Locally modular geometries in homogeneous structures

We show that if M is a strongly minimal large homogeneous structure in a countable similarity type and the pregeometry of M is locally modular but not modular, then the pregeometry is affine over a division ring. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Reactions of [Ru(CO)3Cl2]2 with aromatic nitrogen donor ligands in alcoholic media

The reactions of mono‐ and bidentate aromatic nitrogen‐containing ligands with [Ru(CO)₃Cl₂]₂ in alcohols have been studied. In alcoholic media the nitrogen ligands act as bases promoting acidic behaviour of alcohols and the formation of alkoxy carbonyls [Ru(N–N)(CO)₂Cl(COOR)] and [Ru(N)₂(CO)₂Cl(COOR)]. Other products are monomers of type [Ru(N)(CO)₃Cl₂], bridged complexes such as [Ru(CO)₃Cl₂]₂(N), and ion pairs of the type [Ru(CO)₃Cl₃]^? [Ru(N–N)(CO)₃Cl]⁺ (N–N = chelating aromatic nitrogen ligand, N = non‐chelating or bridging ligand). The reaction and the product distribution can be controlled by adjusting the reaction stoichiometry. The reactivity of the new ruthenium complexes was tested in 1‐hexene hydroformylation. The activity can be associated with the degree of stability of the complexes and the ruthenium–ligand interaction. Chelating or bridging nitrogen ligands suppresses the activity strongly compared with the bare ruthenium carbonyl chloride, while the decrease in activity is less pronounced with monodentate ligands. A plausible catalytic cycle is proposed and discussed in terms of ligand–ruthenium interactions. The reactivity of the ligands as well as the catalytic cycle was studied in detail using the computational DFT methods. Copyright © 2005 John Wiley & Sons, Ltd.     

Bayesian predictive modeling and comparison of oil samples

Statistical comparison of oil samples is an integral part of oil spill identification, which deals with the process of linking an oil spill with its source of origin. In current practice, a frequentist hypothesis test is often used to evaluate evidence in support of a match between a spill and a source sample. As frequentist tests are only able to evaluate evidence against a hypothesis but not in support of it, we argue that this leads to unsound statistical reasoning. Moreover, currently only verbal conclusions on a very coarse scale can be made about the match between two samples, whereas a finer quantitative assessment would often be preferred. To address these issues, we propose a Bayesian predictive approach for evaluating the similarity between the chemical compositions of two oil samples. We derive the underlying statistical model from some basic assumptions on modeling assays in analytical chemistry, and to further facilitate and improve numerical evaluations, we develop analytical expressions for the key elements of Bayesian inference for this model. The approach is illustrated with both simulated and real data and is shown to have appealing properties in comparison with both standard frequentist and Bayesian approaches. Copyright © 2013 John Wiley & Sons, Ltd.     

Stable Isotopically-Enriched D-Glucose: Strategies to Introduce Carbon,Hydrogen and Oxygen Isotopes at Various Sites

INTRODUCTION

For many years stable isotopes were confined to the chemical laboratory, where they were used primarily to study reaction rates,¹ reaction mechanisms,² and molecular conformation.^3,4 Recently, stable isotopically-enriched molecules have been playing increasing roles in biochemical studies, particularly as probes of in vitro biological metabolism^5,6 and of biomacromolecular structure.^7,8 These modern applications have been stimulated, in large part, by dramatic technical developments in nuclear magnetic resonance (NMR) spectroscopy⁹ and mass spectrometry (MS).¹⁰ Very-high field multinuclear high-resolution NMR in one-, two- and three- dimensions,^11,12 and various volume-selective forms of NMR derived from topical magnetic resonance (TMR) spectroscopy,¹³ have become important analytical methods in the study of biological phenomena. TMR and related techniques such as ISIS (Image Selected In-Vivo Spectroscopy)¹⁴ and Spatially Resolved Spectroscopy (SPARS)¹⁵ are particularly attractive, as they permit high-resolution, volume-specific observations essential to non-invasive metabolic studies of intact organs and organisms. Magnetic resonance imaging (MRI)¹⁶ is now a standard clinical tool for non-invasive diagnosis of disease, especially cancer, in humans. Clinical MRI is confined currently to the observation of water protons in cells, but will likely become multinuclear (e.g., ⁷Li, ¹⁹F, ²³Na, ³¹P, ¹³C) in the future.¹⁷     

Modification of polycarbonate surface properties by nano-, micro-, and hierarchical micro–nanostructuring

Polycarbonate surfaces were patterned with nanopillars, microbumps, or nanopillars superimposed on microbumps. Patterning was achieved by applying nanoporous anodized aluminum oxide (AAO) membranes, microstructured aluminum foil, or anodic alumina on microstructured aluminum as mold inserts in injection molding. The effect of the different-sized structures on properties of the polycarbonate surface was investigated in contact angle measurements with water and oleic acid. The water contact angle increased from 82° on the smooth surface to 139° on the hierarchical micro–nanostructure. The transmittance of the polycarbonate increased with nanopatterning, while the reflection properties of the polycarbonate surface decreased. Reflection was lowest for the nanostructure with 53 nm pillar diameter and 77 nm interpillar distance. Values ranged from 0.6 to 1.1% over the whole wavelength range of visible light, which was 4–5% units lower than the corresponding values for the smooth polycarbonate.     

