Vibrational spectroscopic studies, conformations and ab initio calculations of 3,3,3-trifluoropropyltrichlorosilane

Spectrochimica Acta Part A 61 (2005) 1335–1346

Vibrational spectroscopic studies, conformations and ab initio calculations of 3,3,3-trifluoropropyltrichlorosilane Gamil A. Guirgisa , Anne Hornb , Peter Klaeboeb,∗ , Claus J. Nielsenb a

Department of Chemistry and Biochemistry, College of Charleston, 66 George Street, Charleston, SC 29424, USA b Department of Chemistry, University of Oslo, P.O. Box 1033, Blindern N-0315 Oslo, Norway Received 23 September 2004; accepted 27 September 2004 Dedicated to Professor James R. Durig on the occasion of his 70 years birthday.

Abstract Infrared spectra of 3,3,3-trifluoropropyltrichlorosilane (CF3 CH2 CH2 SiCl3 ) were obtained in the vapour, amorphous and crystalline solid phases in the range 4000–50 cm−1 . Additional spectra in argon matrices at 5.0 K were recorded before and after annealing to 20–36 K. Raman spectra of the compound as a liquid were recorded at various temperatures between 298 and 210 K and spectra of the amorphous and crystalline solids were obtained. The spectra suggested the existence of two conformers (anti and gauche) in the fluid phases and in the matrix. When the vapour was shockfrozen on a cold finger at 80 K and subsequently annealed to 120–150 K, six weak or very weak Raman bands vanished in the crystal. Similar variations were observed in the corresponding infrared spectra after annealing and four very weak IR bands disappeared after crystallization. From intensity variations between 298 and 210 K of three Raman band pairs an average value conf H◦ (gauche–anti) = 6.1 ± 0.5 kJ mol−1 was obtained in the liquid. Annealing experiments indicate that the anti conformer also has a lower energy in the argon matrices. The conformational equilibrium is highly shifted towards anti in the liquid, and the low energy conformer also forms the crystal. The spectra of the abundant anti conformer and the few bands ascribed to the gauche conformer have been interpreted. Ab initio calculations at the HF/6-311G** and B3LYP/6-311G** gave optimized geometries, infrared and Raman intensities and vibrational frequencies for the anti and gauche conformers. The conformational energy differences derived were 11.8 and 9.2 kJ mol−1 from the HF and the B3LYP calculations, respectively. © 2004 Elsevier B.V. All rights reserved. Keywords: Raman and infrared spectra; Ab initio calculations; Silanes; Conformational analysis

1. Introduction 3,3,3-Trifluoropropyltrichlorosilane (CF3 CH2 CH2 SiCl3 ), later to be abbreviated TFPTCS, was synthesized and an infrared and Raman spectroscopic study of this compound has been carried out. As is apparent from Fig. 1, TFPTCS can form anti and gauche conformers due to restricted rotation around the central C C bond, the CF3 and SiCl3 groups form substituents in an ethane. The present compound is closely ∗

Corresponding author. Tel.: +47 22855678; fax: +47 22855441. E-mail address: [email protected] (P. Klaeboe).

1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.saa.2004.09.018

related to n-propyltrifluorosilane (CH3 CH2 CH2 SiF3 ) [1], n-propyltrichlorosilane (CH3 CH2 CH2 SiCl3 ) [2] and to 3,3,3-trifluoropropyltrifluorosilane (CF3 CH2 CH2 SiF3 ) [3] and the vibrational spectra of these compounds have recently been published. In addition to the ethane-type silanes, various molecules containing a central C Si bond have been investigated, most recently chloromethyl methyldichlorosilane [4] and dichloromethyl methyl dichlorosilane [5]. A large number of disilanes [6–8] with Si Si and trisilanes [9,10] with Si Si Si linkages have been synthesized by Hassler and coworkers and the vibrational spectra were reported. The potential functions in ethanes with a central C C bond are different from those

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Fig. 1. The anti and gauche conformers of 3,3,3-trifluoropropyltrichlorosilane (TFPTCS) including numbering of atoms for the definition of symmetry coordinates.

of the silanes with Si C or Si Si central bonds. In the diand tri-silanes there are lower barriers than is the case in ethanes, and the anti conformer is more similar to that of the gauche conformer. In TFPTCS the results of the ab initio calculations, suggest that there are many large shifts between the vibrational bands of the rotamers. However, since the conformational equilibrium is highly displaced towards one conformer (anti) in the present molecule, only the most intense gauche bands could be observed as weak peaks.

2. Experimental 2.1. Sample preparation The sample of 3,3,3-trifluoropropyl trichlorosilane was prepared by the reaction of 3,3,3-trichloropropene and trichlorosilane at 250 ◦ C for 16 h in a steel reactor as previously reported by Tarrant et al. [11]. The sample was first purified by simple distillation under nitrogen and then by trapto-trap distillation. The purity of the sample was checked by infrared and NMR spectroscopy and the results were confirmed by GC–mass spectrometry. 2.2. Raman measurements Raman spectra were recorded using a triple monochromator spectrometer from Dilor (model TR 30) with a cooled photomultiplier and ca. 3 cm−1 spectral resolution. The spectra were excited by an argon ion laser from Spectra-Physics (model 2000) using the 514.5 nm line for excitation with 90◦ excitation geometry. The sample was enclosed in a sealed capillary of 2 mm inner diameter, and low temperature measurements of the liquid and the crystal were carried out in a Dewar cooled with nitrogen gas [12]. Additional Raman spectra of the amorphous and annealed crystalline phases of the sample were measured, deposited on a copper finger and cooled with liquid nitrogen. Raman spectra of the liquid in two directions of polarization were recorded at room temperature. Additional spectra

