Activation/Inhibition Effects during the Coelectrodeposition of PtAg Nanoparticles: Application for ORR in Alkaline Media

PtAg bimetallic nanoparticles for oxygen reduction reaction (ORR) in alkaline media were prepared by pulse electrodeposition (PED). During PED the reduction of Ag⁺ ions predominates, thus an increased Ag content in the co‐deposit is accomplished. The mechanism for this anomalous co‐deposition was elucidated by potential pulse experiments, which revealed that nuclei formation mainly occurs via the reduction of Pt²⁺ ions. The growth of the particles is diffusion controlled leading to the formation of a Ag shell covering a PtAg alloyed region. However, the shell is not growing homogeneously on the PtAg alloy. Hence, regions of the PtAg alloy are exposed, which exhibit an enhanced ORR activity compared to a pure Ag surface.     

Scorpio-Ligand: Synthesis of Biphenyl-Dihydroazepine Phosphoramidite Ligands for Asymmetric Hydrogenation

A novel dihydroazepine-bridged BIPHEP phosphoramidite ligand with an amino acid moiety in the backbone was synthesized and evaluated in the Rh-catalyzed asymmetric hydrogenation. The scorpion tail-like amino acid backbone is capable of hydrogen bond formation and able to shift the rotamer composition of the biphenyl axis with the two scissor-like arms. Pivaloyl-l -valine was studied as chiral selector unit and compared with pivaloylglycine as the achiral reference substance. The enantiomerization barrier of the pivaloylglycine-modified biphenylamide was determined to be ΔG^≠=110 kJ/mol. In the case of pivaloyl-l -valine, the (S_ax) isomer is thermodynamically favored. Due to the relatively high barrier, the ligand is atropisomeric at room temperature and allows the preparative separation of the stereoisomers. The obtained phosphoramidite ligands were separated by chiral HPLC. For the first eluting rotamer, Rh complex ([Rh(cod)(L)₂]BF₄) was generated in situ and examined in the enantioselective hydrogenation of 2-acetamidoacrylate and methyl 2-acetamido-3-phenylacrylate, achieving enantiomeric excesses of up to 94 %.     

Rapid Autoxidation Forms Highly Oxidized RO2 Radicals in the Atmosphere

Gas‐phase oxidation routes of biogenic emissions, mainly isoprene and monoterpenes, in the atmosphere are still the subject of intensive research with special attention being paid to the formation of aerosol constituents. This laboratory study shows that the most abundant monoterpenes (limonene and α‐pinene) form highly oxidized RO₂ radicals with up to 12 O atoms, along with related closed‐shell products, within a few seconds after the initial attack of ozone or OH radicals. The overall process, an intramolecular ROO→QOOH reaction and subsequent O₂ addition generating a next R′OO radical, is similar to the well‐known autoxidation processes in the liquid phase (QOOH stands for a hydroperoxyalkyl radical). Field measurements show the relevance of this process to atmospheric chemistry. Thus, the well‐known reaction principle of autoxidation is also applicable to the atmospheric gas‐phase oxidation of hydrocarbons leading to extremely low‐volatility products which contribute to organic aerosol mass and hence influence the aerosol–cloud–climate system.     

New Acyclic Neutral Phosphorus Sulfides and Sulfide Oxides

A great number of binary neutral phosphorus sulfides was discovered and investigated. However all stable representatives of this family of compounds adopt a polycyclic structure in contrast to their lighter homologues, the nitrogen oxides. Acyclic representatives can be stabilized by adduct formation with a nitrogen base. The bis(pyridine) adduct py₂P₂S₅ of the unstable acyclic phosphorus sulfide P₂S₅ is readily obtained stirring P₄S₁₀ in pyridine at ambient temperature. X‐ray diffraction studies on single crystals of py₂P₂S₅ · 0.5 py ( 1b ) show a N₂O₅ like structure for the P₂S₅ framework. The long P–N distances of 1.86 Å indicate only weak coordination of the pyridine molecules to phosphorus. Single crystal X‐ray diffraction studies on py₂P₂S_4.34O_0.66 ( 2 ) reveal the presence of py₂P₂S₄O ( 3 ) together with py₂P₂S₅ in the crystal. Compound 3 contains the mixed phosphorus oxide sulfide molecule P₂S₄O stabilized as bis(pyridine) adduct. It is readily obtained from pyP₂S₅ by oxidation with KMnO₄ in pyridine. The oxygen atom occupies the bridging position between the two phosphorus atoms. Quantum chemical calculations at the MPW1PW91 level of theory as well as DTA/TG thermal analyses confirm the weak coordination of the pyridine molecules in py₂P₂S₅, py₂P₂S₄O, and py₂P₂S₇ to phosphorus.     

