A spectroscopic study of the structure of trimethylaroxysilanes

Abstract

A number of trimethylaroxysilanes of general formula ArOSi (CH3)3 have been studied by the IR, Raman and UV spectroscopy methods and theoretical conformation analysis. The UV spectra of these compounds in the 200–300 nm region are typical for molecules containing an ArO group and consist of bands attributed to the ππ transitions of the oxygen-conjugated benzene ring. This is confirmed by higher 1600 cm−1 line intensity values in the Raman spectra compared to those for the corresponding benzene-monosubstituted compounds. The intensities of the 1600 cm−1 band in the IR spectra of trimethylaroxysilanes were used for the calculation of the OSi(CH3)3 constant as equal to −0.50 ± 0.03. This points to a donating effect of the trimethylsiloxy group on the π-electron system of the benzene ring. There are indications that the molecules of trimethylaroxysilanes in the liquid and solution states have a conformation with the ArOSi dihedral angle not so high as to prevent oxygen-benzene ring conjugation. Theoretical conformational analysis of trimethyl-p-bromophenoxysilane gave a dihedral angle value of 90°. Additional stabilisation of the conjugated conformation (ϑ < 90°) is assumed to occur in the liquid state through the internal field of the medium. A dependence of the 830, 915, and 930 cm−1 band intensities on the dielectric constants of the medium has been found. The internal rotation potential around the CO bond in trimethylaroxysilanes is suggested to have a “smoothed” appearance, that is a torsion vibration with a large amplitude.