Vibrational spectra, r0 structural parameters, barriers to internal rotation, and ab initio calculations of ClCH2SiH3, Cl2CHSiH3, ClCH2SiF3 and Cl2CHSiF3

Journal of Molecular Structure 922 (2009) 93–102

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Vibrational spectra, r0 structural parameters, barriers to internal rotation, and ab initio calculations of ClCH2SiH3, Cl2CHSiH3, ClCH2SiF3 and Cl2CHSiF3 Gamil A. Guirgis a, Savitha S. Panikar 1,b, Ahmed M. El Defrawy b, Victor F. Kalasinsky c, James R. Durig b,* a

Department of Chemistry and Biochemistry, College of Charleston, Charleston, SC 29424, USA Department of Chemistry, University of Missouri-Kansas City, Kansas City, MO 64110, USA c Division of Environmental Toxicology, Armed Forces Institute of Pathology, Washington, DC 20306, USA b

a r t i c l e

i n f o

Article history: Received 22 December 2008 Accepted 12 January 2009 Available online 23 January 2009 Keywords: Structural parameters Ab initio calculations Chloromethylsilane Dichloromethylsilane Chloromethyltrifluorosilane Dichloromethyltrifluorosilane

a b s t r a c t The infrared (3200–400 cm1) and Raman spectra (3500–40 cm1) of gas/or liquid and solid chloromethylsilane, ClCH2SiH3, dichloromethylsilane, Cl2CHSiH3, chloromethyltrifluorosilane, ClCH2SiF3, and dichloromethyltrifluorosilane, Cl2CHSiF3, have been recorded and complete vibrational assignments are given for all four molecules. To support the spectroscopic study ab initio calculations by the Møller–Plesset perturbation method to second order MP2(full) and density functional theory calculations by the B3LYP method have been carried out. The Raman activities, infrared intensities vibrational frequencies and barriers to internal rotation have been predicted from MP2(full)/6-31G(d) calculations and these theoretical quantities are compared to the experimental ones when available. The r0 Si–H distances for chloromethylsilane were determined to be 1.484 and 1.478 Å for the in-plane and out-of-plane atoms, respectively, from the Si–H stretching frequencies. The adjusted r0 parameters for ClCH2SiH3 have been obtained by combining the ab initio MP2(full)/6-311+G(d,p) predicted values with the 12 previously reported microwave rotational constants. The heavy atom structural parameters for this molecule are: r0(Si–C) = 1.886(3), r0(Cl–C) = 1.791(3) Å with \ClCSi of 109.3(5)°. Estimated r0 parameters are given for the other three molecules. Barriers to internal rotation have been obtained from the torsional frequencies and the values are compared to the predicted values as well as the corresponding ones for the carbon analogues. Ó 2009 Elsevier B.V. All rights reserved.

1. Introduction The barrier to internal rotation of the methyl group of monosubstituted ethane depends significantly on the nature of the substituent. For example, addition of a fluorine atom or atoms on one of the carbon atoms has a relatively small effect on the barrier values i.e. CH3CH2F (V3 = 1171 ± 1 cm1, 3349 ± 4 calmol1) [1]; CH3CHF2 (V3 = 1162 ± 2 cm1, 3323 ± 7 calmol1) [2]; CH3CF3 (V3 = 1105 ± 4 cm1, 3160 ± 11 calmol1) [3]. However, when the substituent is the chlorine atom the change in barrier value is quite significant with the addition of each chlorine atom: CH3CH2Cl (V3 = 1234 ± 3 cm1, 3529 ± 8 calmol1) [4]; CH3CHCl2 (V3 = 1490 ± 3 cm1, 4260 ± 9 calmol1) [5]; CH3CCl3 (V3 = 1784 cm1, 5100 calmol1) [6]. Even a more dramatic effect is found for the allyl halides with the substitution of the different halides where for the allyl fluoride (CH2CHCH2F) molecule the cis conformer is more stable than the gauche form [7] (DH = 60 ± 8 cm1, 172 ± 23 calmol1). However,

* Corresponding author. Tel.: +1 816 235 6038; fax: +1 816 235 2290. E-mail address: [email protected] (J.R. Durig). 1 Taken in part from the dissertation of S.S. Panikar which will be submitted to the Department of Chemistry in partial fulfillment of the Ph.D. degree. 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.01.025

for the corresponding chloride [8] and bromide [9] the gauche form is the more stable conformer with the determined DH values of 178 ± 11 cm1 (509 ± 31 calmol1) and 257 ± 50 cm1 (735 ± 143 calmol1), respectively. A conformational study [10] of vinyl silyl fluoride was recently carried out and the gauche form was found to be the more stable conformer with a DH = 76 ± 7 cm1 (217 ± 19 calmol1) from a temperature study of a krypton solution. This value is nearly equal to the value for the corresponding chloride 78 ± 11 cm1 (222 ± 31 calmol1) [11] and slightly higher than that for the bromide 22 ± 6 cm1 (63 ± 17 calmol1) [12]. As a continuation of these studies of comparison of the halomethylsilanes to the corresponding carbon compounds we initiated a vibrational study of ClCH2SiH3, Cl2CHSiH3, ClCH2SiF3 and Cl2CHSiF3 to determine the r0 structural parameters and barriers to internal rotation along with ab initio calculations for comparison to the experimental values when available. The results of this vibrational and theoretical study are reported. 2. Experimental and theoretical methods Chloromethylsilane was prepared by reducing chloromethyltrichlorosilane with lithium aluminum hydride in dry dibutylether as

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solvent. Similarly, dichloromethylsilane was prepared from the corresponding trichlorosilane. However, both chloromethyltrifluorosilane and dichloromethyltrifluorosilane were prepared from the corresponding trichlorosilanes by using freshly sublimed antimony trifluoride without solvent. Initially, the compounds were purified using the trap-to-trap distillation and further purification was obtained by using a low-temperature, low-pressure fractionation column. The purity of the samples was verified by infrared and NMR spectroscopy. The mid-infrared spectra of the gases and solids (Figs. 1–4) were recorded from 4000 to 300 cm1 on a Perkin–Elmer Model 2000 Fourier transform spectrophotometer which was continuously purged with dry nitrogen. The spectrophotometer was equipped with a nichrome wire source, a Ge/CsI beam splitter, and a DTGS detector. Spectra of the vapors were obtained by using a 15 cmpath cell fitted with CsI windows. The spectra of the solid was obtained by condensing the sample, held at 77 K by boiling liquid nitrogen on a CsI substrate, housed in a vacuum cell fitted with CsI windows. The theoretical resolution used to obtain the spectra of both the gas and the solid was 1.0 cm1. Typically, 256 scans were used for both the sample and reference data to give a satisfactory signal-to-noise ratio. The Raman spectra (Figs. 1–4) were recorded on a Spex model 1403 spectrophotometer equipped with a Spectra-Physics model 2017 argon ion laser operating on the 514.5 nm line. The laser power used was 1.5 W with a spectral bandpass of 3 cm1. The spectra of the liquids were recorded with the samples sealed in Pyrex glass capillaries. The measurements of the Raman frequencies are expected to be accurate to ±2 cm1. All of the observed fundamental bands in both the Raman spectra of the liquids and infrared spectra of the gases and solids along with their proposed assignments are listed in Tables 1–4. Fig. 2. Vibrational spectra of Cl2CHSiH3: infrared; (A) gas, (B) solid, (C) simulated and Raman; (D) liquid, (E) simulated. The asterisk indicates the band intensity reduced by 29%.