Synthesis,Characterization and Fluorescent Properties of Three One‐Dimensional Coordination Polymers Derived from Polypyridyl Ligands

Three one‐dimensional coordination polymers, [MnCl₂(4‐pyterpy)]∙2CHCl₃ ( 1 ), [Mn(NO₃)₂(4‐pyterpy)]∙CHCl₃ ( 2 ) and [Ag(NO₃)(3‐pyterpy)]∙H₂O ( 3 ) (4‐pyterpy = 4′‐(4‐pyridyl)‐2,2′:6′,2″‐terpyridine and 3‐pyterpy = 4′‐(3‐pyridyl)‐2,2′:6′,2″‐terpyridine) were synthesized and characterized by X‐ray diffraction. All three compounds exhibit a rare “head‐to‐tail” coordination of the multidentate ligand, but differ significantly in their polymer chain conformations. Additionally, the fluorescent properties of all three compounds were investigated and show a weak, ligand‐centered fluorescence at 416–418 nm.     

Helfrich repulsion and dynamical phase separation of multicomponent lipid bilayers

Thermal fluctuations of surfactant bilayers in an aqueous solution produce an effective, long-range repulsion that can lead to a continuous unbinding transition. We report on an optical interferometry study of the thermal fluctuations of multicomponent bilayers close to the unbinding transition. We find that, in contrast to the case of single-component bilayers, the thermal fluctuation spectrum of multicomponent bilayers does not agree with a continuous unbinding transition but instead indicates the proximity of an unbinding tricritical point.     

Melting of copper and nickel at high pressure: the role of d electrons

Melting curves of Cu and Ni were measured in the laser-heated diamond cell to 97 GPa (3800 K) and 60 GPa (2970 K), respectively. The temperatures of Cu are in good agreement with recent theoretical calculations. The Cu melting slope (dT/dP) is about 2.5 times steeper than for Ni. The present results confirm the key role d-shell electrons play in determining the temperature dependence of high pressure melting curves in transition metals that have filled or partially filled d electron bands.     

Quantification of mephenytoin and its metabolites 4'-hydroxymephenytoin and nirvanol in human urine using a simple sample processing method

A reliable and easy to use liquid chromatography/tandem mass spectrometry (LC/MS/MS) method without the use of sample extraction was developed for the simultaneous quantification of urinary concentrations of mephenytoin, a standard phenotyping substrate for the cytochrome P450 enzyme CYP2C19, and its phase I metabolites 4'-hydroxymephenytoin and nirvanol. Fifty microL of urine were diluted with a buffered beta-glucuronidase solution and incubated at 37 degrees C for 6 h followed by addition of methanol, containing the internal standard 4'-methoxymephenytoin. The chromatographic separation was achieved using a 100 x 3 mm, 5 micro Thermo Electron Aquasil C18 column with a gradient flow, increasing the organic fraction (acetonitrile/methanol 50:50) of the mobile phase from 10 to 90%. Quantification by triple-stage mass spectrometry (TSQ Quantum, Thermo Electron) was accomplished by negative electrospray ionization in the selected reaction monitoring mode. Linearity was observed for all substances in the concentration range 15-10 000 ng/mL. The lower limit of quantification (LLOQ) was 20 ng/mL for 4'-hydroxymephenytoin and 30 ng/mL for nirvanol and mephenytoin, respectively. Intra- and inter-day inaccuracy did not exceed 9.5% for all substances from LLOQ to 10 000 ng/mL. Intra- and inter-day precision were in the range of 0.8-10.5%. The method was validated according to international ICH and FDA guidelines and successfully applied for phenotyping of Caucasian male volunteers who received an oral dose of 50 mg mephenytoin.     

The first and second cinchona rearrangement. Two fundamental transformations of alkaloid chemistry

Stereochemistry, products, and driving forces of the "first and second Cinchona rearrangement" have been investigated and a unified theory is presented. The first cage expansion affords [3.2.2]azabicyclic alpha-amino ether and is formulated via a configurationally stable bridgehead iminium ion and quasiequatorial nucleophilic attack. The second cage expansion affords beta-functionalized [3.2.2]azabicycles. In this case a nonclassical nitrogen-bridged cation is postulated to account for retention of configuration and potential reversibility of the cage expansion. The second rearrangement is favored for the so-called cinch bases (6'-R = H) in trifluoroethanol. Stereoelectronic factors, electron demand at C9, ground state conformation, and solvent type are crucial in all cases. A two-step protocol for preparing 9-epi-configured Cinchona alkaloids from 9-nat precursors is described.