Cyclische Kieselsäurederivate

Cyclic Silicic Acid Derivatives By reaction of SiCl₄ with oxygen at elevated temperatures a number of chlorsiloxanes consisting of more or less fused rings can be prepared of which those components of low molecular weight can be separated by combining distillation and crystallization. Methyl and ethyl esters of silicic acids are obtained by reaction with methyl or ethyl nitrite, resp. The compounds are characterized by silicon n.m.r. and mass spectroscopy.     

Surface Aggregation of Candida albicans on Glass in the Absence and Presence of Adhering Streptococcus gordonii in a Parallel-Plate Flow Chamber: A Surface Thermodynamical Analysis Based on Acid-Base Interactions

Adhesive interactions between yeasts and bacteria are important in the maintenance of infectious mixed biofilms on natural and biomaterial surfaces in the human body. In this study, the extended DLVO (Derjaguin-Landau-Verwey-Overbeek) approach has been applied to explain adhesive interactions between C. albicans ATCC 10261 and S. gordonii NCTC 7869 adhering on glass. Contact angles with different liquids and the zeta potentials of both the yeasts and bacteria were determined and their adhesive interactions were measured in a parallel-plate flow chamber.Streptococci were first allowed to adhere to the bottom glass plate of the flow chamber to different seeding densities, and subsequently deposition of yeasts was monitored with an image analysis system, yielding the degree of initial surface aggregation of the adhering yeasts and their spatial arrangement in a stationary end point. Irrespective of growth temperature, the yeast cells appeared uncharged in TNMC buffer, but yeasts grown at 37 degrees C were intrinsically more hydrophilic and had an increased electron-donating character than cells grown at 30 degrees C. All yeasts showed surface aggregation due to attractive Lifshitz-van der Waals forces. In addition, acid-base interactions between yeasts, yeasts and the glass substratum, and yeasts and the streptococci were attractive for yeasts grown at 30 degrees C, but yeasts grown at 37 degrees C only had favorable acid-base interactions with the bacteria, explaining the positive relationship between the surface coverage of the glass by streptococci and the surface aggregation of the yeasts. Copyright 1999 Academic Press.     

High-spin structure of yrast-band in 78Kr

Lifetime of levels up to 22⁺, have been measured in ⁷⁸Kr and an oblate shape is assigned to the ground state using the CSM and the configuration dependent shell correction calculations. Calculations further show that ⁷⁸Kr is highly γ-soft nucleus. The experimental Q _t values coupled with theoretical calculations indicate an oblate shape for ⁷⁸Kr at low spins and triaxial shape at higher spins     

Siliciumverbindungen mit starken intramolekularen sterischen wechselwirkungen : XLIX. Disilane und ein disilen mit unsymmetrischer substitution an den siliciumatomen

Treatment of chlorobis(2,4,6-triisopropylphenyl)silane (Ar₂SiHCl) with lithium naphthalenide and subsequent addition of the chlorosilanes R₂SiHCl (R = ⁱPr, ^tBu, Mes) gives the unsymmetrical disilanes R₂HSiSiHAr₂ which are smoothly converted into the 1,2-dichlorodisilanes 8–10. The X-ray structure analysis of the 1,1-dimesityldisilane 10 reveals a staggered conformation with the chlorine atoms disposed in an anti orientation. Whilst the reductive chloride elimination from the 1,1-dialkyldichlorodisilanes 8 and 9 did not give the corresponding disilenes, the same reaction of 10 provided bright yellow crystals of 1,1-dimesityl-2,2-bis(2,4,6-triisopropylphenyl)disilene, the structure of which was confirmed by a complete NMR study.     

1, 4‐Additions of HCl and HBr to a Tetrasilabutadiene: Formation of Unsymmetrically Substituted Disilenes [1]

The reactions of hexakis(2,4,6‐triisopropylphenyl)tetrasilabuta‐1,3‐diene R₂Si=SiR—SiR=SiR₂ ( 1 ) with HCl and HBr, slowly generated from HSiCl₃ or LiBr and CF₃COOH, respectively, furnish the unsymmetrically substituted disilenes R₂XSi—SiR=SiR—SiHR₂, X = Cl ( 2 ), Br ( 3 ), by formal 1,4‐addition of the hydrogen halides to 1 . However, passing gaseous hydrogen halides over the solution of 1 yields the 1,4‐dihalotetrasilanes by two‐fold 1,2‐additions to the double bonds of 1 . The structures of 2 and 3 which crystallize isotypically with one another have been determined by X‐ray crystallography.     

Ammonia and Chlorine Additions to a Tetrasilabuta‐1,3‐diene: Conglomerate versus Racemate Crystallization [1]

Treatment of hexakis(2,4,6‐triisopropylphenyl)tetrasilabuta‐1,3‐diene ( 1 ), R₂Si=SiR–SiR=SiR₂, with ammonia and chlorine furnishes the correspondingly substituted 1,4‐diaminotetrasilane ( 3 ) and 1,2,3,4‐tetrachlorotetrasilane ( 6 ). While product 6 crystallizes as a racemate, 3 forms a conglomerate of enantiomerically pure crystals. The change of colors during the formation of 6 indicates a stepwise reaction sequence.