Raman and infrared vibrational spectra, ab initio calculations and normal coordinate analyses for 1,2-dimethyltetrachlorodisilane and 1,2-dimethyltetrachlorodisilane-d6

Article abstract of DOI:10.1016/S0022-2860(97)00005-7

The Raman spectra of CH₃Cl₂SiSiCl₂CH₃ and CD₃Cl₂SiSiCl₂CD₃ between 3000 and 30 cm^-1 have been recorded at various temperatures. The infrared spectra at ambient temperature were recorded from 3000 to 50 cm^-1. Both isotopomers exist as a mixture of gauche and anti rotamers at room temperature. The energy difference E(anti)-E(gauche) was determined from the Raman spectra as being -0.9 ± 0.2 kJ mol^-1 for CH₃Cl₂SiSiCl₂CH₃ and -1.2 ± 0.2 kJ mol^-1 for CD₃Cl₂SiSi₂CD₃. Assisted by ab initio calculations, the vibrational spectra have been assigned using C(2h) symmetry for the anti rotamers and C² symmetry for the gauche rotamers. Scaled and unscaled harmonic frequencies and harmonic symmetry force constants have been reported for both rotamers, and normal coordinate analyses have been carried out. Potential energy distributions have been reported.

Full text of DOI:10.1016/S0022-2860(97)00005-7

MOLSTR 9756
Journal of Molecular Structure 412 (1997) 83–95
Raman and infrared vibrational spectra, ab initio calculations
and normal coordinate analyses for 1,2-dimethyltetrachlorodisilane
and 1,2-dimethyltetrachlorodisilane-d6
a
b
b
c,
Margot Ernst , Karla Schenzel , Anke J a¨ hn , Karl Hassler *
a
Institute of Theoretical Chemistry, Karl Franzens University, Mozartgasse 14, A-8010 Graz, Austria
b
Institute of Analytical Chemistry, Martin Luther University, Weinbergstraße 16, D-06120 Halle, Germany
Institute of Inorganic Chemistry, University of Technology, Stremayrgasse 16, A-8010 Graz, Austria
c
Received 13 June 1996; revised 12 December 1996; accepted 19 December 1996
Abstract
The Raman spectra of CH Cl SiSiCl CH and CD Cl SiSiCl CD between 3000 and 30 cm have been recorded at various
−
1
3
2
2
3
3
2
2
3
−
1
temperatures. The infrared spectra at ambient temperature were recorded from 3000 to 50 cm . Both isotopomers exist as a
mixture of gauche and anti rotamers at room temperature. The energy difference Eanti − Egauche was determined from the Raman
−1
−1
spectra as being − 0.9 Ϯ 0.2 kJ mol for CH Cl SiSiCl CH and − 1.2 Ϯ 0.2 kJ mol for CD Cl SiSiCl CD . Assisted by
3
2
2
3
3
2
2
3
2
ab initio calculations, the vibrational spectra have been assigned using C2h symmetry for the anti rotamers and C symmetry
for the gauche rotamers. Scaled and unscaled harmonic frequencies and harmonic symmetry force constants have been
reported for both rotamers, and normal coordinate analyses have been carried out. Potential energy distributions have
been reported. ᭧ 1997 Elsevier Science B.V.
Keywords: Conformational isomerism; Vibrational spectroscopy; Ab initio calculation; Normal cordinate analysis; Silane
1. Introduction
HI SiSiI H [6] as well as Me SiSiMe SiMe SiMe
2 2 3 2 2 3
[7]. In all these cases, two conformers were observed.
By analogy with 1,2-disubstituted ethanes it is
The variations of the dipole moments of disilanes of
the type Me SiXSiXMe with temperature (X is H, F,
reasonable to assume that 1,2-disubstituted disilanes
will also exist as mixtures of two conformers, gauche
and anti. ED studies of HBr SiSiBr H [1], HI SiSiI H
2
2
Cl, Br, I, OMe, SMe [8]) also supply strong evidence
for the existence of two rotamers. Molecular
mechanics calculations for the tetramethyldihalodisi-
lanes predict that the anti conformers will be more
2
2
2
2
and H ISiSiIH2 [2], Me ClSiSiClMe [3] and
2
2
2
H SiSiH SiH SiH [4] confirm this expectation. The
3
2
2
3
−1
vibrational spectra of MeH SiSiH Me have also been
stable by roughly 1 kcal mol [9].
2
2
interpreted as a superposition of the spectra of both
rotamers [5]. Other disilanes that have been studied
by vibrational spectroscopy are HBr SiSiBr H and
This work is the first of a series of forthcoming
papers dealing with the infrared and Raman vibra-
tional spectra of 1,2-dimethyldisilanes MeX Si-
2
2
2
SiX Me and 1,1,2,2
−
tetramethyldisilanes
2
*
Corresponding author. Fax: +43 316 816591.
Me XSiSiXMe (X is H, F, Cl, Br, I). Conformational
2
2
0
022-2860/97/$17.00 ᭧ 1997 Elsevier Science B.V. All rights reserved
PII S0022-2860(97)00005-7

