Conformational stability, vibrational spectra and ab initio calculations for chloromethyl methyl silane and chloromethyl methyl difluorosilane

Article abstract of DOI:10.1016/S0022-2860(98)00771-6

The infrared spectra of gaseous and solid chloromethyl methyl silane, CICH₂SiH₂CH₃, and chloromethyl methyl difluorosilane, CICH₂SiF₂CH₃, were recorded. Additionally, the variable temperature infrared spectra of the sample dissolved in liquid krypton and/or xenon were obtained. Both the trans and gauche conformers are present in the fluid phases but only the trans conformer is present in the solid for both molecules. The enthalpy difference between the two conformers for chloromethyl methyl silane was determined to be 180 ± 15 cm^-1 (2.16 ± 0.18 kJ./mol) and for chloromethyl methyl difluorosilane 236 ± 15 cm^-1 (2.82 ± 0.17 kJ/mol) in xenon with the gauche conformers more stable for both molecules. The enthalpy value for the silane in krypton is 181 ± 18 cm^-1 (2.16 ± 0.09 kJ/mol). Ab initio calculations were carried out at the RHF/6-31G* and MP2/6-31G* levels for both molecules from which structural parameters and conformational stabilities were determined.

Full text of DOI:10.1016/S0022-2860(98)00771-6

Journal of Molecular Structure 480–481 (1999) 147–151
Conformational stability, vibrational spectra and ab initio
calculations for chloromethyl methyl silane and chloromethyl
methyl difluorosilane
a,
a
a
b
*
Gamil A. Guirgis , Yasser E. Nashed , James B. Robb II , Peter Klaeboe ,
a
a
Pengqian Zhen , James R. Durig
a
University of Missouri-Kansas City, Department of Chemistry, 5100 Rockhill Road, Kansas City, MO 64110-2499, USA,
b
University of Oslo, Department of Chemistry, PO Box 1033, 0315 Oslo, Norway
Received 2 October 1998; accepted 19 October 1998
Abstract
The infrared spectra of gaseous and solid chloromethyl methyl silane, ClCH
2 2 3
SiH CH , and chloromethyl methyl difluor-
osilane, ClCH SiF CH , were recorded. Additionally, the variable temperature infrared spectra of the sample dissolved in liquid
2
2
3
krypton and/or xenon were obtained. Both the trans and gauche conformers are present in the fluid phases but only the trans
conformer is present in the solid for both molecules. The enthalpy difference between the two conformers for chloromethyl
Ϫ1
methyl silane was determined to be 180 ^ 15 cm (2.16 ^ 0.18 kJ/mol) and for chloromethyl methyl difluorosilane 236 ^
Ϫ1
1
5 cm (2.82 ^ 0.17 kJ/mol) in xenon with the gauche conformers more stable for both molecules. The enthalpy value for the
Ϫ1
silane in krypton is 181 ^ 18 cm (2.16 ^ 0.09 kJ/mol). Ab initio calculations were carried out at the RHF/6-31G* and
MP2/6-31G* levels for both molecules from which structural parameters and conformational stabilities were determined.
᭧
1999 Elsevier Science B.V. All rights reserved.
Keywords: Conformational stability; Ab initio calculations; Chloromethyl methyl silane; Difluorosilane
1
. Introduction
was not possible to make meaningful comparison with
the estimated values. For the corresponding chlorosi-
lane, ClCH SiH CH , the stable conformer in the
Previously we [1] investigated the far infrared and
2
2
3
low frequency gas phase Raman spectra of the 1-halo-
crystalline state was initially found to be the trans
form but in the liquid state it was estimated that the
enthalpy difference was zero between the two confor-
mers [2]. In the vapor state the infrared bands of the
conformer pairs are so badly overlapped that measure-
ments of the relative intensities of the bands was not
possible [2]. Therefore, to determine the conformer
stability in the fluid states we carried out a tempera-
ture dependent infrared spectral investigation of
chloromethyl methyl silane dissolved in liquid
xenon [3]. As a continuation of these studies, we
propane molecules, CH CH CH X (X ˆ F, Cl, Br)
3
2
2
and from the observed asymmetric torsional transi-
tions of both the trans and gauche conformers the
potential functions governing the conformer intercon-
versions were predicted. However, the experimental
determined values had such large uncertainties, that it
*
Corresponding author. Tel.: ϩ 1-816-235-1136; fax: ϩ 1-816-
2
35-5191.
E-mail address: durigj@umkc.edu (G.A. Guirgis)
0
022-2860/99/$ - see front matter ᭧ 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-2860(98)00771-6

