Journal of Molecular Structure 482–483 (1999) 571–577
Infrared and Raman spectra, conformations and ab initio
calculations of chloromethyl methyldifluorosilane
V. Aleksa a,1, K. Herzog a, R. Salzer a, A. Gruodis b,1, P. Klaeboe b,*, C.J. Nielsen b,
G.A. Guirgis c, J.R. Durig c
a
Institute of Analytical Chemistry, University of Technology, Dresden, D-01062 Dresden, Germany
b
Department of Chemistry, University of Oslo, PO Box 1033, 0315 Oslo, Norway
c
Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA
Abstract
Chloromethyl methyldifluorosilane (CH2Cl–CH3SiF2) was synthesized for the first time and the infrared spectra were
recorded in the vapour, amorphous and crystalline solid phases in the MIR and FIR regions. Additional MIR spectra were
obtained for the compound isolated in argon and nitrogen matrices at 15 K. The spectra of chloromethyl methyldifluorosilane
showed the existence of two conformers – anti and gauche – present in the vapour and in the liquid. Raman spectra of the liquid
were recorded at various temperatures between 298 and 163 K, giving DH (gauche–anti) equal to ca. 0.2 kJ mol 21.
By careful annealing of the amorphous solid formed by depositing the vapour on a cold CsI or silicon window (infrared) or
Cu finger (Raman) at 80 K, two different crystals were formed. One of these, obtained after annealing to 150 K, contained the
gauche conformer, the other after annealing to ca. 110 K contained molecules in the anti conformer. These experimental data
make a very reliable assignment of the conformer bands possible.
Ab initio calculations were performed using the gaussian 94 program with the HF/6-311G* basis set and gave optimized
geometries, infrared and Raman intensities and scaled vibrational frequencies for the anti and gauche conformers. The
conformational energy derived was 5.9 kJ mol 21 with gauche being the low energy conformer. q 1999 Elsevier Science
B.V. All rights reserved.
Keywords: Conformations; Vibrational spectra; Halosilanes; Ab initio calculations
1. Introduction
A number of halosilanes with conformational equilibria have been investigated in our laboratories. They
are analogous to the corresponding substituted
ethanes and should, as a result of restricted rotation
* Corresponding author. Tel.: 1 47 22855678; fax: 1 47
22855441.
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe)
1
Permanent address: Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania.
around a Si–C central bond, exist as a mixture of an
anti conformer with Cs symmetry and two spectroscopically equivalent gauche conformers with no
symmetry (C1). It is our aim to determine the infrared
and Raman spectra of these molecules and combine
the results with ab initio quantum chemical calculations in order to interpret the spectra and to evaluate
the structure, conformational energies and torsional
barriers for these molecules.
Chloromethyl
methyldifluorosilane
(CH2Cl–
CH3SiF2), later to be abbreviated CMDFS, was
synthesized for the first time. In the present
0022-2860/99/$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
PII: S0022-286 0(98)00678-4
572
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 571–577
Fig. 1. Raman spectra below 1500 cm 21 of chloromethyl methyldifluorosilane (CMDFS) as a liquid at room temperature in two directions of
polarization.
communication we shall present our preliminary
results for CMDFS following similar studies of the
related
molecule
chloromethyl
methylsilane
(CH2Cl–CH3SiH2) [1].
In the process of studying CMDFS it was observed
that this compound formed two different crystals at
80 K depending upon the annealing temperature, one
in which the crystal contained the anti, the other the
gauche conformer. This is a rare occurrence, sometimes observed when the energy difference between
the conformers is small. However, two crystals, each
consisting of a different conformer, have recently
been observed independently in two related silanes:
dichlorodifluoromethylsilane [2] and bromomethyl
dimethylsilane [3]. Very complete assignments of
the vibrational spectra to each of the conformers are
possible in these cases.
2. Experimental
The sample of CMDFS was synthesized by the
reaction of chloroethyl trichlorosilane with freshly
sublimed antimony trifluoride. The reaction was
carried out at room temperature for 30 min
without solvent. The sample was subsequently
purified at a low temperature, low pressure fractionation column, and the purity was checked by mass
spectrometry.
The infrared spectra of CMDFS were recorded in
the following spectrometers: Bruker models 88, 66
and 113v (vacuum bench) and Perkin-Elmer model
2000. Spectra of the vapour were recorded in cells
of 10 cm, 20 cm and 1 m path lengths while the amorphous and crystalline solids were studied in various
cryostats cooled with liquid nitrogen. The sample was
mixed with argon or nitrogen (1 : 1000 and 1 : 500)
and slowly condensed on a CsI window, cooled with a
closed cycle system from Leybold to ca. 15 K and
annealed to temperatures in the range 20–38 K.