8
4
M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
compositions as well as energy differences between
the rotamers will be measured by variable temperature
Raman spectroscopy, and ab initio calculations of
the vibrational spectra and of the harmonic force fields
for anti and gauche rotamers will be reported. The
harmonic force fields will be used in normal coordi-
nate analyses for these molecules, including the CD3
isotopomers.
Ph CD SiSiCD Ph : 1189m, 1155m, 1105sh,
2 3 3 2
1098vs, 1068ms, 1027s, 996 s, 985vs, 975sh, 917w,
858w, 781ms, 760w, 740vs, 726w, 700vs,b, 680 sh,
653w, 639vs, 612vs, 594vs, 546m, 511m, 472vs,
459vs, 408vs, 362vs, 317 w, 279vs
Cl CH SiSiCH Cl : Approx. 0.1 g of AlCl was
2
3
3
2
3
added to a solution of 5.0 g of Ph CH SiSiCH Ph in
2
3
3
2
20 ml of dry benzene and a constant stream of dry,
gaseous HCl was passed through it. The temperature
of the reaction mixture rose immediately, and the
process was continued until the solution had cooled
to room temperature. CH Cl SiSiCl CH was sepa-
2. Experimental
3
2
2
3
2.1. Syntheses
rated by distillation (b.p. 97ЊC/94 mbar [11]). For
CD Cl SiSiCl CD , the procedure was identical.
3
2
2
3
Both CH Cl SiSiCl CH and CD Cl SiSiCl CD
3
3
2
2
3
3
2
2
were prepared from the tetraphenyldisilanes
Ph CH Si) and (Ph CD Si) , cleaving off the
2.3. Spectra
(
2
3
2
2
3
2
phenyl groups with hydrogen chloride. The tetraphe-
Infrared spectra in the range between 3000 and
−1
nyldimethyldisilanes were prepared from Ph ClSi-
250 cm have been measured with a Perkin Elmer
883 spectrometer as films of the pure liquids between
CsBr plates. Far-FTIR spectra of the pure liquids have
been recorded with a Bruker IFS 66 spectrometer
equipped with a 6-mm mylar beamsplitter between
polyethylene plates. The Raman module of the IFS
66 spectrometer was used for recording the Raman
spectra (lNd:YAG = 1064 nm, 350 mW). The samples
of CH Cl SiSiCl CH and CD Cl SiSiCl CD have
2
SiClPh with CH MgBr or CD MgBr, respectively.
2
3
3
Chlorosilanes are easily hydrolysed by water. All
manipulations were therefore carried out under an
atmosphere of dry nitrogen. Solvents were dried
over potassium and distilled in an N atmosphere
2
prior to use.
Ph CH SiSiCH Ph : To a solution of 5.0 g (11.4
2
3
3
2
mmol) of (Ph ClSi) (prepared according to Ref. [10])
2
2
3
2
2
3
3
2
2
3
in 50 ml of dry diethyl ether (Et O), a solution of 22.8
been condensed into 1 mm capillary tubes that were
sealed under N2 atmosphere. Low-temperature
Raman spectra have been obtained using a variable
temperature assembly constructed by Bruker. Cooling
is effected by the flow of cold nitrogen gas across the
sample. The rate of flow of cold gas is controlled by
the amount of current supplied to a heater immersed in
liquid nitrogen. The temperature at the 1 mm Raman
tube was monitored with a K-type thermocouple.
2
mmol of CH MgBr in 50 ml of dry Et O was added
3
2
dropwise. After completion, the reaction mixture was
refluxed for three hours. Toluene (300 ml) was added
and all diethyl ether was removed by distillation. The
salts were filtered off by suction (Ph MeSiSiMePh is
2
2
not sensitive to air or moisture). After removal of the
solvent on a rotavapor, the solid residue of Ph MeSi-
2
SiMePh was recrystallized twice from a mixture of
2
toluene/n-heptane (1:1). The yield was 3.37 g (75%).
The purity of the samples was checked by infrared as
29
29
well as Si NMR spectroscopy (d( Si) = − 21.6 ppm
3. Vibrational spectra
relative to TMS). Infrared spectra are given in the
−
1
range Ͻ 1200 cm as nujol mulls.
The infrared and Raman spectra of the liquids and
the Raman spectra of the solids (173 K) are summar-
ized in Table 1 and selected portions of the Raman
spectra are given in Fig. 1 and Fig. 2. Spectra of
CD Cl SiSiCl CD have not been previously reported
2
.2. Infrared spectra
Ph CH SiSiCH Ph : 1188m, 1155m, 1105vs,
2
3
3
2
3
2
2
3
1
7
4
100sh, 1068ms, 1027s, 997s, 977w, 917w, 854w,
87vs, 761vs, 734vs, 715vs, 700s, 662s, 618mw,
75 vs, 464vs, 419vs, 368vs, 291vs.
in the literature. Infrared spectra of liquid CH Cl Si-
3 2
−1
SiCl CH in the range between 3000 and 300 cm
2
3
have been reported by H o¨ fler [12], as well as Raman

