calculated spectrum of the syn
conformer; (D) calculated spectrum of the gauche conformer.
of the ¯uid phases. The two bands at 521 and
513 cm 21 in the infrared spectrum of the gas which
are assigned as the C±H out-of-plane bending modes,
demonstrate the presence of conformers, where by
repeated annealing of the amorphous solid only the
lower frequency band remains. Other bands which are
observed in the infrared and Raman spectra of the
¯uid phases and amorphous solid but not in the spectrum of the annealed solid are observed at 962, 710,
607, 366, 272, 228 and 201 cm 21.
These data clearly indicate that there are two
conformers present in the ¯uid phases at ambient
temperatures but only one rotamer remains in the
polycrystalline solid. The band at 710 cm 21 is the
only band predicted by ab initio calculations in this
region (703 cm 21) and it can de®nitely be assigned to
the Si±C stretch of the gauche conformer. Since this
band is drastically decreased in the intensity in the
infrared spectra of the argon and nitrogen matrices
and annealed solid, it can be concluded that the syn
Table 4
Observed and calculated wavenumbers for gauche and syn conformers of dimethyl vinyl ¯uorosilane
Description
A0 n1
n2
n4
n5
n6
n7
n8
n9
n 10
n 11
n 12
n 13
n 14
n 15
n 16
n 17
n 18
Ab
initio a
Fixed
IR
scaled b int. c
Raman dp
Obs e
d
d
act
ratio
PED
Ab
Fixed IR
initio a scaled b int. c
Raman dp
Obs e
d
d
act
ratio
PED
3282
3113
15.5
134.4
0.14
3062
99S1
3289
3120
10.4
134.0
0.14
3071
99S1
3206
3042
7.1
70.6
0.67
3019
40S2,41S3,10S6
3207
3042
3.4
68.2
0.73
3019
94S2
3203
3039
8.4
76.1
0.57
3000
54S3,44S1
3207
3042
10.2
64.8
0.63
3000
96S3
3201
3037
14.6
81.1
0.75
2975
53S4,20S5,10S6
3197
3033
14.4
140.6
0.75
2975
95S4
3200
3036
2.1
139.9
0.73
2970 1
54S5,19S4,18S7
3201
3037
11.7
51.0
0.75
2970 1
85S5,14S7
3198
3034
3.9
74.2
0.72
2970 1
77S6,18S4
3195
3031
0.4
146.8
0.68
2970 1
94S6
3187
3108
3023
2948
9.4
0.4
33.7
146.2
0.72
0.01
2957
2916
75S7,24S5
61S8,39S9
3182
3105
3019
2946
7.3
1.3
14.6
217.5
0.75
0.01
2957
2916
86S7,13S5
100S8
3105
2946
0.6
68.2
0.01
2916
61S9,39S8
3104
2945
0.4
0.4
0.75
2916
100S9
1680
1534
1526
1594
1455
1447
11.2
9.3
4.1
24.8
3.0
26.8
0.14
0.47
0.74
1601
1446 1
1442 1
61S10,33S15
49S11,45S12
51S12,44S11
1677
1525
1531
1591
1446
1453
11.8
1.9
11.8
30.9
2.3
32.2
0.16
0.75
0.75
1599
1442 1
1446 1
1521
1443
1.9
17.5
0.75
1425
79S13,17S14
1519
1441
1.2
13.3
0.75
1425
43S13
1517
1439
0.7
8.9
0.71
1421
76S14,17S13
1515
1438
0.9
16.2
0.75
1421
93S14
1489
1413
23.1
31.5
0.41
1413
65S15,24S10
1486
1410
18.3
30.8
0.34
1409
68S15,24S10
1379
1308
33.5
0.8
0.59
1263 1
97S16
1377
1307
30.6
0.8
0.43
1262 1
98S16
1374
1303
58.0
1.2
0.75
1257 1
96S17
1372
1302
56.4
1.0
0.75
1257 1
97S17
1324
1256
2.9
13.3
0.37
1244
59S18,25S20
1323
1255
1.3
14.5
0.35
1244
