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R. Zink, K. Hassler / Spectrochimica Acta Part A 55 (1999) 333–347
(29Si-NMR: SiMe3Si*H3; Si: −16.2 ppm, Si*:
−97.7 ppm, JSiH=182.2 Hz.)
field parameters that were fitted to represent the
observed frequencies in the best possible way. In
the present work all normal coordinate analyses
were performed on the basis of ab initio calcula-
tions providing us with a more realistic picture of
off-diagonal symmetry force constants and vibra-
tional coupling patterns. In particular, the genera-
tion of ab initio symmetry force constants and
their possible transferability to similar, but larger
silanes containing several CMe3 or SiMe3 groups
served as another impetus for this work. For
example, the assignment of SiSi stretching modes
in the spectra of methylated (poly)silanes is more
or less unambiguous since these modes appear as
strong bands in the vibrational spectra due to the
absence of kinetic coupling with vibrations of the
methyl fragment. In contrast, the assignment of
wSiSi in tert-butyl substituted silanes is a clumsy
task requiring the use of normal coordinate analy-
ses since deformation modes of the CC3 fragment
are mixed with SiSi stretching modes implying
that the latter lose intensity. So it has not yet been
possible to unambiguously assign the vibrational
spectra of the highly interesting compound
[Si(CMe3)2]3. This should become realistic with a
set of reliable transferable ab initio symmetry
force constants.
1
2.1.2. 1,1,1-Trifluoro-2,2,2-trimethyldisilane
A solution of 1.8 g (8.7 mmol) of SiMe3SiCl3 in
20 ml of hexachloro-1,3-butadiene was added
dropwise to a suspension of 4.0 g (38.7 mmol) of
ZnF2 in 40 ml of hexachloro-1,3-butadiene.
SiMe3SiF3 was continuously condensed into a
flask cooled by liquid nitrogen at a pressure of
about 200 mmHg. (29Si-NMR: SiMe3Si*F3;=Si:
1
−22.2 ppm, Si*: −58.4 ppm, JSiF=383.3 Hz,
2JSiF=30.6 Hz.)
2.1.3. 1,1,1-Trichloro-2,2,2-trimethyldisilane and
1,1,1-triiodo-2,2,2-trimethyldisilane
A total of 4.0 g (12.0 mmol) of SiMe3SiPh3
(prepared from Ph3SiK and Me3SiCl) was dis-
solved in 20 ml of benzene. A small amount of
AlCl3 (typically 0.1 g) was added and dry gaseous
HCl or HI was passed through the solution. The
completeness of the reaction was indicated by the
disappearance of the sharp IR band at 1427 cm−1
being characteristic for Si–Ph groups. Subse-
quently benzene was removed by evaporation un-
der reduced pressure and n-heptane was added to
precipitate the aluminum salts. Solids were filtered
off and n-heptane was evaporated under reduced
pressure. The remaining liquid of SiMe3SiCl3 was
purified by fractionation (b.p. 65°C/40 Torr). The
product solidified in the cooler and was melted
into a flask.
2. Experimental
2.1. Synthesis
The solid residue of SiMe3SiI3 was recrystal-
lized twice from n-heptane giving colorless crys-
tals. (29Si-NMR: SiMe3Si*Cl3; Si: −6.4 ppm, Si*:
18.5 ppm; 29Si-NMR: SiMe3Si*I3; Si: −0.9 ppm,
Si*: −99.8 ppm.)
The silanes CMe3SiX3 (X=H, F, Cl, Br, I)
were prepared according to methods described in
the literature [6]. For the 1,1,1-trimethyldisilanes
SiMe3SiX3 improved syntheses have been devel-
oped. To the authors’ best knowledge the synthe-
sis of SiMe3SiI3 has not been reported so far.
2.1.4. 1,1,1-Tribromo-2,2,2-trimethyldisilane
6 g (18.0 mmol) of SiMe3SiPh3 was reacted with
an excess of liquid HBr in a sealed glass bomb
over a period of 10 days at room temperature.
Subsequently benzene and unreacted HBr were
removed by evaporation under reduced pressure.
The solid residue was purified by sublimation
(40°C/0.1 Torr). (29Si-NMR: SiMe3Si*Br3; Si: −
2.1 ppm, Si*: −4.7 ppm.)
2.1.1. 1,1,1-Trimethyldisilane
A solution of 3.24 g (15.6 mmol) of SiMe3SiCl3
in 30 ml of di(n-butyl)ether was added dropwise
to an ice-cooled suspension of 0.89 g (23.4 mmol)
of LiAlH4 in 50 ml of di(n-butyl)ether. SiMe3SiH3
was continuously condensed into a flask cooled by
liquid nitrogen at a pressure of about 200 mmHg.