Spectra and structure of silicon containing compounds

of holecular o Eisevier Scientific

Journal

Structure, 49 (1978) Publishing Company,

l-6 Amsterdam

-

Printed in The Netherlands

SPECTRA AND STRUCTURE OF SILICON CONTAINING CCMPOUNDS Part IX. Microwave spectrum of methoxytrifluorosilane

J. D. ODOM, Department (Received

E. J. STAMPF, of Chemistry,

Y. S. LI and J. R. DURIG University

of South

Carolina, Columbia,

S.C. 29208

(U.S.A.)

25 January 1978)

ABSTRACT The microwave spectra of CH,OSiF, and CD,OSiF, have been recorded in the region 18.5-40.0 GHz. Only R-branch a-type transitions were observed. The rotational constants have been found to be: B = 2265.68 + 0.07 and C = 2248.71 f 0.07 MHz for CH,OSiF, and B = 2051.17 + 0.06 and C = 2034.61 f 0.06 MHz for CD,OSiF,. These data were found to be consistent with r(Si-G) = 1.56 2 0.01 A and LCOSi = 132 it 1.5”. The Si-G bond distance is somewhat shorter than the corresponding bond in similar compounds. INTRODUCTION

The low frequency Raman spectra of gaseous disiloxane and disiloxane-d6 have recently been reported [l] and a series of Q-branches for each molec-ule were observed which were attributed to the “double jumps” of the anharmonic skeletal bending mode. These Q-branches were assigned on the basis of a double minimum potential function which gave a barrier to linearity of 112 cm-’ for the “light” molecule with an average equilibrium skeletal angle of 149 f 2”. This angle is about 5” larger than the angle reported for this molecule from an electron diffraction study [2]. The good agreement must be considered fortuitous since at room temperature more than 80% of the molecules are in the linear configuration. An electron diffraction study has recently been reported [3] for methoxytrifluorosilane where the LSiOC was reported to have a value of 131.4 f 3.2”. Because of this relatively large angle with the possibility of a low frequency, large amplitude bending motion, it was felt that shrinkage could have had a pronounced effect on the electron diffraction data. Additionally the Si-0, C-O and Si-F distances contributed to a single peak in the radial distribution curve so that it was difficult to refine these three distances and their associated amplitudes simultaneously. Thus, in the electron dYfi=action refinements the Si-0 distance was fixed at a value of 1.58 A. Therefore, in order to provide additional structural information we have undertaken a microwave study of methoxytrifluorosilane and methoxytrifluorosilane-ds. The results of this study are reported herein.

EXI'ERfMEIWAL All preparative work was carried out in a conventional high-vacuum system employing gre-aseless stopcocks or under a dry nitrogen atmosphere. Methoxytrichlorosilane was prepared from SK& (Alfa) and methanol according to the procedure of Airey and Sheldrick 141. In a similar manner, methoxyt~c~oros&me-d3 was prepared from SiC14 and CD30H (Columbia Organ&) [ 31. Several different synthetic routes to methoxytrifluorosilane were attempted. Direct methanolysis of SiF4 [5] produces small amounts of the expected product CH30SiF3. Methanol (53 mmol) and SiF4 (60 mmol) were condensed into a 3-l glass reaction vessel. The materials were a.llowed to warm to room temperature during which time a cloud of white vapor evolved. The materials were then frozen to -196°C and again allowed to warm. This freeze-melt process was repeated three times and the contents of the reaction vessel were separated by means of cold-column vacuum fractionation 161. The yield of CH,OSiF, was 2.5 mmol or 4.2% based on the starting quantity of SiF4. Greater quantities of CH30SiF3 were prepared by direct fluorination of CH30SiC13 with freshly sublimed SbF3. Fluorination proceeded extremely rapidly at temperatures of -20 to -25°C. Volatile products were separated on the cold-column and were identified by their IR spectra [4]. Two major products, CHsOSiF3 and SiF4, were obtained in approximately equal amounts. The microwave spectra were investigated using a Hewlett-Packard Model 8460A MRR spectrometer in the R-band frequency region from 26.5-40.0 GHz. The Stark cell was modulated with a square wave of frequency 33.3 kHz. All frequency measurements were taken with the Stark cell packed with Dry Ice and are believed to be accurate’to +0.2 MHz. RESULTS

