Wave propagation in finite structures due to transient loading with application to pyroshock simulation

Pyroshocks are transient motions of a structure caused by e.g. explosion of ordnance devices. Prior to their launching space modules have to be tested for fulfillment of strict requirements concerning the expected acceleration during a space mission. Since, in practical applications, this is still based on empirical knowledge and time consuming testing procedures, an analytical way to face this problem is the modelling of a pyroshock test device for the simulation of mechanical impacts on plates carrying the module to be tested. From the mechanical point of view, the problem is described by the wave propagation in a disk or a plate carrying a point mass and being exposed to a punctual load caused by the impact of a sphere. The displacement field is calculated semi-analytically by the use of a modal approach, HERTZian contact theory as well as the GALERKIN procedure. In order to systematically determine the optimum setup leading to a desired acceleration signal an evolution strategy is used as well as comparisons with experimental and FEM results have been made. The present procedure allows for a better and faster simulation and predictability of pyroshock tests. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Condensation of Rare Earth Pyrazolates: The Dimeric,Trimeric and Polymeric Units [Gd2(Pz)6(PzH)4], [Nd3(Pz)9(PzH)2] and $^{1}_{\infty}\rm [Eu(Pz)_{2}(PzH)_{2}]$

Solvent free high‐temperature oxidations of rare earth metals with the heterocycle pyrazole as well as in low to non‐coordinating solvents were investigated to isolate intermediate stages between monomeric and polymeric pyrazolates of the lanthanides. Reaction conditions were tuned according to simultaneous DTA/TG and temperature dependent X‐ray powder diffraction experiments on known monomeric and polymeric pyrazolates, that gave rise to the idea that further structure intermediates could be isolated. Reactions in 1,2,3,4‐tetrahydroquinoline gave the dimeric complex [Gd₂(Pz)₆(PzH)₄](PzH)(Tech) ( 1 ) as well as the triangular complex [Nd₃(Pz)₉(PzH)₂](PzH)(Tech)₂ ( 2 ). The solvent free melt synthesis resulted in a new polymeric form of ( 3 ) (pyrazole, PzH = C₃H₃NNH; pyrazolate anion, Pz^? = C₃H₃NN^?; 1,2,3,4‐tetrahydroquinoline, Tech = C₉H₁₃N). All three compounds contain coordinating pyrazolate amide groups and pyrazole molecules the latter decreasing in numbers upon condensation of the building units. According to simultaneous DTA/TG/MS investigations the condensation process can be identified with the release of pyrazole molecules. 1 consists of dimeric molecules containing trivalent gadolinium with a C.N. of eight. The two gadolinium atoms show different coordination polyhedra. Only σ coordination and bridging is found for 1 . 2 consists of trimeric molecules containing trivalent neodymium. The neodymium atoms also exhibit different coordination polyhedra with C.N.s of eight and nine. Both π and σ coordination is found for 2 , the π coordinating pyrazolate ligands acting as lids of the triangular units. Topological analysis of the electron localization function (ELF) for 2 calculated at the scalar‐relativistic DFT level reveals only weakly covalent π donor η⁵‐Pz–Nd interactions compared to the stronger covalent σ donor Pz–Nd interactions. The topological analysis of both, the ELF and the electron density reveals no significant differences of the respective charges of the Nd atoms. 3 exhibits a one‐dimensional chain structure with Eu^II and a C.N of ten. It can thus be addressed the β form of the referring formula with a new arrangement of the coordinating ligands. Like the α form 3 shows σ and π coordination of pyrazole and pyrazolate ligands. Simultaneous DTA/TG analysis reveals that the low‐temperature α form shows a phase transition into the β form between 110 °C and 130 °C. The three compounds were investigated by low‐temperature single crystal X‐ray analysis, Mid IR and Far IR spectroscopy.     

Characterization and catalytic-hydrogenation behavior of SiO2-embedded nanoscopic Pd, Au, and Pd-Au alloy colloids

Colloids embedded in a silica sol-gel matrix were prepared by using fully alloyed Pd-Au colloids, and pure Pd and Au colloids stabilized with tetraalkylammonium bromide following a modified sol-gel procedure with tetrahydrofuran (THF) as the solvent. Tetraethoxysilicate (TEOS) was used as the precursor for the silica support. The molar composition of the sol was TEOS/THF/H2O/HCl = 1:3.5:4:0.05 for the bimetallic Pd-Au and TEOS/THF/H2O/HCl = 1:4.5:4:0.02 for Pd and Au monometallic systems. After refluxing, the colloid was added as a 4.5 wt % solution in THF for Pd-Au, 10.2 wt % solution in THF for Pd and 8.4 wt % solution in THF for Au at room temperature. The gelation was carried out with vigorous stirring (4 days) under an Ar atmosphere. Following these procedures, bimetallic Pd-Au-SiO2 catalysts with 0.6 and 1 wt % metal, and monometallic Pd- and Au-SiO2 catalysts with 1 wt % metal were prepared. These materials were further treated following four different routes: 1) by simple drying, 2) in which the dried catalysts were calcined in air at 723 K and then reduced at the same temperature, 3) in which they were directly reduced in hydrogen at 723 K, and 4) in which the surfactant was extracted using an ethanol-heptane azeotropic mixture. The catalysts were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption measurements, solid-state 1H, 13C, 29Si-CP/MAS-NMR spectroscopy, powder X-ray diffraction (XRD), small angle X-ray scattering (SAXS), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and 197Au M?ssbauer spectroscopy. The physical characterization by a combination of these techniques has shown that the size and the structural characteristics of the Pd-Au colloid precursor are preserved when embedded in an SiO2 matrix. Catalytic tests were carried out in selective hydrogenation of 3-hexyn-1-ol, cinnamaldehyde, and styrene. These data showed evidence that alloying Pd with Au in bimetallic colloids leads to enhanced activity and most importantly to improved selectivity. Also, the combination of the two metals resulted in catalysts that were very stable against poisoning, as was evidenced for the hydrogenation of styrene in the presence of thiophene.     