1
48
G.A. Guirgis et al. / Journal of Molecular Structure 480–481 (1999) 147–151
Table 1
Temperature and intensity ratios for the conformational study of chloromethyl methyl silane in liquid xenon
T (ЊC)
T (K)
1000/T (K)
I
685/I700
Ϫ ln K
I
738/I700
Ϫ ln K
Ϫ 70
Ϫ 75
Ϫ 80
Ϫ 85
Ϫ 90
Ϫ 95
Ϫ 100
203
198
193
188
183
178
173
4.808
5.051
5.181
5.319
5.464
5.618
5.780
0.4587
0.4759
0.4895
0.5058
0.5260
0.5561
0.5701
0.7795
0.7426
0.7144
0.6816
0.6424
0.5868
0.5619
181 ^ 6
1.159
1.211
1.242
1.281
1.339
1.388
1.454
Ϫ 0.1479
Ϫ 0.1916
Ϫ 0.2168
Ϫ 0.2473
Ϫ 0.2921
Ϫ 0.3278
Ϫ 0.3747
180 ^ 5
a
DH
a
Average DH ˆ 180 ^ 18 (2.16 ^ 0.20 kJ/mol) with the gauche conformer more stable.
have investigated the temperature dependent FT-IR
spectra of the sample dissolved in liquid krypton. It
was hoped that the enthalpy difference between the
conformers in krypton along with that in xenon could
be used to predict the value in the vapor state [4].
For comparison purposes, we have also carried out
vibrational studies of the corresponding silicon–
fluoride compound, ClCH SIF CH . The infrared
in liquified krypton and/or xenon 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
Ϫ1
collected at 1.0 cm resolution averaged and trans-
formed with a boxcar truncation function.
2
2
3
spectra of the gas and solid, as well as the sample
dissolved in liquid xenon were recorded. Addition-
ally, ab initio calculations at the level of MP2/6-
3. Conformational stability
3
1G* were carried out to predict optimized geome-
The determination of the conformational stability is
difficult for both molecules since most of the funda-
mentals for each conformer are predicted to be near
coincident. Nevertheless, it is quite clear from the
spectral data for both ClCH SiH CH , and
tries, conformation stabilities, harmonic force fields,
infrared intensities, and vibrational frequencies of
ClCH SiF CH . The results of these spectroscopic
2
2
3
and theoretical studies are reported therein.
2
2
3
ClCH SiF CH , that conformers are present in the
2
2
3
fluid phases.
For ClCH
2. Experimental
2
SiH CH , infrared bands with maxima at
2 3
Ϫ1
6
85 and 624 cm in the spectrum of the amorphous
The sample of chloromethyl methyl silane was
solid, which are also evident in the spectrum of the
gas, are absent from the spectrum of the annealed
solid. Similarly there are bands at 738 and 580 cm
prepared by the reduction of commercially available
chloromethyl methyl dichlorosilane with lithium
aluminum hydride in dry n-butyl ether. The sample
of chloromethyl methyl difluorosilane was also
prepared from chloromethyl methyl dichlorosilane
by fluorination with freshly sublimed antimony
trifluoride. Purification of both samples was
performed with a low-temperature, low-pressure frac-
tionating column, and the purity of the samples was
checked by recording the mass and the mid-infrared
spectrum of the gases. The samples were stored under
vacuum at low temperature.
Ϫ1
which also disappear from the spectrum of the amor-
phous sample when the sample is annealed to a poly-
crystalline solid. Similar results are also found for the
liquid. Therefore, in both the spectra of the gas and
liquid there is clear evidence for the existence of two
conformers. The ab initio calculations indicate that
the bands, which disappear, are as a result of the
gauche conformer so the trans conformer is the
rotamer that is stable in the solid.
Ϫ1
The conformer pairs at 685/700 and 738/700 cm
The mid-infrared spectra of the samples dissolved
with the first listed frequency owing to the gauche