Raman spectra were recorded with two spectrometers from Dilor (model XY 800 with a CCD camera
and a monochannel model RT 30), excited by an
argon ion laser using the 514.5 nm line. The sample
was enclosed in a capillary of 2 mm inner diameter
cooled with cold nitrogen gas [4] and additional
Raman spectra of the amorphous and annealed crystalline phases were measured when the sample was
deposited on a copper finger, cooled with liquid
nitrogen.
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 571–577
573
Fig. 2. Raman spectra of CMDFS in the range 790–660 cm 21 at 80 K: solid curve, unannealed amorphous solid; dashed curve annealed to
150 K, crystal II; dotted curve, annealed to 110 K, crystal I.
3. Results and discussion
3.1. Raman spectral results
A Raman spectrum of the liquid below 1500 cm 21
in two directions of polarization is given in Fig. 1.
Two detailed Raman spectra recorded at 80 K are
presented in Figs. 2 and 3 covering the regions 790–
660 and 400–180 cm 21, respectively. The three
curves show the Raman spectra of the following solids
Fig. 3. Raman spectra of CMDFS in the range 400–180 cm 21 at 80 K: solid curve, unannealed amorphous solid; dashed curve annealed to
150 K, crystal II; dotted curve, annealed to 110 K, crystal I.
574
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 571–577
Fig. 4. MIR spectra of CMDFS in the range 950–600 cm 21 at 80 K: solid curve, unannealed amorphous solid; dashed curve annealed to 150 K,
crystal II; dotted curve annealed to 120 K, crystal I.
Fig. 5. FIR spectra of CMDFS in the range 400–80 cm 21 at 80 K: solid curve, unannealed amorphous solid; dashed curve annealed to 150 K,
crystal II; dotted curve annealed to 120 K, crystal I.
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 571–577
575
Fig. 6. MIR spectra of CMDFS in an argon matrix (1 : 1000) at 15 K in the range 950–600 cm 21, solid curve, unannealed; dashed curve,
annealed to 31 K.
at 80 K: an unannealed amorphous solid, a solid
which was obtained after annealing to 150 K
containing crystal II, and a solid achieved after
annealing to 95–110 K containing crystal I. It is
immediately apparent from these Raman spectra that
there are large differences between the three curves
and the bands present in the amorphous solid which
are frequently present either in crystal II or in crystal I,
but sometimes in both. It is quite obvious from these
spectra that each crystal contains one separate
conformer. In most cases the bands from one
conformer are separated from those of the other as
clearly seen in Fig. 3 at 215 (I), 224 (II), 253 (I),
261 (I), 278 (II), 312 (II), 323 (II), 334 (I), 337 (II),
365 (I), 368 (II) and 370 cm 21 (I). Many of the bands
present in crystal I and II are quite close together and
would undoubtedly have been attributed to overlapping fundamentals if two different crystals had not
been formed. As will be shown later, there is an excellent agreement between the Raman spectra and those
recorded in the infrared regarding the bands in the
amorphous solid and the two crystalline solids.
The spectrum of the liquid was recorded at six
temperatures between 295 and 183 K and very small
intensity variations were observed. Attempts were
made to calculate the enthalpy difference between
the conformers, employing the three band pairs:
699/685, 374/319 and 211/223 cm 21. Measuring the
peak heights a DH (gauche–anti)
ˆ 0.2 ^
0.3 kJ mol 21 was calculated. As all the Raman spectra
had a fluorescent background which could not be
reduced by distillations, and none of these bands
was particularly strong, the experimental uncertainty
was large.
3.2. Infrared spectral results
Complete vapour spectra in which some bands had
well resolved rotational A and C fine structure were
recorded in the MIR and FIR regions. The vapour was
condensed on a CsI window at 80 K and a series of
spectra were recorded in which the sample was
annealed and recooled to 80 K. In other series the
spectra were recorded at the annealing temperature
without recooling to 80 K. As examples, part of the
MIR region (950–600 cm 21) displaying the amorphous and the two crystalline solids I and II is
shown in Fig. 4 whereas part of the FIR region
(400–80 cm 21) is given in Fig. 5. The low temperature spectra observed for CMDFS in the infrared were
576
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 571–577
Fig. 7. MIR spectra of CMDFS in nitrogen matrix (1 : 1000) at 15 K in the range 950–600 cm 21, solid curve, unannealed; dashed curve,
annealed to 31 K.
in good agreement with those observed in the Raman
cryostat since corresponding bands of crystal I and
crystal II were observed in both spectra.