Fig. 1. Vibrational spectra of ClCH2SiH3: infrared; (A) gas, (B) solid, (C) simulated and Raman; (D) liquid, (E) simulated.

The LCAO-MO-SCF restricted Hartree–Fock calculations were performed with the Gaussian-03 program [13] using Gaussiantype basis functions. The energy minima with respect to the nuclear coordinates were obtained by the simultaneous relaxation of all the geometric parameters using the gradient method of Pulay [14]. Calculations were carried out by the Møller–Plesset perturbation method [15] to second order with valence and core electron correlation up to the 6-311+G(2df,2pd) basis set. The density functional theory (DFT) calculations were restricted to the B3LYP method [16,17]. In order to obtain a complete description of the molecular motions involved in the fundamental modes, normal coordinate analyses have been carried out. The force fields in Cartesian coordinates were obtained with the Gaussian 03 program [13] from the MP2(full)/6-31G(d) calculations. The internal coordinates used to calculate the B-matrix are given in Tables 5–8. The B-matrix elements [18] were used to convert the ab initio force field from Cartesian coordinates into the force field in designated internal coordinates. The ab initio force constants were used to reproduce the ab initio predicted vibrational frequencies which are listed in tabular form for the four silanes. Subsequently, scaling factors of 0.88 for the CH stretches, 1.0 for heavy atom bends and torsions and 0.90 for all other modes were used, along with the geometric average scaling factors for interaction force constants, to obtain the fixed scaled force fields and the resultant wavenumbers (Tables 1–4). A set of symmetry coordinates was used (Table 9) to determine the corresponding potential energy distributions (P.E.D.s) which are given for each molecule.

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Fig. 3. Vibrational spectra of ClCH2SiF3: infrared; (A) gas, (B) solid, (C) simulated and Raman; (D) liquid, (E) simulated. The asterisks indicate the band intensities reduced by 35%.

Fig. 4. Vibrational spectra of Cl2CHSiF3: infrared; (A) gas, (B) solid, (C) simulated and Raman; (D) liquid, (E) simulated.

Table 1 Calculated and observed frequencies (cm1) for chloromethylsilane. Sym Vib No. Approx. description block

A0

A0 0

a b c

m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17 m18

CH2 sym. stretch SiH3 antisym. stretch SiH3 sym. stretch CH2 deformation CH2 wag SiH3 antisym. deformation SiH3 sym. deformation CCl stretch SiC stretch SiH3 rock SiCCl bend CH2 antisym. stretch SiH3 antisym. stretch CH2 twist SiH3 antisym. deformation CH2 rock SiH3 rock torsion

MP2/6-31 G(d)a

MP2/6-311+G(d,p)

Freq. Fixed IR int. Raman scaled act. dp

Freq. IR int. Raman [24] act.

3148 2335 2311 1508 1285 986 964 814 748 571 203 3214 2344 1190 983 866 524 163

2953 2215 2192 1431 1219 936 914 772 710 571 203 3015 2224 1129 932 821 497 163

8.3 87.0 0.11 3124 11.0 111.5 101.1 101.3 0.25 2336 72.0 157.6 124.4 163.4 0.11 2314 116.2 207.6 9.2 13.6 0.74 1448 7.2 8.0 4.3 2.1 0.15 1257 5.8 2.6 60.2 23.1 0.75 995 62.8 17.2 265.5 11.1 0.74 970 286.2 6.8 5.3 10.3 0.66 817 3.8 10.5 17.7 10.5 0.34 747 21.2 9.3 29.1 14.7 0.36 570 32.8 14.6 2.7 0.8 0.53 193 2.6 0.8 1.4 64.0 0.75 3186 1.6 64.0 145.4 57.3 0.75 2342 115.7 68.0 2.4 10.4 0.75 1170 1.5 6.6 64.3 26.3 0.75 988 68.4 22.4 48.5 6.2 0.75 848 46.1 3.3 7.9 2.9 0.75 508 8.8 3.0 0.5 0.3 0.75 154 0.5 0.2

Raman dpc liquid

IR

2960 2198 2176 1416 1187 930 930 753 724 564 199 2994 2198 1109 930 816 508 161

[27]

Gas

Solid

2954 2168 2197 1409 1176 932 922 755 727 564 201 2980 2168 1111 932 818 512 161

2949.6 2189.6 2166.3 1409.7 1182.5 930.8 925.3 765.7 719.2 557.1 – 2991.3 2195.5 1109.9 941.2 817.8 509.6 161.0

2949 2190 2174 1400 1178 896 920 756 714 554 – 2999 2182 1116 941 810 517 –

2942 – 2172 1402 1178 936 936 755 712 550 – 2989 – 1107 936 810 – –

0.03 – 0.07 0.57 0.07 0.76 0.76 0.63 0.04 0.10 – 0.73 – 0.78 0.76 – – –

P.E.D.b

100S1 62S2, 38S3 62S3, 38S2 100S4 98S5 98S6 98S7 70S8, 28S9 34S9, 46S10, 12S11 44S10, 31S9, 25S8 90S11 100S12 100S13 92S14 99S15 61S16, 37S17 60S17, 33S16 100S18

Scaled ab initio calculations with factors of 0.88 for CH stretches, 0.9 for all other modes except torsion and heavy atom bends using MP2/6-31 G(d) basis set. Symmetry coordinates with P.E.D. contribution less than 10% are omitted. Ref. [24].