M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
85
Table 1
Infrared and Raman spectra (cm )] of CH
−1
3
Cl
2
SiSiCl
2
CH
3
and CD
3
Cl
CD
Ir (l)
2
SiSiCl
2 3
CD
CH
3
Cl
2
SiSiCl
2
CH
3
3
Cl SiSiCl
2
2
CD
3
Ir (l)
Ra (l)
Ra (s)
Ra (l)
Ra (s)
2
2
976 mw
906 mw
2978 vs
2903 s,p
2978 m
2901 s
2784 w
2230 vw
2120 w
2229 s
2121 vs
2229 vs
2119 vs
2
784 vw
1
986 m
1974 w
1973 mw
1
1
603 w
584 mw
1174 vvw
1174 mw
1035 mw
1
400 vs
256 vs
1403 m
1255 w
1406 m
1019 ms
1021 w
978 s
990 s
1021 m
978 s
990 s
1
399 m
1
1254 mw
997 s
9
7
80 w,sh
75 w
985 vw
982 vw
852 vvw
8
7
7
48 w
89 vs
65 vs
781 vw
705 sh
708 m
711 s
7
50 sh
7
31 vs
743 m,b
745 m
676 vs
665 w,b
668 ms
628 w
6
6
42 vs
10 s
6
5
80 vvw
65 m
680 vvw
561 m
602 s
550 sh
532 vs
605 w
552 w
552 vw
536 m
5
50 vvs,b
545 m
5
35 m
535 m
520 ms
520 vs
375 vs
4
4
77vs
66 vs
480 sh
462 vs
3
88 vs,p
390 vs
373 vs,p
366 vs,p
357 m
236 vw
225 m
216 m
203 m
178 m
3
2
80 ms
43vs
382 vs,p
63 vw
366 mw
355w
234s
224s
215sh
3
363 vw
359 m
236 vw
225 vs
216 w
206 vs
245 vvw
2
31 vs
2
1
1
28sh
95sh
81s
226 m,b
190 mw,b
165 s
225 s
170 s
177s
1
65 sh
1
45w
4w
141 s
144 s
133 s
150 vs
135 vs
151 vs
138 vs
133 vs
120 m
90 w
1
33 s
128 mw
9
8
5 w
85 vw
84 w,b
−
1
spectra from 3000 to 70 cm , but no hint of the exist-
ence of rotational isomerism has been mentioned.
The wavenumbers reported by H o¨ fler agree with
SiC bonds are in the anti or in gauche position, respec-
tively (Fig. 3).
The distribution of the normal modes between
the symmetry species as well as the IR and Raman
activities will be
−1
ours within Ϯ 3 cm in most cases.
,2-Dimethyltetrachlorodisilane and its d isotopo-
1
6
mer have 14 atoms with 36 fundamental vibrations.
C2h : G = 11A (Ra; p) +7B (Ra) +8A (IR) +10B (IR)
g
g
u
u
The molecular symmetry will be C2h or C if the two
2

8
6
M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
3 2 2 3
Fig. 1. Raman spectrum of solid (173 K) and liquid (298 K) CH Cl SiSiCl CH .
SiCH rocking vibrations usually fall into the range
3
C : G = 19A(IR, Ra, p) +17B(IR, Ra)
2
−1
between 850 and 760 cm and are shifted by approx.
−1
The spectra of the anti rotamer will follow the rule of
mutual exclusion, as the molecule has an inversion
centre. For the gauche rotamer, all fundamentals
are allowed to be infrared- and Raman-active. The
numbering of the fundamentals is included in
Table 3, Table 4, Table 5 and Table 6.
150–170 cm upon isotopic substitution [13]. SiCD3
rocking vibrations are thus expected to be between
−1
600 and 700 cm .
For CH Cl SiSiCl CH , the rCH vibrations lie
3
2
2
3
3
well above the SiC stretching modes. The strong IR
−1
−1
absorptions at 765 and 789 cm (Ra: 781 cm ) are
thus assigned as the rocking fundamentals, and the
Because of the small mass of the H atom, n , n , d
as
s
as
−1
−1
and d CH /CD constitute genuine group vibrations
strong bands at 731 cm (IR) and 745 cm (Ra) as
s
3
3
that are easily recognised and assigned. They will
not be discussed further, as our interest focuses on
the vibrations and force constants of the CSiCl Cl
SiC skeleton. As can be seen from the potential energy
distributions (see Section 6), rCH3 vibrations are
the SiC stretching modes. For the d isotopomer, nSiC
6
−1
−1
is at 665 cm (Raman) and 676 cm (IR). The rCD3
fundamentals, which are kinetically coupled with
SiC stretching modes if they belong to the same
irreducible representation (see Section 6), are lower
in energy. They gain Raman intensity because of the
vibrational coupling. Four strong IR absorptions at
2
2-
also group vibrations, but rCD vibrations are not.
3
The rocking fundamentals are therefore regarded as
being partially skeleton vibrations.
−1
676, 642, 610 and 602 cm as well as three Raman