were obtained at eight different temperatures between 298 and 210 K. From these spectra of the liquid, the enthalpy difference, conf H◦ , between the conformers was calculated. Since the high energy conformer (gauche) was present in very low concentrations, suitable gauche bands were difficult to localize. The compound crystallized spontaneously at ca. 200 K and Raman spectra of the crystal were recorded at ca. 180 K. Moreover, TFPTCS was condensed on a copper finger at 80 K and formed an amorphous solid. After annealing to 130 K and re-cooling to 80 K the sample, appeared perfectly crystalline. TFPTCS was dissolved as a saturated solution in cyclohexane and in approximately 50% concentration in carbon tetrachloride and acetonitrile, and the Raman spectra were subsequently recorded. Unlike the results obtained for 3,3,3-trifluoropropyltrifluorosilane [3], no significant variations in the band intensities with solvent polarity were detected, probably due to the very weak gauche bands. 2.3. Infrared measurements The infrared spectra were recorded on various Fourier transform spectrometers in the middle infrared region (MIR) employing a Bruker spectrometer IFS-66 (4000–450 cm−1 ) and a Perkin–Elmer model 2000 (4000–450 cm−1 ). A vacuum spectrometer from Bruker (model IFS-113 v) was used in the far infrared region (FIR) (600–50 cm−1 ). All the spectrometers had DTGS detectors. Beamsplitters of Ge/KBr were employed in the MIR region, and a metal mesh beam splitter, covering the range 700–50 cm−1 was used in the FIR region. The vapour was studied in cells with KBr (10 cm) and polyethylene windows (19 cm) with 1.0 and 2.0 cm−1 resolution, respectively. Moreover, the vapour was deposited on a CsI window of a MIR cryostat and on a wedge shaped window of silicon in a FIR cryostat, both cooled with liquid nitrogen. An amorphous, but partly crystalline solid was formed on the window, but after annealing to ca. 180 K, the sample appeared completely crystalline from visual inspection. Moreover, the peaks were frequently shifted and they appeared sharper than those from the amorphous phase. A

G.A. Guirgis et al. / Spectrochimica Acta Part A 61 (2005) 1335–1346

Fig. 2. Raman spectum of TFPTCS as a liquid at ambient temperature in the range 1500–100 cm−1 .

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Fig. 3. Raman polarization spectra of TFPTCS in the range 1500–800 cm−1 of a liquid sample at ambient temperature.

few weak IR bands at 1164, 929, 754 and 614 cm−1 apparently vanished after annealing, as did their counterparts in the Raman spectra. The sample of TFPTCS was diluted with argon (1:1000). The mixture was deposited on a CsI window at 5.0 K of a Displex cryostat from APD (model HS-4) with a three-stage cooling system. The MIR spectrum of the unannealed matrix was first recorded at 1.0 and 0.5 cm−1 resolution and subsequently annealed to 15 and 20 K to observe reorientations in the matrices. Further annealing to temperatures in the 20–38 K range was carried out and the matrix was re-cooled to 5.0 K before recording.

3. Results 3.1. Raman spectral results A Raman spectrum of TFPTCS as a liquid at ambient temperature between 1500 and 100 cm−1 is presented in Fig. 2. Spectra in two directions of polarization are given in Figs. 3 (1500–800 cm−1 ) and 4 (900–100 cm−1 ). A Raman spectrum of a crystalline solid at 80 K, annealed to 130 K in a Raman cryostat, is presented in Figs. 5 (1500–800 cm−1 ) and 6 (800–50 cm−1 ). A few weak Raman bands, which were present in spectra of the liquid vanished in spectra of the crystalline solid. The bands of the crystal were shifted relative to those in the liquid and they were sharper. In a few cases 1444/1433, 1215/1210 and 779/776 cm−1 the modes observed in the crystal were split into close-lying peaks. Crystal spectra of TFPTCS were also recorded when the sample was cooled with cold nitrogen vapour in a Miller–Harney cell [12]. The Raman spectrum appeared to be the same as that obtained in the cryostat after shock cooling the vapour on a window at 80 K and subsequent anneal-

Fig. 4. Raman polarization spectra of TFPTCS in the range 900–100 cm−1 of a liquid sample at ambient temperature.

Fig. 5. Raman spectra of TFPTCS in the range 1500–800 cm−1 at 80 K of an unannealed sample (dashed line) and a crystalline sample annealed to 130 K (solid line).

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In order to determine the enthalpy difference from the variable temperature spectra, the bands of the different conformers should both have reasonably high intensities. Moreover, they should be situated on a flat background and not overlap other bands in the spectra. More important, they must be “pure”, meaning that their intensities should be due to one conformer only, with no contribution, either from fundamentals or from combination bands of the other conformer. These criteria were not at all fulfilled in the present spectra, since the anti bands had invariably much higher intensities than the gauche bands. Moreover, the gauche bands were observed as low intensity shoulders on the prominent anti bands. Considerable discrepancies therefore occurred between the conf H ◦ values obtained from different band pairs. The intensities of each band pair were fitted to the Van’t Hoff equation Fig. 6. Raman spectra of TFPTCS in the range 800–50 cm−1 at 80 K of an unannealed sample (dashed line) and a crystalline sample annealed to 130 K (solid line).