A preliminary calculation with assumed structural parameters for methoxytrifluorosilane showed that the symmetry plane is formed by the principal axes a and c. One may therefore expect both a- and c-type transitions. This calculation also indicated that the molecule is very close to a prolate symmetric rotor with the principal axis cz making a very small angle to the SiO and the CO bonds. If the CO, SiF3 and SiO bond moments are making the major contribution to the resultant dipole moment of this molecule, one may expect predominately a-type transitions for methoxytrifluorosilane. The observed low _esolution spectra of C;-130SiF3 and CD30SiF3 are shown in Fig. 1-B and l-A, respectively. These spectra confirmed our previously stated expectations. The two most intense broad bands were assigned f’rom their spacing as the 8 + 7 and 7 -+ 6 a-type transitions. Prom this assignment, the 6 +- 5 a-type transition was also located; it was of moderate intensity. Besides these three main bands there are other sharp lines with comparable intensity with respect to the assigned bands. Some of these lines were identified as due to the rotational transitions of methanol which probably appeared as one of the

3

I

I

I

I

I

I

!

I

38.0

30.0

I

(GHZ) FRE::NCY Fig. 1. The microwave

spectrum

of (A) CD,OSiF,

(B) CH,OSiF,

from 26.5-40.0

GHz.

decomposed products from the sample in the wavegui
Because of the difficulty in observing the c-type transition for methoxytrifluorosilane, it is suggested that the dipole moment component along the c-axis is very smah in comparison with pa. If the dipole moment (1.31 D) of dimethylether [7 ] , is treated as a vector sum of two bond moments in the direction of the C-O bonds, the bond moment p(C-O), may be obtained from the relationship, 1.31 = 2p(C-O) cos 1,12 (111”43’), to be 1.67 D, where 111” 43’ is the COC angle in dimethylether [ 71. Similarly, the SiO bond moment, p(SiO), may be calculated from the dipole moment (0.24 D) [S] and the SiOSi angle (149”) [l] in disiloxane to be 0.39 D. As an approximation, the dipole moments of methoxytllfluorosilane are treated as the

4 TABLE

1

Rotational transitions (MHz) of Methoxytrifluorosilane Transition

6 16 + 6 IS + 6 2.I + 7 i, + 7 16 + 7 25 + 8 18 8 I? + 8 26 9 19 + 9 18 + 9 2, +

CH,OSiF;

5,s 51, 52, 61, 61s

62, 71, 71, 72, 81, S,, 82,

Walculated

in the ground vibrational state

CD,OSiF,

u(obs)

u(calc)a

27035.2 27138.2 27086.0 31541.4 31659.8 31599.8 36048.0 36183.2

27035.5 27137.3 27085.9 31541.6 31660.3 31599.9 36048.6 36183.3

from the rotational

constants

u(0b.s.)

u(calc.)=

28542.2 28657.0 28603.0 32619.3 32751.5 32690.9 36695.2 36845.2 36777.7

28541.8 28657.6 28603.2 32618.9 32751.3 32690.4 36695.9 36844.9 36777.9

listed in Table

2

vector sum of p(CO), p(Si0) and the moment equal to that of trifluorosilane, 1.27 D [9 ] , acting in the opposite direction to the r.L(SiO).By adapting the structural parameters, r(C0) = 1.40 BL,r(SiF) = 1.559 A, LFSiO = 110.7” and LSiOC = 131.9”, as listed in Table 3, the dipole moment components are calculated to be pa = 1.67 D and pc = 0.14 D. Thu+ the small value of ~1, in comparison to p, suggests weak c-type transitions. The molecular structure of methoxytrifluorosilane has been determined by the electron diffraction method [3]. However, the unresclved peak in the radial distribution curve as contributed by the C-O, SiO and Si-F distances makes it difficult to refine these three bond distances simultaneously. Also, the similar distances between Si and C, F and 0, as well as F and F contribute to another overlapped peak. These similar distances made it necessary to make TABLE

2

Rotational

A B C fa Ib

I,

constants

(MHz)

and moments

of inertia (PA’)=

CH,OSiF,

CD,OSiF,

4092b 2265.68 + 0.07 2248.71 f 0.07 123.5b 223.064 224.747

3996b 2051.17 +_ 0.06 2034.61 + 0.06 126.5b 246.392 248.397

aConversion factor: with r(SiF) = 1.559

505391 ,%

MHz UR ’ _ bCalculated

of methoxytrifluorosilane

from the structure

listed in Table 3

5

TABLE3 Structural ca.lculationsofmethoxytrifluorosilane(distances in &angles indegrees) Assumed

Calculateda

r(CO)

r(SW

LFS~O

r(CH)

LHCO

r(Si0)

