The infrared and Raman spectra, ab initio calculations and spectral assignments of cyclopropylmethyl dichlorosilane (c-C3H5)SiCl2CH3

Vibrational Spectroscopy 56 (2011) 136–145

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The infrared and Raman spectra, ab initio calculations and spectral assignments of cyclopropylmethyl dichlorosilane (c-C3 H5 )SiCl2 CH3 Peter Klaeboe a,∗ , Anne Horn a , Claus J. Nielsen a , Gamil A. Guirgis b a b

CTCC, Department of Chemistry, University of Oslo, P.O. Box 1033, 0315 Oslo, Norway Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424, USA

a r t i c l e

i n f o

Article history: Received 4 November 2010 Received in revised form 24 January 2011 Accepted 25 January 2011 Available online 2 February 2011 Keywords: Infrared and Raman spectra Conformations DFT calculations Cyclopropylmethyl dichlorosilane

a b s t r a c t Raman spectra of cyclopropylmethyl dichlorosilane (c-C3 H5 )SiCl2 CH3 as a liquid were recorded at 293 K and polarization data were obtained. Additional Raman spectra were recorded at various temperatures between 293 and 163 K, and intensity changes of certain bands with temperature were detected. No crystallization was ever obtained in the Raman cryostat in spite of extensive annealing. The infrared spectra have been studied as a vapour, as an amorphous solid at 78 K and as a liquid in the range 600–100 cm−1 . No infrared bands present in the vapour or liquid seemed to vanish upon cooling, and the sample never formed crystals on the CsI window of an infrared cryostat. The compound exists a priori in two conformers, syn and gauche, and the experimental results suggest an equilibrium in which the gauche conformer has 1.64 kJ mol−1 lower enthalpy than syn in the liquid, leading to 20% syn at ambient temperature. Most of the syn bands were situated close to the corresponding gauche bands and it was difficult to obtain reliable H values. B3LYP calculations with various basis sets and the CBS-QB3 and G2 and G3 models were employed, yielding the conformational enthalpy difference H (syn–gauche) between 2.6 and 3.4 kJ mol−1 . Infrared and Raman intensities, polarization ratios and vibrational frequencies for the syn and gauche conformers were calculated. Instead of scaling the calculated wavenumbers in the harmonic approximation, calculations from B3LYP/cc-pVTZ were derived in the anharmonic approximation. In most cases these values were in good agreement with the experimental results for 38 observed modes of the gauche and 8 modes of the syn conformer with a deviation of ca. 1%. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Various techniques such as gaseous electron diffraction, X-ray crystallography, microwave, infrared, Raman and NMR spectroscopy have been employed to study the conformations of substituted cyclopropanes. More than 30 years ago two conformers (syn and gauche) were observed in the vapour and liquid states of cyclopropanecarboxaldehyde [1], cyclopropanecarboxylicacid chloride [2] and fluoride [3], cyclopropylmethylketone [4], cyclopropylcarboxylic acid [5], cyclopropylcarbinol [6] and cyclopropylethanol [6]. However, in cyclopropylcarboxamide [5,7], cyclopropylphosphine [8] and cyclopropylamine [9] only one conformer was detected, indicating a large enthalpy difference between the conformers. More recently, substituents like ethyl [10]; cyanomethyl, –CH2 C N [11] and ethynyl, –C C–H [12] attached to the cyclopropane ring have been investigated by variable temperature

∗ Corresponding author. Tel.: +47 22855678; fax: +47 22855441. E-mail address: [email protected] (P. Klaeboe). 0924-2031/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2011.01.006

technique, revealing a predominant gauche conformer in the conformational equilibrium at room temperature. In chloromethyl cyclopropane [13], infrared studies in condensed xenon gave 88% of the gauche form at room temperature with an enthalpy difference of 3.3 kJ mol−1 . Other related molecules containing the cyclopropyl group are cyclopropylmethylsilane [14], cyclopropylmethylgermane [15] and ethylcyclopropane [10] which contain a silicon, a germanium and a carbon atom, respectively. The conformations of these three molecules have all been studied by vibrational spectroscopic technique [10,14,15] revealing an equilibrium between a dominating gauche and a less abundant syn conformer. In order to continue the conformational studies of substituted cyclopropanes, cyclopropylmethyl dichlorosilane (cC3 H5 )SiCl2 CH3 later to be abbreviated CPDCS, was synthesized and a vibrational spectroscopic study was carried out. As is apparent from Fig. 1, CPDCS can form syn and gauche conformers due to restricted internal rotation around the central C–Si bond. The compound is related to cyclopropylmethylsilane [14] except for the two chlorine atoms attached to the central silicon. If the silicon atom in CPDCS is interchanged with germanium, cyclopropylmethyl

P. Klaeboe et al. / Vibrational Spectroscopy 56 (2011) 136–145

Fig. 1. The syn and gauche conformers of cyclopropylmethyl dichlorosilane (CPDCS).

dichlorogermane will be formed, and this related compound is presently being studied in these laboratories.

137

90◦ illumination mode, employing a polarizer and a scrambler between the sample and the monochromator. Except for very weak and/or overlapping bands, fairly comprehensive depolarization data were obtained. However, since the syn conformer is present at ca. 20% concentration at room temperature, it is difficult to distinguish between the Raman bands of the a and a modes of this conformer. Raman spectra of the liquid, including depolarization measurements were primarily recorded at room temperature. Additional spectra were obtained for each 5◦ between 293 and 123 K in a capillary tube of 2 mm inner diameter. The tube was surrounded by a Dewar, cooled by gaseous nitrogen evaporated from a liquid reservoir [17]. From these spectra, the enthalpy difference conf H◦ between the conformers in the liquid was calculated. Various gauche/syn band pairs were attempted for determining the enthalpy difference, giving rather different values. The vapour of CPDCS was condensed on a copper finger at 78 K. An amorphous phase was formed, and the bands were quite similar to those of the liquid. Many annealing experiments were tried, but the sample never crystallized in any of the temperature ranges employed.

