Molecular structure, force field and vibrational spectra of tetramethoxysilane

Journal of Molecular Structure, 244 (1991) 193-202 Elsevier Science Publishers B.V., Amsterdam

193

MOLECULAR STRUCTURE, FORCE FIELD AND VIBRATIONAL SPECTRA OF TETRAMETHOXYSILANE

I.S. IGNATYEV, A.N. LAZAREV, T.F. TENISHEVA and B.F. SHCHEGOLEV The Institute for Silicate Chemistry, Academy of Sciences 199034, Makarov Quay 2, Leningrad (U.S.S.R.) (Received 18 June 1990)

ABSTRACT IR and Raman spectra of Si ( OCH3)4 and Si (OCD, ), are reported and the peak assignments discussed. The equilibrium molecular geometry calculated by ab initio gradient methods agrees with the structure flattened along the S, axis, established by an electron gas diffraction study. The initial force field model, a composite of the ab initio force fields of Si (OH), and H,SiOCHs molecules, reproduces the experimental spectrum rather well, with the exception of the 1200-1000 cm-’ region. The addition of interaction force constants between non-adjacent SiO and CO stretching coordinates enables a fairly good description of vibrations in this frequency range to be made. Ab initio force constant calculations confirm the existence of these terms in the tetramethoxysilane force field.

INTRODUCTION

The structure of tetramethoxysilane has been investigated by vibrational spectroscopy [ 1,2] and electron diffraction [ 31. In the latter study it was determined that the molecule has S, symmetry, caused by the rotation of methoxy groups around the Si-0 bonds. It was concluded in refs. 2 and 4 that there is free rotation around these bonds, although in an electron diffraction study [ 31 it was assumed that there are no free rotations of methoxy groups and a value of 64” for the dihedral angle between the OSiO and SiOC planes was used in the refinement. Our interest in this molecule is determined largely by the fact that it is the simplest model for a SiOl tetrahedron occurring in a network of partially covalent bonds, such as exists in the quartz structure, that is available for direct experimental investigation. In addition its small size allows the application of ab initio MO methods in its study. Ab initio force fields for the SiOl tetrahedron obtained earlier for the SiOi- ion or Si(OH)4 molecule [5,6] cannot be checked by experimental vibrational data, and thus Si(OCH,), appears to be one of the simplest mole-

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cules containing the Si04 moiety for which theoretical frequencies can be compared with experimental data. EXPERIMENTAL

AND COMPUTATIONAL DETAILS

Raman spectra of Si ( OCH3 )4 and Si ( OCDB)4 in the liquid phase were recorded on a Coderg PH-0 spectrometer with 6328 A He-Ne excitation. IR spectra of thin amorphous films, obtained by vapour condensation on the surface of a stainless steel mirror cooled by liquid nitrogen, were recorded for better resolution of IR bands in several spectral regions. Ab initio calculations were performed with the help of Pulay’s TEXAS proTABLE 1 Description of internal coordinates in Si(OC), fragment No.” 1

2 3 4

Si04(A,)=1/2(r,+r,+r,+r,) SiO,(Fh)=1/2(r,+r,-r,-rr,) SiO,(F~)=1/2(r,-r,+r,-r,) SiO,(F~)=1/2(r,-r,-r,+r,)

5 6

d(E’)=1/J12(2~12+2~34-~13-~Y23-~24-~14)

7

w;)=ll&%,-%) d(G) =1/J%,

-%)

d(F;“) =1/&a,,

--14)

8 9 10-13 14-17

d(E”)=1/2((y,3-~23+~24-~14)

CO (rs, r6, r7, rs)

SO2 (oh, ah, ff37,od

“For numbering of bonds see Fig. 1.

Fig. 1. Configuration of Si(OCH,),

molecule.

195

gram [ 71, with a basis set of 3-3-21G for silicon, 4-21G for carbon and oxygen atoms, and 21G for hydrogen atoms [ 8,9]. Internal local symmetry ( Td) coordinates were used for the SiO, tetrahedron, although its symmetry is lowered in the molecule discussed (Table 1) . Local symmetry coordinates (C,,) were employed for bending vibrations of the 0CH3 group. (For their description see ref. 10. ) Residual forces on internal coordinates were less than 0.004 mdyn, with the exception of torsions around Si-0 bonds; which were about 0.0002 mdyn. DISCUSSION OF RESULTS

In Table 2 the optimized geometry of tetramethoxysilane is compared with its experimental rg structure [ 31 and theoretical structures of H3SiOCH3 [ 121 and Si (OH) 4 molecules optimized using the same basis set. These theoretical TABLE 2 Theoretical optimized F, and experimental molecular parameters rg of Si (OCH3)4 compared with molecular parameters of HsSiOCH, and Si(OH), HaSiOCH,

Bond lengths (A) SiO co CH CHt CHt

1.681 1.438 1.078 1.084 1.084

Bond angles (degrees) OSiO (X2) _ OSiO (X4) SiOC 128.2 OCH 107.4 OCHt 111.2 HCHt 109.1 HtCHt 108.9 _ OSiO/COSi SiOC/OCH Net charges Si 0 C “Ref. 12. bRef. 3.

