Silicon model compounds, separation

Figure 4. Separation of silicon model compounds with silicon detection (Si chan-nel ICP-AES) as a function of column packing. Key to packing left, polar amino cyano (PAC) middle, cyano (CN) and right, silica. Key to compounds 1, sSiC CSis 2, (CeHs)tSi(OCtH,)t 3, Si(OOCCHs) 4, sSi(NCtHs) 5, HexamethyU disilazane and 6, t Si(OH)t.

All studied model compounds can distinctly be divided into three groups (Table VII). The first group is composed of substances in which the sulfur, selenium or cyclopentadienyl anion acts as an anionic center. They exist only in open betaine forms, and their PES do not contain local minima corresponding to cyclic isomers. The second group contains compounds with arsonium cationic and oxide anionic centers and silicon and germanium betaines with arsonium and amide centers. They exist as cyclic isomers and their PES have no local minima corresponding to the open forms. Finally, the third group consists of six studied compounds with phosphonium cationic and oxide or amide anionic centers and arsonium-imide betaine. Their PES have minima for both cyclic and open forms separated by low barriers. 

Figure 4 shows as an example the results of the extrapolation in the system SiC4 N , discussed above. The method works as well for systems having C and O or N and O bound to silicon in Eq. I 5 has to be replaced by the 8si values of the corresponding model compounds. For the system Si-C-O, where the peaks for SiC4 0 sites in ceramics are well separated and therefore 8si could... 

The extracted fractions were esterified with either BF3-MeOH reagent or diazomethane and analyzed by GLC. Gas liquid chromatography (GLC) was conducted with a Perkin-Elmer Sigma 3 equipped with flame ionization detector. Separations were obtained on a Hewlett Packard 12 m x 0.2 mm i.d. capillary column coated with methyl silicon fluid (OV-101). The temperature was maintained at 80°C for 2 min then programmed from 80 to 220°C at 8°C/min. The injector temperature was 250°C. Mass spectra were obtained on a Hewlett Packard model 5995 GC-MS mass spectrometer, equipped with a 15 m fused silica capillary column coated with 5% phenyl methyl silicone fluid. Spectra were obtained for major peaks in the sample and compared with a library of spectra of authentic compounds. 

This model accounts for the previously observed differences, which probably originated from different degrees of (nano)disproportionation and phase separation. On condensing molecular (gaseous) SiO onto cold surfaces, cyclic compounds have been identified, in which the silicon atoms are two-coordinate (two oxygen atoms). The further steps are hypothetical, but some "nano-disproportionation" must occur, which starts to separate Si-O and Si-Si bonds. 

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