Magnetic properties of UT2Si2 and UT2Ge2 (T = Co,Ni, Cu) intermetallic systems

Neutron diffraction and magnetization measurements indicate that, at low temperatures, long-range magnetic order is present in UCO₂Si₂, UNi₂Si₂, UCu₂Si₂, UNi₂Ge₂, and UCo₂Ge₂. UCo₂Si₂ and UNi₂Ge₂ are simple collinear antiferromagnets of +-+- type, UCu₂Si₂ a simple collinear ferromagnet. In UNi₂Si₂, a magnetic phase transition from a LSW type structure to collinear antiferromagnetism of +-+- type was found, while in UCu₂Ge₂, the antiferromagnetic structure of ++-- transforms into collinear ferromagnetism. Crystal structure and magnetic parameters are given. No magnetic moment on transition metal ions was found within the accuracy of a powder neutron diffraction experiment. The stability of particular magnetic ordering schemes is discussed in terms of an isotropic RKKY mechanism.     

Communiols A-D: new mono- and bis-tetrahydrofuran derivatives from the coprophilous fungus Podospora communis

Communiols A-D (1-4), new tetrahydrofuran and bis-tetrahydrofuran derivatives, have been isolated from cultures of the coprophilous fungus Podospora communis, and identified by spectroscopic methods.     

A new 1,4-phenanthraquinone from Menispermum dauricum

A new 1,4-phenanthraquinone was isolated from the roots of Menispermum dauricum. The structure was elucidated as 7-hydroxy-3,6-dimethoxy-1,4-phenanthraquinone on the basis of spectral evidence.     

Ruthenium porphyrin catalyzed intramolecular carbenoid C[bond]H insertion. Stereoselective synthesis of cis-disubstituted oxygen and nitrogen heterocycles

[reaction: see text] A ruthenium porphyrin-catalyzed stereoselective intramolecular carbenoid C[bond]H insertion is described. Using [Ru(II)(TTP)(CO)] as catalyst, aryl tosylhydrazones are converted to 2,3-dihydrobenzofurans, 2,3-dihydroindoles, and beta-lactams in good yields and remarkable cis selectivity (up to 99%). Enantioselective synthesis of 2,3-dihydrobenzofurans is also achieved with [Ru(II)(D(4)-Por*)(CO)] as catalyst, and up to 96% ee is attained.     

On Some Three-Color Ramsey Numbers

 In this paper we study three-color Ramsey numbers. Let K _i,j denote a complete i by j bipartite graph. We shall show that (i) for any connected graphs G ₁, G ₂ and G ₃, if r(G ₁, G ₂)≥s(G ₃), then r(G ₁, G ₂, G ₃)≥(r(G ₁, G ₂)−1)(χ(G ₃)−1)+s(G ₃), where s(G ₃) is the chromatic surplus of G ₃; (ii) (k+m−2)(n−1)+1≤r(K _1,k , K _1,m , K _n )≤ (k+m−1)(n−1)+1, and if k or m is odd, the second inequality becomes an equality; (iii) for any fixed m≥k≥2, there is a constant c such that r(K _k,m , K _k,m , K _n )≤c(n/logn), and r(C _2m , C _2m , K _n )≤c(n/logn) ^m/(m−1) for sufficiently large n. Received: July 25, 2000 Final version received: July 30, 2002 RID="*" ID="*" Partially supported by RGC, Hong Kong; FRG, Hong Kong Baptist University; and by NSFC, the scientific foundations of education ministry of China, and the foundations of Jiangsu Province Acknowledgments. The authors are grateful to the referee for his valuable comments. AMS 2000 MSC: 05C55     

Effect of preparation and metal content on the introduction of Fe in BEA zeolite, studied by DR UV-vis, EPR and Mössbauer spectroscopy

Fe-containing SiBEA zeolites were prepared by a two-step postsynthesis method: creation of vacant T-sites by dealumination of tetraethylammonium BEA zeolite with nitric acid and then impregnation of the resulting SiBEA zeolite with an aqueous solution of Fe(NO₃)₃. X-ray diffraction shows that iron is incorporated in SiBEA at lattice sites. The presence of Fe in its oxidation state +3 and at isolated tetrahedral sites for low metal content, was demonstrated by diffuse reflectance UV-vis, EPR and Mössbauer spectroscopy. For high iron content, diffuse reflectance UV-vis and Mössbauer spectra revealed the additional presence of extra-lattice FeO_x oligomers and superparamagnetic Fe-oxyhydroxide. Mössbauer spectroscopy identified superparamagnetic Fe-oxyhydroxide as the main phase when basic conditions are used for the preparation.     

Two novel coordination polymers [Sm2(Pzdc)3(H2O)]x · 2xH2O and [Nd2(Pzdc)3(H2O)]x · 2xH2O: Synthesis, structure, and photoluminescent properties

Coordination polymers, [Sm₂(Pzdc)₃(H₂O)] _x · 2xH₂O (I) and [Nd₂(Pzdc)₃(H₂O)] _x · 2xH₂O (II), were obtained by hydrothermal reactions with 2,3-pyrazinedicarboxylic acid (H₂Pzdc) and the salts nitrates, and characterized by single crystal X-ray structure, thermogravimetric analysis, element analysis and infrared spectroscopy. The X-ray crystal diffraction data indicates that two complexes crystal in a monoclinic system with the space group P2₁/c, of the three dimensional framework. The Pzdc ligand in the complexes I and II adopts tetradentate, hexadentate, and heptadentate bridging modes. The influences of coordination modes of the Pzdc ligand on the superstructural diversity is discussed. The photoluminescent data suggest that the ligands act as efficient “antennas” sensitizing the luminescence of the Sm³⁺ ion. Complex I exhibits strong fluorescent emission bands in the solid state at room temperature.