High resolution infrared spectra of methyl silane

Spectrochimica Acta, 1962. Vol. 18, pp. 115 to 122. Pergamon Press Ltd.

Printed in Northern Ireland

High resolution infrared spectra of methyl silane* M. RANDIC! Institute “Rudjer BoSkovi6”, Zagreb, Yugoslavia (Received

4 September

1961)

Abstract-High resolution infrared spectra of gaseous methyl silane were obtained and a complete assignment of vibrational frequencies has been made. The methyl rocking and silyl deformation bends were distinguished with the help of the Coriolis zeta constants. Normal coordinates and zeta constants were calculated on the basis of a simple diagonal potential function.

Introduction INFRARED spectra of methyl silane have been previously studied under NaCl prism resolution by KAYE and TANNENBAUM [l] in connexion with the investigation of a series of related alkylsilanas. Few bands, characteristic of the silyl group, were assigned. KOVALOV has recently calculated potential constants, using the experimental data of KAYE and TANNENBAUM on the series (CH,),SiH,_, and the results of this work [2]. High-resolution spectra of CH,SiH, have not been studied Here we report the results of a high resolution study, concentrating before. mainly on the use of Coriolis coupling constants to confirm the vibrational assignment.

Experimental The sample of methyl silane was supplied by Dr. EBSWORTH (Cambridge University). It was prepared by reducing trichloromethyl silane with lithium aluminium hydride in diethyl ether solution. The high-resolution grating instrument built in the Colloid Science Department, Cambridge, some years ago and improved lately [3] was used for obtaining spectra. It was a Pfund-type spectrometer with 12-cm diameter paraboloidal mirrors and with an aperture of f/5. Lead sulphide and lead telluride photoconductive cells were used as detector above 2000 cm-l, and a Hilger-Schwartz thermocouple for lower frequencies. A set of gratings was available: NPL replicas with 2400 lines/in. blased at 10 y, 15,000 lines/in., 4800 lines/in. and 720 lines/in. gratings. Some spectra were recorded with the instrument at the Kings College, London,

High resolutionspectra The band at 2930 cm-l constants are

is the only parallel

band resolved

and the rotational

B = 0.3649cm-l B-B" = 0*0003cm-l * Submitted to the European Congress on Molecular Spectroscopy at Bologna, September 1959. [l] S. KAYE snd S. TANNENBAUM, J. Org. Chem. 18, 1750 (1953). [2] I. F. KOVALOV, Zhur. Opt. Spekt. 4,560 (1958). [3] D. J. MARAIS. Thesis, University of Cambridge (1968). 116

I

2880

I

I I

2860

I

I

2980

I

2970

Fig. 2. Fundamental perpendicular band at 2983 cm-l, deg8nerat8 CH stretching.

I

2990

I

3000

I

Fig. 1. Fundamental parallel band at 2927 cm-l, symmetrical CH stretching.

2870

3010

I

2960

I

I

2850

cd

cm-’

s

High resolution infrared spectra of methyl silane

117

The band has double Q-branch (Fig. 1) separated by 1.24 cm-l. A possible origin of the observed doubling of the band is a coupling with the torsional vibration brought through the interaction of vibration and rotation. Coriolis forces will mix the two modes, and the resulting mode will have a torsional component, which, being efficient in tunnelling the potential barrier hindering internal rotation, will result in splitting of the band [4]. The corresponding doubling of parallel SiH stretching band could not be clearly detected, due to severe overlapping of the perpendicular component of the SiH stretching vibration. All perpendicular bands have been resolved, except the CH, deformation band which was obscured by strong atmospheric water absorption. In addition a combination band at 1070 cm-l was resolved. However, the assignment of rotational Q sub-branches was possible for the following bands only: 2980 cm-l, 2170 cm-l and 870 cm-l. In all these cases the alternation of intensity of Qbranches strong-weak-weak, characteristic of molecules with three-fold rotational axes, was observed, though the alternation was not pronounced. The rotational structures of these bands are represented by the following quadratic expressions: Q1< = 2983.0 + 2.79OK -

