Inductively coupled plasma compared with direct current arc spectrometry for analysis of minor elements in aluminium baths

A validation study was carried out in order to evaluate the efficiency of inductively coupled plasma-optical emission spectrometry (ICP-OES) for the analysis of minor elements (manganese, chromium, copper, iron, and titanium) in aluminium alloys. Aluminium casting samples were obtained by adding compressed powder compacts of each alloying element and aluminium (minitablets) to aluminum baths in a laboratory crucible furnace. Digestion of solid samples was performed using concentrated HCI and H202 35% (v/v) previous to analysis by ICP-OES without any matrix separation. This solution-based method was validated considering direct current arc spectrometry as the reference method based on direct analysis without any pretreatment of the solid samples considered. Univariate statistical procedures were carried out, for which precision <3% and trueness of the analytical results were taken into account.     

Hydrogen Isotope Exchange Catalyzed by Ru Nanocatalysts: Labelling of Complex Molecules Containing N-Heterocycles and Reaction Mechanism Insights

Ruthenium nanocatalysis can provide effective deuteration and tritiation of oxazole, imidazole, triazole and carbazole substructures in complex molecules using D₂ or T₂ gas as isotopic sources. Depending on the substructure considered, this approach does not only represent a significant step forward in practice, with notably higher isotope uptakes, a broader substrate scope and a higher solvent applicability compared to existing procedures, but also the unique way to label important heterocycles using hydrogen isotope exchange. In terms of applications, the high incorporation of deuterium atoms, allows the synthesis of internal standards for LC-MS quantification. Moreover, the efficacy of the catalyst permits, even under subatmospheric pressure of T₂ gas, the preparation of complex radiolabeled drugs owning high molar activities. From a fundamental point of view, a detailed DFT-based mechanistic study identifying undisclosed key intermediates, allowed a deeper understanding of C−H (and N−H) activation processes occurring at the surface of metallic nanoclusters.     

用Lewis酸或Brnsted酸催化剂和N2汽提法使木糖脱水为糠醛（英文）

The activity of Lewis (Nb2O5) and Br nsted (Amberlyst 70) acid catalysts for the cyclodehydration of xylose to furfural was studied. The nature of the acidity resulted in significant changes in the reaction mechanism. Lewis acid sites promote the formation of xylulose, while Br nsted acid sites are required to further dehydrate the sugar to furfural. Amberlyst 70 in water/toluene at 175 ℃ showed lower activity but gave a higher furfural yield. Using N2 as the stripping agent considerably improved the furfural yield and product purity in the stripped stream. Catalyst stability was also studied.     

Exploring the Influence of Intermolecular Interactions in Prebiotic Chemistry Using Laser Spectroscopy and Calculations

One of the most fascinating questions in chemistry is why nature chose CGAT as the alphabet of life. Very likely, such selection was the result of multiple factors and a long period of refinement. Here, we explore how the intermolecular interactions influenced such process, by characterizing the formation of dimers between adenine, theobromine and 4-aminopyrimidine. Using a combination of mass-resolved excitation spectroscopy and DFT calculations, we determined the structure of adenine-theobromine and 4-aminopyrimidine-theobromine dimers. The binding energy of these dimers is very close to the canonical adenine-thymine nucleobases. Likewise, the dimers are able to adopt Watson-Crick conformations. These findings seem to indicate that there were many options available to build the first versions of the informational polymers, which also had to compete with other molecules, such as 4-aminopyrimidine, which does not have a valid attaching point for a saccharide. For some reason, nature did not select the most strongly-bonded partners or if it did, such proto-bases were later replaced by the nowadays canonical CGAT.     

Multicentered effective group potentials: ligand-field effects in organometallic clusters and dynamical study of chemical reactivity

A new multicentered effective group potential (EGP) is obtained for η⁶-benzene. Applications on $[\hbox{Ru}_{4}(\hbox{H})_{4}(\hbox{C}_{6}\hbox{H}_{6})_{4}]^{n+}$ clusters (n = 0 or 2) are in excellent agreement with reference DFT studies in terms of geometries, energies and electronic structures. In particular, the small singlet–triplet energy difference (3.8 kcal mol^?1) in [Ru₄(H)₄(C₆H₆)₄]²⁺ is very well reproduced. This new EGP is nevertheless not free from the limitations associated to this first generation of molecular pseudopotentials. A cautious analysis of the nature and exact role of this EGP is made, which provides new directions for the elaboration of the next generation of EGPs. In addition, the η⁵-cyclopentadienyl EGP has been used to perform a constrained dynamical simulation for the reaction of Cp₂LaH with H₂. The energy conservation during the simulation as well as the activation barrier extracted from the simulation clearly demonstrate the good behavior of this EGP in the context of molecular dynamics. Anharmonic effects on this reaction are underlined, further demonstrating the high accuracy of the potential energy surface obtained with EGPs. From a more general point of view, such EGPs are expected to provide accurate albeit low-cost ligand-field effects in organometallic clusters or nanoparticles and to allow dynamical studies at the surface of such compounds.     