58S18,26S20
1062
1053
995
948
916
1008
999
944
900
869
20.5
15.6
38.7
174.7
163.0
0.5
1.1
3.1
0.5
1.8
0.74
0.75
0.64
0.74
0.72
1013 p
1008
966
883
848
58S19,37S30
60S20,29S18
98S21
45S22,36S25
70S23
1063
1050
999
947
919
1007
996
947
898
872
27.0
20.1
127.5
129.8
178.0
0.4
1.3
3.3
0.9
1.7
0.75
0.75
0.64
0.64
0.69
1013 p
1006
969
879
850
62S19
57S20,30S18
99S21
45S22,30S25
71S23
62S10,31S15
55S11,40S16
55S12,40S18
263
n 19
n 20
n 21
n 22
n 23
Syn
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
n3
CH2 antisymmetric
stretch
CH3 antisymmetric
stretch
CH3 antisymmetric
stretch
CH3 antisymmetric
stretch
CH2 symmetric
stretch
CH3 antisymmetric
stretch
CH stretch
CH3 symmetric
stretch
CH3 symmetric
stretch
CyC stretch
CH2 deformation
CH3 antisymmetric
deformation
CH3 antisymmetric
deformation
CH3 antisymmetric
deformation
CH3 antisymmetric
deformation
CH3 symmetric
deformation
CH3 symmetric
deformation
CH in-plane
bend
CH2 twist
CH2 wag
CH2 rock
SiF stretch
CH3 rock
Gauche
264
Table 4 (continued)
Description
Gauche
Ab
initio a
Fixed
IR
scaled b int. c
Raman dp
Obs
act d
ratio d
842
799
114.4
0.9
0.67
799
n 25
n 26
n 27
n 28
822
797
741
727
779
756
703
689
11.6
0.2
37.9
2.5
2.1
2.1
3.6
4.8
0.74
0.75
0.52
0.74
620
588
0.7
16.6
Raman dp
Obs e
d
d
act
ratio
PED
846
802
147.4
1.4
0.75
799
34S24,38S28,
13S33
766
736
711
694
823
797
723
725
781
757
686
688
13.9
1.0
22.9
3.1
2.1
1.7
5.4
5.4
0.75
0.75
0.75
0.43
772
736
694
694
45S25,43S22
61S26,25S24
41S27,36S29,11S23
50S28,22S26,26S24
0.06
601
56S29,30S27
626
594
0.0
14.3
0.01
606
45S29,34S27
25S30,12S36,
32S19,21S38
46S31,27S33
43S32,33S36
40S33,11S31,25S37
28S34,33S32
51S35,21S34,14S36
18S36,29S34,34S35
40S37,18S31,13S35,
13S33,14S38
76S38,14S37
68S39,28S30
530
507
21.8
5.4
0.75
515
404
255
282
199
177
155
175
383
241
269
189
174
149
167
19.1
7.0
11.4
0.4
0.1
0.5
0.1
4.4
1.7
1.1
1.3
4.1
0.5
0.3
0.25
0.75
0.43
0.73
0.75
0.75
0.72
394 p
267 p
273 p
1958
175 p
151 X
170 p
33S30,30S19,
16S38
42S31,41S36
71S32
68S33,10S37,14S38
82S34
76S35,10S36
36S36,23S35,27S31
74S37
152
78
152
78
0.0
0.1
0.0
8.4
0.75
0.75
±
101 #
92S38
62S39,28S30
540
518
21.2
6.2
0.68
527
373
271
257
221
189
174
166
353
257
244
210
189
172
159
10.3
13.6
6.5
0.6
0.4
0.0
0.2
3.6
1.0
1.4
1.6
2.6
1.4
0.4
0.48
0.67
0.71
0.52
0.75
0.69
0.66
365 p
273 p
267 p
218
1958
175 p
164 p
n 38 methyl torsion
n 39 asymmetric torsion
158
70
158
70
0.0
0.1
0.1
7.7
0.69
0.75
±
n 31
n 32
n 33
n 34
n 35
n 36
n 37
Ab
Fixed IR
initio a scaled b int. c
22S24,41S28,
15S26,
11S33
46S25,45S22
57S26,30S24
41S27,13S23,29S29
46S28,34S24,17S26
CH3 rock
CH3 rock
SiC stretch
SiC2 antisymmetric
stretch
SiC2 symmetric
stretch
C±H out-of-plane
bend
CCSi bend
SiC2 wag
SiC2 rock
SiC2 deformation
methyl torsion
CSiF bend
SiC2 twist
n 30
PED
95 #
Calculated with the MP2/6-31G(d) basis set.