ing to 130 K. Possibly, as many as six weak or very weak Raman bands, present in the spectrum of the liquid, disappeared after crystallization. Some of them were interpreted as fundamentals of the high-energy conformer, particularly when the corresponding IR bands also vanished in the crystal spectra. Examples are the Raman bands at 924, 793, 752 and 442 cm−1 . The large energy difference calculated between the anti and gauche conformers (see below) leaves no doubt that the anti conformer has the lower energy and is present in the crystal while the gauche conformer exists in a low concentration in the fluid phases. The low number of gauche fundamentals observed in TFPTCS is in contrast to the results of the related molecule 3,3,3trifluoropropyltrifluorosilane [3]. In this compound as many as 22 fundamentals appeared as separate anti and gauche bands at different wave numbers, whereas 14 of the vibrational modes of one conformer overlapped those of the other. Undoubtedly, the bulky substituent SiCl3 of TFPTCS compared with the SiF3 of 3,3,3-trifluoropropyltrifluorosilane made the gauche conformer much less favourable in TFPTCS. Raman spectra of the liquid were recorded at eight temperatures between 296 and 210 K. Intensity variations with temperature of certain bands relative to neighbouring peaks were observed. They were interpreted as a displacement of the conformational equilibrium. The Raman bands, which vanished upon crystallization, belong to one conformer, and they were paired with other bands (often neighbours), which remained in the crystal. However, it is a priori quite uncertain, if the corresponding liquid bands are characteristic of only one conformer or if they belong to overlapping bands of both conformers. In some cases the results of the force constant calculations strongly suggest that they may originate from one conformer only.

ln{Ianti (T )/Igauche (T )} = −conf H ◦ /RT + constant where Ianti /Igauche is the ratio in peak heights or integrated areas and it is assumed that conf H◦ is constant with temperature. Both peak heights and integrated band areas were attempted for determining band intensities. Integrated band areas should in principle be preferable for these calculations, since effects due to changing band widths with temperature would be avoided. However, it was found for the band pairs in Fig. 7 that a better fit to a straight line was achieved by employing peak heights. In spite of careful curve resolution and determination of band areas, the calculations based upon band areas invariably showed a larger scatter than when peak heights were employed. Therefore, peak heights were chosen for the calculations. The following four pairs of bands were tried in the Van’t Hoff plots: 468/442, 774/752, 774/442 and 774/793 cm−1 (liquid), in which the bands in the denominator vanished in the crystal (gauche conformer). The three former band pairs

Fig. 7. Van’t Hoff plots of three anti/gauche band pairs 468/442, 774/752 and 774/442 cm−1 of TFPTCS in the temperature range 298–210 K.

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Fig. 8. MIR spectra with resolution 1.0 cm−1 of TFPTCS as a vapour in a 10 cm cell; 1500–500 cm−1 , 8 and 2 Torr pressure.

Fig. 9. FIR spectra with resolution 2.0 cm−1 of TFPTCS as a vapour in a 19 cm cell in the range 670–50 cm−1 , 12 and 4 Torr pressure.

gave rise to the Van’t Hoff plots, which are shown in Fig. 7. They gave rise to the conf H◦ (gauche–anti) values 6.9, 4.1 and 7.2 kJ mol−1 , respectively, whereas the latter pair gave completely erratic values. Therefore, the average value from the three former band pairs was chosen for the pure liquid of TFPTCS, conf H◦ (gauche–anti) = 6.1 ± 0.7 kJ mol−1 . This conf H◦ value can be compared with the conf H◦ (gauche–anti) values measured for 3,3,3trifluoropropyltrifluorosilane [3] 3.4 ± 0.3 kJ mol−1 , for n-propyltrifluorosilane [1] 2.1 ± 0.2 kJ mol−1 and for n-propyltrichlorosilane [2] 4.6 ± 0.3 kJ mol−1 . In all these three compounds the anti conformer was the low energy conformer. It is not surprising that the bulky substituent SiCl3 in TFPTCS has a much larger repulsion towards the CF3 in the gauche conformer than the corresponding SiF3 substituent in 3,3,3-trifluoropropyltrifluorosilane [3]. Moreover, the SiF3 group has a smaller repulsion towards CH3 than does SiCl3 in the gauche conformer as is apparent from the increasing enthalpy values of 2.1 and 4.6 kJ mol−1 in n-propyltrifluorosilane [1] and n-propyltrichlorosilane [2], respectively. From the measured: conf H◦ (gauche–anti) = 6.1 ± 0.7 kJ mol−1 a simple calculation, taking into account the statistical weights of 1 and 2 for anti and gauche, respectively, gives ca. 97% anti and 3% gauche in the liquid at room temperature. This is in agreement with the spectral results, since the assigned gauche components appear as weak shoulders or when resolved as very weak bands compared to the bands of the anti conformer.

rotational constants because of the heavy substituents CF3 and SiCl3 , and no rotational vapour contours were observed. Middle infrared region (MIR) spectra of TFPTCS as a liquid capillary between KBr plates are presented in Fig. 10 (1500–500 cm−1 ) and these wave numbers are included in Table 1. Spectra of the amorphous solid (solid line) and the sample annealed to ca. 130 K and recorded at 80 K are given in Fig. 11. Small frequency shifts were observed between the spectra of the unannealed and the annealed samples. The IR bands, which vanished after annealing, were all weak or very weak as expected from the estimated concentrations of less than 3% gauche in the vapour phase and slightly more in the liquid. The MIR spectra of the amorphous (solid line) and the solid annealed to 130 K (dotted line) both recorded at 80 K are shown in Fig. 11. Only small changes were observed between the spectra, but the peaks of the annealed spectrum appeared sharper and slightly shifted relative to the amorphous spec-

3.2. Infrared spectral results Infrared spectra of TFPTCS as a vapour in the range 1500–500 cm−1 at 8 and 2 Torr pressure in a 10 cm cell with resolution 1.0 cm−1 are presented in Fig. 8, while FIR spectra of the vapour in the range 670–50 cm−1 at pressures of 12 and 4 Torr are shown in Fig. 9. The molecule has quite small

Fig. 10. MIR spectra of TFPTCS (1500–500 cm−1 ) as a neat liquid (capillary between KBr plates).