G.A. Guirgis et al. / Journal of Molecular Structure 480–481 (1999) 147–151
149
Table 2
Table 3
Temperature and intensity ratios for conformational stability deter-
mination of chloromethyl methyl silane dissolved in liquid krypton
Temperature and intensity ratios for conformational stability deter-
mination of chloromethyl methyl difluorosilane dissolved in liquid
xenon
a
a
T (ЊC)
1000/T (K)
I701/I685
Ϫ ln K
I
701/I740
Ϫ ln K
T (ЊC)
1000/T (K)
I
775/I765
Ϫ ln K
Ϫ 105 59.47
Ϫ 110 61.29
Ϫ 115 63.23
Ϫ 120 65.29
Ϫ 125 67.49
Ϫ 130 69.85
Ϫ 135 72.38
Ϫ 140 75.10
Ϫ 145 78.03
Ϫ 150 81.20
0.625
0.672
0.749
0.808
0.844
0.910
0.941
1.021
1.105
1.199
Ϫ 0.469 4.088
Ϫ 0.396 4.703
Ϫ 0.287 4.444
Ϫ 0.212 4.753
Ϫ 0.169 5.039
Ϫ 0.094 5.270
Ϫ 0.060 5.612
1.408
1.548
1.491
1.558
1.617
1.662
1.724
1.805
1.880
1.943
163 ^ 10
Ϫ 55
Ϫ 60
Ϫ 65
Ϫ 70
Ϫ 75
Ϫ 80
Ϫ 85
Ϫ 90
Ϫ 95
Ϫ 100
4.58
4.65
4.80
4.92
5.05
5.18
5.31
5.46
5.61
5.78
0.518
0.543
0.578
0.585
0.606
0.638
0.697
0.728
0.756
0.753
0.657
0.611
0.549
0.537
0.501
0.449
0.361
0.318
0.280
0.283
0.020
0.100
0.182
6.081
6.560
6.985
a
DH
199 ^ 10
a
Ϫ1
a
Ϫ1
Average value: DH ˆ 181 ^ 8 cm (2.16 ^ 0.09 kJ/mol)
with the gauche conformer more stable.
Enthalpy value: DH ˆ 236 ^ 15 cm (2.82 ^ 0.17 kJ/mol)
with the gauche conformer more stable.
conformer were used to determine the enthalpy differ-
ence between the conformers by the temperature
dependent infrared spectra of krypton and xenon [3]
solutions. These data are listed in Tables 1 and 2 and it
should be noted that the values in the krypton and
xenon [3] solutions are essentially the same. We
believe this indicates that the value in the vapor is
Ϫ1
1
80 ^ 10 cm with the gauche rotamer more stable.
Similar data have also been obtained for
ClCH SiF CH in xenon solution. We utilized the
2
2
3
Ϫ1
conformer pair 775/765 cm where the first listed
frequency is assigned to the gauche conformer. It is
clear from the observed spectra (Fig. 1) that the
gauche conformer is the more stable rotamer. The
data are listed in Table 3 and the determined enthalpy
Ϫ1
value of 236 ^ 15 cm is significantly higher than
that for the corresponding hydrogen analogue. This is
consistent with the ab initio calculations, which
predict that the value should be nearly twice as large.
4
. Ab Initio calculations
Ab initio calculations were carried out at both the
RHF/6-31G* and MP2/6-31G* levels from which
structural parameters (Table 4) and conformational
stabilities were obtained. For both molecules the
gauche conformers are predicted to be more stable,
which is consistent with the experimental data. The
predicted energy differences from the MP2/6-31G*
Fig. 1. Temperature dependence of the mid-infrared spectrum of
chloromethyl methyl difluorosilane dissolved in liquid xenon.

