UDC 535.338.42

PHYSICS

I. F. KOVALEV, L. A. OZOLIN, M. G. VORONKOV

PRINCIPAL PARAMETERS OF THE VIBRATIONAL RAMAN SPECTRA OF CYCLIC POLYDIMETHYLSILOXANES

(Presented by Academician I. V. Obreimov, 15 XI 1967)

This paper presents the results of measurements of frequencies, integral and peak intensities, degrees of depolarization, and half-widths of lines in the Raman spectra of liquid cyclic polydimethylsiloxanes \([(\mathrm{CH}_3)_2\mathrm{SiO}]_n\) \((n = 4—7)\). Corrected relative scattering coefficients have been calculated in the scales \(5b'^2 + 7g'^2(S)\) and \(5b'^2 + 13g'^2(R)\), as well as the components of the tensor of the derivative of the polarizability.

It is very important to elucidate the regularities in the behavior of the parameters of the Si—O bond as a function of ring size, and also in comparison with linear polydimethylsiloxane chains. They should reflect features in the geometry of the molecule and in the distribution of the electron cloud within it, and indicate possible changes in these factors. Until now, in applying spectroscopic methods to the study of the physicochemical properties of \([(\mathrm{CH}_3)_2\mathrm{SiO}]_n\), only the frequencies of spectral lines and qualitative intensities have been considered \((^{1–6})\).

In the present work the experimental studies were carried out on a DFS-12 instrument by the photoelectric method at a scanning rate of \(0.05—0.07\ \mathrm{cm}^{-1}/\mathrm{sec}\). The samples were studied in the liquid phase at room temperature. The \(802\ \mathrm{cm}^{-1}\) line of cyclohexane was chosen as the standard. For control, the spectra were also photographed with an ISP-51. The calculation of the true parameters was performed using the methods of Bazhulin and Sushchinskii \((^{7,8})\).

The values of the spectral characteristics obtained for the fundamental vibrations are presented in Table 1. The interpretation of the frequencies was carried out taking into account earlier studies of the vibrational spectra \((^{4–6})\). In the spectrum of each compound one can readily distinguish a line lying in the region \(480—495\ \mathrm{cm}^{-1}\) and belonging to the totally symmetric stretching vibration \(\nu_s(\mathrm{Si—O})\) of the Si—O—Si bridge. It is very intense and completely polarized. The frequency of this vibration, on going from the tetramer to more complex rings, increases within a range of \(15\ \mathrm{cm}^{-1}\). The scattering coefficient \(S\) increases in the molecule \(D_6\) \((D = (\mathrm{CH}_3)_2\mathrm{SiO})\) by a factor of 1.5 in comparison with \(D_4\), and in \(D_7\) in comparison with \(D_5\) by 10%. The half-width of the \(\nu_s(\mathrm{Si—O})\) line varies from 19 to \(16\ \mathrm{cm}^{-1}\). In the molecules \(D_4\), \(D_5\), and \(D_6\), \(S\) (or \(R\)) increases in proportion to the number of Si—O—Si units in the ring. In \(D_7\) this parameter correspondingly increases if one takes its summed value for the lines 495 and \(523\ \mathrm{cm}^{-1}\).

Dielectric studies of cyclosiloxanes \((^{9,10})\) showed that they, like linear chains, possess a dipole moment. The ring form at \(n > 3\) is not planar, which is confirmed by X-ray structural and spectroscopic analysis \((^{11,12})\). In both linear and cyclic molecules, considerable internal mobility is observed. In siloxane bridges this phenomenon is associated with internal rotations (“nonplanar” deformation vibrations) and the nonrigidity of the angle

Table 1

Principal parameters of the Raman lines of cyclic polysiloxanes

Notes. 1. Reproducibility of the results in the range 5–10%; in parentheses are indicated less precise values.
2. $\delta(D)$ — vibrations associated with deformation angles of the cyclic framework; $\nu+\delta_{\perp}$ — combinations of the vibration $\nu_s(\mathrm{Si}-\mathrm{O}-\mathrm{Si})$ and “out-of-plane” vibrations of the cycle.
3. In parentheses $(^{3},\,^{4})$ are additionally recorded lines lying in the region $\tilde{\nu}<90\ \mathrm{cm}^{-1}$.
4. Discussion of the parameter $P$, carried out, for example, in $(^{13})$.

