Sr[ReO4]2: The First Single Crystals of an Anhydrous Alkaline-Earth Metal Meta-Perrhenate

Colorless single crystals of Sr[ReO₄]₂ were obtained from halide melts at 1123 K in open corundum crucibles. X-ray diffraction revealed that Sr[ReO₄]₂ crystallizes in the monoclinic space group P2₁/n with the lattice parameters a = 627.31(4) pm, b = 1004.56(7) pm, c = 1271.25(9) pm and β = 97.118(3)° for Z = 4. The crystal structure contains a unique Sr²⁺-cation site surrounded by eight crystallographically different oxygen atoms forming distorted bicapped trigonal prisms. All corners of these [SrO₈]^14– polyhedra (d(Sr–O) = 259–268 pm) are shared with tetrahedral meta-perrhenate units [ReO₄]^– (d(Re–O) = 166–173 pm) formed from two crystallographically different Re⁷⁺ cations surrounded by four O^2– anions each, building up the three-dimensional mosaic-like structure of Sr[ReO₄]₂. Single-crystal Raman data confirm the presence of two different kinds of symmetry-free meta-perrhenate units [ReO₄]^– and match well with results known from literature.     

<Emphasis Type="Italic">H</Emphasis><Superscript>1</Superscript>-boundedness of Riesz transforms and imaginary powers of the Laplacian on Riemannian manifolds

We prove that the linearized Riesz transforms and the imaginary powers of the Laplacian areH ¹-bounded on complete Riemannian manifolds satisfying the doubling property and the Poincaré inequality, whereH ¹ denotes the Hardy space onM.     

One-pot synthesis of vinylisoxazolidines from simple hydroxylamines and conjugated carbonyls

Herein is reported the highly chemo- and regioselective synthesis of 3-vinyl-4-formyl and 3-vinyl-5-formylisoxazolidines from enals, hydroxylamines and dipolarophiles under thermal conditions. The reaction works in high yields for a large array of substituted enals and a variety of dipolarophiles. The reaction provides the respective isoxazolidines with high chemoselectivity, stereospecificity and diastereoselectivity without significant purification. The substitution pattern on the dipolarophile directs the regioselectivity of the reaction to provide either 3,4- or 3,5-substituted isoxazolidine isomers. This method provides access to a wide variety of highly substituted, stereochemically dense isoxazolidine scaffolds from the selective reaction of the three proposed components.     

Synthesis of Conformationally Restricted Carbocyclic Nucleosides: The Role of the O(4′)-Atom in the Key Hydration Step of Adenosine Deaminase

Conformationally restricted carbocyclic nucleosides with either a northern(N)-type conformation, i.e., N-type 2′-deoxy-methanocarba-adenosine 8 ((N)MCdAdo), or a southern(S)-type conformation, i.e. S-type 2′-deoxy-methanocarba-adenosine 9 , ((S)MCdAdo), were used as substrates for adenosine deaminase (ADA) to assess the enzyme's preference for a fixed conformation relative to the flexible conformation represented by the carbocyclic nucleoside aristeromycin ( 10 ). Further comparison between the rates of deamination of these compounds with those of the two natural substrates adenosine (Ado; 1 ) and 2′-deoxyadenosine (dAdo; 2 ), as well as with that of the conformationally locked nucleoside LNA-Ado ( 11 ), which, like the natural substrates, has a furanose O(4′) atom, helped differentiate between the roles of the O(4′) anomeric effect and sugar conformation in controlling the rates of deamination by ADA. Differences in rates of deamination as large as 10000 can be attributed to the combined effect of the O(4′) atom and the enzyme's preference for an N-type conformation. The hypothesis proposed is that ADA's preference for N-type substrates is not arbitrary; it is rather the direct consequence of the conformationally dependent O(4′) anomeric effect, which is more efficient in N-type conformers in promoting the formation of a covalent hydrate at the active site of the enzyme. The formation of a covalent hydrate at the active site of ADA precedes deamination. A new and efficient synthesis of the important carbobicyclic template 14a , a useful intermediate for the synthesis of (N)MCdAdo ( 8 ) and other conformationally restricted nucleosides, is also reported.     

An elementary analysis of a procedure for sampling points in a convex body

In this paper we describe a new method for proving the polynomial-time convergence of an algorithm for sampling (almost) uniformly at random from a convex body in high dimension. Previous approaches have been based on estimating conductance via isoperimetric inequalities. We show that a more elementary coupling argument can be used to give a similar result. © 1998 John Wiley & Sons, Inc. Random Struct. Alg., 12, 213–235, 1998     

Simulation of micro discharges for the optimization of energyrequirements for removal of NOx from exhaust gases

Experimental and theoretical investigations to remove NO_x from oxygen rich engine exhaust gases using dielectric barrier discharges are conducted in our laboratory. To explain experimental results in different reactor geometries, a two-dimensional fluid model using an unstructured grid was implemented for the simulation of transient microdischarges. As an example, simulation results of simultaneously occurring microdischarges in a packed bed reactor are presented in this paper     

H1–L1 Boundedness of Riesz Transforms on Riemannian Manifolds and on Graphs

We prove that the Riesz transforms are bounded from H ¹ to L ¹ on complete Riemannian manifolds and on graphs with the doubling property and the Poincaré inequality.