Scaling factors 0.9 for stretching and bending coordinates and 1.0 for torsional coordinates.
c
Calculated infrared intensities in km/mol from the MP2/6-31G(d) calculation.
d
Ê 4/amu and dp ratios, from RHF/6-31G(d) calculation.
Calculated Raman activities in A
e
Frequencies are obtained from the annealed spectrum of the nitrogen matrix except for the ones with ( p), ( X), ( 1), (8), and ( #) signs are taken from infrared gas, infrared solid, argon
matrix, Raman liquid and Raman solid, respectively.
b
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
n 24 CH3 rock
n 29
a
Syn
e
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Table 5
Temperature and intensity ratios for conformational study of
dimethyl vinyl ¯uorosilane dissolved in liquid xenon
T (8C)
1000/T (K)
I710/I695
2ln K
255
260
265
270
275
280
285
290
295
2100
DH a
4.58
4.69
4.80
4.92
5.05
5.18
5.31
5.46
5.61
5.78
2.8438
2.9173
2.8899
2.9631
3.0203
3.0587
3.0263
3.0630
3.1828
3.0814
21.1647
21.1651
21.1774
21.1725
21.1398
21.1794
21.1999
21.12601
21.2459
21.2813
53 ^ 9
a
DH ˆ 53 ^ 9 cm 1 (0.64 ^ 0.10 kJ/mol) with
conformer the more stable form.
the gauche
conformer remains in the annealed solid with a minor
amount of the gauche conformer (Figs. 5±7). Further
support for this conclusion is found from the assignments for the CH2 rock and CCSi bend which are
observed at 966 and 370 cm 21 for the gauche
conformer. Similarly, the other listed bands, which
disappear upon solidi®cation and annealing, are all
assigned to the gauche conformer and they will be
discussed latter. Therefore, all the spectral data indicate that the syn form is the stable conformer in the
annealed solid.
Variable temperature studies of the infrared spectra
of DVFS dissolved in liquid xenon (Table 5) and
krypton (Table 6) were conducted to determine the
265
enthalpy difference between the two stable conformers. An important advantage to this temperature
study is that the conformer peaks are better resolved
and the area under them is more easily measured than
bands observed in the infrared spectrum of the gas.
Infrared spectral data from 4000 to 400 cm 21 were
obtained at different temperatures between 255 to
21008C for the xenon solution and between 2105
and 21508C for the krypton solution. The spectral
changes in lique®ed krypton of the pairs at 962/965,
710/695 and 522/513 cm 21 are shown in Fig. 11.
From all spectral data from lique®ed xenon and
krypton solutions we observed increases in the intensity of the infrared bands assigned to the gauche
conformer as the temperature decreases. This clearly
con®rms the stability of the gauche rotamer over the
syn conformer in these rare gas solutions.
In order to obtain the enthalpy difference, ten
spectral data points were obtained over the temperature range 255 to 21008C for the xenon solution and
2105 to 21508C for the krypton solution. The intensities of each conformer pair were ®t to the equation
2ln K ˆ …DH=RT† 2 …DS=R† where K is the intensity
ratio (Ig/Ic) and it is assumed that DH is not a function
of temperature. Using a least squares ®t of the slope of
the line, a DH value of 53 ^ 9 cm 21 was obtained
from the 710/695 cm 21 bands from the xenon data.