2. Experimental 2.1. Preparation

2.3. Infrared measurements

The sample of CPDCS was prepared by two procedures. In the first method, the sample was obtained in two steps. Firstly, cyclopropylbromide reacted with magnesium in dry ether using the Grignard method. This product was subsequently coupled with methyltrichlorosilane in dry ether under dry nitrogen gas to obtain cyclopropylmethyl dichlorosilane which was separated from the ether under reduced pressure. The sample was purified by trap-totrap distillation to isolate the highly volatile methylsilane. Another sample was prepared from dimethoxy methylvinylsilane using the Simmons–Smith procedure [16], followed by reaction with hydrogen chloride to obtain the product, which was purified by similar procedures as the first sample. The authenticity of the two samples was verified by comparing the infrared and Raman spectra with the predicted ones, and analyses of the 1 H and 13 C NMR data.

The mid infrared spectra (MIR) were recorded on two Fourier transform spectrometers: a Bruker spectrometer Vertex 80 (4000–400 cm−1 ) and a Perkin-Elmer model 2000 (4000–400 cm−1 ). Additional far infrared (FIR) spectra were obtained using the Vertex 80 spectrometer (600–100 cm−1 ) in the FIR set-up. All the spectrometers had DTGS detectors. Beamsplitters of Ge coating on KBr substrate were employed in the MIR region, and a multilayer coating on Mylar was used in the FIR region. The vapour was studied in cells with CsI windows (10 cm path) in the MIR region and polyethylene widows (18 cm path) in FIR. The spectrometer was flushed with dry nitrogen gas to diminish the absorption of water vapour in the FIR region. The vapour was deposited on a CsI window, cooled with liquid nitrogen, but as expected no crystallization was observed on the CsI window. FIR spectra of CPDCS as a liquid were recorded in liquid cells of polyethylene (throw away cells) of ca. 0.2 mm path.

2.2. Raman measurements The Raman spectra were recorded with a multichannel spectrometer from Horiba (Jobin Yvon) model T 64000 employing both single and triple monochromator detection. The spectrometer was fitted with a CCD detector, cooled to ca. 130 K. The spectra were excited by a Milennia Pro diode-pumped (Nd:YVO4 crystal) laser from Spectra-Physics (Model J 40), using 90◦ scattering geometry, adjusted to give approximately 50 mW of the 532 nm line on the sample. The silicon and two chlorine atoms in the molecule lead to relatively high electron density, and a large scattering cross section was observed. The spectrometer was applied with different optical set ups: (1) a single monochromator using a notch filter, a triple monochromator with (2) additive or (3) subtractive collection. A higher signal to noise ratio was achieved with the single monochromator, the triple additive collection gave slightly higher resolution, while the triple subtractive set-up allowed the spectra to be recorded at wavenumbers closer to the exciting line. However, the different systems of light collection gave quite similar spectra. In the liquid sample the low wavenumber modes were recorded to 60 cm−1 with the triple subtractive mode, compared to 120 cm−1 with the single monochromator and the notch filter. The high sensitivity of the CCD detector gave spectra with large signal/noise ratio, and the spectra were recorded at ca. 1 cm−1 resolution. Most spectra were recorded in the range 3500–100 cm−1 within 5 min. Polarization measurements were carried out in the

3. Results 3.1. Raman spectral results Raman spectra of CPDCS as a liquid at ambient temperature in the ranges 3200–700 and 800–50 cm−1 are presented in Figs. 2 and 3. A Raman spectrum of the amorphous sample deposited on a Cu finger at 78 K in a cryostat was quite similar to that recorded of the liquid at ambient temperature, but the bands appeared with lower band widths. Raman spectra of the liquid were recorded at 32 temperatures (every 5◦ ) in two series of experiments between 293 and 123 K. The two spectral ranges 1050–500 and 500–130 cm−1 are given in Figs. 4 and 5, in which the solid curves show the Raman spectra at 293 and the dashed curves show those recorded at 123 K. Various minor intensity variations with temperature of Raman bands relative to neighbouring bands were observed. They were interpreted as a displacement of the conformational equilibrium. The calculations with various basis sets (see below) give an enthalpy difference between the conformers amounting to 2.6–3.4 kJ mol−1 (see below), with gauche being the more stable conformer. In agreement with these calculations, the bands being reduced in intensities, predicted as syn bands, are invariably weak in Figs. 4 and 5, and often appear as shoulders on the more intense

P. Klaeboe et al. / Vibrational Spectroscopy 56 (2011) 136–145

Intensity

Intensity

138

3000

2500

2000

1500

Wavenumber / cm

1000

500

-1

400

300

Wavenumber / cm

200 -1

Fig. 2. Raman spectrum of CPDCS as a liquid at 293 K in the range 3200–750 cm−1 .

Intensity

Fig. 5. Raman spectra of CPDCS at 293 (solid line) and at 123 K (dashed line) in the range 500–130 cm−1 .

800

700

600

500

400

300

Wavenumber / cm

200

100

-1

Fig. 3. Raman spectrum of CPDCS as a liquid at 293 K in the range 800–60 cm−1 .