$\mathrm{Si}-\mathrm{O}-\mathrm{Si}$. The mentioned feature of the molecules under consideration is clearly manifested in the distribution of scattering intensities in the spectrum. The share of deformation vibrations of the framework $\mathrm{C}_x\mathrm{Si}_y\mathrm{O}_z$ in cyclic compounds accounts for a significant total value, varying within the limits of 130–170 units. Linear polydimethylsiloxane chains are characterized by greater freedom of deformation. If in octamethylcyclotetrasiloxane $D_4$ each $\mathrm{Si}-\mathrm{O}-\mathrm{Si}$ unit corresponds to a value $S$ of 40 units, then in hexamethyldisiloxane it is 100 units. $(^{13})$ The frequency of the symmetric stretching vibration $\nu_s(\mathrm{Si}-\mathrm{O}-\mathrm{Si})$ in $[(\mathrm{CH}_3)_3\mathrm{Si}]_2\mathrm{O}$ is higher by $39\ \mathrm{cm}^{-1}$.

The antisymmetric stretching vibration $\nu_{as}(\mathrm{Si}-\mathrm{O}-\mathrm{Si})$ appears in the Raman spectrum as a very weak line lying in the region 1030–1070 $\mathrm{cm}^{-1}$. In the infrared absorption spectrum it is registered in the interval 1060–1090 $\mathrm{cm}^{-1}$ and is very intense. The possible position of the vibrations $\nu(\mathrm{Si}-\mathrm{O}-\mathrm{Si})$ in compounds of various classes is discussed in the review $(^{14})$.

The vibrations in the $\mathrm{CH}_3$ groups are sufficiently isolated from one another. This is confirmed by the course of the intensities in the series of molecules considered. The symmetric line $\nu_s(\mathrm{C}-\mathrm{H})$ gives a fraction of the scattered light two times greater than the antisymmetric $\nu_{as}(\mathrm{C}-\mathrm{H})$. In both the first and the second cases the scattering coefficients increase linearly with an increase in the number of methyl groups in the molecule. The deviation from linearity is no more than 5%. Additivity of the characteristics is also manifested fairly well for the internal and external deformation vibrations $\delta(\mathrm{CH}_3)$.

In accordance with the selection rules, the intense polarized line 710–715 $\mathrm{cm}^{-1}$ should be assigned to the totally symmetric stretching vibration $\nu_s(\mathrm{Si}-\mathrm{C})$. This line is characteristic in frequency, intensity, degree of depolarization, and half-width in all the compounds considered. To $\nu_{as}(\mathrm{Si}-\mathrm{C})$ belongs the depolarized line 792–799 $\mathrm{cm}^{-1}$. For the corresponding vibration the scattering $S$ also increases on going to more complex cyclosiloxanes, but no strict linear dependence is observed for the total intensity. The half-width of the antisymmetric line increases systematically as the ring becomes more complex, which may be explained by an increase in the mutual influence between this vibration and the ring vibrations.

The depolarized line 685–691 $\mathrm{cm}^{-1}$

cannot be regarded as belonging only to \(\nu_s(\mathrm{Si—C})\). This vibration is, to a large extent, an asymmetric torsional vibration of the methyl groups.

The low frequencies 147–452 cm\(^{-1}\) correspond to deformation vibrations of the skeleton. Their assignment to normal vibrations should be regarded as preliminary. In \(D_5\) and \(D_6\) the lines at 194 and 190 cm\(^{-1}\) are anomalously broad and, apparently, double.

The authors express their deep gratitude to I. V. Obreimov for his attention to and interest in the work.

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

Received
14 XI 1967

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