UDC 535.338.42

PHYSICS

V. A. ARBUZOVA, I. F. KOVALEV, E. N. TIKHOMIROVA,
M. G. VORONKOV, L. A. ZHAGAT

INTEGRAL INTENSITIES

IN THE INFRARED ABSORPTION SPECTRA

OF METHYLETHOXYSILANES \((\mathrm{CH}_3)_n\mathrm{Si}(\mathrm{OC}_2\mathrm{H}_5)_{4-n}\) \((n = 0,1,2,3)\)

(Presented by Academician I. V. Obreimov, 14 IV 1966)

1. For tetraethoxysilane and methylethoxysilanes, data on infrared absorption spectra are available in the literature, but only the frequencies of the principal bands have been measured \((^{1-7})\). In spectroscopic studies of tetraalkoxysilanes, much attention has been attracted by the central group \(\mathrm{SiO}_4\). A similarity has been noted in the structure of \(\mathrm{Si}(\mathrm{OR})_4\), which include this group, and silicates. The effects of the influence on the \(\mathrm{SiO}_4\) group of different radicals bonded to oxygen are of interest.

In order to clarify more thoroughly the features of the \(\mathrm{SiO}_4\) group and of the \(\mathrm{Si—O—C}\) bridge, we have studied in detail the integral intensities, absorption coefficients at the maximum, and half-widths of the bands in infrared vibrational absorption spectra. A calculation of the force constants for the tetrahedral skeleton of the molecules was also carried out, and electro-optical parameters were estimated.

1. Dimethyldiethoxysilane was obtained by the reaction of tetraethoxysilane with polydimethylsiloxane in the presence of \(\mathrm{KOH}\) \((^{8})\). Trimethylethoxysilane and methyltriethoxysilane were synthesized simultaneously by an analogous route from the cohydrolysis product of methyltrichlorosilane and trimethylchlorosilane (1 : 3). All compounds \((\mathrm{CH}_3)_n\mathrm{Si}(\mathrm{OC}_2\mathrm{H}_5)_{4-n}\) were purified by distillation on an efficient column over metallic sodium.

The spectra were recorded with an IKS-14 instrument in solutions of \(\mathrm{CCl}_4\) and \(\mathrm{CS}_2\), and also as the pure liquid, in the range \(400—3000\ \mathrm{cm}^{-1}\). The scanning rate for the LiF, NaCl, and KBr prisms was, respectively, 12; 5 and \(1—3.3\ \mathrm{cm}^{-1}/\mathrm{min}\). The band parameters were calculated using the methods described in \((^{9,10})\). The complete instrumental function was determined as the sum of the spectral expression of the geometrical slit width and the diffraction resolution limit.

1. The assignment of frequencies to normal vibrations and the parameters of the principal bands are given in Table 1. In interpreting the spectra we used the data obtained by us on the frequencies, intensities, and degrees of depolarization of lines in the spectra of combination scattering, as well as calculations carried out for the molecular skeleton. The totally symmetric stretching vibrations \(\nu_s(\mathrm{Si—O})\) in all compounds correspond to very weak bands lying in the range \(605—660\ \mathrm{cm}^{-1}\). The frequency and intensity of the vibration \(\nu_s(\mathrm{Si—O})\) systematically increase when methyl groups at silicon are replaced by ethoxyl groups. The antisymmetric vibrations \(\nu_{as}(\mathrm{Si—O})\) in methylethoxysilanes correspond to bands in the infrared absorption spectra that are very intense and change little in frequency, \(790—820\ \mathrm{cm}^{-1}\). The intensity of the \(\nu_{as}(\mathrm{Si—O})\) band, within the experimental error, increases linearly in going from trimethylethoxysilane to tetraethoxysilane. In this direction there is also observed, both in the infrared spectra and in the combina-

tional scattering a considerable broadening of the bands \(\nu_s(\mathrm{Si—O})\) and \(\nu_{as}(\mathrm{Si—O})\) is observed. These effects are due to interaction of the polar \(\mathrm{Si—O—C_2H_5}\) bonds and to a change in molecular symmetry. The silicon—oxygen bond becomes stronger: the force constants \(K_q(\mathrm{Si—O})\), corresponding to stretching of this bond, successively take the values 6.00; 7.76; 8.87 and 9.14 \((10^6\ \mathrm{cm}^{-2})\).