Intensity

gauche bands. The Raman bands which are enhanced at lower temperatures belong to one conformer, and they were paired with other bands (often neighbours) which diminish in intensity upon cooling. The low intensities of the proposed syn bands and the frequent

1000

900

800

700

Wavenumber / cm

600

500

-1

Fig. 4. Raman spectra of CPDCS at 293 (solid line) and at 123 K (dashed line) in the range 1050–500 cm−1 .

small wavenumber differences between the syn and gauche bands make the results uncertain. 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. The intensities of each band pair were fitted to the van’t Hoff equation: ln{Igauche (T)/Isyn (T)} = −conf H/RT + constant; where Igauche /Isyn 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. The following seven pairs of bands were first attempted in the van’t Hoff plots: 462/479, 901/306, 315/306, 640/479, 640/306 and 901/479 cm−1 (liquid) employing peak heights, giving the widely different values 0.37, 1.04, 1.49, 1.19, 1.74 and 0.87 kJ mol−1 , respectively (293–123 K). Firstly, the band pairs involving the assumed gauche band 901 (21 ) and those of the syn band 479 cm−1 (a 18 ) were both discarded. Comparing the calculated anharmonic wave numbers for the syn and gauche conformers in Tables 2 and 3, respectively, it appears than both had calculated values at 890 and 467 cm−1 . Thus, it seems likely that both the 901 and 467 cm−1 peaks are not pure gauche and syn bands as anticipated, but have contributions from both conformers. Therefore, they are obviously not suitable for derminining conf H◦ and expected to give misleading values as is apparent from the van’t Hoff plots. Moreover, all the van’t Hoff plots had a curved appearance with a more horizontal branch at low temperatures (below 173 K). The physical reason for this was probably that a large flow of cold nitrogen gas was necessary to maintain the low temperatures, and the turbulent gas flow and the condensation of water on the Dewar prevented reliable Raman intensity data to be recorded. If the plots were restricted to the temperature range 273–163 K and the lowest range 163–123 K was discarded, fairly linear correlations were obtained. The plots in Fig. 6 were restricted to the band pairs 315/306 and 640/306 cm−1 , since no other syn band than 306 cm−1 . From peak heights the two lower plots in Fig. 6 gave conf H values 1.49 and 1.74 kJ mol−1 (293–163 K). The band pair 315/306 cm−1 was also treated with integrated band areas, employing a mixed Gaussian/Lorenzian curve fitting in the Grams software. At lower temperatures the bands decrease in band widths and the Lorenzian contribution will increase relative to the Gaussian. Although the 306 cm−1 band overlapped to some extent the band at 315 cm−1 , the curve fitting had a very small residual background as demonstrated in Fig. 7. It gave a conf H equal to 1.70 kJ mol−1 and the smooth background with negligible residual intensity suggested

P. Klaeboe et al. / Vibrational Spectroscopy 56 (2011) 136–145

139

1.2

1.0

Transmottance

INIgauche / I syn

0.8

0.4

0.5

0 0.0 3200

0.004

0.005 -1

T /K

3000

2800

0.006

1400

1200

1000

Wavenumber / cm

800

600

-1

-1

Fig. 6. van’t Hoff plots of the syn/gauche band pairs 315/306 cm−1 (peak heights, middle plot; intergrated band areas, top plot) and 640/306 cm−1 (peak heights, bottom plot) of CPDCS in the temperature range 293–163 K.

a reliable curve resolution. The three plots gave an average value of conf H (syn–gauche) = 1.64 kJ mol−1 , which might be considered as the lower limit since the syn band 306 cm−1 band may have a small contribution from the neighbouring gauche band 315 cm−1 . To be discussed below, the calculations employing a variety of basis sets gave considerably higher values for H, lying between 2.6 and 3.4 kJ mol−1 . 3.2. Infrared spectral results A MIR spectrum of CPDCS as a vapour in the range 3200–2750 and 1500–450 cm−1 at the full pressure of 10 Torr in a 10 cm cell with resolution 1.0 cm−1 is presented in Fig. 8. A FIR spectrum between 600 and 100 cm−1 with 2.0 cm−1 resolution in a 18 cm cell having the full pressure of 10 Torr is presented in Fig. 9. Some vapour contours with resolved P, Q and R peaks were observed in the spectra both for the assumed gauche and syn conformers, but in other cases apparent fine structure is attributed to close-lying conformer peaks. Another possibility is contribution of the 35 Cl and 37 Cl isotopes to the vibrational spectra, although the isotopic split-

Fig. 8. Mid infrared (MIR) vapour spectra of CPDCS in the ranges 3200–2750 and 1500–450 cm−1 with resolution 1 cm−1 , at the maximum pressure of ca. 10 Torr at room temperature in a 10 cm cell.

ting is expected to show smaller shifts (considering the moments of inertia) than those observed in this molecule (Table 1). MIR spectra of CPDCS deposited on a CsI window at 78 K are shown in Fig. 10 (1350–850 cm−1 ) and Fig. 11 (900–450 cm−1 ). In spite of many attempts of annealing at different temperatures, the sample was invariably amorphous at these temperatures. A FIR spectrum of the sample, recorded at room temperature in a liquid cell of polyethylene with ca. 0.2 mm thickness in the range 600–200 cm−1 is given in Fig. 12. A complete list of the infrared and Raman bands for CPDCS are listed in Table 1. 3.3. Quantum chemical calculations DFT calculations were performed using Gaussian 03 programs [18]. To allow an easy comparison with the series of halosilanes studied earlier, the basis sets 6-311G*, and cc-pVXZ (X = D, T, and Q), were employed. The minima on the potential surface were found by relaxing the geometry. The Cartesian coordinates, the structural parameters for the syn and gauche conformers resulting from B3LYP/cc-pVTZ calculations and the symmetry coordinates are given in Tables S3–S5 (Supporting information). The conformational energy difference, conf H obtained from the B3LYP calculations at 298 K was 3.0 kJ mol−1 for cc-pVDZ, 2.7 for cc-pVTZ

Transmittance

Intensity

1.0

0.5

0.0

340

330

320

310

300

290

-1

Raman shift / (cm ) Fig. 7. Curve resolution of the band pair 315/306 cm−1 of CPDCS in Raman spectra of the liquid at 298 K, employing Lorenzian/Gaussian curve fitting in the Grams quick fit program.