Band Contour A

B

40 1 88 68 – 28 78 90 – 58 55 – – – – – – –

60 99 12 32 100 72 22 10 100 42 45 – – – – – – –

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Table 2 Calculateda and observed frequencies (cm1) for dichloromethylsilane. Sym. block Vib. No. Approx. description

Ab initio Fixed scaledb IR int. Raman act. dp

A0

3189 2362 2329 1232 982 957 846 744 528 313 236 2346 1296 976 806 633 193 159

A0 0

a

b c

m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17 m18

CH stretch SiH3 antisym. stretch SiH3 sym. stretch CH bend SiH3 antisym. deformation SiH3 sym. deformation CCl2 sym. stretch SiH3 rock SiC stretch CCl2 deformation CCl2 wag SiH3 antisym. stretch CH bend SiH3 antisym. deformation CCl2 antisym. stretch SiH3 rock CCl2 twist Torsion

2991 2240 2210 1169 932 908 802 706 501 313 236 2225 1230 926 765 601 193 159

2.4 71.2 108.0 59.4 73.0 185.8 1.9 6.0 68.6 23.6 266.0 9.4 9.2 9.4 20.5 7.3 19.5 12.4 1.7 4.9 6.0 2.0 132.7 73.6 11.4 7.9 65.7 20.9 31.9 3.0 57.5 16.2 0.0 0.3 0.0 0.2

0.31 0.61 0.03 0.65 0.75 0.74 0.71 0.38 0.20 0.46 0.72 0.75 0.75 0.75 0.75 0.75 0.75 0.75

IR [24]

[27]

Gas

Solid

2990 2197 2224 1140 923 942 748 718 511 305 220 2197 1205 923 807 617 180 177

2979 2214 2191 1196 943 923 754 687 507 306 219 2191 1140 923 803 612 180 170

2981.2 2212.2 2187.6 1146.3 942.0 924.0 808.5 710.6 513.1 – – 2191.3 1198.4 919.0 747.9 611.8 – 151.1

2981 2213 2185 1143 933 926 796 715 514 – – 2194 1192 889 730 622 – –

Raman dp liquid

P.E.D.c

2978 – 2183 1145 935 915 805 711 512 300 228 2183 1197 915 735 608 182 –

100S1 94S2 94S3 91S4 97S5 94S6 40S7, 44S9, 11S10 60S8, 15S11, 11S7 32S9, 32S7, 30S8 64S10, 16S7, 14S11 65S11, 20S10, 10S9 100S12 100S13 98S14 79S15, 17S16 78S16, 23S15 72S17, 26S18 74S18, 24S17

0.19 – 0.10 0.52 0.77 – 0.73 0.06 0.03 0.11 0.62 0.10 0.76 – 0.72 0.77 0.75 –

Band Contour A

C

81 3 96 89 30 91 90 – 46 22 91 – – – – – – –

19 97 4 11 70 9 10 100 54 78 9 – – – – – – –

MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions (P.E.D.s). Scaled frequencies with scaling factors of 0.87 for CH and 0.90 for all other modes except torsion and heavy atom bends. Symmetry coordinates with P.E.D. contribution less than 10% are omitted.

The simulated infrared spectra were predicted from the MP2(full)/6-31G(d) calculations. Infrared intensities were calculated based on the dipole moment derivatives with respect to the Cartesian coordinates. The derivatives were taken from the ab initio calculations transformed to normal coordinates by: (olu/oQi) P = j(olu/oXj) Lij, where Qi is the ith normal coordinate, Xj is the jth Cartesian displacement coordinate, and Lij are elements of transformation matrix between the Cartesian displacement coordinates and normal coordinates. The infrared intensities were then calculated with (Np)/(3c2) [(olx/oQi)2 + (oly/oQi)2 + (olz/oQi)2]. The predicted infrared spectra were obtained and are shown for ClCH2SiH3 (Fig. 1C), Cl2CHSiH3 (Fig. 2C), ClCH2SiF3 (Fig. 3C) and

Cl2CHSiF3 (Fig. 4C). The corresponding experimental spectra for the gases are shown in Figs. 1A, 2A, 3A and 4A, respectively, and there is excellent correspondence between the observed infrared spectra of the gases and the simulated spectra for all four molecules. Also to further support the vibrational assignments, we have simulated the Raman spectra from the fixed scaled ab initio MP2(full)/631G(d) results. The evaluation of Raman activity by using the analytical gradient methods has been developed [19–22]. The activity Sj can be expressed as: Sj = gj ð45a2j þ 7b2j Þ, where gj is the degeneracy of the vibrational mode j, aj is the derivative of the isotropic polarizability, and bj is that of the anisotropic polarizability. To obtain the

Table 3 Calculateda and observed frequencies (cm1) for chloromethyltrifluorosilane. Sym block Vib No. Approx. description

A0

A0 0

m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17 m18

CH2 sym. stretch CH2 deformation CH2 wag SiF3 antisym. stretch SiF3 sym. stretch CCl stretch SiC stretch SiF3 sym. deformation SiF3 antisym. deformation SiF3 rock SiCCl bend CH2 antisym. stretch CH2 twist SiF3 antisym. stretch CH2 rock SiF3 antisym. deformation SiF3 rock Torsion

Ab initio Fixed scaledb IR int. Raman act. dp

3151 1506 1303 1029 926 808 682 406 339 240 117 3216 1193 1039 781 323 208 53

2956 1429 1236 976 879 767 647 406 339 240 117 3017 1132 986 741 323 208 53

7.2 16.7 14.6 172.4 153.7 25.7 10.0 83.0 29.6 1.1 1.9 0.0 7.8 189.5 7.7 17.2 0.2 3.8

83.1 12.9 2.2 0.6 4.7 5.3 11.3 1.0 1.3 1.5 0.3 60.0 10.0 0.6 1.9 0.4 0.2 0.8

0.11 0.74 0.49 0.75 0.27 0.70 0.03 0.75 0.47 0.55 0.74 0.75 0.75 0.75 0.75 0.75 0.75 0.75

IR

P.E.D.c

Raman d

d

[27]

Gas

Solid Liquid

dp

2955 1397 1199 990 900 732 664 403 337 242 123 3003 1112 990 767 337 242 65

2954.3 1399.7 1199.7 989.1 903.2 767.8 666.1 403.4 340.0 226.2 119.0 – 1114.7 989.1 729.5 326.0 – 62.1

2960 1390 1200 972 897 754 661 402 341 226 105 – 1113 972 733 317 224 81

p 0.5 – – 0.2 0.6 p 0.2 0.3 0.5 dp dp 0.6 – p p – –

2955 1395 1198 982 897 760 658 403 343 242 123 3003 1114 982 – 308 242 –

100S1 100S2 95S3 94S4 53S5, 28S7 67S6, 26S5 45S7, 19S6, 18S5 66S8, 14S11 62S9, 19S8 32S10, 29S9, 21S11 55S11, 43S10 100S12 92S13 90S14 80S15 88S16 81S17, 10S16 99S18

Band Contour A

B

90 58 52 15 88 99 93 94 3 100 – – – – – – – –

10 42 48 85 12 1 7 6 97 – 100 – – – – – – –

a MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions (P.E.D.s). b Scaled ab initio calculations with scaling factors of 0.88 for CH stretches, 0.9 for all other modes except torsion and heavy atom bends. c Symmetry coordinates with P.E.D. contribution less than 10% are omitted. d Ref. [28].