The pair at 962/965 cm 21 was not resolved suf®ciently to be measured in the temperature range of
the xenon measurements and, therefore, not utilized
in the calculation. Since the signal-to-noise ratio was
relatively low in the range of 600±500 cm 21, we were
Table 6
Temperature and intensity ratios for conformational study of dimethylvinyl ¯uorosilane dissolved in liquid krypton
T(8C)
1000/T (K)
I962/I965
2ln k
I695/I710
2ln k
I51/I513
2ln k
2105
2110
2115
2120
2125
2130
2135
2140
2145
2150
DH a (cm 21)
5.95
6.13
6.32
6.53
6.75
6.99
7.23
7.51
7.80
8.12
2.5385
2.7410
2.7051
2.8240
2.7941
2.8300
2.7710
2.8501
2.8876
2.9420
20.9316
21.0083
20.9951
21.0382
21.0275
21.0403
21.0192
21.0473
21.0604
21.0791
32 ^ 7
3.2050
3.2062
3.2460
3.2300
3.1262
3.2525
3.3200
3.5256
3.4760
3.6014
21.1647
21.1651
21.1774
21.1725
21.1398
21.1794
21.1999
21.12601
21.2459
21.2813
39 ^ 8
1.9853
2.0637
2.1420
2.3272
2.4425
2.4469
2.4657
2.3350
2.4022
20.6858
20.7245
20.7617
20.8447
20.8930
20.8948
20.9025
20.8480
20.8764
a
Average DH ˆ 44 ^ 7 cm 21 (0.53 ^ 0.09 kJ/mol) with the gauche conformer the more stable form.
69 ^ 21
266
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
Fig. 11. Temperature dependent infrared spectra of dimethyl vinyl ¯uorosilane.
unable to measure DH from the pair at 522/513 cm 21.
Utilizing the krypton data for the above three
conformer pairs (Fig. 11), DH values of 32 ^ 7,
39 ^ 21 and 69 ^ 21 cm 21 were obtained with an
average value of 44 ^ 7 cm 21 (0.53 ^ 0.09 kJ/mol)
with the gauche form the more stable conformer.
Additional infrared spectra of DVFS were recorded
in argon and nitrogen matrices (1:500 and 1:1000)
deposited at 5 and 15 K; the spectra in the argon
matrices are given in Fig. 6, whereas detailed spectra
in a nitrogen matrix are presented in Fig. 7. Supposedly, the conformational equilibrium of the vapor
phase is maintained when the gas mixture is shock
frozen on the CsI window at 5 or 15 K, provided the
barrier to conformational equilibrium is above 3 and
5 kJ mol 21, respectively. When the matrices were
annealed below 20 K some small spectral changes
occurred, which were interpreted as a relaxation of
DVFS in the matrix lattice. The samples were subsequently annealed for 10 min at every 3 K between 20
and 37 K for argon and 20 and 34 K for the nitrogen
matrices before being recooled to 5 K and the spectra
recorded.
Prominent changes were observed when the
samples were annealed to ca. 34 K for argon and
32 K for nitrogen. Certain bands were enhanced,
others diminished in intensities after annealing
which are interpreted as a displacement of the conformational equilibrium. Qualitatively, the same intensity changes occurred in the spectra of both matrices,
but they were more prominent in the spectra obtained
from the argon matrix. As we shall see, the infrared
bands which vanished or were reduced in intensity in
the spectra of the crystal were enhanced in the
matrices after annealing, whereas those present in
the crystal spectra were reduced in intensities. These
bands are indicated with arrows pointing upwards or
downwards, respectively, in Table 1. The observed
annealing temperatures (34 and 32 K) suggested that
the conformational barrier is ca. 9 kJ mol 21 from the
curves given by Barnes [19]. However, it can be seen
from the spectral data shown in Figs. 5±7 that no
infrared bands disappeared completely in the spectra
of the matrices after annealing. Therefore the enthalpy
difference between the conformers must be quite low
in both matrices since an equilibrium was maintained
at the annealing temperatures at 32±34 K. A rough
estimate using a simple Boltzmann distribution
suggests an enthalpy difference of 17±33 cm 21 in
the matrices which is in agreement with the results
from the xenon and krypton solutions in con®rming
that the gauche conformer is the more stable form.