M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
87
3 2 2 3
Fig. 2. Raman spectrum of solid (173 K) and liquid (298 K) CD Cl SiSiCl CD .
−
1
lines at 711, 628 and 605 cm , are assigned as rCD
rotamer and five for the gauche conformer. In the
liquid, five Raman bands are observed, and two of
them disappear completely in the solid state. Assisted
by ab initio calculations, the three persisting bands at
3
modes. Even with the help of the ab initio calcula-
tions, no unambigous assignment of the rCD modes
3
is possible.
−1
The SiCl and SiSi stretching modes of chlorinated
561, 535 and 390 cm are assigned as belonging to
the anti rotamer. The two disappearing bands at 545
−1
disilanes usually lie between 350 and 600 cm . A
striking feature of these molecules is the strong kinetic
coupling between nSiSi and nSiCln modes, as
observed for instance for Si Cl [14]. nSiSi is shifted
to higher, nSiCl to lower wavenumbers. A simplified
normal coordinate analysis for CH Cl SiSiCl CH
3
12] yielded the result that the Raman lines at
88 cm and 565 cm have to be described as a
0:50 mixture of nSiSi (A ) and n SiCl (A ). Three
Raman-active stretching modes (nSiSi (A ), n SiCl
−1
and 382 cm then clearly belong to the gauche con-
former (see Section 4).
For the anti conformer, five Raman-active and five
IR-active deformation modes involving the heavy
atoms are predicted. For the gauche rotamer, all ten
deformations are allowed to be Raman- and IR-active.
As can be seen from Fig. 1 and Fig. 2, five strong
Raman bands are observed in the solid state for
CH Cl SiSiCl CH and also for its d isotopomer
2
6
n
3
2
2
[
3
5
−1
−1
g
s
2
g
g
s
2
3
2
2
3
6
−1
(
A ), and n SiCl (B )) are expected for the anti
(h : 231, 225, 170, 144 and 133 cm ; d : 225, 206,
6 6
g
as
2
g

8
8
M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
van’t Hoff’s equation. The bands at 388 and
−
1
3
3
73 cm belong to the anti conformers, those at
−1
82 and 366 cm to the gauche conformers. As can
be clearly seen from Fig. 1 and Fig. 2, the anti
conformer is more stable.
In Fig. 4 and Fig. 5, the logarithms of the intensity
ratios (as determined from deconvolutions of the
bands) are plotted against the reciprocal temperature,
Fig. 3. Newman projection of anti and gauche 1,2-dimethyltetra-
chlorodisilane.
giving energy differences E − E
kJ mol and − 1.2 Ϯ 0.2 kJ mol for the h and d
of − 0.9 Ϯ 0.2
anti
gauche
−1
−1
6
6
isotopomers, respectively. The two values agree satis-
factorily within the error limits of Ϯ 0.2 kJ mol .
−1
−1
1
51, 138 and 133 cm ). They are assigned as belong-
ing to the anti rotamers.
Methyl torsional modes are expected to give bands
with rather small intensities around 150 cm . They
have not been observed because of their small inten-
sities, as was also the case with the SiSi torsion.
They are considerably smaller than the DE values
calculated with empirical force field methods (−1.23
kcal mol = −5.15 kJ mol [9])
−1
−1
−1
5
. Ab initio calculations
4. Rotational isomerism
The ab initio calculations that have been performed
during this work are part of a series of studies of
methylated disilanes MeX SiSiX Me and Me XSi-
SiXMe (X is H, F, Cl, Br and I). To make the entire
series feasible from the viewpoint of computational
cost, a fairly simple computational scheme was used
throughout. The core electrons of all heavy atoms
The variations in the intensity ratios of the Raman
2
2
2
−1
bands at 388/382 cm (CH Cl SiSiCl CH ) and at
2
3
2
2
3
−1
3
73/366 cm (CD Cl SiSiCl CD ) with temperature
3
2
2
3
have been used to determine the energy differences
between the anti and gauche conformers using the
−1
3 2 2 3
Fig. 4. Logarithm of the intensity ratio of the Raman bands at 388 and 382 cm of CH Cl SiSiCl CH as a function of reciprocal temperature.

M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
89
−1
Fig. 5. Logarithm of the intensity ratio of the Raman bands at 373 and 366 cm of CD
3
Cl
2
SiSiCl
2
CD
3
as a function of reciprocal temperature.
were represented by an effective core potential (ECP);
the ECPs for Br and I are relativistic ECPs. The ECPs
and basis sets use the parameters fitted by Stevens et
al. [15]. A valence double-zeta basis with polarisation
functions on all atoms was used. Geometries were
optimised on the level of RHF calculations; the har-
monic frequencies at the equilibrium positions were
also obtained at the RHF level.
SiC bond, posesses the smaller silicon carbon bond
distance.
Table 3, Table 4, Table 5, Table 6 give the calcu-
lated and observed fundamental vibrations of anti and
gauche CH Cl SiSiCl CH and CD Cl SiSiCl CD .
3
2
2
3
3
2
2
3
A single scale factor of 0.92 was used for all com-
pounds and all vibrations. Potential energy distribu-
tions (see Section 6) were calculated by transforming
the ab initio force field defined in cartesian coordi-
nates into a force field defined with symmetry coordi-
nates. As the calculated CSiSiC torsion angle of the
anti conformer is 179.23Њ, the point group C2h was
used for the construction of the symmetry coordinates.
According to the correlation tables, A and A vibra-
The calculated ab initio structural parameters of
anti and gauche CH Cl SiSiCl CH are summarised
3
2
2
3
in Table 2. As was pointed out in [16] for ClSi H , a
2
5
chlorine substitution into Si H influences the fre-
2
6
quency of the SiH bonds gauche to the chlorine
atom, but has no effect on the SiH bond in the trans
position. The same gauche effect is observed with 1,2-
dimethyltetrachlorodisilane. For the gauche rotamer,
the SiCl bonds that have two Cl atoms in the gauche
position (see Fig. 3) are shorter by 1.09 pm than the
SiCl bonds that have one Cl atom in the anti and one
in the gauche position. In the anti conformer, the
SiCl bond length is in between the two values for
the gauche conformation. A small effect is also calcu-
lated for the SiC bond lengths. Anti CH Cl Si-
g
u
tions of point group C2h combine into A type
vibrations of point group C2.
C
2h
C
A
B
A
B
2
Ag
→
→
→
→
B
A
g
u
3
2
SiCl CH , with two chlorine atoms gauche to the
Bu
2
3