LCOSi

1.382 1.392 1.400 1.410 1.420 1.382 1.392 1.400 1.410 1.420

1.559

110.7 110.7 110.7 110.7 110.7 110.7 110.7 110.7 110.7 110.7

1.097 1.097 1.097 1.097 1.097 1.097 1.097 1.097 1.097 1.097

109.6 109.6 109.6 109.6 109.6 109.6 109.6 109.6 109.6 109.6

1.565 1.564 1.562 1.560 1.558 1.554 1.552 1.551 1.549 1.548

133.0 132.3 131.9 131.3 130.7 133.0 132.4 131.9 131.2 130.6

1.559 1.559

1.559 1.559 1.570 1.570 1.570 1.570 1.570

ar(Si-0) =1.56 +_0.01,~COSi= 132 + 1.5'coversthe range ofvalues.

some assumptions in order to refine the structural parameters [ 33. In the present microwave study, only limited information can be obtained for determining the molecular structure for methoxytrifluorosilane, We have fixed the FSiO, HCO angles and the C-H distance and made a series of assumptions for the C-O and Si-F bond lengths in order to obtain a least-squares fit of the structural parameters to the observed rotational constants B and C for CH3QSiF3 and CD,OSiF,, respectively. The results of the calculations are summarized in Table 3. The assumed C-O distances have covered the value (1.39 A) obtained from the microwave study for dimethylether [7]. Since the Si-F bond was one of the best parameters determined by the electron diffraction study [3], it served as the basis for our assumption on r(Si-F) in these structural calculations. There is no way of determining the best set from our assumption. Fortunately, it is found from Table 3 that the r(Si-0) and LCOSi do not change appreciably with the various assumptions for the r(C0) and r(SiF) distances. Within experimental error, the COSi angle (m-132”) is essentially the same as the value (131.4 + 3.2”) obtained from the diffraction method [3]. Howeva, the SiO distance (‘~1.56 A) is significantly smaller than the vaiue (1.580 A) reported in the previous structural investigation [ 3 1. In comparison to the corresponding values in disiloxane (1.634 A) [2] and in perfluorodisiloxane (1.58 A) [lo] our v&e also appears to be rather small. Such a small Si-0 distance may be interpreted in at least two ways. A charge induction effect caused by the attached electronegative fluorine atoms on only one side of the molecule could contribute to a short Si--O bond distance. Another effect which must be considered is the possibility of n-bonding between a lone pair on oxygen and the empty d-orbit& on silicon. The electronegative fluorine atoms would tend to contract the silicon d-orbit& and the increased LCOSi would lend itself to effective overlap of the proper orbitals, thus increasing the SiO bond order.

Methoxytrifluorosilane is expected to have a low frequency SiF3 torsional anode but no vibrationally excited states could be identif%d. Additionally no splitting caused by the Cl& internal rotation was observed. Thus, no information could be obtained on the barriers to internal rotation in this molecule.

The authors gratefully acknowledge the financial support given to this study by the National Science Foundation by Grant CHE76-23542. REFERENCES

1 J. FLDurig, M. J. Flanagan and V. F. K&x&sky,

J. Chem. Phyr., 66 (1977) 2775. 2 A. Aimenningen, 0. Bastiansen, V. Ewing, K. Hedberg and M. Traetteberg, Acta Chem. Stand., 17 (1963) 2455. 3 W. Airey, C. GlideweU, A. G. Robiette and G. M. Sheldrick, J. MoL Strut%, 8 (1971) 413. 4 W. Airey and G. M. Sheldrick, J. Inorg. Nucl. Chem., 32 (1970) 1827. 5 J. L Margrave, K. G. Sharp and P. W. Wilson, Inorg. Nucl. Chem. Lett., 5 (1969) 995. 6 D. F. Sbriver, ‘“The Manipulation of Air-Sensitive Compounds”, McGraw-Hill, New York, N.Y., 1962, p. 91. 7 U. Blukis, P. H. Kasai and R. J. Myers, J. Chem. Phys., 48 (1963) 2753. 8 R. Varma, A. G. MacDiarmid and J. G. Miller, Inorg. Chem., 3 (1964) 1754. 9 S. N. Ghosh, R Trambarulo and W. Gordy, J. Chem. Phys., 21 (1953) 308. 10 W. Airey, C. Glideweii, D. W. H. Rankin, A. G. Robiette, G. M. Sheldrick and D. W. J. Cruiclcshank, Trans. Faraday Sot., 66 (1970) 551.