Table 1

Parameters of the principal absorption bands in the infrared spectra*
\[ (\mathrm{CH}_3)_n\mathrm{Si}(\mathrm{OC}_2\mathrm{H}_5)_{4-n}\quad (n=0,1,2,3) \]

* Notes:
1. Designations: \(\nu\) — stretching vibration, \(\delta\) — deformation vibration, \(\rho\) — rocking vibration, \(s\) — symmetric band, \(as\) — antisymmetric, medium — band of medium intensity, weak — weak, very weak — very weak.
2. The reproducibility of the results in measurements in solutions of different concentrations was within an average accuracy of up to 5%. Less accurate values, obtained for overlapping bands, are indicated in parentheses.

The bands assigned to the symmetric and antisymmetric stretching vibrations \(\nu(\mathrm{C—O})\) lie in the region \(1080\text{–}1110\ \mathrm{cm}^{-1}\), are close to one another in frequency, and retain their position in the spectrum in all the compounds considered. The half-width of these bands changes almost not at all.

In this connection, as a first approximation one may take the group $\mathrm{C_2H_5}$ as a single “atom” and regard the bridge $\mathrm{Si—O—C_2H_5}$ as a single whole.

It should be noted that the vibration $\nu(\mathrm{Si—O})$ is strongly split into symmetric and antisymmetric components in the systems $\mathrm{RSiOSiR'}$ as compared with $\mathrm{ROSiOR'}$. In the former, the interval in frequency positions reaches $500\ \mathrm{cm}^{-1}$ ($^{7,11}$); in the latter it is approximately $200\ \mathrm{cm}^{-1}$. Apparently, in the first case a considerable resonance interaction of the vibrations of the groups $\mathrm{RSi—O}$ and $\mathrm{O—SiR'}$ occurs through the oxygen atom.

The stretching vibrations $\nu_s(\mathrm{C—C})$ and $\nu_{as}(\mathrm{C—C})$ are observed in the region $940$—$960\ \mathrm{cm}^{-1}$. In the IR spectrum, a strong broad asymmetric band is recorded in this region, resulting from the superposition of these vibrations.

The vibrations $\nu(\mathrm{C—H})$ are quite characteristic in intensity. Each of the $\mathrm{CH_3}$ and $\mathrm{C_2H_5}$ groups accounts for a definite share in the total intensity of the $\mathrm{C—H}$ frequencies, and this share is preserved in passing from one compound to another.

1. Using experimental data on the intensities of the antisymmetric vibrations $\mathrm{Si—O}$ and $\mathrm{Si—C}$, we have attempted to calculate electro-optical parameters. The problem was solved in the first approximation of the bond-optical scheme ($^{12}$) for the molecular skeleton. It was assumed that the electro-optical parameters are identical in all the compounds, that the derivatives of the dipole moment of the $\mathrm{Si—O}$ bond with respect to changes in the angles adjacent to it are equal, and that the corresponding derivatives for the $\mathrm{Si—C}$ bond may be neglected. As a result, the following values were obtained, accurate to the significant digit:

$$ \partial\mu_1/\partial q_1 - \partial\mu_1/\partial q_1' = 3.88,\qquad \partial\mu_1/\partial\alpha - \partial\mu_1/\partial\alpha' = 2.04, $$

$$ \partial\mu_2/\partial q_2 - \partial\mu_2/\partial q_2' = 0.90\ \mathrm{(D/\AA)}, $$

$$ \mu_1 = \mu(\mathrm{Si—O}) = 2.14,\qquad \mu_2 = \mu(\mathrm{Si—C}) = 1.13\ \mathrm{(D)}. $$

The calculated values have only an approximate significance, since at present it does not appear possible to measure intensities in the low-frequency region of the spectrum and thereby obtain a more complete set of initial experimental data, or to take into account the correction associated with the change in the light field acting on the molecule in a condensed medium as compared with the gaseous state.

Saratov State
Pedagogical Institute

Institute of Organic Synthesis
Academy of Sciences of the Latvian SSR

Received
11 VII 1966

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