Upon crystallization, bands at 969, 876, 711, 607,
366, 228 and 201 cm 21 presented in the Raman
spectra of the liquid and amorphous phases signi®cantly diminish and/or disappeared. These bands are
due to the second conformer and seven spectra data
points of the liquid were recorded between 19 and
21208C in order to obtain the enthalpy difference.
Band pairs at 712/696 and 371/396 cm 21 were
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
21
selected to obtain DH. The band at 696 cm might
have a contribution from the other conformer. A series
of van't Hoff plots based on measured peak heights
were obtained giving DH values of 17 and 33 cm 21,
respectively, for the pairs mentioned above with the
average value of 25 ^ 15 cm 21 (0.30 ^ 0.18 kJ/mol)
also with the gauche form being the more stable
conformer.
5. Vibrational assignment
The conformational analysis of DVFS shows that
the molecule exists in two stable conformations in the
¯uid phase. The syn conformer has Cs symmetry and
the 39 fundamentals will span the irreducible representation 23 A 0 and 16 A 00 , whereas for the gauche
conformer with C1 symmetry all the fundamentals
belong to species A. Since most of the observed
bands in the infrared spectrum of the vapor and the
Raman bands of the liquid are common to both the syn
and gauche conformers, the band contours and the
polarization ratios are of limited help in the spectral interpretation. For the sake of similarity, the
fundamentals of both the gauche and syn conformers have been numbered consecutively, instead
of the conventional numbering of the modes
belonging to species A 0 before those of A 00 in
the syn conformer. Guided by the assignments of
the similar normal modes for dimethyl vinyl chlorosilane [7] and also by the calculated spectral
intensities and predicted wavenumbers from ab
initio calculations, we propose the vibrational
assignments listed in Table 1.
The assignments of the carbon±hydrogen modes
have been previously reported [7] for dimethyl vinyl
chlorosilane, and with only minor wavenumber shifts,
they remain essentially the same for DVFS. The
spectra of these two compounds look similar down
to about 1000 cm 21. In the region below 1000 cm 21,
a few features were observed in the infrared and
Raman spectra of the gas or liquid which disappear
upon crystallization. The C-type Q-branches located
at 969 and 964 cm 21 in the infrared spectrum of the
gas are assigned to the CH2 rocks with the syn
conformer having the higher wavenumber. The intensity of this band increases with decreasing the
temperature of the krypton (Fig. 11) solution
267
con®rming that the gauche form is the more stable
conformer in the gas phase. The Si±C stretch is
assigned to the C-type Q-branch at 710 cm 21 for the
gauche form and to the shoulder at 694 cm 21 for the
syn conformer where the former has almost disappeared from the spectrum of the crystalline solid.
The later one is also assigned to the SiC2 antisymmetric stretch for the both conformers. It should be
noted that the gauche conformer has a minor contribution to this band but we used it as a conformer band
(Fig. 11) for the enthalpy determination. The CH outof-plane bending conformer pair is observed in the
infrared spectrum of the liquid krypton solution at
521 and 513 cm 21 for the gauche and syn conformers,
respectively. The CCSi bending mode observed at
365 cm 21 for the gauche form is evident in the far
infrared spectra of the gas and amorphous solid but
disappears from the spectrum upon annealing (Fig. 6)
the sample.
