Journal of Molecular Structure 482–483 (1999) 627–632
A. Gruodis a, c
, D.L. Powell a,1
, P. Klaeboe a, *, C.J. Nielsen
V. Aleksa c a
, G.A. Guirgis b
, J.R. Durig b
, a
Department of Chemistry, University of Oslo, PO Box 1033, 0315 Oslo, Norway b
Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110-2499, USA c
Department of General Physics and Spectroscopy, Vilnius University, Vilnius 2734, Lithuania
Abstract
The infrared spectra of 1,1,1-trifluoropropyl trifluorosilane were recorded in the vapour, amorphous and crystalline solid phases in the range 4000–50 cm
2 1
. Additional MIR spectra of the compound isolated in argon and nitrogen matrices at 5 K before and after annealing to temperatures in the range 15–35 K were obtained. Raman spectra of the liquid were recorded at various temperatures between 298 and 163 K. Spectra of the amorphous solid on a copper finger were recorded and spectra of crystalline solids cooled by different techniques were obtained. The spectra showed the existence of two conformers – anti and gauche – present in the vapour and in the liquid. Approximately five Raman bands present in the fluid phases vanished upon crystallization, but only small changes occurred in the corresponding infrared spectra. From intensity variations with temperature in the Raman spectra of the liquid, the enthalpy difference was determined to be
D
H(gauche-anti)
ˆ
3.2 kJ mol
2
1
.
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 was calculated to be 5.3 kJ mol
2 1 with anti being the low energy conformer.
q
1999 Elsevier Science
B.V. All rights reserved.
Keywords: Conformations; Vibrational spectra; Halosilanes; Ab initio Calculations
1. Introduction
1,1,1-Trifluoropropyl trifluorosilane (CF
3
CH
2
–
CH
2
SiF
3
) to be abbreviated TFTFS, was synthesized, and the infrared and Raman spectra were investigated.
This molecule is an ethane derivative and can form anti and gauche conformers because of restricted rotation around the central C–C bond. It is structurally
* Corresponding author.
E-mail address: peter.klaboe@kjemi.uio.no (P. Klaeboe)
1
Permanent address: Department of Chemistry, The College of
Wooster, Wooster, OH 44691, USA.
related to 2-chloroethyl trifluorosilane (CH
2
Cl–
CH
2
SiF
3
) which is also being studied in these laboratories [1] and is also similar to 1,1,1,4,4,4-hexafluoro butane [2] and to butane itself [3,4]. In the present investigation the vapour, amorphous and crystalline samples of TFTFS were recorded in the middle
(MIR) and far infrared (FIR) regions, and the matrix isolation technique was employed to obtain spectra of the compound trapped in argon and nitrogen matrices.
Raman spectra of the liquid, including polarization measurements, were obtained and spectra of the liquid were recorded at different temperatures. Raman spectra of TFTFS as a crystal were observed using
0022-2860/99/$ - see front matter q
1999 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 2 8 6 0 ( 9 8 ) 0 0 6 8 3 - 8

628 A. Gruodis et al. / Journal of Molecular Structure 482–483 (1999) 627–632
Fig. 1. Raman spectra of 1,1,1-trifluoropropyl trifluorosilane (TFTFS) as a liquid in two directions of polarization (1500–600 cm
2
1
).
different cooling techniques. Moreover, the conformational energies, the structure, the force constants and infrared and Raman intensities were calculated by ab initio methods. Our preliminary data are given in the present communication.
2. Experimental
The sample of TFTFS was prepared for the first time by reaction of the corresponding chloro compound with freshly sublimed antimony trifluoride
Fig. 2. Raman spectra of TFTFS as a liquid (295 K) (solid line) and crystal at 205 K (dashed line) in the range 1000–100 cm
2
1
.

A. Gruodis et al. / Journal of Molecular Structure 482–483 (1999) 627–632 629
Fig. 3. Raman band pair 669/635 cm
2
1 at 288, 273, 257 and 243 K.
at room temperature for one hour. The compound was purified in a low temperature, low pressure fractionation column and the purity was checked by mass spectrometry.
Raman spectra were recorded with a spectrometer from Dilor (TR 30), excited by a Spectra-
Physics model 2000 argon ion laser using the
514.5 nm line. The sample was placed in a capillary of 2 mm inner diameter and low temperature measurements of the liquid and the crystal were carried out in a Dewar cooled with cold nitrogen gas. 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.
The infrared spectra were recorded on one of the following FT-IR spectrometers: Bruker models 66, 88 and 113v (vacuum instrument) and Perkin-Elmer model 2000. The vapour spectra were recorded in cells of 10 cm (CsI windows) and 20 cm (PE windows) path lengths whereas the amorphous and crystalline solids were studied in MIR and FIR cryostats cooled with liquid nitrogen. The sample of
TFTFS was mixed with argon or nitrogen (1:1000) and slowly condensed on a CsI window, cooled with a closed cycle system from APD to ca. 5 K and annealed to temperatures in the range 10–35 K.
3. Results and discussion
3.1. Raman spectral results
A Raman spectrum of the liquid at ambient temperature in two directions of polarization in the range 1500–600 cm
2
1 is presented in Fig. 1. Spectra of the liquid at 295 K and a crystal at 205 K are given in Fig. 2. Bands at 875 w, 669 s, 470 w and 398 w cm
2 1 of the liquid vanish after crystallization. Moreover, the components of the doublet at 250 w and
246 s cm
2 1 change intensities after crystallization and the weak shoulders at 1130 and 1088 cm
2
1 may disappear in the spectrum of the crystal. These bands are assigned to the second conformer which is absent in the crystal. The number of vanishing bands is quite small, revealing that most of the fundamentals of one conformer overlap those of the other and the conformational equilibrium is one-sided.
Raman spectra of the liquid were recorded between
298 and 223 K and the enthalpy difference
D
H between the conformers was calculated using the band pair 669/635, plotted at four temperatures in
Fig. 3. and in which the band at 669 cm
2 1 vanished in the crystal. The van’t Hoff plots from 10 temperatures gave the value 3.2
^ 0.3 kJ mol
2 1
. Attempts were made to employ the pairs at 398/424 and 470/