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Table 1 Infrared and Raman spectral data for trifluoropropyltrichlorosilane (CF3 CH2 CH2 SiCl3 ) Infrared Vapour

Raman Liquid

Amorphous

295 Kb

Ar matrix 32 Ka 5.0 Kb

Liquid

80 Kb

Crystal 130 Ka 80 Kb

295 Kb

2990 vw 2961 w

2979 w 2964 w

2982 vw 2955 w

2981 w 2954 w

2981 vw 2955 w

Interpretation Crystal 130 Ka 80 Kb

anti

2997 vw,D 2968 s,P

2993 w 2967 s} 2963 w, sh

A ν22 A ν23

295 Kb

2955 w 2924 vw 2910 w 2868 vw

2915 w

2915 w

2951 vw,sh 2917 m

2928 s,P

2928 s

A ν 1

2871 vw

2870 vw

2873 vw

2884 w,P

A ν 2

1450 s 1436 m

1486 w 1448 m 1436 m

2865 vw 1486 vw 1448 m 1436 w 1433 vw

2833 vw,P

1451 s 1439 m

2819 vw 1487 vw 1448 m 1436 w

2885 vw} 2878 vw 2837 w

1451 m,P? 1431 m,bd,P

1451 m 1444 w} 1433 vw

A ν 3 A ν 4

1410 m

1429 vw 1405 m

1405 w

1403 m 1389 w 1375 s

1407 m,P

1405 s

A ν 5

1375 s

1403 m 1390 w 1375 s

1377 w,P?

1378 w

A ν24

1320 s 1295 vw

1319 s 1297 w

1320 s 1297 w 1275 m 1272 w 1265 vs 1249 m

1322 m,P 1298 vw

1320 s

A ν 6

1257 s

1268 w,P

1271 w A ν 7

1232 w 1224 m 1213 vs

1237 vw 1218 s,P?

1215 s} 1210 w

A ν25

1142 m,bd,D?

1133 m

A ν 8

2924 w 2879 w 2822 vw

1378 s 1325 s 1295 w,sh 1282 w,sh 1269 vs 1241 w,sh

1219 vs 1188 vw,sh 1175 vw? 1151 vs 1131 vw

1377 s↑ 1374 s↓ 1321 s↑ 1298 vw 1298 vw 1269 vs↑ 1265 vs 1235 vw↓ 1222 w↑ 1219 w↑ 1216 s} 1214 vs

1144 vs

1266 vs

1233 w 1214 vs

1215 vs

1162 w 1138 vs,bd

1188 w 1164 w 1138 vs

1108 vw

1138 m↑ 1131 v↓ 1119 vw

1107 vw

1106 vw

1082 s

1075 s

1073 vs

1071 vs

∼1040 vw 1031 s

1032 m↑ 1029 m 1026 m 920 vw↑ 900 vs 893 vw 844 s↑ 840 s

1029 s 927 vw 901 s

1030 s 1013 w 929 w 900 s

838 s

838 s

795 w 774 s

796 w 771 s

753 w 709 s

754 vw 709 s

648 s 610 vw 598 vs

572 vs

928 vw 903 s 843 s 797 w 778 s 740 w 712 s 650 s 607 vs

582 vs

779 m↑} 777 s 712 s↑} 710 s 650 s 617 w↑? 606 vs 603 s↑ 580 vs 575 w↑

1188 w * 1139 m 1129 vs

gauche

1130 m 1106 m 1091 w 1075 vs 1070 vs 1057 w 1027 s 1013 w * 901 s 893 vw 838 s

1076 m,D

1071 m

A ν26

1030 s,P

1027 s

A ν 9

924 vw,P 899 m,D

* 898 m

A ν27

838 vs,P

839 vs

A ν10

793 w,P 774 s,P?

* 779 m} 776 m * 709 vw

A ν28

794 w 782 w} 780 s * 710 vs

752 w,P 707 w,P?

649 s 614 vw 602 vs 593 m,sh

649 s * 601 vs

645 w,P 614 vvw 597 s,P?

648 w

A ν12

596 vs

A ν13

577 vs 566 m,sh

577 vs

572 s,D?

576 m 566 s

A ν29

ν27

A ν11 ν11

ν12

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Table 1 (Continued) Infrared Vapour 295 Kb 554 m 551 m 540 w,sh 512 vw 470 s

419 w 398 w 365 vw,br 325 vw,br 289 s 226 m 205 vw? 160 m 148 vw

81 vw 72 w 61 w

Raman Ar matrix 32 Ka 5.0 Kb

Liquid

Amorphous

295 Kb

566 vw 555 m

553 m

470 s 467 m↑ 465 m

468 s 463 s

Liquid

80 Kb

Crystal 130 Ka 80 Kb

553 m

508 vw 469 s 463 m

anti

295 Kb

Crystal 130 Ka 80 Kb

549 m

552 m,P

550 s

A ν14

539 vw

538 m,D?

