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PHYSICS

I. V. SHEVCHENKO, I. F. KOVALEV, M. G. VORONKOV, E. Ya. LUKEVITS

PRINCIPAL SPECTRAL PARAMETERS OF THE RAMAN-SCATTERING LINES OF TETRAMETHYLSILANE AND METHYLCHLOROSILANES

(Presented by Academician I. V. Obreimov, May 31, 1968)

By the photoelectric method we have studied the Raman spectra of the compounds \((\mathrm{CH}_3)_4\mathrm{Si}\), \((\mathrm{CH}_3)_3\mathrm{SiCl}\), \((\mathrm{CH}_3)_2\mathrm{SiCl}_2\), and \(\mathrm{CH}_3\mathrm{SiCl}_3\). The fundamental frequencies, integral and peak intensities, half-widths, and degrees of depolarization of the lines were measured. In the scales \(5b'^2 + 7g'^2\) and \(5b'^2 + 13g'^2\), corrected relative scattering coefficients (standard intensities) were calculated. Values of the invariants of the tensors of polarizability derivatives were obtained. Taking into account the local-field correction, calculated on the basis of refractive-index dispersion data, the absolute cross sections of spontaneous Raman scattering of light were calculated for the methylchlorosilanes in the case of the exciting mercury line 4358 Å and the ruby-laser light 6943 Å. Measurements of the spectral characteristics were carried out in the liquid phase at a temperature of 25–30°. The 802 cm\(^{-1}\) line of cyclohexane was taken as the standard. The procedure of our investigation has been described previously \((^1)\). The results are given in Tables 1–3.

The interpretation of the normal-vibration frequencies (Table 1) is based on calculations of the potential functions of molecules of this homologous series \((^{2-4})\)*. Difficulties are associated with the assignment of the 700 and 761 cm\(^{-1}\) lines in the spectrum of trimethylchlorosilane. Which of them belongs to the antisymmetric vibration \(\nu_{\mathrm{as}}(\mathrm{Si}—\mathrm{C})\)? The possible ambiguity in assigning these lines is discussed in \((^5)\). Data on integral intensities in the Raman spectra do not provide complete clarity in answering the question posed. Both lines are depolarized and weak in Raman scattering. Studies in infrared absorption show that, in the 600–765 cm\(^{-1}\) region in the gas phase, three bands appear: 637, 699, and 764 cm\(^{-1}\). The absolute intensities of these bands are respectively 120, 70, and \(170 \cdot 10^{-9}\) cm\(^2\)/molecule·sec. The 637 cm\(^{-1}\) band undoubtedly belongs to the symmetric stretching of the Si—C bond. Of the two remaining bands, in accordance with the selection rules, the vibration \(\nu_{\mathrm{as}}(\mathrm{Si}—\mathrm{C})\) should be assigned to 764 cm\(^{-1}\). It has a greater intensity than 637 cm\(^{-1}\). The 699 cm\(^{-1}\) band is almost 2 times weaker and, apparently, is composite.

The principal fraction of the scattered light in the compounds studied (70–90%) is due to vibrations of the methyl groups, with 60–80% due to stretching vibrations of the C—H bonds. Replacement in tetramethylsilane of \(\mathrm{CH}_3\) groups by chlorine atoms strongly affects the parameters of the stretching vibrations of Si—C and even, in part, the properties of C—H. The lines \(\nu_s(\mathrm{Si}—\mathrm{C})\), with accumulation of chlorine atoms at silicon, increase in frequency from \(\sim 600\) to 760 cm\(^{-1}\). Their intensity \(S\) systematically decreases and, on going from \((\mathrm{CH}_3)_4\mathrm{Si}\) to \(\mathrm{CH}_3\mathrm{SiCl}_3\), decreases by a factor of 7. The force coefficient \(K_Q(\mathrm{Si}—\mathrm{C})\) increases from 4.62 to \(5.29 \cdot 10^5\) cm\(^{-2}\). The observed facts are explained by a great—

* In \((^{2-4})\) references are given to other works devoted to spectroscopic studies of the compounds \((\mathrm{CH}_3)_n\mathrm{SiCl}_{4-n}\).

