Regiochemistry of S-alkyl-substituted alkynes in Pauson-Khand reactions. Is a correlation with X-ray data and charge distribution calculations of the Co(2)(CO)(6)-alkyne complexes possible?

[reaction: see text] Pauson-Khand reactions (PKR) of RSCtbd1;CR' (6-10) yielded in all cases as the main product the regioisomer with the alkyl-S group disposed alpha to the CO group (16a-22a). Correlation of these results with X-ray data and charge distribution calculations of the corresponding dicobalthexacarbonyl complexes proves that a recently postulated "trans effect" in these complexes is not suitable for predicting the regiochemical outcome of the PKR unambiguously.     

Schlüsselreaktionen einer synthese des sibirosamins

[2,3]-Sigmatropic rearrangement and vic. $\text{cis}$-oxyamination of 3-$\text{C}$-methyl-ald-m-enopyranosides are the key steps for the synthesis of a sibirosamine precursor.     

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Direct imaging of Cu dimer formation, motion, and interaction with Cu atoms on Ag(111)

We have investigated the formation and motion of copper adatoms and addimers on Ag(111) between 6 and 25 K with low-temperature scanning tunneling microscopy. The presence of atoms and dimers alters the motion of atoms and dimers via the long-range interaction mediated by the electrons in the two-dimensional surface state band. Above 16 K, dimers show quantum rotor behavior with altered rotational behavior in the presence of an additional adatom. The most favorable diffusional motion of the dimer is identified in combination with molecular dynamics calculations to be a zigzag out-of-cell motion starting above 24 K.     

Characterization and control of phase fluctuations in elongated Bose–Einstein condensates

Quasi-one-dimensional Bose–Einstein condensates (BECs) in elongated traps exhibit significant phase fluctuations even at very low temperatures. We present recent experimental results on the dynamic transformation of phase fluctuations into density modulations during time of flight and show the excellent quantitative agreement with the theoretical prediction. In addition we confirm that, under our experimental conditions, in the magnetic trap density modulations are strongly suppressed even when the phase fluctuates. We also discuss our theoretical results on control of the condensate phase by employing a time-dependent perturbation. Our results set important limitations on future applications of BECs in precision atom interferometry and atom optics, but at the same time suggest pathways to overcome these limitations. Received: 17 August 2002 / Published online: 15 January 2003 RID="*" ID="*"Corresponding author. Fax: +49-511/762-3023, E-mail: Helge.Kreutzmann@ITP.uni-hannover.de     

Syntheses and solid state structures of cyclic diynes with two chalcogen centres--a competition between weak interactions

In this paper we report the synthesis of cyclic diynes of the general formula mX2n. The letters m and n indicate the lengths of chains between two C triple bond C-X units where X indicates S, Se, or Te. The lengths of the bridges vary between m=4-8 and n=2-6. Structural investigations by means of X-ray diffraction reveal for most systems a chair-like conformation in the solid state. For 5S(2)2, 5S(2)3, 7S(2)5, 5Se(2)2 and 5Te(2)3 tubular structures were encountered. These structures can be traced back to weak XX or weak C-Hpi interactions.     

Quantum chemical and RRKM calculations of reactions in the H/S/O system

Quantum chemical calculations at the MRCI/aug-cc-pV5Z level are used to describe the conversions between HSO, HOS, H + SO, S + OH and O + SH on the doublet H/S/O potential energy surface. An RRKM analysis of this multiple-well system was carried out in the temperature range 300-2000 K between 0.1 and 10 atm. At these pressures, the stabilization reaction H + SO → HSO or HOS is at the low pressure limit, and stabilization from S + OH and O + SH was not detected. The reactions S + OH → H + SO and O + SH → H + SO were found to be barrierless and very fast at room temperature (4 × 10¹⁴ and 1.5 × 10¹⁴ cm³ mol⁻¹ s⁻¹, respectively). The reaction channel O + SH → S + OH is two orders of magnitude slower than the more exothermic O + SH → H + SO reaction, although a second pathway involving direct H-abstraction (O + SH → S + OH) on the quartet surface appears as a minor channel at high temperatures.     

Untersuchung pharmazeutischer Pr?parate

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Zur Bestimmung des Schwefels im Eisen

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