Si(OCHs),

+ 1.175 -0.872 -0.136

1.640 1.418 1.080

120.6

1

111 108

1.646 1.435 1.078 1.082 1.083

113.7 107.4 131.9 107.4 110.9 109.2 108.9 83.8 5.4

+2.144 -0.926 -0.120

Si(OH),

1.613 1.414

1.643

1.120

115.5 106.0 122.3

114.2 107.2

111 108 64 0

-

88.1 _

+2.066 - 0.964

196

structures are included in this table in order to facilitate the analysis of structural changes in the SiO, moiety of Si (OH), when hydrogens are replaced by methyl groups and changes in the structural parameters of the SiOCH, fragment of H,SiOCH3 under the same H-0CH3 substitution. Theoretical geometries reproduce with sufficient accuracy the changes observed in Si-0 and C-O bond lengths upon H-OCH, substitution in the SiH, group. These changes can be linked to the significant growth of calculated positive net charge on silicon in Si ( OCH3)+ The potential function governing rotation around Si-0 bonds is rather flat, however, the energy minimum was found at a value of the dihedral angle between the 0-Si-0 and Si-O-C planes (@) close to 84” (as in ref. 3 these torsional angles were calculated with respect to the eclipsed periplanar configuration). It should be noted that rotations around Si-0 bonds are strongly correlated with Si-0 bond length and 0-Si-0 angle values. This can be rationalized in terms of oxygen lone-pair interactions, assuming that these interactions may strongly affect equilibrium values of 0-Si-O-C dihedral angles and 0-Si-0 angles in the SiO, tetrahedron. A comparison of SiO, fragment geometry in optimized Si(OCH,), and Si (OH), structures shows very small changes in the geometrical parameters of this group. The similarity of SiO, geometry allows us to assume that the same similarity exists between force fields of the SiO, moiety in these molecules. Thus, since the complete force constants calculation of tetramethoxysilane involves too much computer time, the force field of this molecule was composed in the following way. Ab initio force field of the SiO, fragment in the Si(OH)4 molecule (4-21G basis set) was transferred to the Si (OCH, ), molecule while that of the SiOCH, group was taken from the scaled force field of this group in H3SiOCH3 (421(0* ) G) [lo]. Values of force constants describing rotation around Si-0 and C-O bonds were also transferred from this molecule. Adoption of this force field for Si(OCH,)4 was based on the assumptions that all interactions between SiOCH3 groups are concentrated within the silicon-oxygen tetrahedron and that other interactions between these groups are negligible. Two scale constants, one for SiO stretching coordinates and another for SiO, bending coordinates, were introduced to fit calculated frequencies with the force field constricted as described above. This force field (scale constants were found to be equal to unity) gives a satisfactory description of the 3001000 cm-l and 1200-3000 cm-l frequency ranges, although it failed to yield the correct vibrational shapes in the v, SiOC region. The complex group of bands in the spectra of Si ( 0CH3)* in this region can be identified with v,, SiOC (A, B, E) andp CHB vibrations. (Table 3, Figs. 2a, 3a). The latter can be divided as usual into two groups p’CH3 and p”CHB, according to their orientation relative to the SiOC plane. The out-of-plane

197 TABLE 3 Experimental and calculated frequencies of Si ( 0CH3)4* Raman (liquid)

IR (-1900)

Symmetry

V,.k (cm-‘)

Potential energy distribution (% %)