539 s

A ν30

468 s 463 m

468 vvs,P

469 vvs

A ν15

460 vw,P 442 w,P

*

397 m 372 m

397 m,P 365 vw,D

394 m 368 vw

A ν16 A ν31

286 s 227 m 220 m

286 vs,P 227 w,D 222 w,P? 188 vw,sh 162 m,D

286 s 227 m 217 m

A ν17 A ν32 A ν18

165 s

A ν33

148 m,P 134 w,sh 115 w,br,D?

152 s 134 s 115 vw 85 w 79 w 62 vw 53 vw 45 vw?

A ν19 A ν20 A ν34 A ν21 A ν35 A ν36

441 vw 419 w? 397 m 372 m 329 vw

Interpretation

186 vw 166 m} 163 m 149 vw 134 vw

147 w 134 w

82 m 65 w 61 w 53 vw

82 m 64 w 61 w 52 vw

166 m

gauche

ν15

Abbreviations; s, strong; m, medium; w, weak; v, very; sh, shoulder; br, broad; P, Q, R and A, B, C, denote band contours; the arrows pointing upwards and downwards, respectively, represent bands being enhanced or diminished in the matrix spectra after annealing; asterisks denote bands vanishing in the crystal; P, polarized; D, depolarized. a Annealing temperature. b Recording temperature.

Fig. 11. MIR spectra (1500–450 cm−1 ) of TFPTCS as an unannealed solid at 80 K (solid line) and a crystalline solid annealed to 130 K (dashed line) recorded at 80 K.

trum. Possibly, also the spectrum of the unannealed sample was partly crystalline. The spectra reveal that only the weak or very weak bands at 1164, 929, 754 and 614 cm−1 of the vapour and amorphous phases vanished in the crystal. Sim-

ilar spectra were recorded in FIR, and spectra of a sample annealed to 160 K (solid line), 170 K (dashed line) and 190 K (dotted line) are shown in Fig. 12. As is apparent, negligible changes occurred after annealing, but apparently part of the sample disappeared from the window, making the absorption bands weaker. Probably the sample was completely or partially crystalline before annealing. Moreover, the gauche bands were too weak to be observed with certainty. Additional infrared spectra of TFPTFS were recorded in an argon matrix with mixing ratios 1:1000, deposited at 5.0 K. Supposedly, the conformational equilibrium of the vapour phase is maintained when the gas mixture is quickly frozen on the CsI window at 5.0 K, provided that the barrier to conformational equilibrium is above 3.5 kJ mol−1 [13]. Small spectral changes occur after annealing to 20 K. They are interpreted as site effects due to relaxation of TFPTCS in the matrix cages. At higher annealing temperatures (22–38 K) small spectral variations, which might be correlated with conformational changes, were observed in the matrix spectra. Matrix spectra in argon in the regions 1500–950 cm−1 and 950–430 cm−1 are presented before annealing (upper curve) and after annealing to 32 K (lower curve) in Figs. 13 and 14, respectively. Certain small intensity changes were observed

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Fig. 12. FIR spectra (660–40 cm−1 ) of TFPTCS as apparently crystalline solids, annealed to 160 K (solid line), 170 K (dashed line) and 190 K (dotted line), all spectra were recorded at 80 K.

Fig. 14. MIR spectra (950–430 cm−1 ) of TFPTCS in an argon matrix (1:1000), deposited and recorded at 5 K, upper curve, unannealed, lower curve, sample annealed to 32 K.

3.3. Quantum chemical calculations after annealing. As is apparent from Table 1, these bands are marked with arrows pointing upwards and downwards, respectively. However, none of the bands observed in the argon matrix spectra of TFPTCS seemed to belong to the high-energy gauche conformer. Therefore all the bands, which increased or decreased in intensities after annealing apparently belong to the anti conformer. The small intensity changes observed after annealing were probably not connected with conformational effects. The observed wave numbers for the infrared and Raman bands of TFPTCS in the various states of aggregation are listed in Table 1. The bands vanishing in the crystalline solids are marked with asterisks.

Fig. 13. MIR spectra (1500–950 cm−1 ) of TFPTCS in an argon matrix (1:1000), deposited and recorded at 5 K, upper curve, unannealed, lower curve, sample annealed to 32 K.

Hartree–Fock quantum chemical calculations were performed using the Gaussian-98 programs [14] To allow an easy comparison with the series of halosilanes studied earlier, the standard basis set 6-311G** was employed in the present study. Additional DFT calculations were also performed with the B3LYP functional. The minima on the potential surface were found by relaxing the geometry. Bond distances and angles for both the anti and gauche conformers of TFPTCS have not been presented for the sake of brevity. The conformational energy difference was calculated to be 11.8 kJ mol−1 (HF) and 9.2 kJ mol−1 (DFT) with anti being the low energy conformer. The potential energy curve for torsion around the central C C bond of TFPTCS is given in Fig. 15, calculated at the B3LYP/6-311G** approximation. A very similar curve was obtained in the HF/6-311G* calculations. The curves reveal a barrier to gauche–anti conversion

Fig. 15. Potential curve for TFPTCS as a function of torsional angle (CF3 )CH2 CH2 (SiCl3 ) calculated at the DFT B3LYP/6-311G** level.