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G.A. Guirgis et al. / Journal of Molecular Structure 922 (2009) 93–102 Table 4 Calculateda and observed frequencies (cm1) for dichloromethyltrifluorosilane. Sym. block Vib. No. Approx. description

Ab initio Fixed scaledb IR int. Raman act. dp

IR [29]

Gas

A0

3187 1290 1052 1040 816 648 432 340 231 210 114 1255 939 801 351 306 134 36

2981 1193 1003 941 792 624 428 340 238 212 – 1173 913 743 347 300 – –

2980.9 2913 2978 2979 1190.2 1188 1194 1193 1009.8 989 1005 1004 1000.3 981 986 990 791.9 788 784 787 623.7 623 623 624 424.5 414 419 421 336.1 331 335 336 234.3 233 233 232 206.2 213 211 210 115.7 126 123 121 1171.3 1170 1172 1171 911.0 906 906 907 743.7 734 736 734 343.1 346 335 347 296.6 298 300 300 138.9 147 150 148 – 63 53 100*

m1 m2 m3 m4 m5 m6 m7 m8 m9 m10 m11 m12 m13 m14 m15 m16 m17 m18

A0 0

a

b c *

CH stretch CH bend SiF3 antisym. stretch SiF3 sym. stretch CCl2 sym. stretch SiC stretch SiF3 sym. deformation CCl2 deformation SiF3 antisym. deformation CCl2 wag SiF3 rock CH bend SiF3 antisym. stretch CCl2 antisym. stretch SiF3 antisym. deformation SiF3 rock CCl2 twist Torsion

2990 1224 998 987 774 615 432 340 231 210 114 1177 891 760 351 306 134 36

3.6 68.1 8.1 8.8 166.8 0.4 163.2 0.8 40.2 1.6 12.5 8.9 96.9 1.7 27.2 4.4 0.2 1.4 0.6 2.0 1.4 0.2 18.5 4.8 126.0 5.4 64.8 5.7 22.2 1.2 11.6 1.3 2.4 0.3 0.1 0.4

0.30 0.75 0.75 0.75 0.61 0.00 0.48 0.33 0.75 0.72 0.75 0.75 0.19 0.75 0.75 0.34 0.75 0.75

P.E.D.c

Raman Solid [29]

Band Contour

Liquid 100S1 86S2 93S3 55S4, 36S5 35S5, 26S6, 21S8 25S6, 24S4, 18S5, 15S10 56S7, 24S8, 12S10 25S8, 28S10, 27S9, 11S5 67S9, 18S8 20S10, 16S6, 20S7, 40S11 55S11, 18S10, 11S6 100S12 97S13 96S14 63S15, 36S16 36S16, 38S17, 20S15 65S17, 23S16, 11S15 100S18

A

C

97 94 2 98 100 93 100 48 6 66 4 – – – – – – –

3 6 98 2 – 7 – 52 94 34 96 – – – – – – –

MP2(full)/6-31G(d) ab initio calculations, scaled frequencies, infrared intensities (km/mol), Raman activities (Å4/u), depolarization ratios (dp) and potential energy distributions (P.E.D.s). Scaled frequencies with scaling factors of 0.88 for CH and 0.90 for all other modes except torsion and heavy atom bends. Symmetry coordinates with P.E.D. contribution less than 10% are omitted. 2m18.

polarized Raman scattering cross sections, the polarizabilities are incorporated into Sj by multiplying Sj with (1  qj)/(1 + qj), where qj is the depolarization ratio of the jth normal mode. The Raman scattering cross sections and calculated wavenumbers obtained from the Gaussian 03 program [13] were used together with a Lorentzian band shape function to obtain the simulated Raman spectra. Comparison of experimental Raman spectra of the liquids and the calculated ones are shown in Figs. 1E, 2E, 3E and 4E for ClCH2SiH3, Cl2CHSiH3, ClCHSiF3 and Cl2CHSiF3, respectively.

3. Vibrational spectra and structural parameters Several vibrational studies have been reported for the chloromethyl and dichloromethyl silanes [23–27] whereas more limited vibrational studies have been reported for the two trifluorosilane molecules [27–29]. A rather complete vibrational assignment of all four molecules has been reported [27] by utilizing previously obtained [30] force fields for CH3SiX3 where X = H, F and Cl with extensions for the ClCH2- and Cl2CH-moieties along with previ-

Table 5 Structural parameters (Å and degree), rotational constants (MHz), dipole moments (debye) and centrifugal distortion constants (kHz) for chloromethylsilane. Structural parameters

Internal coordinates

RHF631G(d)

MP2(full)/631G(d)

MP2(full)/6311+G(d,p)

B3LYP/6311+G(d,p)

r(Si–C) r(Cl–C) r(Si–H4,5) r(Si–H6) r(C–H7,8) \ClCSi \H4;5 SiC \H6 SiC \H7;8 CSi \H4 SiH5 \H4;5 SiH6 \H7;8 CCl \H7 CH8 s H6SiCCl A B C |la| |lb| |lc| |lt| DJ DK DJK dJ dK

R1 R2 R3, R4 R5 R6, R7

1.901 1.800 1.472 1.477 1.081 111.2 109.7 107.8 111.8 110.2 109.6 106.8 108.0 180.0 22,311 3096 2856 1.680 1.324 0.000 2.139 1.60 179.39 16.26 0.21 3.21

1.895 1.791 1.482 1.486 1.092 111.0 109.5 108.2 111.5 110.2 109.7 107.5 107.7 180.0 22,199 3130 2885 1.611 1.331 0.000 2.090 1.75 182.33 17.49 0.24 3.84

1.889 1.789 1.473 1.477 1.091 111.0 109.3 108.2 111.3 110.2 109.9 107.6 107.7 180.0 22,340 3142 2897 1.629 1.307 0.000 2.088 1.93 202.74 20.64 0.27 4.19

1.899 1.822 1.482 1.487 1.090 111.2 109.7 107.9 111.9 110.2 109.7 106.7 108.2 180.0 22,043 3066 2828 1.479 1.229 0.000 1.923 1.86 210.64 20.88 0.25 3.69

a

w /1, /2 /3 h1, h2 d1 d2, d3 r1, r2

r3

Ref. [34], see text for assumptions made in obtaining the different structures.

