Conformational Control of Polysilanes: Use of CH2 Spacers in the Silicon Backbone
By the reaction of a number of oligosilyl potassium compounds with (trimethylsilyl)chloromethane, derivatives containing the (trimethylsilyl)methyl substituent were prepared. Using X-ray single-crystal structure analysis and UV spectroscopy the conformational properties of some of the compounds were studied. It was found that the (trimethylsilyl)methylated examples exhibit UV absorption properties which correspond to lower energy transitions in comparison to those of analogous trimethylsilylated molecules. The influence of this effect decreases, however, with increasing chain lengths.
The preparation of α,ω-dibromopermethylpolysilanes and characterization of these and related chloropermethylpolysilanes by mass spectrometry
The series of α,ω-dibromopermethylpolysilanes, Br(SiMe2)nBr, (n = 2−6) has been prepared by the action of bromine on dodecamethylcyclohexasilane. The mass spectra of these compounds, of the corresponding chlorine compounds Cl(SiMe2)nCl (n = 2−6), and of the 1-chloropermethylpolysilanes Cl(SiMe2)Me (n = 2, 3, 4, 6) have been recorded at 14 and 70 eV. For compounds with n > 3, the formation of the ions Si3Me6X+ and Si4Me8X+ (X = Cl, Br) is strongly favored, with these ions being major or base peaks in the spectra. It is proposed that the stability of these ions is due to cyclization and bridging:
Turbulent boundary layer wall‐pressure fluctuations downstream from an abrupt change in surface roughness
Spectra of wall‐pressure fluctuations beneath a turbulent boundary layer have been investigated immediately downstream from an abrupt change in surface roughness, from slightly rough to very rough. Over the measurable frequency band (400 Hz to 7 kHz), the spectra in the first 150 mm scale on wall variables. This indicates that the motions responsible for the pressure fluctuations measured in this region are very close to the wall. Farther downstream this scaling procedure breaks down. It is suggested that the initially thin internal layer behaves independently of the flow external to it until it has grown appreciably and only then interacts with the outer boundary layer. It was found that, for all frequencies measured, the dimensional spectral level decreases initially then rises to a maximum at 250 mm from the roughness change before decreasing once more. The position of the maximum is close to where the internal layer ceases to have a distinct edge.