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 (Me2Si=CH2), THIOFORMALDEHYDE (H2C=S) AND DIMETHYLSILANTHIONE (Me2Si=S)

Article abstract of DOI:10.1016/S0022-328X(00)99104-6

The products of very low pressure pyrolysis (VLPP) of hexamethylcyclotrisilthiane (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 (Me2Si=S) (IV).The starting material was recovered on pyrolysis of II.Thermal decomposition of III involves three intermediate unsaturated compounds: dimethylsilaethylene (Me2Si=CH2) (V) and thioformaldehyde (H2C=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 diemr 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 Angstroem and 4.72 mdyn/Angstroem, respectively.Calculation of normal vibrations resulted in the following values of vibrational frequencies of dimethylsilanthione (cm^-1): 884 ν_s(SiC2) (A₁), 735 ν_as(SiC2) (B₂), 626 ν_s(Si=S) (A₁), 200 < CSiC (A₁).