Microwavea

Adjusted r0

Structure I

Structure II

Best structure

1.894 1.783 1.477 1.477 1.096 109.3 108.3 108.3 109.3 110.6 110.6 – 107.5 – – – – – – – – – – – – –

1.884 1.793 1.477 1.477 1.096 109.3 108.3 108.3 109.4 110.6 110.6 – 107.5 – – – – – – – – – – – – –

1.889 (10) 1.788 (10) 1.477 (5) 1.477 (5) 1.096 (10) 109.3 (3) 108.3 (5) 108.3 (5) 109.3 (5) 110.6 (5) 110.6 (5) – 107.5 (5) – 21759.2 3204.1 2938.1 – – – – 4.00 – – 0.50 –

1.886 (3) 1.791 (3) 1.478 (3) 1.484 (3) 1.091 (2) 109.3 (5) 109.3 (5) 108.2 (5) 111.3 (5) 110.2 (5) 109.9 (5) 108.6 (5) 107.7 (5) 180.0 21758.9 3203.2 2939.1 – – – – – – – – –

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Table 6 Structural parameters (Å and degree), rotational constants (MHz) and dipole moment (debye) for dichloromethylsilane. Structural parameters

Internal coordinates

RHF6-31G(d)

MP2(full)/6-31G(d)

MP2(full)/6-311+G(d,p)

B3LYP/6-311+G(d,p)

Estimated r0

r(Si–C) r(C–H3) r(Si–H7,8) r(Si–H6) r(C–Cl4,5) \H3 CSi \H7;8 SiC \H6 SiC \Cl4;5 CSi \H7 SiH8 \H6 SiH7;8 \H3 CCl4;5 \Cl4 CCl5 s Cl4CSiH3 A B C |la| |lb| |lc| |lt|

R1 R2 R3, R4 R5 R6, R7

1.916 1.076 1.471 1.468 1.785 112.3 107.8 108.5 110.6 110.3 111.2 106.1 111.0 118.3 3096 3027 1617 1.952 0.000 1.074 2.228

1.908 1.090 1.482 1.478 1.783 112.3 108.0 108.0 110.2 110.4 111.2 106.4 111.4 118.4 3090 3064 1627 1.880 0.000 1.092 2.174

1.903 1.089 1.473 1.470 1.781 112.2 108.0 108.0 110.2 110.6 111.2 106.4 111.4 118.3 3094 3078 1631 1.894 0.000 1.058 2.169

1.914 1.087 1.482 1.478 1.809 113.0 107.8 108.4 110.5 110.3 111.2 105.7 111.3 118.2 3012 3008 1589 0.000 1.725 0.947 1.968

1.900 1.093 1.480 1.477 1.783 112.2 108.0 108.0 110.2 110.6 111.2 106.4 111.4 118.3 3087 3081 1631 – – – –

w /1, /2 /3 h1, h2 d1 d2, d3 r1, r2

r3

Table 7 Structural parameters (Å and degree), rotational constants (MHz) and dipole moment (debye) for chloromethyltrifluorosilane. Structural parameters

Internal coordinates

RHF6-31G(d)

MP2(full)/6-31G(d)

MP2(full)/6-311+G(d,p)

B3LYP/6-311+G(d,p)

Estimated r0

r(Si–C) r(Cl–C) r(Si–F4,5) r(Si–F6) r(C–H7,8) \ClCSi \F4;5 SiC \F6 SiC \H7;8 CSi \F4 SiF5 \F4;5 SiF6 \H7;8 CCl \H7 CH8 s F6SiCCl A B C |la| |lb| |lc| |lt|

R1 R2 R3, R4 R5 R6, R7

1.862 1.791 1.567 1.571 1.081 111.6 111.7 108.7 110.9 108.6 108.0 107.6 107.9 180.0 3880 1363 1347 0.097 2.105 0.000 2.107

1.855 1.786 1.591 1.595 1.092 111.2 111.5 108.7 110.7 108.6 108.2 108.2 107.7 180.0 3779 1362 1348 0.243 2.169 0.000 2.183

1.848 1.783 1.591 1.596 1.091 110.7 111.7 109.3 110.5 108.1 107.9 108.6 107.9 180.0 3796 1371 1356 0.659 2.337 0.000 2.428

1.857 1.811 1.597 1.602 1.090 111.4 112.1 109.0 110.8 108.1 107.6 107.7 108.3 180.0 3766 1339 1325 0.678 2.147 0.000 2.252

1.845 1.786 1.565 1.570 1.092 110.7 111.7 109.3 110.5 108.1 107.9 108.6 107.9 180.0 3906 1385 1369 – – – –

w /1, /2 /3 h1, h2 d d2, d3 r1, r2

r3

Table 8 Structural parameters (Å and degree), rotational constants (MHz) and dipole moment (debye) for dichloromethyltrifluorosilane. Structural parameters

Internal coordinates

RHF6-31G(d)

MP2(full)/6-31G(d)

MP2(full)/6-311+G(d,p)

B3LYP/6-311+G(d,p)

Estimated r0

r(Si–C) r(C–H3) r(Si–F7,8) r(Si–F6) r(C–Cl4,5) \H3 CSi \F7;8 SiC \F6 SiC \Cl4;5 CSi \F7 SiF8 \F6 SiF7;8 \H3 CCl4;5 \Cl4 CCl5 s Cl5CSiH3 A B C |la| |lb| |lc| |lt|

R1 R2 R3, R4 R5 R6, R7

1.879 1.077 1.565 1.561 1.779 111.0 109.2 111.4 110.4 108.4 109.3 106.5 111.7 118.0 1820 1204 890 0.049 0.000 1.429 1.430

1.872 1.091 1.590 1.586 1.778 111.2 109.3 111.1 109.9 108.5 109.4 106.9 112.1 118.1 1792 1209 892 0.237 0.000 1.479 1.498

1.865 1.090 1.590 1.587 1.777 111.3 109.9 111.2 109.4 108.1 108.9 107.3 112.3 118.3 1796 1217 896 0.493 0.437 1.568 1.701

1.877 1.088 1.595 1.591 1.801 111.5 109.9 111.6 110.0 107.7 108.8 106.5 112.2 118.0 1766 1189 875 0.708 0.000 1.362 1.535

1.861 1.090 1.565 1.561 1.782 111.3 109.9 111.2 109.4 108.1 108.9 107.3 112.3 118.3 1815 1228 900 – – – –

w /1, /2 /3 h1, h2 d1 d2, d3 r1, r2

r3

G.A. Guirgis et al. / Journal of Molecular Structure 922 (2009) 93–102 35

Table 9 Symmetry coordinates of chloromethylsilane.