Electron localization function in full-potential representation for crystalline materials

The electron localization function (ELF) is implemented in the first-principles, all-electron, full-potential local orbital method. This full-potential implementation increases the accuracy with which the ELF can be computed for crystalline materials. Some representative results obtained are presented and compared with the results of other methods. Although for crystal structures with directed bonding only minor differences are found, in simple elemental metals, there are differences in the valence region, which give rise to different ELF topologies.     

Expedient five-step synthesis of SIB-1508Y from natural nicotine

Altinicline (SIB-1508Y), an anti-Parkinson's agent, was prepared in five steps from natural nicotine in 32% overall yield via a regioselective substitution of the pyridine ring of (S)-nicotine.     

Molecular-recognition properties of a water-soluble cucurbit[6]uril analogue

The molecular-recognition properties of the cucurbit[6]uril analogue (1) in aqueous buffer (sodium acetate, 50 mM, pH 4.74, 25 degrees C) toward a variety of guests including alkanediamines (6-12), aromatics (14-32), amino acids (33-36), and nucleobases (37-42) were studied by fluorescence spectroscopy. For the alkanediamines studied (H2N(CH)nNH2, n = 6, 7, 8, 9, 10, 11, 12), the association constants increase as the length of the alkane (n) is increased. Host 1 is capable of forming strong complexes with guests containing aromatic rings with association constants (Ka) ranging from 10(2) to 10(6) M(-1) as a result of the favorable pi-pi interactions that occur between host 1 and the aromatic rings of the guest when bound in the cavity of 1. Biologically relevant guests such as amino acids and nucleobases are also bound in the cavity of 1 with Ka values ranging from 10(3) to 10(6) M(-1). Consequently, cucurbit[6]uril analogue 1 functions as a versatile fluorescent sensor for the presence of a wide range of chemically and biologically important substances in aqueous solution including nitroaromatics, neurotransmitters, amino acids, and nucleobases.     

Combinatorial synthesis and directed evolution applied to the production of alpha-helix forming antimicrobial peptides analogues

Antimicrobial peptides (AMPs) are effector molecules of innate immune systems found in different groups of organisms, including microorganisms, plants, insects, amphibians and humans. These peptides exhibit several structural motifs but the most abundant AMPs assume an amphipathic alpha-helical structure. The alpha-helix forming antimicrobial peptides are excellent candidates for protein engineering leading to an optimization of their biological activity and target specificity. Nowadays several approaches are available and this review deals with the use of combinatorial synthesis and directed evolution in order to provide a high-throughput source of antimicrobial peptides analogues with enhanced lytic activity and specificity.     

2D supramolecular structures of a shape-persistent macrocycle and co-deposition with fullerene on HOPG

Two 2D supramolecular structures of macrocycle 1 and 1/C60 have been obtained on HOPG by self-assembly under ambient conditions and investigated by high-resolution STM. The monolayers of 1 are characterized by structures with perfect ordering over relatively large areas. In the case of 1/C60, the size of the macrocycle 1 and the presence of two individual bithiophene units per ring lead in the final superstructure to a 1:2 stoichiometry. The fullerenes are not trapped at the graphite surface inside the macrocyclic holes but are located around the periphery of the bithiophene units. This clearly shows that the donor-acceptor interaction between C60 and the electron-rich units of the ring is the dominant factor for the structure formation.     

Near-UV induced interstrand cross-links in anthraquinone-DNA duplexes

Anthraquinone (AQ) has been extensively used as a photosensitizer to study charge transfer in DNA. Near-UV photolysis of AQ induces electron abstraction in oligonucleotides leading to AQ radical anions and base radical cations. In general, this reaction is followed by the transport of base radical cations to sites of low oxidation potential, that is, GG, and conversion of G radical cations to DNA breaks. Here, we show that AQ also produces interstrand cross-links in DNA duplexes. About half of the cross-links collapse to single strands in hot piperidine treatment. The structure of stable interstrand cross-links was deduced by MS, NMR, and sequence substitution. The cross-links consist of a covalent link between the methyl group of T on one strand with either C6 or C7 of AQ on the other strand. The formation of interstrand cross-links decreased in O2 compared to deoxygenated solutions. In the presence of O2, the yield of breaks at GG doublets was 10-fold greater than that of cross-links for end tethered AQ, while cross-links exceeded breaks for centrally located AQ. The formation of stable cross-links can be explained by initial charge transfer from T to excited AQ, deprotonation of T radical cations, and condensation of the latter species with AQ radicals. These studies reveal a novel pathway of damage in the photolysis of AQ-DNA duplexes.     

Cationic shape-persistent macrocycles: the unexpected formation of a nano-size supramolecular dimer

Shape-persistent macrocycles with a rigid pyridyl core and a flexible oligo-alkyl corona aggregate to some extent in nonpolar solvents to form large (tubular) aggregates. To increase the assembling tendency, the intraannular pyridyl groups of the macrocycles were alkylated. Unexpectedly, the quarternized macrocycles show no tendency at all to form tubular micellar-like structures but form well-defined dimers, as determined by X-ray and dynamic light scattering.