9
0
M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
Table 2
Structural parameters (pm, deg.) for anti and gauche CH
3
Cl
2
SiSiCl
2
CH
3
Parameter
anti CH
3
Cl
2
SiSiCl
2
CH
3
gauche CH
237.38
3
Cl
2
2 3
SiSiCl CH
r(SiSi)
r(SiC)
r(SiCl)
r(CH)
ЄSiSiC
ЄSiSiCl
ЄClSiCl
ЄCSiCl
ЄCSiSiC
ЄClSiSiCl
ЄClSiSiCl
ЄHCSi
ЄHCH
237.05
187.05
207.53
109.88
115.50
107.38
107.53
109.44
179.23
187.29
206.62 ; 207.71
a
b
109.93; 109.88; 109.87
114.08
a
b
109.50 ; 106.94
108.25
109.16 ; 108.75
a
b
69.34
45.35; − 71.72
− 71.72; 171.21
110.57; 109.92; 110.27
108.48; 108.85; 108.69
64.01; 179.34
110.27; 109.93
108.90; 108.73
a
Bond distance and bond angle with two Cl atoms in the gauche position.
Bond distance and bond angle with one Cl atom in the gauche and one in the trans position.
b
Identical symmetry coordinates were therefore
used for the gauche conformer.
by them can be obtained from the authors upon
request.
6. Normal coordinate analysis
7. Discussion
Normal coordinate analyses (NCA) were per-
For disilanes, energy differences between con-
−1
formed for both anti and gauche MeCl SiSiCl Me
formers are of the order Ϸ 0–10 kJ mol , with the
set of available experimental data being very small.
Table 7 summarises experimental DE values that have
been reported so far. Obviously, ED experiments do
not allow DE values to be obtained with any accu-
racy, and no estimated error limits are given for
the dipole moment measurements. Therefore, the
only reliable DE values are derived from vibrational
spectroscopy. For both molecules that have been
investigated by temperature-dependent Raman spec-
troscopy, the anti conformers are more stable. This
contradicts the ‘‘gauche effect’’ [18] stating that
the more stable structure is the one which has the
maximum number of gauche interactions between
adjacent electron pairs and/or polar bonds. As SiCl
bonds are more polar than SiC bonds, and SiC
bonds are more polar than SiSi bonds, the gauche
structures are predicted to be more stable because
they possess three gauche interactions between the
most polar bonds (Fig. 6).
2
2
using the FG-formalism [17]. The cartesian coordi-
nates obtained for the optimized geometries were
input into the G-matrix program, together with a set
of 37 internal coordinates (bond lengths and bond
angles). Torsional coordinates were omitted from
the calculations. The 37 internal coordinates were
used to form symmetry coordinates. With the symme-
try force constants obtained from ab initio calcu-
lations, the pure ab initio frequencies were
reproduced to within Ϯ 1 cm , the small deviations
being attributed to rounding errors and to the neglect
of the torsional vibrations.
The calculated potential energy distributions are
presented in Tables 3–6. As was expected from a
previous simplified NCA [12], strong vibrational cou-
pling occurs between SiSi and SiCl stretching modes,
but also within the skeletal deformational modes.
Upon isotopic substitution, rCD3 modes become
involved in the coupling patterns.
−1
Detailed tables of the symmetry coordinates
that have been used and of the force fields defined
However, the results of the dipole moment
measurements (Table 7) are in accordance with the