In the amorphous and crystalline states the infrared
and Raman bands from 258 to 286 cm 21 are assigned
to SiC2 wag and SiC2 rock. The ab initio calculations
predict very weak infrared bands as well as weak
Raman lines for the normal modes below 225 cm 21
and the assignments of these remaining ®ve fundamentals n 34 ± n 38, are less certain as shown in Table
1. The Raman bands at 225 and 209 cm 21, appearing
as shoulders and their corresponding very weak
infrared counterparts, are assigned to the SiC2 deformation for the gauche. However, the intense peak at
195 cm 21 in the Raman spectrum of the liquid is
assigned to both the SiC2 deformation for the syn
conformer and the methyl torsion for the gauche
form. The n 36 and n 35 0 modes are observed as a very
weak infrared band at 175 cm 21 in the spectrum of the
vapor for the gauche and syn conformers, respectively. The weak infrared band of the vapor at
170 cm 21 corresponds to the infrared and Raman
bands around 178 cm 21 in the condensed phases and
is attributed to n 37 of the syn rotamer and the very
weak infrared band at 164 cm 21 (spectrum of the
gas) is assigned to the gauche for the same normal
mode. There are no bands observed for n 38, the methyl
torsion. The asymmetric torsional mode is assigned to
the two bands at 101 and 95 cm 21, which were
observed in the Raman spectrum of the liquid and
calculated at 78 and 70 cm 21 for the syn and gauche
conformers, respectively.
268
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
6. Discussion
In the present study of DVFS, neither the infrared
band conformer of the gas nor the depolarization ratio
of the liquid was helpful in the determination of the
conformational stability of this molecule. Additionally, the calculated energies for the syn and gauche
rotamers derived from the ab initio calculations have
large uncertainties, but the force constants and the
wavenumbers after appropriate scaling usually give
good agreement with those of the observed fundamentals. Within the group frequency regions for the CH3
and CH2 stretching and deformation vibrations, the
calculated wavenumbers for the syn and gauche
fundamentals are usually separated by less than
5 cm 21 (Table 2). Below 1450 cm 21 there are 12
instances in which the fundamentals of the syn and
gauche conformers are separated by more than
5 cm 21. The largest shifts are calculated for the six
fundamentals n 27, n 28, n 30, n 31, n 32 and n 37 in which
they should be larger than 10 cm 21. As expected, most
of the observed syn/gauche conformer separations
predicted pairs are located approximately with the
wavenumber.
The following observed band pairs from the Raman
spectrum of the liquid: 971/963, 883/864, 712/696,
621/613, 371/396, 275/265, 225/209 and 195/
187 cm 21 in which the high frequency bands vanished
(or were reduced in intensity) in the infrared and
Raman spectra of the annealed solid are correlated
with the calculated scaled wavenumbers of the syn
and gauche conformers, respectively (Table 4). It
was found from all eight of these pairs that the high
frequency bands can qualitatively be ®tted with the
predicted wavenumbers of the gauche form and the
remaining bands to those of the syn conformer
whereas the opposite interpretation is not feasible.
The experimental and calculated frequencies are
also in reasonable agreement for the n 21, n 27, n 28,
n 31, n 33, n 34 and n 35 fundamentals. Two band pairs
have less convincing assignments where for n 22 the
observed difference is 19 cm 21 and that calculated
only 1 cm 21 and n 30 which was assigned to overlapping bands at 515 cm 21 although a difference of
11 cm 21 was predicted. However, we feel that there
are compelling reasons to attribute the vanishing
bands to the gauche conformer, which means that
the syn conformer remains in the crystal even though
the gauche conformer has lower energy than the syn
rotamer in the rare gas solutions, the liquid state and
also in the matrices.
In addition to the band pairs assigned to the syn and
gauche conformers on the basis of spectral changes on
crystallization, described above, the infrared spectra
in argon and nitrogen matrices can frequently give
clues to close lying conformer pairs. Thus, the band
pairs attributed to n 10, n 15, n 17, n 19, n 23, n 25, n 29 and
n 30 observed at 1601, 1411, 1254, 1013, 848, 772, 521
and 515 cm 21, respectively, can all be tentatively
assigned to separate conformer bands. As apparent,
each of these pairs of bands are characterized by
intensity changes in one or both matrices after
annealing and they are indicated with arrows in
Table 1. The bands, which are enhanced after
annealing, are assigned to the gauche form, whereas
those that diminish in intensity to the syn conformer.
