Spectra methyl siliconates

Chlornitrofen and nitrofen conditions for GC/MS column, cross-linked methyl silicone capillary (12 m x 0.22-mm i.d., 0.33- am film thickness) column temperature, 60 °C (1 min), 18 °C min to 265 °C inlet, transfer line and ion source temperature, 260, 200 and 200 °C, respectively He gas column head pressure, 7.5 psi injection method, splitless mode solvent delay, 3 min electron ionization voltage, 70 eV scan rate, 0.62 s per scan cycle scanned mass range, m/z 100-400. The retention times for chlornitrofen and nitrofen were 11.8 and 11.3 min, respectively. The main ions of the mass spectrum of chlornitrofen were at m/z 317, 319 and 236. Nitrofen presented a fragmentation pattern with the main ions at m/z 283, 202 and 285. ... 

The wine sample is extracted by ethyl acetate and the organic solution is dried with magnesium sulfate. The GC/MS analysis of the extract can be performed using a fused silica methyl silicone column and the following oven temperature program 60 °C isotherm for 2 min, then raised to 250 °C at 6°C/min. Identification of analyte is based on the library mass spectrum shown in Fig. 8.16 (Chisholm and Samuels, 1992). 

Figure 6. Mass spectrum of perdeutero fatty acid (8 0) (as methyl ester) from run No. 5-44. The spectrum was taken as the compound was eluted from a glass column (1.8 m long X 0.4-cm i.d.) packed with 1% methyl silicone on diatomaceous earth and ionized by electron impact at 70 eV in a Hewlett-Packard 5730A gas chromatograph-mass spectrometer combination.

Figure 4. Si NMR spectrum of methyl siliconates. Conditions 10-s pulses 20 s...

The diphenyl derivative is more stable than the radical anion of its carbon derivative, 9,9-diphenylfluorene, but, after extended periods of reduction, the spectrum of the biphenyl radical anion begins to grow in intensity. The 5,5-dimethyl derivative appears to be stable under these conditions. The enhanced stability of the silicon derivative might be due to stabilization of the carbon-silicon bonds by delocalization of charge into available d-orbitals 81). Methyl proton hyperfine splitting observed for the anion radicals of the 5,5-dimethyl- and 5,5-diethyldibenzosilole has been cited as evidence for d-7r interaction (56). 

The compound we use as a reference sample is usually tetramethylsilane, TMS. This is silane (SiH with each of the hydrogen atoms replaced by methyl groups to give Si(CH3)4- The four carbon atoms attached to silicon are all equivalent and, because silicon is more electropositive than carbon, are fairly electron-rich (or shielded), which means they resonate at a frequency a little less than that of most organic compounds. This is useful because it means our reference sample is not bang in the middle of our spectrum ... 

A powerful technique for investigating these different species is 29Si-NMR (see Fig. 8.1). Different lines can be observed in the 29Si-NMR spectrum of a silicate solution corresponding to the differently positioned 29Si nuclei in the (poly)sili-cate ions. The highest values for the chemical shift (8 = -71.5 ppm with respect to tetra methyl silane) are found for the monomeric silicate units while the resonances of fully condensed silicon atoms (i.e. Si-(0-Si)4 tetrasiloxy silane) are to be found at the lowest values for the chemical shift (8 = -110 ppm). 

In the first two cases, fc-sequences are present in the NMR spectrum due to partial inversion of ini-tial dichloro-derivative. All polymers with different tacticity types displayed high yields, except for the polymer of cis-tactic structure, the yield of which is much lower due to intramolecular cyclizati-on reaction, which induces formation of tricyclosiloxane (in the case of methyl groups at silicon atoms). 

Diphenyl(trimethylsilyl)phosphine is a colorless liquid that yellows upon standing. Like diphenylphosphine, it has a very bad odor and is reactive toward water and oxygen. The 1H n.m.r. spectrum shows a doublet (./p h = 4.9 Hz.) which is assigned to methyl protons on silicon at 8 = —0.14 p.p.m. (TMS) and a complicated multiplet from 8 = —7.0 to —8.6 p.p.m. which can be assigned to the phenyl protons. 

Fig. 3 depicts a MALDI-MS spectrum (expanded plot) of linear siloxanes with methyl and hydroxyl end groups. It shows that linear siloxanes with methyl end groups may be differentiated from diols (the molar mass difference is 4 daltons) in the range 1.000-1.500 Da. Of course, there is no baseline separation between these specimen because of silicon and carbon isotope distribution in silicon-organic compounds. Smaller amounts of cyclosiloxanes, also present in the sample, are clearly separated from methyl end-capped components. 

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