Crystal structure of the ceramic superconductor BaPb0.75Bi0.25O3

The ceramic superconductor BaPb_0.75Bi_0.25O₃ is monoclinic atT<430 K with space groupI2/m. Lattice constants and the atomic coordinates result from 20 reflection groups (12<2<120°, =1.1267 Å) collected with a high resolution powder diffractometer using a synchrotron radiation source and a full neutron powder pattern (6<2<140, =1.909 Å). Synchrotron and neutron data were simtultaneously refined with SIMREF, a new profile refinement program, based on the Rietveld method. Group theoretical analysis of the distortion field leading from the ideal perovskite structure toI2/m exhibitsR ₄ ⁺ symmetry. A comparison is made with the corresponding distortions in nonsuperconducting BaBiO₃ where three different symmetry types are identified. The space groupI2/m found for BaPb_0.75Bi_0.25O₃ allows only one type of averaged (Pb/Bi)O₆ octahedron in the structure: this may support valence fluctuations and hence contribute to the mechanism of superconductivity in this particular compound.     

Cyclic homology of differential operators,the Virasoro algebra and aq-analogue

We show how methods from cyclic homology give easily an explicit 2-cocycle on the Lie algebra of differential operators of the circle such that restricts to the cocycle defining the Virasoro algebra. The same methods yield also aq-analogue of as well as an infinite family of linearly independent cocycles arising when the complex parameterq is a root of unity. We use an algebra ofq-difference operators andq-analogues of Koszul and the Rham complexes to construct these quantum cocycles.     

Über die lokale Zetafunktion von Shimuravarietäten. Monodromiefiltration und verschwindende Zyklen in ungleicher Charakteristik

Ohne Zusammenfassung

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On the local zeta function of Shimura varieties. Monodromy filtration and vanishing cycles in unequal characteristic

     

Synthesis and structure determination from powder data of the first organically templated tin(IV) phosphate: MIL-76 or Sn3(IV)O2(H2O)(HPO4)4 x [H2N-C2H4-NH2]2.5 x [H2O]2

The first organically templated tin(IV) phosphate has been isolated and its structure solved from powder X-ray diffraction data; it exhibits a one-dimensional inorganic network built up from chains of trimers of tin(IV) octahedra on which phosphate tetrahedral groups are grafted interacting with water molecules and organic moieties.     

Ultrafast fragmentation and ionisation dynamics of ammonia clusters

The formation of protonated and unprotonated ammonia cluster ions is studied by femtosecond two colour two photon pump-probe techniques applied to (NH₃) _n and (ND₃) _n clusters withn up to 8. The fourth harmonic (～ 200 nm, 6.2 eV, 160 fs) of a Ti: Sapphire laser pulse is used to excite the clusters in a state corresponding to theà state of NH₃ while the third harmonic (267 nm, 4.65 eV) is used for the subsequent ionisation step. Employing a combination of the optical Bloch equations for the excitation process and rate equations for the cluster dynamics we calibrate the zero time delay and carefully analyse the time dependence of the pump-probe signal. Several distinct intermediate steps in the time evolution can be distinguished, having characteristic time constants ranging from 40 fs to over 100 ps. They are discussed in a consistent scheme for the excitation, ionisation and protonation dynamics, accounting also for characteristic differences observed between deuterated and undeuterated species. A particularly remarkable time dependence of the homogeneous (NH₃) ₂ ⁺ cluster ion signal is interpreted as a fingerprint of internally protonated neutral precursors of the type NH₃NH₂NH₄.     

Abbau von Palustrin zu (−)-Dihydropalustraminsäure ((2R, 6S, 1′S)-[6-(1′-Hydroxypropyl)-2-piperidyl]essigsäure) und Struktur von Palustrin und Palustridin. Mitteilung über Schachtelhalmalkaloide

Degradation of palustrin to (?)-dihydropalustramic acid ((2R,6S,1′S)-[6-(1′-hydroxypropyl)-2-piperidyl]acetic acid), and the structure of palustrin and palustridin The structure of the macrocyclic alkaloid palustrin is shown to be 1a . Its piperidine unit can be obtained as (?)-dihydropalustramic acid ( 6a ) by the following sequence of degradation reactions (Scheme 1): catalytic hydrogenation of 1a followed by methylation and Hofmann degradation provides the allyl base 4 . the regioselectivity of the Hofmann elimination is explained by intramolecular proton abstraction at C(3) by C(18)-O^?. Catalytic reduction of 4 and subsequent acidic hydrolysis yielded 6a and N, N-dimethylputrescine (?N,N-dimethyl-1,4-butanediamine; 7 ). Loss of the N-alkyl group in the formation of 6a occurs during the catalytic hydrogenation step. This interpretation is supported by the results of model experiments. The position of the double bond in 1a is deduced from the IR. spectrum of the bromo-δ-lactone 19 prepared by treatment of 1a with N-bromosuccinimide at pH 4 (Scheme 3). Some of our previously published results on the degradation of dihydropalustrin ( 2a ) are obviously at variance with the newly proposed structure for palustrin ( 1a ). They can easily be explained by assuming a partial hydrogenolysis of the C(17)-N(1) bond during the preparation of dihydropalustrin from palustrin. Periodate cleavage of dihydropalustramic acid methyl ester ( 6b ) liberates propionaldehyde, which can be trapped by working at pH 7.5 (Scheme 2); at lower pH values it condenses rapidly with the simultaneously generated 3,4,5,6-tetrahydropyridine derivative 15 . The structure of the condensation product is proposed to be 16 on the basis of the isolation of its hydrogenation product, an isomeric dihydropalustramic acid ( 17 ).     

An Efficient Strategy to Orthogonally Protected (R)- and (S)-alpha-Methyl(alkyl)serine-Containing Peptides via a Novel Azlactone/Oxazoline Interconversion Reaction

A novel strategy for the synthesis of (R)- and (S)-alpha-methyl(alkyl)serine-containing peptides is presented. Using (S)-phenylalanine cyclohexylamide 6 as chiral auxiliary, the optically pure azlactones (R)- and (S)-2 were synthesized via a novel azlactone/oxazoline interconversion reaction (Figures 3 and 6). These azlactones constitute fully protected and activated synthetic equivalents of (R)- and (S)-alpha-methylserine and can be directly incorporated into peptides without further protective group manipulations. Like other alpha,alpha-dialkylated glycines, optically pure alpha-alkylserines can be used to stabilize beta-turn and alpha-helical conformations in short peptides.     

Optimization of molecular geometries in the PCILO and CNDO/2 formalism. The barrier to internal rotation in thioacetaldehyde

The barrier to internal rotation in thioacetaldehyde was investigated within the PCILO and CNDO/2 framework using standard and optimized geometries. The optimized geometries give for the barrier of PCILO a value closer to the experimental one (1.06 kcal mol^?1) whereas the CNDO/2 results yield a qualitatively wrong conformational behavior.     

Synthesen von Heterocyclen, 121. Mitt.: Chinolizine und Indolizine V: Eine Synthese von 4-Methyl-2-chinolizinonen

Zusammenfassung Die Reaktion von 2-Pyridylessigsäurederivaten (1a, 1b) mit Diketen (2) in sied. Eisessig liefert in 1-Stellung substit. 4-Methyl-2-chinolizinone.

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Syntheses of heterocycles, CXXI: Quinolizines and indolizines. A synthesis of 4-methyl-2-quinolizinones

The reaction of 2-pyridylacetic acid derivatives (1a, 1b) with diketene (2), gives 4-methyl-2-quinolizinones.