A

0

A00

a

99

Description

Symmetry coordinatea

CH2 sym. stretch SiH3 antisym. stretch SiH3 sym. stretch CH2 deformation CH2 wag SiH3 antisym. deformation SiH3 sym. deformation CCl stretch SiC stretch SiH3 rock SiCCl bend CH2 antisym. stretch SiH3 antisym. stretch CH2 twist SiH3 antisym. deformation CH2 rock SiH3 rock Torsion

S1 = R6 + R7 S2 = 2R5  R3  R4 S3 = R5 + R3 + R4 S4 = 4r3  h1  h2  r1  r2 S5 = h1 + h2  r1  r2 S6 = 2d1  d2  d3 S7 = d1 + d2 + d3  /1  /2  /3 S8 = R2 S9 = R1 S10 = 2/3  /1  /2 S11 = w S12 = R6  R7 S13 = R3  R4 S14 = h1  r1  h2 + r2 S15 = d2  d3 S16 = h1+r1  h2  r2 S17 = /1  /2 S18 = s1

Not normalized.

ously reported spectral data. By the use of ab initio predictions the fundamental frequencies can usually be obtained to better than 1% and the potential energy distributions will predict the mixing of the normal modes. We [31] have shown that ab initio MP2/6311+G(d,p) calculations predicted the r0 structural parameters for more than 50 carbon–hydrogen distances better than 0.002 Å compared to the experimentally determined values from isolated C–H stretching frequencies which were compared [32] to the previously determined values from microwave studies. It has also been shown [33,34] that similar ab initio calculations predict the SiC distances for a number of organosilanes to a relatively small error from the experimental value. The SiH distance can be obtained from the frequency of the SiH stretch [35]. Finally, we have found [7] that good structural parameters can be obtained by adjusting the structural parameters obtained from the ab initio calculations to fit the rotational constants obtained from the microwave experimental data. In order to reduce the number of independent variables, the structural parameters are separated into sets according to their types. Bond lengths in the same set keep their relative ratio. Also, bond angles and torsional angles belonging to the same set keep their differences in degrees. This assumption is based on the fact that the errors from ab initio calculations are systematic. Therefore, it should be possible to obtain ‘‘adjusted r0” structural parameters for chloromethylsilane utilizing the microwave rotational constants from the 12 reported values [36] for the four isotopomers studied.

Cl molecule. The lower frequency band at 719 cm1 is exceptionally mixed with 46% SiH3 rock and 34% SiC stretch (Table 1). The 557 cm1 band has similar mixing with 44% SiH3 rock, 31% SiC stretch and 25% CCl stretch. The band contours of the 719 and 557 cm1 are exactly as predicted (see Fig. 5) whereas the 765 cm1 band does not appear to be 90% A-type. In the microwave investigation [36] of ClCH2SiH3 the molecules with 35Cl, 37Cl, 13C, 29Si as well as those with deuterium were investigated and with the assignment of 7–10 lines for each isotopomer rotational constants were determined with centrifugal distortion constants DJ (4.0 kHz) and dJ (0.5 kHz). Two different substitution structural values were reported. Since the carbon atom was too close to one of the axes it gave imaginary values for the coordinate ‘‘a” which was set to zero. These structural parameters (structure I) are listed in Table 5. Attempts to improve the results by utilizing the product of inertia failed so the CH2 triangle was moved to reduce the value of Rmiai to zero. The results of this computation are also given in Table 5 as structure II along with the average of the values obtained by these two methods. Also, it should be noted that the SiH3 moiety was assumed to be symmetric with equal SiH distances. It is believed that improved r0 structural parameters can be obtained by adjusting the predicted parameters from the MP2(full)/6311+G(d,p) calculations so that the resulting rotational constants agree with the experimentally determined microwave rotational constants from the four isotopomers with differently substituted heavy atoms. Additionally, the CH distances were taken from the predicted values from the MP2(full)/6-311+G(d,p) calculations and the SiH distances were obtained from the stretching frequen-

3.1. Chloromethylsilane (ClCH2SiH3) In the two complete assignments [24,25] of ClCH2SiH3 there are some significant differences in the assignments for the three SiH3 stretches and three SiH3 deformations as well as CCl and CSi stretches. In the initial study [24], all three of the SiH3 deformations were assigned to a single band by assuming they were accidently degenerate whereas in the later study [27] the SiH3 symmetric deformation was assigned 10 cm1 lower than the two antisymmetric modes (Table 1). It was hoped that the predicted infrared band contours and relative intensities of the infrared bands and Raman lines could provide sufficient information to distinguish these alternatives. The P.E.D. clearly shows that the higher frequency band at 765 cm1 is mainly the CCl stretch (70% S8) with only 28% the SiC stretch. Also supporting this assignment is the predicted spectrum of the 37Cl isotopomer which is 4 cm1 below the band for the

Fig. 5. Predicted pure A-, B- and C-type infrared contours for chloromethylsilane.

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Table 10 Comparison of rotational constants (MHz) obtained from modified ab initio MP2(full)/ 6-311+G(d,p) predictions, experimental values from microwave spectra, and the adjusted r0 structural parameters for chloromethylsilane. Isotopomer

Rotational constant

MP2(full)/6311+G(d,p)

Experimental

ClCH2SiH3

A B C A B C A B C A B C

22340.2 3142.1 2896.9 22271.1 3064.5 2829.7 21651.3 3141.9 2884.9 22305.8 3094.0 2855.4

21759.2 3204.1 2938.1 21692.9 3125.1 2870.4 21080.4 3204.4 2925.5 21726.8 3155.4 2896.5

37

ClCH2SiH3

Cl13CH2SiH3

ClCH229SiH3

a

a

Adjusted r0

| D|

21759.8 3203.6 2938.7 21693.0 3124.7 2871.0 21077.6 3203.6 2926.0 21726.8 3154.8 2897.1

0.6 0.5 0.6 0.1 0.4 0.6 2.8 0.8 0.5 0.0 0.6 0.6

Ref. [33].