Synthesis and Molecular Structure of a Hexaaminodistannane Comprising two Identical Triaminotin Subunits

Reaction of the previously characterized lithium stannate [HC{SiMe₂N(4‐CH₃C₆H₄)}₃SnLi(thf)₃] ( 1 ) with AgCl afforded the corresponding distannane [HC{SiMe₂N(4‐CH₃C₆H₄)}₃Sn]₂ ( 2 ) as the product of an oxidative coupling in good yield. Its [2, 2, 2]bicyclooctane‐related cage structure, comprising the trisilylmethane unit and the triamido‐tin fragment, as well as the Sn‐Sn bond (2.8204(4)Å) were established by single crystal structure analysis: Space group group P2₁/n, Z = 2, lattice dimensions at 120(2) K: a = 13.6515(3), b = 15.8391(3), c = 16.3522(3)Å, β = 102.7760(10)°, R₁ = 0.0347.     

Highly luminescent octanuclear Au(I)-Cu(I) clusters adopting two structural motifs: the effect of aliphatic alkynyl ligands

Reactions of the homoleptic (AuC(2)R)(n) precursors with stoichiometric amount of diphosphine ligand PPh(2)C(6)H(4)PPh(2) (P^P) and Cu(+) ions lead to an assembly of a new family of bimetallic clusters [Au(6)Cu(2)(C(2)R)(6)(P^P)(2)](2+) (type I; R=9-fluorenolyl (1), diphenylmethanolyl (2), 2,6-dimethyl-4-heptanolyl (3), 1-cyclohexanolyl (4), Cy (5), tBu (6)). In the case of R=1-cyclohexanolyl, a structurally different complex [Au(6)Cu(2)(C(2)C(6)H(11)O)(6)(P^P)(3)](2+) (7, type II) could be obtained by treatment of 4 with one equivalent of the diphosphine, while for R=isopropanolyl only the latter type of cluster [Au(6)Cu(2)(C(2)C(3)H(7)O)(6)(P^P)(3)](2+) (8) was detected. Steric bulkiness of the alkynyl ligands and O···H-O hydrogen bonding are suggested to play an important role in stabilizing the type I and type II cluster structural motif, respectively. All the complexes exhibit intense photoluminescence in solution with emission parameters that depending on the geometrical arrangement of the octanuclear metal core. The clusters 1-4 and 6 show single emission band in a blue region (469-488 nm) with maximum quantum yield of 94% (4), while structurally different 7 and 8 emit yellow-orange (590 nm) with unity quantum efficiency. The theoretical DFT calculations of the electronic structures have been carried out to demonstrate that the metal-centered triplet emission within the heterometallic core plays a key role for the observed phosphorescence.     

Modification of olefin polymerization catalysts. I. Mechanism of the interaction between AIEt3 and silyl ethers

The interaction between AlEt₃ and silyl ethers, Ph_nSi(OMe)_4-n (n = 0–3), was followed by ¹³C- and ²⁹Si-NMR techniques in conditions close to those typical for an olefin polymerization reaction with supported Ziegler–Natta catalysts (A1Et₃:silyl ether ratios from 1 to 10, temperature range 25–75°C). A1Et₃ and silyl ethers form instantaneously at ambient temperature a donor-acceptor complex, which is stable at a 1:1 molar ratio. In the presence of excess A1Et₃ the complex decomposes via a mechanism consisting, in the case of PhSi(OMe)₃, of five consecutive steps: alternating complexation and ether reductions with the formation of alkylated silyl ethers, Ph(Et)_nSi(OMe)_3-n (n = 1,2), and dialkyl-aluminum alkoxides, (Et₂A1OMe₃)_n (n = 2,3). The rate of decomposition was enhanced by the increasing number of methoxy groups present in the silyl ether, heating, or a high A1Et₃:silyl ether ratio. The decomposition was not inhibited by the presence of 1-hexene.     

The active site of cellobiohydrolase Cel6A from Trichoderma reesei: the roles of aspartic acids D221 and D175

Trichoderma reesei cellobiohydrolase Cel6A is an inverting glycosidase. Structural studies have established that the tunnel-shaped active site of Cel6A contains two aspartic acids, D221 and D175, that are close to the glycosidic oxygen of the scissile bond and at hydrogen-bonding distance from each other. Here, site-directed mutagenesis, X-ray crystallography, and enzyme kinetic studies have been used to confirm the role of residue D221 as the catalytic acid. D175 is shown to affect protonation of D221 and to contribute to the electrostatic stabilization of the partial positive charge in the transition state. Structural and modeling studies suggest that the single-displacement mechanism of Cel6A may not directly involve a catalytic base. The value of (D2O)(V) of 1.16 +/- 0.14 for hydrolysis of cellotriose suggests that the large direct effect expected for proton transfer from the nucleophilic water through a water chain (Grotthus mechanism) is offset by an inverse effect arising from reversibly breaking the short, tight hydrogen bond between D221 and D175 before catalysis.     

Molecular structures of magnesium dichloride sheets and nanoballs

The structures and relative stabilities of (MgCl(2))(n)() sheetlike clusters and nanoballs were studied by quantum chemical methods. The sheets as discrete molecules were studied up to Mg(100)Cl(200). Their stabilities increase systematically as a function of the size of the sheet. Periodic ab initio calculations were performed for (001) monolayer sheets of alpha- and beta-MgCl(2), beta-sheet being slightly favored. Nanoballs were constructed from Archimedean polyhedra, producing tetrahedral, octahedral, and icosahedral symmetries, and were studied up to Mg(60)Cl(120). Nanoballs prefer to take the shape of truncated cuboctahedron (Mg(48)Cl(96)). Comparisons to sheetlike clusters and periodic calculations suggest that magnesium dichloride nanoballs are stable.