DFT study of the ring opening polymerization of ε-caprolactone by grafted lanthanide complexes: 2--Effect of the initiator ligand

The influence of the initiator ligand on the Ring Opening Polymerization (ROP) of ε-caprolactone by lanthanide complexes grafted on silica have been investigated by means of density functional theory (DFT) calculations. Three different initiator ligands (alkyl, dialkylamido and borohydride) and three grating modes (mono-grafted, bi-grafted or bi-grafted after breaking of a Si-O-Si bridge) have been considered. This study highlights that lanthanum grafted complexes (alkyl, amide or borohydride) are active in lactone polymerization. In any case the reaction process is demonstrated to be similar to the one found for homogeneous catalysts. However, even if the different grafting modes are energetically equivalent for the ε-caprolactone ROP initiation reaction, some differences are observed according to the ligand involved in the initiation reaction. In agreement with experimental data, grafted lanthanide amides rapidly polymerise the ε-caprolactone. The grafted alkyl lanthanum complexes are also predicted to be very efficient catalysts. The borohydride is thus predicted to be the least efficient due to the difficulties in the ring opening. Indeed, the rate-determining step is the nucleophilic attack for the methyl and dialkylamido ligands (occuring with a low barrier) whereas it is the ring opening for the borohydride ligands (highest barrier) and the formation of -CH(2)C(=O)(X) (X = CH(3) or NMe(2)) terminal group is more favorable than that of a -CH(2)OBH(2) end group.     

Effects of long‐term exposure to elevated CO2 conditions in slow‐growing plants using a 12C‐enriched CO2‐labelling technique

Despite their relevancy, long‐term studies analyzing elevated CO₂ effect in plant production and carbon (C) management on slow‐growing plants are scarce. A special chamber was designed to perform whole‐plant above‐ground gas‐exchange measurements in two slow‐growing plants (Chamaerops humilis and Cycas revoluta) exposed to ambient (ca. 400 µmol mol^?1) and elevated (ca. 800 µmol mol^?1) CO₂ conditions over a long‐term period (20 months). The ambient isotopic ¹³C/¹²C composition (δ¹³C) of plants exposed to elevated CO₂ conditions was modified (from ca. ?12.8‰ to ca. ?19.2‰) in order to study carbon allocation in leaf, shoot and root tissues. Elevated CO₂ increased plant growth by ca. 45% and 60% in Chamaerops and Cycas, respectively. The whole‐plant above‐ground gas‐exchange determinations revealed that, in the case of Chamaerops, elevated CO₂ decreased the photosynthetic activity (determined on leaf area basis) as a consequence of the limited ability to increase C sink strength. On the other hand, the larger C sink strength (reflected by their larger CO₂ stimulatory effect on dry mass) in Cycas plants exposed to elevated CO₂ enabled the enhancement of their photosynthetic capacity. The δ¹³C values determined in the different plant tissues (leaf, shoot and root) suggest that Cycas plants grown under elevated CO₂ had a larger ability to export the excess leaf C, probably to the main root. The results obtained highlighted the different C management strategies of both plants and offered relevant information about the potential response of two slow‐growing plants under global climate change conditions. Copyright © 2008 John Wiley & Sons, Ltd.     

Highly Stable trans‐Cyclooctene Amino Acids for Live‐Cell Labeling

trans‐Cyclooctene groups incorporated into proteins via non‐canonical amino acids (ncAAs) are emerging as specific handles for bioorthogonal chemistry. Here, we present a highly improved synthetic access to the axially and the equatorially linked trans‐cyclooct‐2‐ene isomers ( 1 a , b ). We further show that the axially connected isomer has a half‐life about 10 times higher than the equatorial isomer and reacts with tetrazines much faster, as determined by stopped‐flow experiments. The improved properties resulted in different labeling performance of the insulin receptor on the surface of intact cells.