2979 w dp 2951 s pb 2912 VWdp 2849 vs pb 14781466 sh s dp

2980 m

-4&E

3 x 2999

100 CH

A,B,E 4&E

3x2957 3 x 2909

100 CH 100 CH

ARE

3 x 1492

70 dk, 27 d,

1460 m dp

1455 sh

B,E

2 x 1472 1470 3 x 1460 1212 2 x 1203 2x1185 1184 1118 1111 1091 829 819

72 d,, 26 d :. 77 d,, 22 d:, 95 d& 72 r’, 12 CO, 11 SiOl (A,) 84 r’ ,8 SiOl (Fz ) 94 r” 93 r” 52 CO, 26 SiO, (A,), 20 r’ 46 SiO, (F,), 42 CO 52 CO, 42 SiOl (F,), 6 r’ 52 CO, 40 SiOl (F2) 45 CO, 45 SiO, (F2) _

2957 mb 2914 m 2848 mb 1474 sh 1463 m

1

A

{ A,B,E 1196 m pp

1194 vs

1161 m dp

1163 VW

1115 s p 1094 m dp

-

846 m dp 820 sh m dp 642 vs p 440 w pp? 423 VW?dp 406 w dp 379 sh? pp? 309 w pp? 202mp

1082 vs 1072 vs 843 vs 827 s 814 s 648 w 428 m

A

1B,E -IA,B E A E B E B

623 445

61 SiOl (A,), 38 CO 73 d (F,), 7 d (E)

413 s

411

60 d (F,), 25 SiOC,

312”

321 282 226 172 114

49 d (E), 86 d (E), 96 SiOC, 59 SiOC, 56 SiOC,

_

30 SiOC, 14 d (Fz) 81C0 2 d (E) 19d (F,), 177CO 38 d (E)

“Calculated frequencies (cm-‘) rC0: 114, 107, 104; rSi0: 53, 43,37. bFermi resonance components. “From the gas phase spectrum.

vibrations (p”CH3) do not couple with any other vibration in this region and can easily be identified with bands near 1160 cm-’ (IR and Raman). In-plane rockings in methoxy group-containing molecules usually interact with CO stretching vibrations. These modes can be assigned to IR and Raman bands near 1195 cm-l. The intensity of the IR band can be explained by significant mixing of rocking modes with v,, SiOC. The other component of “resonance” between p’ CH3 and vas SiOC is split into three bands. One is a strong and polarized Raman band at 1115 cm-’ that should be unequivocally ascribed to the A-species vibration, with a significant

Fig. 2. IR absorption spectra of solid films at - 190” of (upper) Si(OCH3), and (lower) Si(OCD3),I between 1500 and 400 cm-‘.

contribution from the SiOC asymmetric stretching mode. Other v,,SiOC modes belonging to E and B species are identified as strong IR bands at 1082-1072 cm-‘. In spectra of the Si ( OCDB), molecule (Figs. 2b, 3b) p CD, frequencies shift to the 900-1000 cm-’ range and Y,, SiOC modes now couple with S, CD3 (Table 4). However, among the various components of vasSiOC, one belonging to the A species (under S, symmetry) is easily identified with a strong and polarized Raman band at 1152 cm-‘. Corresponding vasSiOC modes in E and 23species are ascribed to very strong IR bands at significantly lower frequencies, 1107 and 1094 cm-‘. Thus, for this molecule, as for Si(OCH3)4, a relation exists between the symmetric and asymmetric components of v,, SiOC vibrations, i.e. v,, SiOC (A) > vas SiOC (B, E). This relationship, however, can hardly be obtained in the normal coordinate calculations using the field composed from the force fields of Si(OH), and H$SiOCH,. Any variation of the scale factors for SiO and CO stretching coordinates leads to an inverse relation between the vas SiOC component frequencies. The only way to obtain the proper relationship between vas SiOC frequencies is to let the diagonal SiO stretch force constantsf,, andf,, (A, and F2 under Td symmetry), vary independently and to assume independent variation of off-diagonal terms describing interactions between Al and F2 stretching coordinates of the central fragment with CO stretches (fl,10andf,,,,). (Table 5). In the force field considered in terms of internal coordinates (individual SiO stretches) this corresponds to variation of the fsio-fsio, sio relation obtained in ab initio force field calculations, and to the introduction of an fsio, co term that describes the interaction of SiO and CO groups that have no common atoms. This “long range” interaction contradicts the local character of the proposed force field. The best fit to experimental frequencies was obtained with fsio, co

199

200 TABLE 4 Experimental and calculated frequencies of Si( OCD3)48 Raman (liquid ) 2230 s dp 2135 s pb 2075 vsp” 1152 mp 1118mdp

IR (-190”)

Symmetry

pfdc (cm-‘)

Potential energy distribution (% %)