M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
91
Table 3
Observed and calculated wave numbers and potential energy distributions ( Ͼ 10%) for anti CH
3
2 2 3
Cl SiSiCl CH
Species
Vibr. no.
Description
Ab initio
Unscaled
Obs.
PED
Scaled
A
g
(Ra)
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
1
n
n
d
d
asCH
3
3281.8
3185.6
1543.2
1399.6
891.9
778.7
404.9
596.1
165.7
134.4
246.3
3282.5
1543.5
863.7
574.8
229.4
146.1
167.7
3281.8
1542.5
864.7
587.2
203.1
70.1
3019
2931
1420
1288
821
716
372
548
152
124
227
3020
1420
795
529
211
134
154
3019
1419
796
540
187
64
2978
2903
1403
1255
–
100(1)
100(2)
94(3)
100(4)
75(5)
85(6)
32(7),38(8),21(9)
56(8),11(5),35(7)
36(9),16(5),11(7),55(11)
51(10),13(7),22(9),20(11)
31(11),17(9),37(10)
100(12)
94(13)
85(14)
105(15)
92(16),17(14)
99(17)
2
s
CH
3
3
asCH
3
4
s
CH
3
5
rCH
nSiC
3
6
745
388
565
170
133
231
2978
1403
781
535
225
144
–
2976
1400
789
550
195
–
7
nSiSi
SiCl
dSiSiC
8
n
s
2
9
10
11
12
13
14
15
16
17
18
19
dSiCl
gSiCl
n
2
2
B
A
g
(Ra)
asCH
asCH
3
3
d
rCH
asSiCl
3
n
2
rSiCl
tSiCl
CH
2
2
3
torsion
–
u
(IR)
n
asCH
asCH
3
100(19)
94(20)
84(21)
100(22)
45(23),21(21),38(24)
72(24),67(23)
–
n20
n21
d
3
rCH
asSiCl
3
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
n
2
rSiCl
tSiCl
CH
2
2
3
torsion
166,8
23.9
153
22
–
–
SiSi torsion
–
B
u
(IR)
n
n
d
asCH
asCH
asCH
3
3
3
3282.5
3185.3
1543.0
1398.3
841.5
766.5
501.0
257.8
193.8
87.9
3020
2930
1420
1286
774
705
461
237
178
81
2978
2903
1403
1255
765
731
466
243
181
–
100(27)
100(28)
94(29)
100(30)
85(31)
87(32)
94(33)
66(34),14(31),11(35)
85(35),11(34)
99(36),30(34)
dCH
rCH
3
3
nSiC
SiCl
dSiSiC
n
s
2
dSiCl
gSiCl
2
2
concept of the ‘‘gauche effect’’, as the anti conforma-
tion seems to be successively stabilised with increas-
ing mass of the halogen atom (and decreasing
polarity of the SiX bond). Unfortunately, the number
of accurate experimental data is far too small to
deduce any systematic trends.
the values predicted by normal coordinate analy-
−1
ses. For Si Cl and for Si Me , 2.40 N cm [14]
2
6
2
6
−1
and 1.65 N cm [19] are reported in the literature.
A linear interpolation between these two values gives
an estimated SiSi force constant of 2.15 N cm for
MeCl SiSiCl Me, which compares with a scaled
(0.92) ab initio force constant of 1.69 N cm . In a
recent RHF/6-31G* calculation for Si Me [20], a
−1
2
2
−1
The force constants of the gauche conformer do not
differ very much from those of the anti conformer.
2
6
−1
The SiSi stretching force constants (2.00 N cm for
anti, 1.99 for gauche 1,2-dimethyltetrachlorodisilane,
unscaled) are identical, but considerably smaller than
SiSi stretching force constant of 1.303 was reported,
which is also considerably smaller than the value
obtained from the NCA. For disilane, Si H , however,
2
6

9
2
M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
Table 4
Observed and calculated wave numbers and potential energy distributions ( Ͼ 10%) for anti CD
3
Cl
2
SiSiCl
2
CD
3
Species
Vib. no.
Description
Ab initio
Unscaled
PED
Scaled
Obs.
A
g
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
1
n
n
d
d
asCD
3
2431.9
2287.4
1114.8
1108.1
750.6
708.7
389.4
556.9
148.5
134.3
239.7
2431.9
1113.6
692.9
556.7
212.8
142.1
119.9
2432.1
1113.4
695.5
566.8
181.2
69.4
2237
2104
1026
1019
691
652
358
512
137
124
221
2237
1024
638
512
196
131
110
2238
1024
640
521
167
64
2229
2121
1021
1021
708
665
366
520
150
135
225
2229
1021
628
520
203
120
–
100(1)
100(2)
85(3)
2
s
CD
3
3
asCD
3
4
s
CD
3
78(4),20(6)
33(5),17(6),20(7)
50(6),10(4),17(5),18(8)
33(7),11(6),34(8),17(11)
48(8),33(5),22(7)
56(9),22(5),35(11)
52(10),12(7),24(9),19(11)
22(11),24(9),41(10)
100(12)
5
rCD
nSiC
3
6
7
nSiSi
SiCl
dSiSiC
8
n
s
2
9
10
11
12
13
14
15
16
17
18
19
dSiCl
gSiCl
n
2
2
B
A
g
asCD
asCD
3
3
d
96(13)
rCD
asSiCl
3
67(14),18(15),12(16)
90(15),15(14)
94(16),20(14)
102(17)
n
2
rSiCl
tSiCl
CD
2
2
3
torsion
–
u
n
asCD
asCD
3
2230
1035
642
532
177
–
100(19)
96(20)
n20
n21
d
3
rCD
asSiCl
2
3
64(21),21(22)
82(22),19(21)
44(23),28(21),40(24)
72(24),68(23)
–
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
n
rSiCl
tSiCl
CD
2
2
3
torsion
117.3
23.2
108
21
–
–
SiSi torsion
–
B
u
n
n
d
d
asCD
3
2432.4
2287.6
1114.1
1106.2
656.8
709.1
490.8
80.6
2238
2105
1025
1018
604
652
452
74
2230
2120
1035
1019
602
676
462
–
100(27)
100(28)
92(29)
85(30)
s
CD
3
asCD
3
s
CD
3
dCD
nSiC
SiCl
dSiSiC
3
79(31)
68(32),12(30)
87(33),10(32)
101(34),25(36)
79(35),17(36)
62(36),16(35)
n
s
2
dSiCl
gSiCl
2
191.2
240.0
176
221
177
224
2
values for the SiSi stretch obtained by normal coordi-
nate analyses agree very well with those calculated
at the MP2/6-31G** level. The differences are of
two conformers, reflecting the gauche effect. For
gauche 1,2-dimethyltetrachlorodisilane, the force
constants of the SiCl bonds that have two adjacent
gauche SiCl bonds (these will be called ‘‘inner’’
−1
the order of 0.07–0.14 N cm [20], which is a
much better correlation and must be attributed to the
inclusion of configuration interactions. The ab initio
calculations do, however, predict an increase in the
SiSi stretching force constants in the series Si Me Ͻ
−1
bonds further on, compare Fig. 6) are 2.65 N cm
(scaled by 0.92), whereas the value for the ‘‘outer’’
−1
bonds is 2.55 N cm . The inner bonds are also shorter
by 1.09 pm (Table 2). The interaction force constants
f(SiCl/SiCl) (same Si atom) are identical (0.144 and
0.142 for anti and gauche, respectively), as are all inter-
2
6
Si H Ͻ Si Me Cl , paralleling the results of normal
2
6
2
2
4
coordinate analyses [21].
SiCl stretching force constants are different for the
−1
actions separated by the SiSi bond (0.04 N cm ).