600

500

400

300

Wavenumber / cm

200

100

-1

Fig. 9. Far infrared (FIR) vapour spectra with resolution 1 cm−1 of CPDCS in the range 600–100 cm−1 at the maximum pressure of ca. 10 Torr at room temperature in a 18 cm cell.

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Table 1 Infrared and Raman spectral dataa for cyclopropylmethyl dichlorosilane (c-C3 H5 SiCl2 CH3 ). Infrared Vapourb (293 K)

Raman Liquidb (293 K)

3097 m, R 3093 s, Q 3087 m 3019 s 2998 m, R 2995 s, Q 2986 s 2980 m 2941 w 2938 m, br

3082 m 3071 m, sh 3004 s

3081 s, P 3072 s, D 3005 vs, P

3084s 3073 s 3007 vs

1 2 3

2978 s, P

2979 s

5

2916 vw, sh 2906 vs, P 1457 m 1435 m, P 1400 w, D 1390 m, P 1306 w 1294 m, P 1280 vw 1260 w, P↑

2939 w 2909 vs

7 8 9 10 11 12

Syn

1434 m 1402 m 1379 w a 8 1297 m a 9

Gauche

13 14

1266 w 

a 10

1208 w, P 1187 vs, P

1209 w 1188 vs

1102 w↑

1101

1040 s 1018 m 970 m 922 m

1042 m, P 1018 w, P↑ 971 w↑ 921 vw, sh

1041 w 1018 vw 934 vw

19 20 Comb Comb

902 vs

901 s, D↑

905 s

21

847 s 828 s 806 s

847 m, D↑ 828 w, sh, D↑ 807 vw↑

849 s 834 m 808 vw

22 23 24

790 s 782 s

788 w, sh 784 w, P↑

786 w

25 26

751 s 743 vw, sh 722 w 709 w, sh↓ 683 w, br

753 m↑ 742 w, sh, P↓ 721 w, br, P↑ 718 vw, sh 685 w, P↓

752 w 743 vw, sh 724 vw 684 w

Comb

644 s

640 s, P↑

642 s

28

548 vs, br 480 m 461 s 417 vw, sh 392 w

598 w 562 vw, sh 540 vvs, br 480 m 460 s 416 vw 392 w

601 m, P↑ 566 m, P 541 w, D↑ 479 m↓ 462 vvs, P↑ 413 vw, P↑ 389 m, P↑

606 vw 564 w 542 w

Comb Comb 29

351 s

349 s

350 s, P↑

350 s

315 w

315 w 296 w 259 vvw 222 m 198 s, P 186 m 183 w, sh 137 w, br 122 vw, sh?

315 vs, P 306 m, sh↓ 265 w, D 221 s, P 201 s 186 vs, P

315 s 305 vw, sh 269 vw 223 s

912 s, R 907 vs, Q 902 s, P 851 vw, sh 830 m 808 s, Q 801 s, R 791 m, Q 786 m 745 w, sh 741 m

678 vw 647 w, R 642 m Q 637 w, P 610 vw

398 vw, br 354 w, R 350 m, Q 343 w, P 316 vw

Amorph.b (78 K)

1208 w 1187 m 1166 m 1099 s, sh 1092 m, br↓ 1085 s

967 m

561 vs 484 m 467 m

Liquidb (293 K)

4 2979 s 6 2938 m 2927 m 1458 m, D 1431 w 1401 w 1393 w 1304 w, P 1296 s 1284 m 1260 s

1412 m 1397, br 1302 w 1298 m 1292 w 1272 s, Q 1266 s, P 1195 vw 1191 w 1165 vw 1114 s 1094 w, sh 1072 vw 1048 vw, R 1044 w, Q

Interpretation

Amorph.b (78 K)

221 vw, br

223 m

186 vw 182 vw

184 m 182 w

a  29

18

1085 vw

144 m, br

15 16 17

Comb a 15 a 16

a 18

30 Comb Comb

462 vs 419 vw 392 w

187 s

27

a  35

32 Comb 33 34 35 36 37 38

a Abbreviations: s, strong; m, medium;w, weak; v, very; sh, shoulder; sp, sharp, bd, broad; P, polarized; D, depolarized in the Raman spectra; P, Q and R signify band contours in the infrared vapour spectra; arrows pointing upwards and downwards signify bands which increase and decrease in intensities at lower temperatures. b Recording temperature.

P. Klaeboe et al. / Vibrational Spectroscopy 56 (2011) 136–145

141

1.2 8

-1

6

E /kJ mol

Transmittance

0.9

0.6

4

2

0.3

0 0

1300

1200

1100

1000

Wavenumber / cm

900

-1

1.2

Transmittance

0.9

0.6

0.3

800

700

600

Wavenumber / cm

500

-1

Fig. 11. Mid infrared (MIR) spectra of CPDCS in the range 900–450 cm−1 in a cryostat at 78 K, resolution 1 cm−1 .

and 2.6 for cc-pVQZ with gauche being the low energy conformer. A value of 3.4 kJ mol−1 was obtained from CBS-QB3 [19], 2.7 from G2, 3.3 for G3 [20] and 2.8 for MP2//G2 calculations. Thus, these calculated conf H values (with gauche being the low energy con-

0.9

Transmittance

200

300

Dihedral angle /deg

Fig. 10. Mid infrared (MIR) spectra of CPDCS in the range 1350–850 cm−1 in a cryostat at 78 K, resolution 1 cm−1 .