2241 sh m 2228 m 2139 Wb 2077 & 1148 sh ? -

A&E A,B,E

3x2225 3x2190

99 CD 100 CD

-4&E

3 x 2084

99 CD

A E B E A B A

1147 1129 1119 1115 1108 1102 1072 2x1071

54 CO, 32 SiO, (A,) 50 SiO, (Fa), 29 d,, 18 CO 92 d,, 5 SiO, (F, ) 63 d,, 21 CO, 10 SiO( (F2) 82 d,, 21 CO, 10 SiOl (Fa) 50 Si04 (F2 ) ,46 CO 92 d:, 94 d:,

1107 vs 1099 m dp

1094 vs

1071 s dp

1080 sb 1070 sh

917 s, pp

924 s

907 m dp

904 m

798 m dp 786 sh dp 611 vsp 419 w dp 391 w, dp 365 sh, pp 290 m, dp 184 m, p

801 s 779 s 619 VW 418 w 395 m

I

1

B, E 4&E W

3 x 1058

A B A E E B A B E

928 918 908 907 905 793 783 591 435 394

B A A E B

304 275 203 156 101

1

2x

100 d: 84 r’, 1OCO 90 F' ,8 Co 98 r” 98 r” 98r” 45 CO, 34 SiO, (F,), 9 r’ 40 CO, 39 SiO, (F,), 8 r’ 57 SiO, (A,), 38 CO 74 d (F,), 7 d (E), 10 SiOC 60 d (F,), 21 SiOC 49 d (E), 28 SiOC, 12 d (Fz) 87d (E),6rCO 96 SiOC 61 SiOC, 18 d (F2) 52 SIOC, 37 d (E)

“Calculated frequencies (cm -‘) rC0: 90,80, 76; rSi0: 43,36,32, bFermi resonance components.

70.13lmdynA-l (adjacentc-OandSi-Obonds) andfsio,co=-0.138mdyn A-’ (with no common atoms). To check whether such interactions between non-adjacent bonds really exist in the force field of Si ( OCHB) 4,two lines of force constant matrix ( SiO stretching coordinates 1 and 2 (Table 1) ) were calculated theoretically. Computed 1’&&” %V&WZG&Wz7z&$&F&z I:- 4 .&57?&V&V. valuesd+slU,cQ cQW&.%~%!z&WXr& A- ’ ) . However, its absolute value is twice as small as the values obtained in our tit of theoretical frequencies to experimental data. It should be noted that underestimation of the interaction force constant between adjacent C-O bonds or Si-0 and C-O bonds in COC or SiOC bridges is, as stated in ref. 10, the

201 TABLE 5 Force constants of Si(OC), skeleton (mdyn A-‘, mdyn, mdyn A)” No.

Force constant

No.

Force constant

7.089

F

- 0.339

6.657 0.642 0.906 1.238 1.168 4.384 0.430 0.014 0.190

F 1. 10 F 2. 10 F 2, 12 F 1, 14 F 2, 14 F 2.16 F IO, 14

-0.402 -0.141 0.134 -0.134 0.141 0.141 -0.141 0.178

F::,7Fu

“For numbering of coordinates see Table 1.

most serious drawback of ab initio force field calculations which use split valence sets lacking polarization d-functions on oxygen atoms. The influence of d-functions on the value of non-adjacent SiO-CO interactions was not studied for tetramethoxysilane, but it may be supposed that this kind of force constant is also sensitive to their absence. Significant bond-bond interaction force constants for non-adjacent bonds have been met in quantum chemical calculations of some molecules, e.g. aromatic compounds, where their existence could be correlated with peculiarities of the electronic sub-system [ 111. The physical origin of non-adjacent SiOCO interaction in tetramethoxysilane deserves special investigation, with particular attention given to the probable role of interactions between oxygen lone-pairs along tetrahedral edges. It is worth noting that the introduction of a similar interaction term into an a-quartz force field considerably improves the results of calculations of its phonon spectrum and dynamical properties as was shown by A.P. Mirgorodsky.

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202 9 10 11 12

M.S. Gordon, J.S. Binkley, J.A. Pople, W.J. Pietrow and W.J. Hehre, J. Am. Chem. Sot., 104 (1982) 2797. I.S. Ignatyev, J. Mol. Struct., 172 (1988) 139. G. Fogarasi and P. Pulay, in J.A. Durig (Ed.) Vibrational Spectra and Structure, Vol. 14, Elsevier, Amsterdam, 1985, p. 125. C. Glidewell, D.W.H. Bankin, A.G. Bobiette, G.M. Sheldrick, B. Beagley and J.M. Freeman, J. Mol. Struct., 5 (1970) 417.