M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
93
Table 5
Observed and calculated wave numbers and potential energy distributions ( Ͼ 10%) for gauche CH
3 2 2 3
Cl SiSiCl CH
Species
Vib. no.
Description
Ab initio
Unscaled
PED
Scaled
Obs.
A
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
1
n
n
n
d
d
d
asCH
asCH
CH
s 3
asCH
asCH
3
3
3281.4
3274.3
3182.7
1542.1
1546.0
1400.8
881.9
861.6
768.7
578.2
397.4
609.8
81.5
197.7
164.9
235.7
139.3
171.7
21.0
3281.3
3275.5
3182.9
1544.8
1542.7
1398.4
863.2
849.9
764.6
589.2
501.4
149.2
239.4
263.5
202.8
75.5
3019
3012
2928
1419
1422
1289
811
793
707
532
366
561
75
182
152
217
128
158
19
3019
3013
2928
1421
1419
1287
794
782
703
542
461
137
220
242
187
69
2978
2978
2903
1403
1403
1255
789
765
743
545
382
565
85
75(1),25(3)
75(2),25(1)
100(3)
88(4)
88(5)
100(6)
71(7)
83(8)
86(9)
84(10),13(12)
35(11),38(12),16(15)
42(12),20(10),28(11)
83(13)
44(14),12(15),19(16),27(17)
25(15),13(7),14(11),40(13)
54(16),17(14)
40(17),32(14),11(15)
–
2
3
4
3
3
5
6
s
CH
3
7
rCH
rCH
nSiC
3
8
3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
n
asSiCl
nSiSi
SiCl
dSiSiC
2
n
s
2
dSiCl
gSiCl
rSiCl
tSiCl
CH
SiSi torsion
n
n
n
d
d
d
2
190
165
226
133
–
2
2
2
3
torsion
–
–
B
asCH
asCH
3
3
2978
2978
2903
1403
1403
1255
789
789
743
545
477
133
226
245
190
–
64(20),35(21)
64(21),36(20)
100(22)
76(23),18(24)
76(24),18(23)
100(25)
77(26)
80(27)
88(28)
100(29)
s
CH
3
asCH
asCH
3
3
s
CH
3
rCH
rCH
nSiC
3
3
n
asSiCl
SiCl
2
n
s
2
94(39)
dSiSiC
52(31),43(32),13(35)
34(32),24(31),20(34)
88(33)
34(34),17(26),36(35)
62(35),30(31),53(34)
–
dSiCl
gSiCl
rSiCl
tSiCl
2
2
2
2
CH
3
torsion
172.6
159
–
Fig. 6. Newman projection of gauche MeCl
indicated by arrows.
2 2 3 2 2 3
SiSiCl Me (left) and gauche Me SiMe SiSiMe SiMe (right) with the gauche interactions