Table 1

Main parameters of the Raman scattering lines of tetramethylsilane and methylchlorosilanes

… by the strong electron-attracting action of the chlorine atoms. In the presence of these atoms, the spectroscopic properties of the Si—C bond change; it becomes stronger. The \(\nu_s(\mathrm{Si—C})\) line in tetramethylsilane is the narrowest. Its half-width is an order of magnitude smaller than that of the other lines. On going to methylchlorosilanes this line broadens. In methyltrichlorosilane its half-width is 4 times greater than in tetramethylsilane. In the direction considered, there also increases the degree—

Table 2

Dispersion of the refractive index of methylchlorosilanes

Table 3

Absolute differential cross sections of Raman scattering of light per molecule, in units of solid angle, for the Stokes lines of methylchlorosilanes (in 10⁻³⁰ cm²)

Notes. \((d\sigma/d\Omega)_{\parallel}\) is the cross section for the case of parallel directions of the polarization vectors of the incident and scattered radiation (wavelength 4358 Å) and scattered radiation; \(d\sigma/d\Omega\) is the total cross section for \(\parallel\) and \(\perp\) polarizations of the incident and scattered radiation. \(d\sigma_T/d\Omega\) is the full differential cross section for the case of exciting natural light at 4358 Å. \((d\sigma/d\Omega)_{\parallel}^{(r)}\) is the scattering cross section for exciting light of a ruby laser, 14,400 cm⁻¹.

* In calculating the cross sections for these lines, the degeneracy factor was not taken into account.

the degree of depolarization of the corresponding wave. The symmetry of the polarizability tensor departs increasingly from spherical.

The intensities of the symmetric and antisymmetric stretching vibrations \(\nu(C—H)\) decrease as the number of methyl groups in the molecule decreases. At the same time, complete additivity is not observed when these quantities are added. As the number of chlorine atoms at silicon increases, the fraction of scattering attributable to each equivalent CH₃ group increases. In work (⁶), using infrared absorption spectra as an example, it was shown that in the general case the dependence of intensities on the number of identical groups in a molecule is very difficult to determine. The influence of chlorine atoms is also manifested in measuring the vibrational frequencies \(\nu(C—H)\). The frequency of the antisymmetric vibration \(\nu_{as}(C—H)\) increases in the considered series of molecules by more than 20 cm⁻¹.

Characteristic of methylchlorosilanes are the stretching vibrations $\nu(\mathrm{Si—Cl})$. The symmetric lines lie in the region 450–470 cm$^{-1}$. They are intense and almost completely polarized. The antisymmetric lines are strongly depolarized and are $\sim 100$ cm$^{-1}$ higher in frequency. The course of the changes in the parameters of the $\nu(\mathrm{Si—Cl})$ lines is also connected with the mutual influence of the corresponding vibrations.

Table 1 gives the dispersion of the refractive index of methylchlorosilanes*. The refractive index is higher for the compound containing the larger number of chlorine atoms. Methylchlorosilanes are characterized by mean dispersion; the dispersion coefficient for them is the same and is equal to 50. With an increase in temperature by 10°, the refractive index decreases by approximately 0.006.

Table 2 presents the absolute differential cross sections of spontaneous Raman scattering of light. The effective-field correction $L$ ($^{7}$) takes values from 2.9 to 3.2. In calculating the cross sections, a transition was made from the 802 cm$^{-1}$ line of cyclohexane to the 459 cm$^{-1}$ line of CCl$_4$. For the latter, the multiplier $5b^{\prime 2} + 7g^{\prime 2}$ was taken to be $13 \cdot 10^{-8}$ cm$^4$/g ($^{8}$). In calculating $(d\sigma/d\Omega)_{\parallel}^{(r)}$, the dependence of the scattering coefficient $S$ on the frequency of the exciting light was neglected.

If the polarization direction of the incident wave parallel to the scattered wave is considered, then the waves $\nu_s(\mathrm{Si—Cl})$ and $\nu_s(\mathrm{C—H})$ have the largest scattering cross sections. With an increase in the number of Si—Cl bonds, these parameters also increase substantially.

We are deeply grateful to Academician I. V. Obreimov for his interest in the work and for his advice.

Saratov Pedagogical Institute
Institute of Organic Synthesis
Academy of Sciences of the Latvian SSR

Received
31 V 1968

CITED LITERATURE

1. I. F. Kovalev, L. A. Ozolin et al.; collection Optics and Spectroscopy, 3, “Nauka,” 1967, p. 301.
2. I. F. Kovalev, Optics and Spectroscopy, 6, 594 (1959).
3. I. F. Kovalev, Optics and Spectroscopy, 10, 707 (1961).
4. I. F. Kovalev, collection Physical Problems of Spectroscopy, 1, Publishing House of the Academy of Sciences of the USSR, 1962, p. 360.
5. A. N. Lazarev, Yu. P. Zhukov, L. L. Shchukovskaya, Inorganic Materials, 3, 2021 (1967).
6. L. A. Gribov. Theory of Intensities in Infrared Spectra of Polyatomic Molecules, Publishing House of the Academy of Sciences of the USSR, 1963.
7. G. Eckhardt, W. G. Wagner, J. Mol. Spectr., 19, 407 (1966).
8. H. W. Schrötter, H. J. Bernstein, J. Mol. Spectr., 12, 1 (1964).

* We are deeply grateful to G. P. Kalyanov and N. I. Kovalenko for measuring the dispersion of the refractive index.