0.008K2

Qk- = 2167.14 + 2.970K - 0.0059K2

Qli = 870.30 + 1.68K + 0.004K2 Other perpendicular bands wore resolved partly. Thus in the 1070 cm-l region fifteen widely spaced Q-branches were recorded, having an average 4.90 cm-l spacing. At the 980 cm-l region ten Q-branches were observed, with the mean spacing of 3.85 cm-l and a few Q-branches of the 540 cm-i band were resolved giving the average spacing of 2.1 cm-l. From the observed splittings of Q-branches and known geometry of the molecule [5] Coriolis zeta constants of all bands (except the C’H, deformation) were obtained (see Table 4). The assignment of all bands is straightforward, the only difficulty is presented with the absorption in the 10 ,U region, where a strong parallel band at 940 cm-l overlaps with absorptions at 1070 cm-l and 980 cm-l, and where the SiH, deformation is expected. The centres of these bands are difficult to locate. From the spectra of disilane and silyl halides one expects the silyl deformation at about 950 crnr. The value 980 cm-r taken here is slightly too high. The spacing of these bands agree with the assignment. Coriolis constants for the two bands are -0.52 and -0.24 respectively, the second value being close to -Q, as found for other methyl and silyl deformation modes. The first band is therefore the overtone of the 540 cm-l SiH, rocking mode, this assignment being supported by zeta constants of the 540 cm-l and 1070 cm-l band, which should satisfy [6] ((2~) = -25(v). Fundamental frequencies with the assignments are listed in Table 1.

Normal co-ordinate calculations In the following we consider the degenerate vibrations only, and we are mainly interested in the calculations of Coriolis zeta constants from normal co-ordinates. [4] M. RAND& Thesis, University of Cambridge (1958). [6] R. W. KILB and L. PIERCE, J. Chem. Phys. 27, 108 (1957). [6] D. R. J. BOYD and H. C. LONCXJET-HIQCXNS, Proc. Roy. Sot. (Loladon)A 213, 55 (1952).

118

M. RANDIC

I

I

I

1100

1080

1060

I

1040 cm'

Fig. 3. Perpendicular overtone of the 540 cm-l silyl rooking in 10 p region.

. I

I

I

1010

1000

990

I

980 cm’

Fig. 4. Part of the fundamental silyl deformation in the region between 1010 and 970 cm-*.

I

I

I

I

890

880

870

860

cnf'

Fig. 5. Fundamental perpendicular band of 870 cm-l, degenerate methyl rooking.

High resolution infrared spectra of methyl silane

119

There are very few calculations of zeta constants, though the method of MEAL and POLO [7] is a aimple extension of GF calculations. The potential assumed is a simple valence potential function with diagonal of ethane [S, 91 and a more recent force constants only. Earlier investigations study of the methyl halides [lo] have shown that E-class frequencies (E, in ethane) Table 1. Fundamental frequencies of methyl silane A class CH SiH CH, SiH, CSi

stretching stretching deformation deformation stretching

~ -,

I

/

cm-l 2927 2170 1260 940 700

E class CH 8iH CH, SiH, CH, SiH,

stretching stretching deformation deformation rocking rocking

2983 2167 1450 980 870 540

can be accounted for by use of a simple diagonal potential. The G matrix was set up by the standard method [ll] (for their expression see Ref. [12]) and the following parameters have been used: /LH = 0.9920

C-H

1.093 A

,I+ = 0.0833

Si-H

1.495 A

f&i = 0.0357

C-Si

1.8668 A

All angles are assumed tetrahedral. Methyl and silyl groups have some characteristic properties, independent of the surrounding; therefore to reduce calculations the groups were considered separately. This was accomplished by the “splitting off” method [ll] usually applied to high frequencies: The reduced G*-matrices are G&I, = Gcn, -

G’(G&-l

G’

G&R, = GSiH, -

G’(Gc,s)-l

G’

Force constants were calculated for the deformation and rocking modes to the nearest fO.005 (in lo5 dyn/cm), the agreement between calculated and experimental frequencies being better than &5 cm-l. The diagonal force constants used, [7] J. H. MEAL and S. R. POLO, J. Chem. Phys. 24, 1119 and 1126 (1956). [S] J. B. HOWARD, J. Chem. Phys. 5, 442 (1937). 191F. STITT. J. Chem. Phws. 7. 297 (19391. [lo] W. T. K&a, I. M. MI&S and B.‘CRAGFORD, Jr., J. Chews. Phys. 2’7, 455 (1957). [Ill E. B. Wr~soa,J. Chem. Phys. 9, 76 (1941). [12] M. RANDI? and I. M. MILLS, J. Chem. Phys. 81, 1681 (1959). c-1