It is characteristic that in each of these pairs the
wavenumber difference in the matrices is small
(between 2 and 8 cm 21) leading to overlapping
bands in the ¯uid phases. Since the bandwidths are
much lower in the matrices the separate bands due to
the syn and gauche conformers can be detected.
Because of matrix, effects frequently encountered
during annealing some of these assignments may be
erroneous. However, if the same general features are
observed in both matrices we can be fairly con®dent
in the experimental results. It should be emphasized
that for the additional band pairs n 21 around 964 cm 21
and n 23 at 710 and 694 cm 21 the bands changed both
during crystallization and on annealing the matrices
and the conclusions should be de®nitive.
The conformational energy difference in DVFS can
be compared with the corresponding value for related
silanes. In dimethyl vinyl chlorosilane [7] CH2y
CHSi(CH3)2Cl, the gauche conformer also has the
lower energy, with DH (syn±gauche) equal to
0.5 ^ 0.1 kJ mol 21 (equal to that of DVFS within
the experimental uncertainty); however, in the present
study the syn form is present in the crystal which is in
contrast to the chloro analogue. The vibrational
spectra of the two conformers are strikingly similar
for DVFS and the corresponding chloro analogue. In
vinylsilyl chloride [5] (CH2yCHSiH2Cl) the syn
conformer was more stable by 1.2 kJ mol 21, in the
liquid state revealing increased stability of the syn
conformer when the two methyl groups are absent.
A. Horn et al. / Journal of Molecular Structure 554 (2000) 251±269
However, the gauche conformer was present in the
crystal [5], as well as in the rare gas solution [6]
which makes this molecule quite different to the situation in DVFS.
A comparison of the structural parameters
predicted by ab initio calculations (Table 2) with all
basis sets indicates little difference in the parameters
upon conformer changes from the gauche to the syn
form. With a given basis sets, for example 6-31G(d)
and 6-3111G(d,p) with full electron correlations, the
Ê and bond angles
bond distances agree within 0.005A
within 18 for the corresponding parameters of the two
rotamers. Even with the larger 6-3111G(2d,2p)
basis set, the differences become insigni®cant for
most of the parameters (Table 2).
Two halogens (F or Cl) and one methyl group
attached to the Si atom leads to methyl vinyl di¯uorosilane (CH2yCHSi(CH3)F2) and methyl vinyl
dichlorosilane (CHyCHSi(CH3)Cl2) both of which
the conformational stabilities have been investigated.
In methyl vinyl di¯uorosilane the gauche conformer
is more stable in the rare gas solution [10], but the syn
form is present in the crystal [9]. The synconformer
was the more stable form in the rare gas solution [20]
and this form was also present in the crystal of vinyl
dichlorosilane [8], demonstrating the effect of the
larger chlorine atom compared to the ¯uorine atom
for these similar molecules. It should be noted that
in the latter molecules the syn conformer has both
halogens in the gauche positions compared to the
CyC bond, whereas in the gauche conformer, one of
the halogens is situated in the syn position of the CyC
bond. Finally, in methyl vinyl silane (CH2yCHSi(CH3)H2) the single methyl group prefers the
gauche conformation [21] with a DH equal to
1.59 ^ 0.13 kJ mol 21 but the syn conformer is present
in the crystal [22]. If the energy difference between
the conformers is low as observed for DVFS, the high
energy conformer, which frequently has the larger
dipole moment, can be preferred in the crystal because
of larger crystallization energies for that conformer
which can overcome the higher conformational
energy.
Acknowledgements
J.R.D. acknowledges the University of Kansas City
269
Trustees for a Faculty Fellowship award for partial
®nancial support of this research.
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