Die Kristallstrukturen von KNdTe4, RbPrTe4 und RbNdTe4 — Untersuchungen zur thermischen Stabilität von KNdTe4 sowie Bemerkungen zu einigen anderen Vertretern der Zusammensetzung ALnTe4 (A = K,Rb, Cs und Ln = Seltenerd‐Metall)

Crystal Structures of KNdTe₄, RbPrTe₄, and RbNdTe₄ — Investigations concerning the Thermal Stability of KNdTe₄ as well as some Remarks concerning Additional Representatives of the Composition ALnTe₄ (A = K, Rb, Cs and Ln = Rare Earth Metal) Of the compounds ALnQ₄ (A = Na, K, Rb, Cs; Ln = Lanthanoid; Q = S, Se and Te) the crystal structures of the three new tellurides KNdTe₄, RbPrTe₄ and RbNdTe₄ were determined by X‐ray single‐crystal structure analysis and of the three additional new ones KCeTe₄, KPrTe₄ and CsNdTe₄ by X‐ray powder diffraction experiments. All six new compounds are isotypic with KCeSe₄. Characteristic for the crystal structure of the compounds mentioned above are layers built from (Q₂)^2— dumbbells in form of 4.3².4.3 nets with embedded cations A⁺ and Ln³⁺ between them, which are coordinated eightfold in form of square‐shaped antiprisms by Q ions. The distances Te‐Te within the dumbbells were found to be 277.8(2) pm for all investigated tellurides. By combination of X‐ray diffraction and DTA measurements it was shown that the compound KNdTe₄ is metastable at ambient temperature with a limited existence range between the temperatures 260 and 498 °C.     

Diastereoselective hydroformylation of 2-substituted allylic o-DPPB-esters-on the origin of 1,2-asymmetric induction

2-Substituted secondary alcohol o-DPPB esters (o-DPPB=ortho-diphenylphosphanylbenzoyl) have been prepared and their o-DPPB-directed diastereoselective hydroformylation examined. It was found that the diastereoselectivity increased as a function of the steric demand of the substituents both at the stereogenic center and in the alkene 2-position. Hydrolytic cleavage of the o-DPPB group afforded-via the lactols 29-the corresponding lactones 30, the relative configurations of the vicinal stereogenic centers of which were ascertainable by 2D-NOESY spectroscopy. In addition, a crystal structure analysis of the hydroformylation product 2 d provided further confirmation of the relative configuration. Replacement of the ester carbonyl group of the o-DPPB by a methylene unit resulted in significantly worse diastereoselectivity in the course of the hydroformylation (34-->35), which indicates a decisive role for the ester carbonyl function. All the experimental observations were combined in a model of the origin of the 1,2-asymmetric induction during the title reaction. The key feature is the consideration of diastereomeric trigonal-bipyramidal rhodium-hydrido-olefin complexes I and II, capable on the basis of the Hammond postulate of acting as good models for the transition states of the selectivity-determining hydrometalation step. Investigation of these complexes by force-field methods indicated good correlation between theoretically predicted and experimentally determined diastereoselectivities.     

‘Non-Koopmans’ States in Stilbene Cations and a Remarkable Example to the «SDT-Equation»: Indeno [2,1-a]indene

The radical cations of indeno [2, 1-a]indene ( 1 ), stilbene ( 2 ) and 3, 5, 3', 5'-tetramethylstilbene ( 3 ) were prepared by γ-irradiation of the neutral precursors in an electron-scavenging matrix at 77 K . Their electronic spectra were recorded and compared to the photoelectron spectra ( PE .) of the neutral precursors. The results show that either the fourth or the fifth excited doublet state of the cations is of «Non-Koopmans» type, with specific doublet energy (D) D (²B_g)=2.74 eV ( 1 ⁺), =2.59 eV ( 2 ⁺), =2.49 eV ( 3 ⁺). Remarkably, 1 ⁺ possesses two electronic states in the 2.7-2.8 eV energy range: ²A_u («Koopmans»-type) and ²B_g («Non -Koopmans»-type). The «SDT»-equation \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm D} = \sqrt {{\rm S} \cdot {\rm T}} $\end{document} approximately connecting excited singlet (S) and triplet (T) states of a neutral alternant system with the excited doublet (D) states of its radical cation - provided e-promotion occurs For all three excited states between the same (paired) orbitals-is satisfyingly exemplified by 1 : S¹ = 3.92 eV and T¹= 2.06 eV for 1 , D^{4 or 5}=2.74 eV for 1 ⁺.     

Critical evaluation of spiking of low-grade ore samples in activation analysis for gold and uranium

In applying non-destructive neutron-activation analysis for gold and uranium in spiked low-grade ore samples, the following extrapolated unspiked concentrations were measured: 0.27₈ ± 0.01₅ ppm gold (chemical assay: 0.20₂ ± 0.02₀ ppm gold); 25.1 ± 1.1 ppm uranium (chemical assay value: 19.5 ±2.0 ppm uranium). Different approaches to the fitting of results, and the influence of spiking non-uniformity, are discussed.     

CdSe and CdSe/CdS nanorod solids

We demonstrate the self-organization of CdSe nanorods into nematic, smectic, and crystalline solids. Layered colloidal crystals of CdSe nanorods grow by slow destabilization of a nanocrystal solution upon allowing the diffusion of a nonsolvent into the colloidal solution of nanocrystals. The colloidal crystals of nanorods show characteristic birefringence, which we assign to specific spherulite-like texture of each nanorod assembly. To demonstrate the general character of nanorod self-assembly technique, CdSe/CdS heterostructure nanorods were organized into highly luminescent superlattices.     

X-ray magnetic circular dichroism of Pseudomonas aeruginosa nickel(II) azurin

We show that X-ray magnetic circular dichroism (XMCD) can be employed to probe the oxidation states and other electronic structural features of nickel active sites in proteins. As a calibration standard, we have measured XMCD and X-ray absorption (XAS) spectra for the nickel(II) derivative of Pseudomonas aeruginosa azurin (NiAz). Our analysis of these spectra confirms that the electronic ground state of NiAz is high-spin (S = 1); we also find that the L(3)-centroid energy is 853.1(1) eV, the branching ratio is 0.722(4), and the magnetic moment is 1.9(4) mu(B). Density functional theory (DFT) calculations on model NiAz structures establish that orbitals 3d(x2-y2) and 3d(z2) are the two valence holes in the high-spin Ni(II) ground state, and in accord with the experimentally determined orbital magnetic moment, the DFT results also demonstrate that both holes are highly delocalized, with 3d(x2-y2) having much greater ligand character.     

Antimalarial activity of n(6)-substituted adenosine derivatives (part 2)

We have investigated the in vitro antimalarial activity of a new series of adenosine derivatives. The results show that N(6)-(1-naphthylmethyl)-5'-deoxy-5'-(amido)adenosines as well as N(6)-(4-phenylbenzyl)-5'-deoxy-5'-(amido)adenosines display significant activity against the malaria-causing parasites, with the sterically demanding bisubstituted species reported being active in most cases in the low-micromolar range. The novel compounds with unusual substitution pattern were obtained applying an efficient convergent polymer-assisted solution-phase (cPASP) synthesis protocol. Thus, we were able to prepare a series of substituted derivatives in parallel that would have been difficult to synthesize by standard techniques. The scope and limitations of the synthetic methodology are discussed.