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Table 2 Observed and calculated fundamental modes of vibration in 3,3,3-trifluoropropyl trichlorosilane A

ν22 ν23 A ν 1 A ν 2 A ν 3 A ν 4 A ν 5 A ν24 A ν 6 A ν 7 A ν25 A ν 8 A ν26 A ν 9 A ν27 A ν10 A ν11 A ν28 A ν12 A ν13 A ν29 A ν14 A ν30 A ν15 A ν16 A ν31 A ν17 A ν32 A ν18 A ν33 A ν19 A ν20 A ν34 A ν21 A ν35 A ν36 A a b c

Obsa

Calc

IR

R

2990 2961 2924 2879 1451 1439 1410 1378 1325 1269 1219 1151 1082 1031 903 843 778 712 650 607 582 554 540 470 398 365 289 226 222c 160 148 134c 115c 81 72 61

3114 3084 3066 3040 1485 1455 1404 1346 1272 1249 1233 1153 1085 1037 918 838 771 730 644 586 561 547 533 456 392 363 286 225 217 164 159 143 103 69 45 30

3 0 8 2 19 9 44 40 273 83 106 166 69 29 81 24 63 46 16 169 147 20 0 59 1 0 13 3 5 1 3 1 0 2 2 0

19 72 49 90 3 8 3 6 1 3 2 2 1 5 1 5 3 0 0 3 3 1 1 7 2 0 5 2 3 2 2 1 0 0 0 0

Dep 0.75 0.75 0.08 0.09 0.59 0.66 0.39 0.75 0.58 0.25 0.75 0.66 0.75 0.35 0.75 0.04 0.59 0.75 0.66 0.72 0.75 0.62 0.75 0.01 0.08 0.75 0.17 0.75 0.75 0.75 0.74 0.51 0.75 0.75 0.75 0.75

PEDb

Description

91 S22 90 S23 96 S4 96 S5 92 S12 84 S13 13 S2 , 57 S14 13 S24 , 67 S28 , 13 S29 29 S2 , 28 S6 , 27 S16 12 S8 , 58 S15 22 S24 , 20 S26 , 38 S29 58 S8 , 11 S20 40 S24 , 25 S28 , 22 S29 84 S1 19 S24 , 41 S26 , 15 S27 , 21 S29 27 S2 , 52 S6 63 S3 18 S26 , 59 S27 52 S16 76 S9 85 S25 10 S16 , 54 S20 76 S32 71 S7 10 S11 , 43 S18 , 26 S20 68 S30 , 16 S32 15 S7 , 14 S11 , 42 S17 , 48 S31 , 20 S33 12 S10 , 21 S17 , 17 S18 , 14 S19 , 25 S21 25 S31 , 59 S33 16 S19 , 63 S21 11 S3 , 34 S10 , 31 S17 , 12 S18 66 S35 , 16 S36 42 S11 , 42 S19 48 S34 , 17 S31 55 S36 , 24 S34 , 14 S35

(CF3 ) CH2 antisym str (SiCl3 )CH2 antisym str (CF3 ) CH2 sym str (SiCl3 ) CH2 sym str (CF3 ) CH2 scissor (SiCl3 ) CH2 scissor (CF3 ) CH2 wag (CF3 ) CH2 twist (F3 )C C(H2 ) str (SiCl3 ) CH2 wag (SiCl3 ) CH2 twist CF3 antisym str CF3 antisym str (H2 )C C(H2 ) str (CF3 ) CH2 rock CF3 sym str (H2 )C Si(Cl3 ) str CH2 (SiCl3 )rock CF3 sym def SiCl3 antisym str SiCl3 antisym str CF3 antisym def CF3 antisym def SiCl3 sym str CF3 antisym def CF3 antisym def SiCl3 sym def SiCl3 antisym def SiCl3 antisym def SiCl3 antisym def SiCl3 antisym def CCC bend CF3 CH2 torsion CCSi bend H2 C CH2 torsion CH2 SiCl3 torsion

Wavenumbers from IR vapour spectra, except when noted. Symmetry co-ordinates are defined in Table 3. From Raman liquid spectra.

of only ca. 3 kJ mol−1 , suggesting that the gauche conformer will be in thermal equilibrium with anti in the fluid phases except at the lowest temperatures of 6–7 K [13].

3.4. Normal coordinate calculations Analytical force constants were derived for each of the two conformers of TFPTCS in the HF and the DFT calculations. The calculated force constants were transformed from Cartesian to symmetry coordinates, derived from a set of valence coordinates. The ab initio calculated wave numbers are invariably larger than the experimental values and they are commonly scaled. In the DFT calculations the agreement with the experimental values was fairly satisfactory without scaling, except in the high frequency region of the C H stretches. The infrared intensities, Raman scattering cross sections and Raman polarization ratios for both conformers were calculated. Since only a few very uncertain gauche bands were detected,

these data are given for the anti conformer in Table 2 while those for the gauche conformer are omitted. The potential energy distribution (PED) is expressed in terms of the symmetry coordinates. The normalized symmetry coordinates are given in Table 3 and have been constructed from a set of valence coordinates, and the numbering of the atoms appears in Fig. 1. Only PED terms larger than 10% have been included in Table 2. The CH2 stretching and scissoring modes, the C C stretch of the central bond and the three SiCl3 stretches are well localized, while the CF3 and SiCl3 deformations and the skeletal deformations are highly mixed. 4. Discussion 4.1. Conformations There is absolutely no doubt that the low energy conformer in TFPTCS is anti. Nearly all the observed IR and Raman

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Table 3 Symmetry coordinates for 3,3,3-trifluoropropyl trichlorosilane (TFPTCS) Description