9
4
M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
Table 6
Observed and calculated wave numbers and potential energy distributions ( Ͼ 10%) for gauche CD
3 2 2 3
Cl SiSiCl CD
Species
Vib. no.
Description
Ab initio
Unscaled
PED
Scaled
Obs.
A
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
n
1
n
n
n
d
d
d
asCD
asCD
3
3
2432.5
2426.2
2286.0
1115.4
1116.2
1106.6
748.9
697.3
696.5
572.3
381.9
550.8
78.3
184.9
149.3
225.4
136.9
121.7
29.4
2432.6
2426.7
2286.1
1112.8
1113.4
1107.5
668.2
694.6
704.4
567.8
490.8
141.0
229.2
250.4
184.2
73.5
2238
2232
2103
1026
1027
1018
689
642
641
527
351
507
72
170
137
207
126
112
27
2238
2233
2103
1024
1024
1019
615
639
648
522
451
130
211
230
169
68
2229
2229
2121
1021
1021
1021
708
642
642
552
373
520
84
71(1),30(2)
70(2),30(1)
100(3)
48(4),28(5),17(6)
67(5),19(4)
62(6),29(4)
30(7),20(9),20(11)
55(8),16(109
42(9),12(5),19(12)
52(10),15(8),12(12)
37(11),33(12)
31(12),25(7),33(10),13(11)
73(13),26(15),28(16),25(17)
37(14),35(16),19(17)
31(15),16(7),14(11),30(13)
48(16),28(14),11(15)
49(17),28(14),11(15)
–
2
3
s
CD
3
4
asCD
asCD
3
3
5
6
s
CD
3
7
rCD
rCD
nSiC
3
8
3
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
n
asSiCl
nSiSi
SiCl
dSiSiC
2
n
s
2
dSiCl
gSiCl
rSiCl
tSiCl
CD
SiSi torsion
n
n
n
d
d
d
2
178
135
216
135
–
2
2
2
3
torsion
–
–
B
asCD
asCD
3
3
2229
2229
2121
1021
1021
1021
610
642
642
536
462
135
203
236
178
–
62(20),38(21)
63(21),39(20)
100(22)
56(23),38(24)
57(24),35(23)
83(25),20(28)
75(269
51(27),10(28),19(29)
59(28),12(25),10(30)
80(29),21(27)
88(30)
52(31),34(32),20(35)
49(32),20(31),10(34)
88(33)
s
CD
3
asCD
asCD
3
3
s
CD
3
rCD
rCD
nSiC
3
3
n
asSiCl
SiCl
2
n
s
2
dSiSiC
dSiCl
gSiCl
rSiCl
tSiCl
2
2
2
2
41(34),24(27),13(32),31(35)
58(35),35(31),52(34)
–
CD
3
torsion
121.7
112
–
Neither their absolute value nor their sign depends on
the dihedral angle between the two SiCl bonds. The
SiC stretching force constants are also slightly influ-
enced by the molecular conformations, as the values
for the trans rotamer (two adjacent SiCl bonds in the
gauche position) and for the gauche rotamer are 2.789
is not the case for the SiC force constants. For hex-
amethyldisilane, H o¨ fler [19] gave a value of 2.88 N
−1
−1
cm , and for octamethyltrisilane, 290 N cm was
reported [23]. The poor correlation for the SiCl
force constants and the much better one for the SiC
force constants is again caused by the neglect of
configuration interactions, as their inclusion should
cause a shortening of the SiCl bond. Atomic distances
for single bonds between two second row elements
are usually calculated too long when configuration
interactions are neglected [24], whereas single bonds
−1
and 2.806 N cm , respectively.
The SiCl force constants calculated by the ab initio
method used in this work are also considerably
smaller than values deduced from normal coordinate
analyses (Si Cl : 2.90 [14], Si Cl : 2.80 [22]), but this
2
6
3
8

M. Ernst et al./Journal of Molecular Structure 412 (1997) 83–95
95
Table 7
Energy differences Eanti − Egauche (kJ mol ) between anti and gauche conformations of disilanes
−1
Molecule
DE
− 2.09 Ϯ 2.51
− 0.84 Ϯ 2.51
1.25 Ϯ 2.51
0.2 Ϯ 1.1
Method
Br
HSiSiHI
IH SiSiH
SiSiH
MeCl SiSiCl
2
HSiSiHBr
2
ED [1]
ED [2]
ED [2]
ED [6]
I
2
2
2
2
I
H
3
2
SiH
2
SiH
Me
3
2
2
− 1.05 Ϯ 0.2
Ra, this work
Dipole moment [8]
ED [3]
Dipole moment [8]
Dipole moment [8]
Dipole moment [8]
Dipole moment [8]
Dipole moment [8]
Dipole moment [8]
Ra [7]
−
2.50
Me
2
ClSiSiClMe
2
2.51 Ϯ 2.09
2.17
−
Me
Me
Me
Me
Me
Me
2
2
2
2
2
3
FSiSiFMe
BrSiSiBrMe
ISiSiIMe
(MeO)SiSi(OMe)Me
(MeS)SiSi(SMe)Me
SiSiMe SiMe SiMe
2
− 2.09
− 4.16
− 6.54
− 10.15
− 2.93
− 2.26 Ϯ 0.15
2
2
2
2
2
2
3
involving first row elements tend to lengthen when
these interactions are included.
E. Hengge, J. Organomet. Chem. 182 (1979) 165. J. Nagy,
G. Zsombok, E. Hengge, W. Veigl, J. Organomet. Chem.
232 (1982) 1.
[9] R. Stolevik, P. Bakken, J. Mol. Struct. 197 (1989) 131.
[
10] W. K o¨ ll, Thesis TU Graz, 1995.
Acknowledgements
[
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12] F. H o¨ fler, E. Hengge, Monatsh. Chem. 103 (1972) 1506.
13] K. Hassler, Spectrochim. Acta 37A (1981) 511.
14] F. H o¨ fler, W. Sawodny, E. Hengge, Spectrochim. Acta 26A
[
[
[
Two of the authors (K.S. and A.J.) thank the Land
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8] J. Nagy, S. Ferenczi-Gresz, E. Hengge, S. Waldh o¨ r,
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[