cies for this group. The resulting r0 parameters after small adjustments to fit the 12 rotational constants are listed in Table 5 and the fit to these rotational constants are given in Table 10. It is believed that these determined bond distances have significantly smaller uncertainties than those previously reported [36] where some were as great as ±0.010 Å. It should be noted that these parameters are in excellent agreement with the parameters obtained by structure II. Some of the errors may arise from the assumption that the SiH bond distances are all equal. 3.2. Dichloromethylsilane (Cl2CHSiH3) There have been two vibrational assignments previously reported [24,27] for dichloromethylsilane. In the first study [24] the two (A0 and A00 ) SiH3 antisymmetric stretches were assigned the same frequency at 2197 cm1 whereas the symmetric stretch was assigned at a higher frequency at 2224 cm1. In the second study [27] the antisymmetric stretches were assumed to be split with the A0 at 2214 and the A00 at 2191 cm1 as accidently degenerate with the symmetric stretch. These discrepancies are, however, resolved in the present study where three different bands are observed for these three stretching modes with the symmetric stretch assigned to the lowest frequency as shown in Table 2. A similar assignment [24,27] was made for the SiH3 deformations whereas in the current study clear bands are observed for the three deformational modes. From the P.E.D.s, we observed extensive mixing of the deformation modes. As an example, the 808 cm1 band assigned to the CCl2 symmetric stretch has been observed to be only 40% S7 with 44% contribution from S9 (SiC stretch) and 11% from S10 (CCl2 deformation). The 513 cm1 band assigned to the SiC stretch was seen to consist of equal percent (32%) of the CCl2 and the SiC stretch and another 30% of the SiH3 rock. The other modes, namely SiH3 rock, CCl2 deformation and CCl2 wag have contributions in the range of 60– 65% of the approximate descriptions indicated with the remaining percents being for the rock, the deformations and the CCl2 stretch. No structural parameters have been previously reported for this molecule; therefore, we utilized ab initio calculations to predict the structural parameters from the RHF6-31G(d), MP2/6-31G(d), MP2/ 6-311+G(d,p) as well as density functional theory, B3LYP6311+G(d,p) calculations (Table 6). The r0 values have been estimated by taking the CH distance from the MP2/6-311+G(d,p) calculated value, the SiH distances by adding 0.007 Å to the SiH predicted ab initio value, along with the slight adjustment (+0.002 Å) to the CCl predicted distance while keeping the angles to the values predicted from the MP2/6-311+G(d,p) calculations.

By this process it is believed that the estimated parameters would be close to the actual values. 3.3. Chloromethyltrifluorosilane (ClCH2SiF3) Unlike the previous two molecules, the vibrational assignment for the chloromethyltrifluorosilane has not been widely studied. In a previous investigation [27], both the SiF3 antisymmetric stretches (A0 and A00 ) have been assigned the same frequency at 990 cm1 which is higher than the value for the corresponding symmetric stretch (900 cm1). In the present study, the ab initio calculation predicts the two antisymmetric stretches to be only 10 cm1 apart at 976 and 986 cm1 whereas the symmetric stretch is predicted at 879 cm1 (Table 3). From the experimentally obtained infrared spectra for the gas and solid, only two bands are distinctly observed for the SiF3 stretches as shown in Fig. 3. The band at the higher frequency of 989 cm1 is assigned to both the antisymmetric stretches essentially making them degenerate. The other band at 903 cm1 is assigned to the symmetric stretch. The CH2 antisymmetric stretch which was predicted at 3017 cm1 has zero predicted infrared intensity, thus, it was not observed in the infrared spectrum. The P.E.D.s for a number of the fundamentals are significantly mixed. The A0 SiF3 rock assigned at 226 cm1 is extensively mixed with only 32% S10 (SiF3 rock), 29% S9 (SiF3 antisymmetric deformation) and 21% S11 (SiCCl bend). The CCl stretch assigned at 767 cm1 contributes only 67% S6 whereas 26% comes from the SiF3 symmetric stretch (S5) which itself is only 53% of the normal mode as assigned. The SiF3 symmetric and antisymmetric deformations are about 66% and 62% contributions, respectively, of these described motions. The SiF3 symmetric stretch is only 53% S5 with 28% contribution coming from the SiC stretch (S7). As was the case for dichloromethylsilane, similarly for chloromethyltrifluorosilane, no structural parameters have been previously reported. However, the structural parameters have been estimated as described for the corresponding silane molecule. For the ClCH2 moiety the estimated parameters were adjusted as found necessary for chloromethylsilane from the MP2/6-311+G(d,p) predicted values. For the SiF3 parameters the RHF predicted values are known to be quite comparable to the experimental values [38]. Therefore, these parameters have been used to estimate the SiF distances. 3.4. Dichloromethyltrifluorosilane (Cl2CHSiF3) There has been two previous vibrational assignments reported [25,29] for Cl2CHSiF3 and there are major differences in the assignments made from our observed spectra and those given earlier. For example, the SiF3 antisymmetric and symmetric stretches for the A0 species is separated by 9.5 cm1 whereas they were previously assigned as separated by 62 cm1. For the two antisymmetric deformations (A0 and A00 ) they were assigned [29] as accidently degenerate at 347 cm1 whereas we have assigned them at 234.3 cm1 (A0 ) and 343.1 cm1 (A00 ) where these modes have 67% S9 and 63% S15 contributions. Another major difference is the relative assignments for the two CCl2 stretches where the symmetric mode is assigned at 791.9 and the antisymmetric stretch at 743.7 cm1 which was reversed in the earlier assignment [29]. The m5 fundamental has extensive mixing with the SiC stretch (26% S6) and the CCl deformation (21% S8) whereas the antisymmetric stretch is nearly a pure mode (96% S14). Finally, the CCl2 twist (A00 ) is assigned at 138.9 cm1 (65% S17) with the SiF3 rock (A0 ) assigned at 115.7 cm1 (55% S11) whereas the assignment for these two modes were reversed in the earlier assignment [29]. There are three modes (m6, m8 and m10) which are extensively mixed with major contributions from three other symmetry coordi-

