The Reaction of Transient 1,1‐Bis(trimethylsilyl)silenes with Tris(trimethylsilyl)silyllithium – Synthesis and Structure of Sterically Congested 1‐Trimethylsilylalkylpolysilanes

Tris(trimethylsilyl)silyllithium ( 3 ) reacted with aldehydes and ketones (molar ratio 2 : 1) according to a modified Peterson mechanism under formation of transient silenes, which were immediately trapped by excess 3 to give the organolithium derivatives (Me₃Si)₃SiSi(SiMe₃)₂C(Li)R¹R² ( 7 ). Hydrolysis of 7 afforded the alkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂CHR¹R² ( 8 ). Depending on the substituents R¹ and R², 7 proved to be rather unstable in THF solution and underwent a rapid rearrangement, involving a 1,3‐Si,C‐trimethylsilyl migration, resulting in the formation of the lithium silanides (Me₃Si)₂Si(Li)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 9 ), which were hydrolized during the aqueous workup to give the H‐silanes (Me₃Si)₂Si(H)Si(SiMe₃)₂C(SiMe₃)R¹R² ( 10 ). Reaction of 9 with chlorotrimethylsilane produced the 1‐trimethylsilylalkylpolysilanes (Me₃Si)₃SiSi(SiMe₃)₂C(SiMe₃)R¹R² ( 11 ). The structures of the products described were elucidated by comprehensive spectral analyses. The results of X‐ray crystal structure analyses, performed for 8 l (R¹ = H, R² = 2,4,6‐(MeO)₃C₆H₂), 10 d (R¹ = H, R² = Mes) and 11 d (R¹ = H, R² = Mes) are discussed and confirm the expected extreme sterical congestion of the molecules.     

Homotopy classification of minimizers of the Ginzburg-Landau energy and the existence of permanent currents

Our objective is to explain the phenomenon of permanent currents within the context of the Ginzburg-Landau model for superconductors. Using variational techniques we make a connection between the formation of permanent currents and the topology of the superconducting sample.Research supported in part by the Fund for the Promotion of Research at the Technion.Research supported in part by a grant from the National Science Foundation.     

A comparison of RF electrode sheath models

Analytic expressions have been found from different time-averaged and dynamic models for the electrode sheath width in a capacitively driven RF discharge. The sheath widths predicted from all the models under consideration are shown to practically coincide if they are expressed in terms of DC sheath voltage and ion current to the RF electrode. However, these models have fundamental differences in their RF discharge scaling laws that result in different theoretical predictions for RF discharge electrical characteristics. Analytic expressions for the sheath width with arbitrary collisionality at moderate RF voltages are found to be in good agreement with experimental measurements made over a wide range of discharge voltage and gas pressure     

Intermolecular [4 + 2]-Cycloadditions of Nitroalkenes with Cyclic Olefins. Transformations of Cyclic Nitronates

Nitrocyclohexene undergoes facile SnCl₄-induced, [4 + 2]-cycloadditions with simple cycloalkenes to produce nitronates. The nitronates can be transformed sterospecifically into a number of other functional groups (alcohol, ketone, oxime, amine) by hydrolytic, reductive, and oxidative processes. The mechanism of the [4 + 2]-cycloaddition is believed to involve formation of a zwitterionic intermediate which can collapse via competing pathways to form the observed products. 1,3-Dipolar cycloadditions of the nitronates are described.     

Gas chromatography for in situ analysis of a cometary nucleus. IV. Study of capillary column robustness for space application

As part of the development of the European Space Agency Rosetta space mission to investigate a cometary nucleus, the selection of columns dedicated to the gas chromatographic subsystem of the Cometary Sampling and Composition (COSAC) experiment was achieved. Once the space probe launched, these columns will be exposed to the harsh environmental constraints of space missions: vibrations, radiation (by photons or energetic particles), space vacuum, and large temperature range. In order to test the resistance of the flight columns and their stationary phases, the columns were exposed to these rough conditions reproduced in the laboratory. The comparison of the analytical performances of the columns, evaluated prior and after the environmental tests, demonstrated that all the columns withstand space constraints, and that their analytical properties were preserved. Therefore, all the selected capillary columns, even having porous layer or chiral stationary phases, were qualified for space exploration.     

USING CELL-FRACTIONATION AND PHOTOCHEMICAL CROSSLINKING METHODS TO DETERMINE THE CELLULAR BINDING SITE(S) OF THE ANTITUMOR DRUG DMP 840

Abstract— In order to understand its mechanism of action we have begun an effort to better define the cellular target of action of the experimental antitumor agent DMP 840 (NSC D640430; (R, R)-2,2'-(1,2-ethanediylbis(imino-(1 -methyl-2,1 -ethanediyl)))-bis(5-nitro-1 H-benz(de)isoquinoline-1,3-(2H)-dione) dimethanesulfonate). Using a combination of gentle cell fractionation procedures and a previously unidentified photochemical crosslinking reaction, we have shown that after the drug is added to cultured Clone A cells, more than 80% of the drug that is found associated with cells partitions to the chromatin-containing structural framework of the cell and that the primary target after crosslinking with 360 nm light is DNA. While DMP 840 photoreacts quite efficiently with purified RNA in vitro , no photoat-tachment of the drug to RNA was observed in cells. In vitro photochemical studies also reveal that while GC-rich DNA is a preferred target for drug interaction, AT-rich DNA is more active in the photochemical crosslinking reaction. These results suggest that DMP 840 probably kills cells by interfering with DNA-metabolic processes, and that the drug and its derivatives are likely to be useful photoactive molecular probes for investigating higher order chromatin structures in cells.     

Eichinvariante Berechnung des Diamagnetismus und der chemischen Verschiebung

The procedure derived in^1–6 was used to calculate the magnetic susceptibilities and the magnetic shielding of a series of diamagnetic molecules (CH₄, C₂H₆, C₂H₄, C₂H₂, and HCN). The model requires only the knowledge of the unperturbed electron density. An incrementary treatment of the susceptibility-and shielding tensors is based on a partition of the electron density into localized parts.The values obtained by this procedure are in good agreement with experimental results in the case of susceptibilities and magnetic protonshieldings. The results for the¹³C-shielding however are not satisfactory.