900

100

Fig. 13. Potential curve for CPDCS derived from B3LYP/6-311G* calculations, in which the geometry was relaxed for fixed values of the C–C–Si–C torsional angle.

former) were situated between 2.6 and 3.4 kJ mol−1 for all the basis sets employed, leading to the average value 2.93 kJ mol−1 . Slightly lower H values between 2.5 and 3.2 kJ mol−1 giving an average of 2.84 kJ mol−1 was calculated with the same basis sets at 0 K. The potential function for torsion around the central C–C bond was estimated in B3LYP/6-311G* calculations in which the geometry was relaxed for fixed values of the C–C–Si–C torsional angle (Fig. 13). The calculations suggest a syn to gauche barrier of 5.2 kJ mol−1 , a gauche to syn barrier of 8.1 kJ mol−1 , and a gauche to gauche barrier of 6.1 kJ mol−1 . Vibrational wavenumbers were obtained for both conformers in B3LYP/cc-pVDZ, cc-pVTZ and cc-pVQZ calculations. There were only minor differences between the results obtained with the triple and quadruple zeta basis sets, suggesting that the B3LYP electron density is described with sufficient accuracy employing the triple zeta basis set. Raman intensities and anharmonic vibrational frequencies were therefore calculated with this basis set. The vibrational wavenumbers of the syn and gauche conformers of CPDCS resulting from the three B3LYP/cc-pVXZ (X = D, T, and Q) calculations are compared in Tables S3 and S4, respectively (Supporting information). Provided an adequate basis set is used in calculations of the normal modes of vibration, the calculated wavenumbers are invariably too high. A common procedure to remedy this discrepancy is to scale the calculated wavenumbers to compensate for, among others, mechanical anharmonicity. In the present study the cubic and quartic force constants were calculated allowing a prediction of the anharmonic vibrational wavenumbers directly. This was done in the B3LYP/cc-pVTZ calculations and the harmonic and anharmonic values for the syn and gauche conformers are listed in Tables 2 and 3, S3 and S4. 3.4. Normal coordinate calculations

0.6

0.3

600

550

500

450

400

350

Wavenumber / cm

300

250

200

-1

Fig. 12. Far infrared (FIR) spectra of CPDCS in the range 600–200 cm−1 as a liquid in a polyetheylene cell of thickness ca. 0.2 mm, resolution 2 cm−1 .

Harmonic force constants were obtained for each of the two conformers of CPDCS in B3LYP/cc-pVTZ calculations. The harmonic, unscaled force constants were transformed from Cartesian to symmetry coordinates, derived from a set of valence coordinates to obtain an approximate description of the normal modes. These calculations were carried out both for the syn and the gauche conformers employing the VIBROT program [21] and the results are listed in Tables 2 and 3. The infrared intensities and Raman polarization ratios obtained in the harmonic approximation, are included in these tables in combination with the experimental wavenumbers, relative intensities and Raman depolarization values. The theoretical Raman intensity, R, which mimics the mea-

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Table 2 Observed and calculated vibrational modes of the syn conformer of dichloromethylsilyl cyclopropane (c-C3 H5 SiCl2 CH3 ). No. 1 23 2 24 3 4 25 5 6 26 27 7 8 9 10 28 29 30 11 12 13 31 32 14 33 15 16 17 34 18 35 19 20 36 21 37 22 38 39 a b c d e f

Sym. spec. 

a a  a a  a a a  a a a  a  a a a a a  a  a  a a a a  a  a a  a a a a  a a  a a a  a a  a a  a 

Harma waven.

Anharm. waven.

3211 3196 3127 3124 3137 3114 3107 3034 1512 1486 1459 1457 1323 1299 1214 1203 1133 1096 1069 1046 918 867 839 819 818 802 742 633 541 473 313 302 218 195 170 164 135 111 52

3059 3044 3022 3008 2982 2967 2966 2938 1472 1443 1429 1433 1292 1278 1192 1171 1092 1064 1041 1021 890 840 815 800 798 786 731 618 533 467 305 299 219 180 166 139 129 94 13

Obs.b waven.

1302 1292 1195 1172

741

484 306f

IRc int. 14 0 13 18 0 2 3 1 1 5 2 2 16 25 8 0 1 3 9 3 92 1 27 38 51 21 56 53 145 48 1 0 9 0 3 0 2 0 0

Rad int.

Dep. ratio

P.E.D.e

Description

115 153 311 79 212 136 104 260 20 42 27 20 21 3 202 16 2 4 18 4 75 137 23 5 0 5 73 113 76 317 116 287 192 157 227 9 270 165 413

0.48 0.75 0.02 0.75 0.21 0.70 0.75 0.00 0.75 0.75 0.75 0.71 0.41 0.04 0.06 0.75 0.75 0.75 0.33 0.67 0.69 0.75 0.75 0.67 0.75 0.75 0.42 0.25 0.75 0.00 0.75 0.11 0.62 0.75 0.68 0.75 0.67 0.75 0.75

98S1 99S2 96S6 99S7 95S5 99S3 99S4 99S8 87S11 94S14 91S13 95S12 52S15 +21S17 94S16 72S17 38S5 +47S29 42S19 +52S28 97S18 72S20 27S15 +27S20 +19S21 +20S22 82S23 74S24 21S19 +29S28 +35S29 65S27 81S25 46S21 +28S22 58S32 15S10 +59S33 80S9 62S10 24S34 +49S35 46S10 +35S36 +24S38 80S30 32S31 +44S34 69S26 +23S36 93S37 15S26 +21S36 +51S38 44S31 +37S35 87S39