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nates. The band at 206.2 cm1 assigned as m10 has only 20% contribution of the CCl2 wag (S10) and 40% S11, the SiF3 motion, but the band at 115.7 cm1 has 55% S11 which makes it more appropriate to refer to this band as the SiF3 rock rather than the higher frequency band. Thus it should be noted that some of the descriptions are only approximate which indicates in some cases the molecular motions are more for bookkeeping than giving the atomic displacements. Since there were no previously reported structural parameters just like the two previous silanes, the structural parameters were estimated from comparable molecules. The SiC distance was decreased by 0.004 Å whereas the CCl distances were increased by 0.005 Å. The CH distance was taken from the MP2/6-311+G(d,p) predicted value and the SiF parameters from the RHF6-31G(d) calculations. 4. Discussion The significant changes in the assignments for several of the fundamentals for all four of the molecules was made possible mainly from the predictions of the ab initio calculations with the frequencies most important followed by the band contours and finally the infrared intensities and Raman activities. The chloromethylsilane molecule illustrates this order well where the predicted frequencies for the A0 species have an average difference of 12 cm1 (0.9%) from the observed values and those for the A00 modes a difference of 14 cm1 (1.1%). The SiH3 antisymmetric stretch (A00 ) must be a pure C-type band whereas the corresponding A0 mode is predicted to be 88% A-type and 12% B-type and the observed spectra (Fig. 6) clearly shows the lower frequency Q-branch is the center of an A-type band. However, for the Cl2CHSiF3 molecule the predicted B-type bands for the A00 modes were difficult to identify since the minimum could not be observed and the two maxima could be interpreted as a Q-branch or the band simply appeared as a broad band without any structure. The predicted fundamental frequencies for the other silane had slightly higher differences of 14 and 17 cm1 for the A0 and A00 modes, respectively, with the major differences for the SiH3 stretches and CH bends. For the corresponding trifluorosilanes the predicted differences are 13 and 10 cm1 for the A0 modes and 10 and 13 cm1 for the A00 modes for the ClCH2SiF3 and Cl2CHSiF3 molecules, respectively. Therefore, good predictions can be obtained for the organosilanes from ab initio MP2/631G(d) calculations with only two scaling factors. As indicated earlier the ‘‘best structure” as well as structure II as reported [36] from the microwave study for ClCH2SiH3 were in good agreement with the adjusted r0 values for the heavy atom distances within 0.003 Å but the uncertainties were ±0.010 Å whereas those of the adjusted values are believed to be not more than ±0.003 Å. The uncertainty on the CH distance was also reported [36] to be ±0.10 Å but the adjusted value has an uncertainty of ±0.002 Å with those for the SiH distances of ±0.003 Å and the angle uncertainties estimated to be no more than ±0.5°. Therefore, it is believed that the adjusted r0 values are as accurate as can be determined for this molecule in the vapor state. In some earlier studies of some organosilanes [37,38] it has been found that the ab initio MP2(full)/6-311+G(d,p) predicted values for the SiC bond are too short by values in the range of 0.003– 0.006 Å with the SiH bond distances predicted too short usually by 0.006 ± 0.002 Å where the values obtained in this study fit these ranges. We have also compared the ab initio MP2(full)/6311+G(d,p) predicted CCl bond distances for a number of organochlorides and found the predicted distances are a few thousandths of an Å shorter which is consistent with the adjustment by an increase of 0.002 Å for the ClCH2SiH3 molecule. With these adjustments we have estimated the rotational constants for Cl2CHSiH3.

Fig. 6. (A) Typical C-type and A-type band contours at 0.5 cm1 resolution; (B) typical B- and C-type band contours at 0.5 cm1 resolution compared (C) to the same bands at 1.0 cm1 resolution.

We have also estimated the rotational constants for the corresponding trifluorosilanes where the small adjustments for the CCl and CSi distances have been made along with the predicted SiF distances obtained from the RHF predicted values. We have found that this level of ab initio calculations with the 6-31G(d) basis set provides SiF distances about 0.020 Å longer than those from the corresponding MP2 calculations where the longer values are relatively close to the experimental values for some substituted trifluorosilanes. The ClCH2SiF3 molecule should provide a good microwave problem where improved structural parameters could be obtained from such a study. It should be noted that the B3LYP/6-311+G(d,p) predicted values for the SiF, CCl and CSi distances are much poorer than those obtained from the corresponding ab initio MP2(full) predicted values. However, for the R-C„C–H molecules the C„C distances are predicted much better with values of 0.010 Å shorter [39,40] which are relatively consistent with experimental values when known. The predicted intensities for the Raman spectra (Figs. 1–4) are remarkably good for all four molecules. There are one or two Raman lines in each of the spectra where there is a quite noticeable difference in the predicted intensities compared to the experimental value. However, in general, these Raman data can be very useful

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for determining vibrational assignments. It is more difficult to assess the quality of the ab initio predicted intensities for infrared spectra of the gas because of the broad bands and particularly for the solid since there can be some crystal effects that change the relative band intensities. Nevertheless, a comparison of the infrared spectra for the four molecules studied with the predicted values indicating considerable utility of these data for aiding vibrational assignments as well as distinguishing some vibrational options. The barrier to internal rotation of the SiH3 rotor has been determined for both ClCH2SiH3 and Cl2CHSiH3. By utilizing the adjusted/ or estimated r0 structural parameters listed in Tables 5 and 6 as well as the frequency for the SiH3 torsional mode, the periodic three-fold barrier, V3 to internal rotation of the SiH3 group can be calculated. The torsional frequency for the SiH3 rotor calculated using the difference band from the SiH stretch is 161.0 cm1 for ClCH2SiH3 and 151.1 cm1 for Cl2CHSiH3. The general equation: V3 = (9/4)Fs is used, where F (cm1) is related to the reduced moment of inertia of the SiH3 top by h/8p2cIr. The dimensionless parameter of the Mathieu equation, s is indirectly determined from the observed frequency. For ClCH2SiH3, the F number has a value of 3.670 cm1 and with a fundamental frequency of 161 cm1 a barrier of 826 cm1 (9.88 kJmol1) is obtained. This value is slightly lower than 892 cm1 (10.67 kJmol1) obtained previously [24] from this laboratory using the F value of 3.48 cm1 at a fundamental observed at 159 cm1. This barrier value is also lower than 927 cm1 (11.09 kJmol1) which was obtained from the torsional splittings observed in the microwave spectrum [36]. We have also calculated the three-fold barrier for the silyl rotor from the ab initio calculations by using the MP2(full)/6-311+G(d,p) basis set and the predicted value for the silyl barrier is 873 cm1 (10.44 kJ mol1). For Cl2CHSiH3, Mathieu’s equation gave a barrier value of 887 cm1 (10.61 kJmol1) from an F number of 3.033 cm1 using the torsional frequency of 151.1 cm1 which is nearly the same as the value obtained from the MP2(full)/6-311+G(d,p) calculation (914 cm1, 10.94 kJmol1). This value is considerably lower than the value obtained previously [24] of 1291 cm1 (15.44 kJmol1) where this difference is primarily due to the higher torsional frequency of 177 cm1 observed in the solid. From a microwave study which was done [41] on methylsilane a barrier of 595 cm1 (7.1 kJmol1) was obtained. This value is significantly lower than the barrier (826 cm1, 9.88 kJmol1) obtained when one chlorine atom is added to the carbon atom. The addition of a second chlorine atom though seems to cause a smaller increase in the barrier value (887 cm1, 10.61 kJmol1). Thus, the chlorine atom has a significant effect on the silyl barrier but not as large as that found in the hydrocarbon analogues. Acknowledgement JRD acknowledges the University of Missouri-Kansas City for a Faculty Research Grant for partial financial support of this research. AME acknowledges the Egyptian Ministry of Higher Education for supporting his research through joint supervision program with UMKC.

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