630 A. Gruodis et al. / Journal of Molecular Structure 482–483 (1999) 627–632
Fig. 4. MIR vapour spectrum (1500–500 cm
2
1
) of TFTFS in a 10 cm cell at 60 (solid line), 20 (dashed) and 6 Torr pressure (dotted).
424 cm
2
1
, but these band pairs gave very unreliable results.
3.2. Infrared spectral results
An infrared vapour spectrum of TFTFS in the region 1500–500 cm
2
1 is presented in Fig. 4, giving well resolved A-type band contours at 422 cm
PR separation equal to 6.5 cm
2
1 contour was observed at 231 cm
2
1
2
1 with
. Another A-type
. Extensive low temperature spectra were recorded in the cryostats fitted with windows of CsI and Si in which the sample had been sprayed onto the window at liquid nitrogen temperature. Very careful annealing experiments
Fig. 5. FIR spectra (500–200 cm
2
1
) of amorphous (solid line) and a crystalline solid of TFTFS at 80 K, annealed to 130 K (dashed).

A. Gruodis et al. / Journal of Molecular Structure 482–483 (1999) 627–632 631
Fig. 6. MIR spectra (1400–800 cm
2
1
) of TFTFS in an argon matrix (1:1000) before (solid line) and after (dashed line) annealing to 32 K.
were carried out and spectra were recorded either at the annealing temperatures or after recooling to 80 K.
The spectra revealed that one crystal was formed after annealing (I), but after recooling an additional crystal was formed (II), both containing the same conformer, but giving bands with slightly different wave numbers such as 1382 (I) and 1378 cm
2
1
(II).
Also, the band intensities varied considerably in the two crystals. Crystal I was obtained after annealing to
112–120 K and the other (crystal II) after annealing to
130–140 K. As the sample which contained 6 fluorine atoms had a high vapour pressure, the sample disappeared from the window at ca. 160 K. The FIR spectral region is shown in Fig. 5 (500–200 cm
2
1
). The
Fig. 7. MIR spectra (1000–500 cm
2
1
) of TFTFS in a nitrogen matrix (1:1000) before (solid line) and after (dashed line) annealing to 32 K.

632 A. Gruodis et al. / Journal of Molecular Structure 482–483 (1999) 627–632 weak bands at 872, 744 and 392 cm
2 1 vanished in the two crystals. It is significant that the 872 and
392 cm
2 1 bands also vanished in the Raman spectra while the strong Raman band at 669 cm
2 1
, disappearing in the crystal and employed in the van’t
Hoff plots, had negligible intensity in the infrared.
Spectra of TCTFS were recorded in argon (Fig. 6) and nitrogen (Fig. 7) (1:1000) matrices at 5 K and gave sharper peaks than the vapour, liquid, amorphous and crystal spectra. Annealing in the 15–20 K range led to small spectral changes as a result of relaxation in the matrix lattice. At higher temperatures (25–
35 K) a few bands in each matrix increased or diminished in intensity after annealing. It was observed that some bands which diminished in intensity after annealing corresponded with those vanishing in the crystals: 1135, 876 and 746 cm
2 1
, while additional bands at 1264, 1208 and 1024 cm
2 1 diminished.
Infrared bands at 960, 955, 639, 555, 421 and
412 cm
2
1 were enhanced after annealing.
Thus, it appears that the conformer which was more stable in the liquid and which was also present in the crystals had lower energy both in the argon and nitrogen matrices. As discussed below, this conformer was probably anti. Bands of the other conformer
(gauche) vanished or were reduced in intensity after annealing. As the conversion from the high to the low energy conformer started at ca. 30 K, the barrier can be estimated [5] to be ca. 8 kJ mol
2 1
.
although it seems a priori very likely that TFTFS like other 1,2-disubstituted ethanes (with the exception of 1,2-difluoroethane) has anti as the more stable conformer. Both conformers are nearly prolate symmetric tops with rotational constants: anti,
2.335, 0.474 and 0.472 and gauche, 2.256, 0.641
and 0.650 GHz.
The ab initio calculated frequencies were scaled with a factor of 0.9 for the fundamentals above
400 cm
2
1 and not scaled for those below. The bands discussed above which could be attributed to the more and the less stable conformers were correlated with the scaled anti and gauche wave numbers from the calculations. The agreement was somewhat better when the more stable conformer was correlated with anti than with gauche. However, at the present time this assignment should not be considered quite conclusive.
Acknowledgements
AG and VA have received fellowships from Det
Norske Videnskaps-Akademi and the Research
Council of Norway, respectively.
References
3.3. Quantum chemical calculations
Quantum chemical calculations using the Gaussian-94 programs with basis function HF/6-311G* was carried out giving the conformational energy difference 5.3 kJ mol
2
1 with anti being the low energy conformer. The large uncertainty in these calculations prevents using this result for the assignments,
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