CH2 antisymmetric stretch CH2 antisymmetric stretch CH2 symmetric stretch CH2 symmetric stretch CH stretch CH3 antisymmetric stretch CH3 antisymmetric stretch CH3 symmetric stretch CH2 deformation CH2 deformation CH3 antisymmetric deformation CH3 antisymmetric deformation CH wag (in-plane) CH3 symmetric deformation Ring breathing CH2 rock CH2 twist CH2 rock CH2 wag CH2 twist Ring deformation Ring deformation CH bend (out-of-plane) CH3 rock CH3 rock CH2 wag C–Si antisymmetric stretch C–Si antisymmetric stretch SiCl2 symmetric stretch SiCl2 symmetric stretch Ring–SiCl2 bend (out-of-plane) Ring–SiCl2 bend (in-plane) SiCl2 wag SiCl2 rock SiCl2 scissor CH3 torsion C–Si–C bend SiCl2 twist Asymmetric torsion

Calculated with B3LYP/cc-pVTZ basis set. The observed wavenumbers are derived from IR vapour spectra, except when noted. Calculated infrared intensities in km/mol. Calculated Raman scattering activities in arbitrary units, see text and Ref. [22]. For definition of symmetry coordinates, see Table S5. From Raman spectra of the liquid.

sured Raman spectrum is related to the Raman scattering activity: Ri = C(L − i )4 × i −1 × Bi −1 × Si , where i is the frequency and Si is the corresponding Raman scattering activity of mode i, L is the laser excitation frequency, Bi = 1 − exp(−h × i × c/kT), and C is a constant [22]. Finally, the wavenumbers derived from the anharmonic calculations are included in Tables 2 and 3 to be directly compared with the experimental values as observed in the infrared and Raman spectra. The PED (potential energy distribution) is expressed in terms of the symmetry coordinates. The symmetry coordinates in Table 5 [14] have been slightly modified and normalized in Table S5, constructed from a set of valence coordinates while the numbering of the atoms appears in Fig. 1. Only PED terms larger than 10% have been included in Tables 2 and 3 and the largest term in PED is also described in terms of valence coordinates in these tables. The CH2 and CH stretching and bending and the Si–Cl stretching modes are reasonably well localized, but the CH2 deformations, Si–C stretches and the skeletal deformations are highly mixed. 3.5. Conformations It can be seen in Tables 1–3 that whereas 38 (all, but the lowest torsional mode) gauche bands have been assigned, only 8 of the syn modes have been tentatively identified. With an experimen-

tal enthalpy difference of 1.64 kJ mol−1 favouring gauche and the statistical weight two of this conformer, the abundance of the syn form is 20% at 298 K based upon the experimental results, compared with approximately 12.5% if the much higher calculated value of H equal to 3 kJ mol−1 (see above) was employed. A comparison of the conformational equilibrium of the parent molecule cyclopropylmethylsilane (c-C3 H5 SiH2 CH3 ) [14] and that of CPDCS, reveals a slightly lower conf H (syn–gauche) value of 1.21 kJ mol−1 in the former molecule, with ca. 23% of the syn form present [14]. Apparently, the two chlorine substituents in CPDCS compared to the hydrogens in cyclopropylmethylsilane lead to small differences in the relative stabilities of the syn and gauche conformers. The barriers to internal rotations of the CH3 SiH2 moiety in cyclopropylmethylsilane was calculated (MP2/6-31G(d)) to be 8.2 kJ mol−1 for gauche to syn, 7.9 for gauche to gauche and 6.7 kJ mol−1 for syn to gauche [14], slightly higher than for CPDCS (Fig. 13, B3LYP/6-311G*), but the different basis sets employed may be significant for the variations. The calculated wavenumbers of the syn and gauche conformers of CPDCS were initially obtained with the B3LYP/cc-pVTZ harmonic calculations. Subsequently, the calculations for both conformers were extended to include cubic and quartic force constants and the anharmonic frequencies were calculated in perturbation calculations.

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Table 3 Observed and calculated vibrational modes of the gauche conformer of dichloromethylsilyl cyclopropane (c-C3 H5 SiCl2 CH3 ). No.

Harma freq.

Anharm. freq.

Obs.b freq.

1 2 3 4 5 6 7 8 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 37 38 39

3218 3204 3133 3129 3120 3119 3108 3035 1508 1478 1459 1455 1323 1299 1218 1204 1128 1100 1071 1043 920 876 839 824 816 799 735 633 542 449 349 310 212 188 175 162 125 114 56

3070 3054 3027 3014 2972 2974 2962 2918 1471 1445 1404 1401 1290 1264 1188 1179 1089 1071 1046 1020 890 850 824 809 799 782 717 623 535 444 348 313 217 194 185 179 132 124 30

3093 3087 3019 2995 2986 2980 2976 2938 1458f 1435f 1412 1397 1298 1272 1191 1165 1114 1072 1044 1018f 907 851 830 808 791 786 720 642 561 467 350 316 221 198f 186 182 137f 122f

a b c d e f

IRc Int. 10 0 10 15 5 4 4 1 0 4 3 3 15 21 9 0 2 2 9 5 76 14 31 58 50 7 58 49 140 29 11 0 5 1 2 0 1 0 0

Rad Int.