M. RANDIC

120

the calculated and observed frequencies and the normal co-ordinates are given in Table 2. In the above approximation each force constant is characterizing effectively one frequency only. A change of any of the force constants by 0.005 shifts Table 2. Methyl

and silyl parts of methyl silane. Force constants, freauencies and normal co-ordinates. CH,-Si

calculated and observed

part of CH,SiH,

Frequencies Force constants

0.12 SiH,--C

-0.21

part of CH,SiH,

Frequencies Force constants

the corresponding frequency by approximately 10 cm-l, while other frequencies are affected by less than 1 cm-l. From the calculated normal co-ordinates we see that CH and SiH stretching modes are almost “pure”, but there is a mixing between deformation and rocking modes, the dominant component contributing about 80 per cent to the resulting vibrational mode. To investigate the coupling of the methyl and silyl groups, and estimate the limits of the approximate separation of CH, and SiH, modes, the CH stretching and SiH stretching frequencies were split off. The force constants used, calculated and observed frequencies, and the normal co-ordinates obtained are shown in Table 3. From the normal coordinates obtained we see that the deformation modes Q2 and Q5 are still localized on the methyl and silyl groups respectively, and are therefore characteristic of these groups. However, the rocking modes are coupled to some extent with the deformations of both groups and are not localized in smaller parts of the molecule. These results are consistent with the experimental data, which proved the rocking frequencies to depend to some extent on the surroundings, while deformation modes are to a high degree independent on their environment. From the known normal co-ordinates one can easily calculate Coriolis zeta constants using the relation [7] c= where

L-1

L-l represents normal co-ordinates

CL”-1

and C is a matrix, the elements of which

121

High resolution infrared spectra of methyl silane Table 3. Methyl

silane force constants, calculated and observed frequencies and normal coordinates after high frequencies are split off Frequencies

Table 4. Coriolis zeta constants (1) for separate methyl and silyl groups, (2) when high frequencies are split, (3) the average values and (4) experimental values

(2)

(1) Q1 CH QZ CH, Q3 CH, Q4 SiH, Q6 SiH, &s SiH

stretching deformation rocking rocking deformation stretching

0.14 -0.38 0.31 0.24 -0.29 0.06

1

0 -0.19 0.35 0.32 -0.21 0

-8 Sum rule

0.08

(3) Mean

(4) Observed

0.07 -0.29 0.33 0.28 -0.25 0.03

0.06 (-0.31)* 0.35 0.24 -0.24 0.00

-~-

1

0.27

/

I

i

0.17

j

I

I

* Calculated from the theoretical value 0.10 of the 5 sum.

I

I

I

540

I

520

I

500 cm"

Fig. 6. Part of fundamental silyl rocking oentred at 540 cm-‘.

Recorded by Perkin-Elmer

21 with KBr prism.

can be determined from the known molecular geometry (for their expression see Ref. [13]). The zeta constants are sensitive to the form of the normal co-ordinates. In Table 4 the calculated zetas are given for the two approximations discussed: (1) separate CH, and SiH, groups and (2) splits off high CH and SiH frequencies. The experimental value of zeta for the methyl deformation mode was estimated from the zeta sum rule. Calculated mean values of zetas are in good agreement with the observed values, the agreement being even better than might be expect&d. [13] M. RAN&.

J. Chem. Phys.

38, (1962).

To be published.

122

M. RANDIC

However, although a simplified force field was used, it is felt that such approximate calculations are justified at present, since more elaborate normal co-ordinate treatments are not sufficiently reliable at the moment. AchowZedgement8-This work is a part of a thesis [4], submitted to the University of Cambridge, England, where more details of methyl silane high resolution spectra are given. I wish to thank Dr. N. SHEPPARDfor the supervision of the work. I also thank Dr. D. J. M~~nrs (Steelenbosch, South Africa) for discussions and help in the work, Prof. W. C. PRICE, Dr. G. R. WILKINSON and Dr. H. MOULD (King’s College, London) for use and help with measurements of some of the spectra on their instrument. Finally I would like to thank the Institute Rudjer BolkoviE, Zagreb, for the scholarship to study abroad and especially Prof. I. Supek for his encouragement.