Definationa

A (H2 )C C(H2 ) stretch (F3 )C C(H2 ) stretch (H2 )C Si(Cl3 ) stretch Sym. CH2 stretch in (CF3 ) CH2 Sym. CH2 stretch in CH2 (SiCl3 ) Sym. CF3 stretch Sym. SiF3 stretch Antisym. CF3 stretch Antisym. SiF3 stretch C C C bend C C Si bend CH2 scissor in (CF3 ) CH2 CH2 scissor in CH2 (SiCl3 ) CH2 wag in (CF3 ) CH2 CH2 wag in CH2 (SiCl3 ) Sym. CF3 deformation Sym. SiCl3 deformation Antisym. CF3 deformation Antisym. SiCl3 deformation Antisym. CF3 deformation Antisym. SiCl3 deformation A Antisym. CH2 stretch in (CF3 ) CH2 Antisym. CH2 stretch in CH2 (SiCl3 ) Antisym. CF3 stretch Antisym. SiCl3 stretch CH2 rock in (CF3 ) CH2 CH2 rock in CH2 (SiCl3 ) CH2 twist in (CF3 ) CH2 CH2 twist in CH2 (SiCl3 ) Antisym. CF3 deformation Antisym. SiCl3 deformation Antisym. CF3 deformation Antisym. SiCl3 deformation (CF3 )CH2 CH2 (SiCl3 ) torsion CF3 CH2 torsion CH2 SiCl3 torsion a

S1 = R5,6 S2 = R1,5 S2 = R6,7
 √ S4 = R5,11 + R5,12 /√2
 S5 = R6,13 + R6,14 / 2
 √ S6 = R1,3 + R1,2 + R1,4 / √3
 S7 = R7,8 + R7,9 + R7,10 / √3
 S8 = 2R1,3 − R1,2 − R1,4 / √6
 S9 = 2R7,8 − R7,9 − R7,10 / 6 S10 = α1,5,6 S11 = α5,6,7 S12 = α11,5,12 S13 = α
13,6,14  S14 =
α6,5,11 + α6,5,12 − α1,5,11 − α1,5,12  /2 S15 = α5,6,14 + α5,6,13 − α7,6,14 − α7,6,13 /2
 √ S16 = α5,1,3 + α5,1,2 + α5,1,4 − α2,1,4 − α3,1,4 − ∆α3,1,2 / √ 6
 S17 = α6,7,8 + α6,7,9 + α6,7,10 + √ α9,7,10 + α8,7,10 + α8,7,9 6
 S18 = 2α5,1,3 − α5,1,2 − α5,1,4 / √6 
S19 = 2α6,7,8 − α6,7,9 − α6,7,10 /√ 6
 S20 = 2α2,1,4 − α3,1,4 − α3,1,2 / √ 6 
S21 = 2α9,7,10 − α8,7,10 − α8,7,9 / 6 S22 S23 S24 S25 S26 S27 S28 S29 S30 S31 S32 S33 S34 S35 S36


 √ = R5,11 − R5,12 /√2 
/ 2 = R6,13 − R6,14 √
 = R1,2 − R1,4 / √2
 =
R7,9 − R7,10 / 2  =
α6,5,11 − α6,5,12 + α1,5,11 − α1,5,12  /2 =
α5,6,14 − α5,6,13 + α7,6,14 − α7,6,13  /2 =
α6,5,11 − α6,5,12 − α1,5,11 + α1,5,12  /2 = α5,6,14 − α5,6,13 − α + α7,6,13 /2
 √ 7,6,14 = α5,1,2 − α5,1,4 / √2
 = α6,7,9 − α6,7,10 /√ 2
 = α3,1,4 − α3,1,2 / √2
 = α8,7,10 − α8,7,9 / 2
 √ = τ1,5,6,7 + ∆τ12,5,6,14 + ∆τ111,5,6,13√/ 3 
= τ6,5,1,3 + τ12,5,1,4 + τ11,5,1,2 / √3 
= τ5,6,7,8 + τ13,6,7,9 + τ14,6,7,10 / 3

See Fig. 1 for numbering of the atoms.

bands belong to this conformer, and only a few weak or very weak possibly belong to the gauche conformer. The fundamentals of the anti conformer are numbered as ν1 –ν21 for the symmetric modes of species A , and ν22 –ν35 for the A modes. The four uncertain gauche modes at 928, 740, 614 and 442 cm−1 were labelled as ν27 , ν11 , ν12 and ν15 in accordance with the numbering of the corresponding anti modes. The bond moments of the C F and Si Cl are much larger than those of the other bonds in TFPTFS. It is therefore expected that the anti conformer with opposite C CF3 and C SiF3 bonds should have a much smaller dipole moment than gauche in which these bonds make approximately a tetrahedral angle. This was supported by the results of the ab initio calculations giving the dipole moments 0.25 and 3.55 D for the anti and gauche conformers, respectively. Therefore, the concentration of the polar gauche conformer should be

higher in the liquid compared to the vapour phase [15], and should increase in solvents of increasing polarity. However, as reported above, no intensity effects could be observed probably due to the very low intensity of the gauche bands. 4.2. Assignments With 14 atoms each conformer has 36 modes of vibration. In the anti conformer with a plane of symmetry (Cs symmetry) the fundamentals will divide into 21 A modes and 15 modes of species A . The assignments for the anti conformer are supported by the results of the ab initio calculations. Both the scaled wave numbers from the HF/6-311G* and those from the unscaled DFT B3LYP/6-311G** calculations were in reasonable agreement with the observations. Exceptions are the high frequency modes connected with CH2 stretch