Dep. ratio

P.E.D.e

Description

96 140 374 141 134 140 125 292 22 39 25 26 25 7 178 16 4 4 20 9 74 101 46 6 2 6 94 90 370 406 136 181 177 211 251 2 247 201 325

0.54 0.74 0.04 0.27 0.60 0.66 0.75 0.00 0.70 0.75 0.75 0.74 0.36 0.11 0.07 0.75 0.72 0.75 0.24 0.75 0.69 0.74 0.74 0.75 0.75 0.50 0.32 0.32 0.74 0.00 0.38 0.28 0.71 0.66 0.63 0.73 0.73 0.74 0.75

97S1 98S2 80S6 +17S7 15S6 +83S7 88S5 91S3 98S4 99S8 86S11 99S14 96S13 96S12 51S15 +22S17 94S16 70S17 43S19 +44S29 40S19 +53S28 97S18 78S20 29S15 +20S20 +22S21 +22S22 84S23 62S24 27S24 +21S28 +24S29 19S21 +20S25 +27S27 50S25 +16S27 40S21 +37S22 56S32 +26S33 37S33 77S9 74S10 30S30 +15S35 +32S36 15S34 +28S35 +16S36 35S30 +24S38 35S31 +34S34 65S26 +15S30 99S37 16S31 +23S36 +30S38 26S31 +30S35 +19S38 84S39

CH2 antisymmetric stretch CH2 antisymmetric stretch CH2 symmetric stretch CH2 symmetric stretch CH stretch CH3 antisymmetric stretch CH3 antisymmetric stretch CH3 symmetric stretch CH2 deformation CH2 deformation CH3 antisymmetric deformation CH3 antisymmetric deformation CH wag (in-plane) CH3 symmetric deformation Ring breathing CH2 wag CH bend (out-of-plane) CH2 rock CH2 wag CH2 twist Ring deformation Ring deformation CH2 rock CH3 rock CH3 rock CH2 wag C–Si antisymmetric stretch C–Si symmetric stretch SiCl2 antisymmetric stretch SiCl2 symmetric stretch Ring–SiCl2 bend (in-plane) SiCl2 rock SiCl2 wag SiCl2 twist SiCl2 scissor CH3 torsion C–Si–C bend Ring–SiCl2 bend (out-of-plane) Asymmetric torsion

Calculated with B3LYP/cc-pVTZ basis set. The observed wavenumbers are derived from IR vapour spectra, except when noted. Calculated infrared intensities in km/mol. Calculated Raman scattering activities in arbitrary units, see text and Ref. [22]. For definition of symmetry coordinates, see Table S5. From the Raman spectra of the liquid.

As is apparent both from Tables 2 and 3 the vibrational modes derived from the anharmonic calculations are with a few exceptions at lower wavenumbers than those from the harmonic approximation both for the syn and gauche conformers. An exception is the syn mode 20 (SiCl2 wag) which is blue shifted one wavenumber, while for the gauche conformer six vibrational modes 32 –37 were calculated at slightly higher wavenumbers in the anharmonic than in the harmonic approximation. In most cases, the anharmonic wavenumbers agreed quite well with the observed values. A comparison between the calculated anharmonic wavenumbers of the syn and gauche fundamentals reveals that in every other case (19) the gauche modes were shifted more than 10 cm−1 compared to the corresponding modes of the syn conformer. In all the other cases the syn–gauche shifts were lower than 10 cm−1 . Obviously, the chances for observing separate syn and gauche fundamentals are best in the former cases. 4. Assignments 4.1. Fundamentals With 14 atoms in CPDCS each conformer has 39 modes of vibration. In the syn conformer with Cs symmetry the fundamentals will

divide into 22 modes of species a and 17 of species a . They are numbered as 1 –22 for species a and 23 –39 for a , while for the gauche conformer the modes are numbered consecutively, 1 –39 . Since the order of the fundamentals sometimes change between the harmonic and the anharmonic calculations (Tables 2 and 3), the latter are made the basis for the numbering. With a few exceptions, the assignments are in good agreement with the results of the B3LYP/cc-pVTZ in the anharmonic calculations. From the experimental results including Raman and IR spectra in the amorphous state, it was concluded that possibly 8 separate syn bands were observed. The actual wavenumbers of the observed and calculated fundamentals are listed in Tables 2 and 3 and a deviation of 1% between the observed and calculated values was derived. Assuming the syn conformer to have ca. 1/4 of the concentration of the gauche conformer, the weak syn bands may be difficult to identify and many syn modes will remain unobserved in the IR and Raman spectra. For this reason only the gauche fundamentals will be discussed in the following. A quick glance at the potential energy distribution (PED) in Tables 2 and 3 reveals that most of the CH, CC, CCl stretching modes, the CH2 scissor and the torsional modes are well localized to one symmetry coordinate. The CH2 wag, twist and rock, CCl2 deformations and skeletal bending modes are mixed between two or more

