Infrared spectroscopic study of very low pressure pyrolysis products of cyclosilthianes and 3,3-dimethyl-3-silathietane isolated in argon matrices. A new source of 1,1-dimethyl-1-silaethylene (Me 2siCH 2), thioformaldehyde (H 2CS) and dimethylsilanthione (Me 2SiS)

Abstract

The products of very low pressure pyrolysis (VLPP) of hexamethylcyclotri-silthiane (I), tetramethylcyclodisilthiane (II) and 3,3-dimethyl-3-silathietane (III) were isolated in Ar matrices and were studied by IR spectroscopy. The only pyrolysis product of I was cyclosilthiane II, a dimer of transient dimethylsilanthione (Me 2SiS) (IV). The starting material was recovered on pyrolysis of II. Thermal decomposition of III involves three intermediate unsaturated compounds: dimethylsilaethylene (Me 2 SiCH 2) (V) and thioformaldehyde (H 2CS) (VI), both isolated in Ar matrix at 10 K, as well as silanthione IV fixed in the matrix in a form of the cyclic dimer II. The latter was also observed in the study of copyrolysis of 1,1-dimethyl-1-silacyclobutane and thietane, being authentic sources of intermediates V and VI. IR spectra of starting compounds I, II and III isolated in Ar matrices were obtained. The theoretical structure of IV and force constant F(SiS) were determined by the CNDO/2 method. With regard to CNDO/2 errors, SiS bond distance and F(SiS) are equal to 1.993 Å and 4.72 mdyn/Å, respectively. Calculation of normal vibrations resulted in the following values of vibrational frequencies of dimethylsilanthione (cm −1): 884 ν s(SiC 2) ( A 1), 735 ν as(SiC 2) ( B 2), 626 ν s(SiS) ( A 1), 200 ⪯ CSiC ( A 1).