1
50
G.A. Guirgis et al. / Journal of Molecular Structure 480–481 (1999) 147–151
Table 4
a
Comparison of the structural parameters, rotational constants, dipole moments and energy for chloromethyl methyl silane and difluorosilane
RHF/6-31G*
gauche SiH
MP2/6-31G*
gauche SiH
b
Parameter
2
gauche SiF
2
trans SiH
2
trans SiF
2
2
gauche SiF
2
trans SiH
2
trans SiF
2
Si
2
–C
–Si
Cl–C
1
1.882
1.905
1.804
1.086
1.085
1.086
1.480
1.475
1.081
1.081
111.4
111.6
110.7
111.2
111.3
106.3
108.4
111.8
112.1
61.5
1.853
1.881
1.799
1.086
1.085
1.086
1.589
1.585
1.081
1.081
115.3
111.2
110.5
111.2
111.1
105.7
108.3
111.4
111.8
52.95
176.9
Ϫ120.1
120.1
122.0
Ϫ124.4
119.5
Ϫ119.6
3472
1.888
1.904
1.801
1.086
1.087
1.087
1.475
1.475
1.081
1.081
109.3
111.3
110.7
111.5
111.5
108.5
108.5
112.0
112.0
180.0
180.0
119.7
Ϫ119.7
120.7
Ϫ120.7
119.3
Ϫ119.3
15550
1705
1.858
1.880
1.794
1.086
1.086
1.086
1.584
1.584
1.082
1.082
112.0
112.7
110.7
111.3
111.3
108.9
108.9
111.0
111.0
180.0
180.0
Ϫ119.8
119.8
Ϫ121.7
121.7
Ϫ120.1
120.1
3714
1.874
1.897
1.794
1.093
1.094
1.093
1.490
1.485
1.092
1.092
110.6
111.1
111.0
110.6
111.0
106.7
108.3
111.6
111.5
59.7
1.843
1.872
1.791
1.093
1.092
1.093
1.613
1.609
1.092
1.092
114.4
110.6
110.8
110.9
110.7
106.0
108.5
111.4
111.3
52.10
175.9
Ϫ120.3
120.3
121.9
Ϫ124.0
120.0
Ϫ119.7
3439
1.880
1.896
1.792
1.093
1.093
1.093
1.485
1.485
1.092
1.092
109.1
111.3
110.8
111.2
111.2
108.4
108.4
111.5
111.5
180.0
180.0
119.8
Ϫ119.8
120.8
Ϫ120.8
119.8
Ϫ119.8
15702
1741
1.849
1.872
1.787
1.093
1.093
1.093
1.608
1.608
1.093
1.093
109.4
109.9
110.8
111.4
111.4
108.3
108.3
112.7
112.7
180.0
180.0
Ϫ119.9
119.9
Ϫ121.7
121.7
Ϫ120.6
120.6
3633
C
3
2
3
H
H
H
X
X
H
H
5
6
7
8
9
–C
1
–C
–C
1
1
–Si
–Si
2
2
10–C
11–C
3
3
C
3
Si
2
C
1
ClC
3
Si
2
H
H
H
X
X
H
H
5
6
7
8
9
C
C
C
1
1
1
Si
Si
Si
2
2
2
3
3
Si
Si
2
C
C
2
10
C
C
3
3
Si
Si
2
11
2
ClC
3
Si
2
C
1
H
H
H
X
X
H
H
A
B
C
5
6
7
8
9
C
C
C
1
1
1
Si
Si
Si
2
2
2
3
3
C
H
H
C
C
3
Ϫ179.8
119.9
Ϫ120.4
120.5
Ϫ122.8
119.2
Ϫ119.4
7763
Ϫ179.8
120.0
Ϫ120.0
120.8
Ϫ122.2
119.8
Ϫ119.6
7681
5
5
Si
Si
2
C
C
1
2
1
10
C
C
3
Si
Si
2
Cl
Cl
11
3
2
2237
1910
1349
1302
1329
1306
2289
1941
1353
1314
1332
1313
1595
1629
͉
͉
͉
͉
m
m
m
m
a
͉
1.683
0.793
0.924
2.077
0.36/8
0.602
1.632
1.778
6.083301
2.552
1.319
0.000
2.873
2.112
3.318
0.000
3.933
6.081164
1.597
0.770
0.917
2.000
1.160
0.639
1.683
1.807
6.935774
2.517
1.333
0.000
2.849
1.756
3.619
0.000
4.023
6.933532
b
͉
c
͉
͉
t
Ϫ(E ϩ 1020)
Ϫ(E ϩ 824)
4.216001
4.215428
126
4.716628
4.717433
176
Ϫ1
DE (cm
)
469
492
a
˚
Bond distances in A, bond angles in degrees, rotational constants in MHz, dipole moments in Debye and energies in Hartrees.
X is either an H or F atom.
b
calculations are slightly higher than the experimental
containing the gauche conformer and one containing
the trans conformer [5].
enthalpy differences. Also, it should be noted that the
predicted dipole moment of the trans conformers are
larger than those for the gauche conformers. This is
consistent with the observations of the trans
conformer of the hydrogen analogue in the crystalline
solid state. However, for the corresponding fluoro
analogue two types of crystals were observed, one
The predicted bond distances between the trans and
gauche conformers for each molecule are very similar
˚
with predicted differences of 0.005 A or less for the
Si–C and C–Cl bonds. Also the only significant angle
difference with the exception of the ClCSiC dihedral
angle is the CSiC angle of the fluoro compound,

G.A. Guirgis et al. / Journal of Molecular Structure 480–481 (1999) 147–151
151
which is 5Њ larger for the gauche conformer. Also it
should be noted that the structural parameters are very
similar between the corresponding conformers for
both molecules except for the Si–C bond distance
and, of course, the silicon–hydrogen and silicon–
fluorine parameters.
[2] M. Hayaski, K. Ohno, H. Murata, Bull. Chem. Soc. Jpn. 46
1973) 797.
(
[3] G.A. Guirgis, Y.E. Nashed, J.B. Robb II, J.R. Durig, J. Mol.
Struct. 446 (1998) 15.
[4] W.A. Herrebout, B.J. van der Veken, J. Phys. Chem. 100 (1996)
9671.
[5] V. Aleksa, K. Herzog, R. Salzer, A. Gruodis, P. Klaeboe, C.J.
Nielsen, G.A. Guirgis, J.R. Durig, J. Mol. Struct., submitted.
References
[1] J.R. Durig, S.E. Godbey, J.F. Sullivan, J. Chem. Phys. 80
(1984) 5983.