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symmetry coordinates. As expected in the syn conformer with a plane of symmetry the vibrational modes are more localized than in the gauche conformer without any symmetry. It appears from Table 3 that mode 20 , has between 20 and 29% contributions from at least four symmetry coordinates like the corresponding mode in cyclopropylsilane [14], and the designation CH2 twist is therefore quite arbitrary. As expected, the observed wavenumbers of the fundamentals in CPDCS are in many cases very similar to those of cyclopropylsilane [14], taking into account that two hydrogens in cyclopropylsilane are substituted with chlorine atoms in CPDCS. As is apparent from Table 1, 10 infrared bands were observed between 3100 and 2900 cm−1 in the vapour and 6 of these were seen in the in the amorphous phase and as counterparts in the Raman spectra. The four CH2 stretches of the cyclopropane ring and the three CH3 stretches of the methyl group of the gauche conformer are assigned in this range. An additional C–H stretch was attributed to the intense IR vapour band at 2986 with a strong, polarized Raman peak at 2978 cm−1 , very close to the corresponding mode at 2975 cm−1 in methylsilylcyclopropane [14]. With 8 hydrogens in CPDCS attached to carbon, we expect 16 vibrational modes mostly involving CH2 scissor, wag twist and rock, CH3 antisymmetric and symmetric deformations and C–H bending in-plane and out-of-plane. These vibrational modes are expected in the range 1500–750 cm−1 as apparent from Tables 2 and 3. Apart from 9 (CH2 scissor) which was observed only in the Raman spectra, all the CH2 deformations were observed both in the IR and the Raman spectra (Table 1), in good agreement with the calculated band intensities (Table 2). Also, the calculated (non-scaled) wavenumbers in the anharmonic calculations agree quite well with the observed values for the gauche conformer. The deviations are ca. 1% for all the modes, but they are only ca. 0.6% for the modes at high and intermediate wavenumbers, 1 –27 . Only the breathing mode of the cyclopropane ring (15 ) was intermingled with the 16 CH2 bending modes. The breathing mode was observed as a weak vapour band at 1191 and a medium intense IR band at 1187 cm−1 in the low temperature solid. In Raman, however, this mode was highly polarized (calculated dep. ratio 0.07) and observed as a very intense peak at 1187 cm−1 as is apparent from Fig. 2. The ring breathing mode of the cyclopropyl moiety is a characteristic group frequency observed in a very small frequency range. This was demonstrated by intense, polarized Raman bands in the following liquids: cyclopropylmethylketone (1198 cm−1 ) [4], methylcyclopropylcarboxylate (1195 cm−1 ) [4], cyclopropylcarboxylic acid (1196 cm−1 ) [5], cyclopropanecarboxamide (1198 cm−1 ) [5], cyclopropylcarbinol (1199 cm−1 ) [6], cyclopropylethanol (1196 cm−1 ) [6], methylsilylcyclopropane (1187 cm−1 ) [14], methylgermylcyclopropane (1191 cm−1 ) [15] and fluoromethylcyclopropane (1200 cm−1 ) [23] compared with 1187 cm−1 in CPDCS. The present results regarding the ring stretching, bending and CH2 modes of cyclopropane give a very good agreement with the values reported in a review by Wurrey and Nease [24]. The C–Si–C antisymmetric stretch (27 ) was assigned to the weak IR and Raman bands around 720 cm−1 in good agreement with the calculated value at 717 cm−1 . A corresponding symmetric C–Si–C stretch (28 ) appeared as a medium intense IR band at 642 cm−1 with P and R rotational fine structure and a strong, polarized Raman counterpart at 640 cm−1 , calculated at 623 cm−1 . The two SiCl2 antisymmetric and symmetric stretching vibrations 29 and 30 were attributed to the IR vapour bands at 561 and 467 cm−1 , respectively, with their Raman counterparts at 541 and 462 cm−1 . While the 29 fundamental was intense in IR and weak in Raman, the opposite was observed for 30 , and the Raman band at 462 cm−1 was the most intense in the entire spectrum (Fig. 3) in agreement with the calculations. These antisymmetric and symmetric vibrational modes were situated close to the SiCl2 antisymmetric and

symmetric stretches in CHCl2 –CH3 SiCl2 [25] (575, 478 cm−1 ) and in CH2 Cl–CH3 SiCl2 [26] (557, 485 cm−1 ). Various mixed SiCl2 scissor, wag, twist and rock and skeletal bending and torsional modes of the gauche conformer are calculated in the range 350–30 cm−1 . Many of these fundamentals are expected to have intense Raman components as listed in the calculated Raman intensities in Table 3. The observed infrared and particularly the Raman bands assigned to the fundamentals 32 –38 (Tables 1 and 3) agree quite well with the values obtained in the anharmonic calculations, although the percent deviations between the observed and calculated values are larger than in the high frequency region (1 –27 ). The CH3 torsion was expected at 182 cm−1 and was attributed to weak, uncertain IR and Raman bands partly overlapping the SiCl2 scissoring mode 35 at 186 cm−1 . An asymmetric torsion predicted around 30 cm−1 was not observed in the spectra. The assignments of the syn conformer bands (Table 2) are much more uncertain since this conformer is present in low concentration. Three weak or very weak IR and Raman bands observed at 1302, 1292 and 1195 cm−1 in the vapour are tentatively assigned to the syn modes a 8 , a 9 and a 10 , although no intensity variations with temperature were observed for these Raman bands. Two Raman bands at 742 and 709 cm−1 diminished in intensities at lower temperatures and were assigned to the syn fundamentals a 15 and a 16 , respectively. Two additional syn fundamentals are attributed to the Raman bands at 479 and 306 cm−1 including their infrared analogues since both of these diminished in intensity at lower temperatures. One of these were applied in the van’t Hoff plots with the conformer pair 315/306 cm−1 (Fig. 7) and 640/306 cm−1 . 4.2. Combination bands Some infrared and Raman bands were observed which were neither fundamental modes in the gauche nor in the syn conformers. These bands were mostly weak or very weak in intensities and were interpreted as binary combinations or overtones, some enhanced by Fermi resonance from neighbouring fundamentals. With 39 fundamental modes for each conformer, the number of binary combinations is very high, and any attempt to interpret these bands are futile. It appears from Table 1 that bands at 967, 745, 610, 413 and 398 cm−1 have Raman components which were enhanced at low temperatures and were therefore interpreted as combination bands of the gauche conformer. The Raman band at 685 cm−1 , however, diminished in intensity at lower temperatures and was attributed to the syn conformer. Finally, no intensity variations were detected for the Raman bands at 921, 566 and 265 cm−1 and they have arbitrarily been assigned to the dominant gauche conformer. The very weak Raman bands around 265 cm−1 are tentatively attributed to the binary combination mode 38 + 37 = 259. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.vibspec.2011.01.006. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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