CMDFS was deposited in argon and nitrogen
matrices at 15 K and the IR spectra were recorded
before annealing and after annealing for 15 min at
24, 27, 29 and 31 K and subsequent recooling to
15 K. Detailed spectra recorded in argon and nitrogen
matrices in the spectral region 950–600 cm 21 are
given in Figs. 6 and 7, respectively. The bands of
the matrix spectra are very sharp, frequently giving
separate peaks for the two conformers when they
nearly coincide in the condensed phases. Some intensity changes were detected after annealing, but the
effects were small, suggesting a low enthalpy difference between the conformers in the matrices also.
Quite small intensity changes after annealing were
observed also for the related silanes chloromethyl
dimethylchlorosilane [5] and chloromethyl dimethyl
fluorosilane [6] owing to the small enthalpy difference
between the conformers.
Supposedly, the conformational equilibrium of the
vapour phase is maintained when the gas mixture hits
the CsI window at 15 K. The high energy conformer
may pass the potential barrier and convert to that a
low energy one when the temperature is raised. It was
generally observed that IR bands which were present
in crystal I diminished and those present in crystal II
were enhanced after the matrices were annealed to ca.
30 K, but some cases were uncertain. As the anti
conformer (with all the three halogens in gauche positions) of CMDFS has a higher dipole moment than the
gauche, the anti conformer should be stabilized in the
liquid compared to the vapour and the matrices. From
the plots given by Barnes [7] neglecting the effects of
matrix viscosity, the barrier height should therefore be
around 8 kJ mol 21.
3.3. Quantum chemical calculations
The
LCAO–MO–SCF
calculations
were
performed using the Gaussian 94 program [8] with
the basis set HF/6-311G*. The conformational energy
derived from this basis set was 5.9 kJ mol 21, with
gauche being the low energy conformer in poor agreement with the experimental DH values for CMDFS.
The force fields in Cartesian coordinates were
converted to internal coordinates in the usual manner
V. Aleksa et al. / Journal of Molecular Structure 482–483 (1999) 571–577
and the diagonal force constants scaled with a factor
of 0.9. A comprehensive table giving the assignments
of the two conformers could be constructed and the
observed wave numbers correlated with those
obtained from the calculations. This comprehensive
table could not be given here for the sake of brevity,
but it will be published in due course. It was quite
definite from these correlations that the conformers
present in crystals I and II were anti and gauche,
respectively. Among the 27 fundamentals expected
for CMDFS we have assigned more than half (17) to
bands in which the anti and gauche bands are clearly
separated. In nearly all of these the anti–gauche shifts
agree with the results of the calculations. For the three
fundamentals n 20, n 21 and n 22, the calculated anti–
gauche separations were larger than 15 cm 21 and the
observed fundamentals were all shifted in the right
direction.
In the related 2-chloroethylsilane (CH2Cl–
CH3SiH2) only one crystal phase was detected,
containing the anti conformer. However, the gauche
conformer was more stable and the enthalpy difference between the conformers was found to be
2.2 kJ mol 21 in liquified xenon [1]. Therefore, the
conformational stability of this molecule changes
considerably when the two hydrogens at the silicon
are substituted with fluorine leading to a larger change
in dipole moments between the conformers.
577
Acknowledgements
VA and AG are grateful to Konferenz der
deutschen Akademien der Wissenschaften (Volkswagenstiftung), Germany and Det Norske VidenskapsAkademi, Norway, respectively, for fellowships.
References
[1] G.A. Guirgis, Y.E. Nashed, J.R. Bobb, P. Klaeboe, V. Aleksa,
J.R. Durig, to be published.
[2] V. Aleksa, P. Klaeboe, A. Horn, C. J. Nielsen, G.A. Guirgis, J.
Raman Spectrosc. 29 (1998) 627.
[3] V. Aleksa, P. Klaeboe, C. J. Nielsen, V. Tanevska, G.A.
Guirgis, Vibrational Spectrosc. (1998) in press.
[4] F.A. Miller, B.M. Harney, Appl. Spectrosc. 24 (1970) 291.
[5] H.M. Jensen, P. Klaeboe, V. Aleksa, C.J. Nielsen, G.A. Guirgis,
Acta Chem. Scand. 52 (1998) 578.
[6] V. Aleksa, P. Klaeboe, A. Horn, C.J. Nielsen, G.A. Guirgis, J.
Mol. Struct. 445 (1998) 161.
[7] A.J. Barnes, J. Mol. Struct. 113 (1984) 161.
[8] Gaussian 94, Revision D.2, M.J. Frisch, G.W. Trucks, H.B.
Schlegel, P.M.W. Gill, B.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B.
Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M.
Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong,
J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox,
J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. HeadGordon, C. Gonzalez, J.A. Pople, Gaussian, Inc., Pittsburgh
PA, 1995.