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in the range 3000–2850 cm−1 , which were invariably calculated at too high wave numbers. The IR and Raman bands assigned as A and A fundamentals are listed in the experimental Table 1. In Table 2 the vibrational modes of the anti conformer are compared with the wavenumbers derived from the B3LYP calculations, whereas those obtained from the HF calculations are omitted for the sake of brevity. The highly localized antisymmetric (ν22 and ν23 ) and symmetric CH2 stretching modes (ν1 and ν2 ) were assigned to vapour bands at 2990, 2961, 2924 and 2879 cm−1 , all with Raman counterparts. They are very close to the corresponding four CH2 stretches in 3,3,3-trifluoropropyltrifluorosilane [3] at 2990, 2966, 2930 and 2888 cm−1 . Also the two CH2 scissoring modes (ν3 and ν4 ) at 1451 and 1439 cm−1 are close to those in 3,3,3-trifluoropropyltrifluorosilane [3] at 1452 and 1431 cm−1 . The CH2 wagging and twisting modes (ν5 and ν24 ) are assigned to the vapour bands at 1410 and 1378 cm−1 although the corresponding Raman band at 1377 cm−1 might be polarized. The intense IR vapour bands at 1325, 1269 and 1219 cm−1 with Raman counterparts of varying intensities are attributed to the modes ν6 , ν7 and ν25 . All these modes are mixed, but have major contributions from C C stretch, CH2 wag and CH2 twist, respectively. The two antisymmetric and the symmetric CF3 stretching modes (ν8 , ν26 and ν10 ) are attributed to the intense IR bands at 1151, 1082 and 843 cm−1 with medium or strong Raman counterparts. These modes are all remarkably similar to the three CF3 stretching modes in 3,3,3trifluoropropyltrifluorosilane [3] situated at 1158, 1088 and 839 cm−1 . The central C C stretch (ν9 ) at 1031 cm−1 was nearly coinciding with the corresponding mode in 3,3,3trifluoropropyltrifluorosilane [3] situated at 1035 cm−1 , while the C Si stretch (ν11 ) at 778 cm−1 was found at 700 cm−1 in the latter compound. Weak or very weak IR and Raman modes around 750 cm−1 vanished in the crystal spectra and were attributed to the gauche mode ν11 , which was calculated to be at 764 cm−1 . The Raman bands at 752 (gauche) and 774 cm−1 (anti) were ratioed in the Miller–Harney calculations (see Fig. 7). The CH2 rocking mode (ν27 ) at 903 cm−1 was found at 891 cm−1 in 3,3,3-trifluoropropyltrifluorosilane [3] in accordance with the delocalization of this mode. Very weak IR and Raman bands around 928 cm−1 in spectra of the vapour, liquid and matrix, which vanished in the crystal, were tentatively attributed to the corresponding gauche fundamental (ν27 ) predicted to lie at 923 cm−1 from the calculations. Three intense IR and Raman bands around 607, 582 and 470 cm−1 were assigned as the antisymmetric (ν13 and ν29 ) and symmetric (ν15 ) SiCl3 stretching modes. These modes are all less than 12 cm−1 separated from the corresponding fundamentals in n-propyltrichlorosilane [2], situated at 598, 578 and 481 cm−1 . As expected the Raman band at 468 cm−1 connected with symmetric SiCl3 stretch is the most intense in the spectrum. The weak band at 442 cm−1 in the Raman spectrum of the liquid disappeared in the crystal spectrum and was assigned to the gauche rotamer (ν15 ). Assumed to be the most intense Raman band of the gauche rotamer, this

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Raman band was chosen to be ratioed with ν15 (anti) and with ν11 (anti) for the Miller–Harney plots (Fig. 7). The four IR vapour bands at 554, 540, 398 and 365 cm−1 were assigned as ν14 , ν30 , ν16 and ν31 in full agreement with the observation of two polarized and two depolarized Raman bands. They were attributed to the CF3 deformation modes in reasonable agreement with the corresponding modes in 3,3,3-trifluoropropyltrifluorosilane [3]. A succession of five IR bands with Raman counterparts were observed at 289, 226, 205, 160 and 148 cm−1 . Although mixed between various symmetry coordinates, they were all predominantly SiCl3 deformation modes: ν17 , ν32 , ν18 , ν33 and ν19 . Since 289 cm−1 was the symmetrical SiCl3 deformation, it is obvious that this mode gave rise to the most intense Raman band at 286 cm−1 . Comparison with the spectra of n-propyltrichlorosilane [2] reveals a close correspondence with the SiCl3 modes in this molecule, situated at 269, 228, 211, 173 and 166 cm−1 . The remaining vibrational modes in TFPTCS had mainly contributions from skeletal bend and from the three torsions. All of these give rise to weak bands both in IR and in Raman in agreement with the calculated intensities, and the spectral interpretations are therefore less reliable. The bands around 134 cm−1 both in the IR and in the Raman spectra, which could not be detected in the vapour, are attributed to a delocalized mode ν20 having the largest contribution from CCC deformation. The bands around 82 cm−1 had major contributions from CCSi bend mixed with SiCl3 deformation. Finally, the three torsional modes of species A are tentatively assigned to the bands at 115, 72 and 61 cm−1 , of which the first was not observed in the IR spectra. They are attributed to the CF3 CH2 , CH2 CH2 and CH2 SiCl3 torsions, respectively. Although most of the IR and Raman bands listed in Table 1 are interpreted as anti fundamentals, a number of weaker bands are left unassigned. They are presumably combination bands or overtones, while some of the weak bands 1175, 928, 797, 740, 614 and 442 cm−1 have tentatively been attributed to the gauche conformer.

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