Probing cooperative effects in bimetallic clusters: indications of C-N coupling of CH4 and NH3 mediated by the cluster ion PtAu+ in the gas phase

The bimetallic cluster ion PtAu+ activates methane in the gas-phase, yielding the carbene PtAuCH2+ which further reacts with ammonia under C-N coupling. In contrast, neither Pt2+ nor Au2+ mediates C-N bond formation. This example demonstrates how bond activation in the gas phase can be tuned by cooperative effects in bimetallic clusters.     

Ultimate state of thermal convection

The ultimate regime of thermal convection, the so-called Kraichnan regime [R. H. Kraichnan, Phys. Fluids 5, 1374 (1962)]], hitherto has been elusive. Here numerical evidence for that regime is presented by performing simulations of the bulk of turbulence only, eliminating the thermal and kinetic boundary layers and replacing them with periodic boundary conditions.     

Analyte response in laser ablation inductively coupled plasma mass spectrometry

The dependence of analyte sensitivity and vaporization efficiency on the operating parameters of an inductively coupled plasma mass spectrometer (ICPMS) was investigated for a wide range of elements in aerosols, produced by laser ablation of silicate glass. The ion signals were recorded for different carrier gas flow rates at different plasma power for two different laser ablation systems and carrier gases. Differences in atomization efficiency and analyte sensitivity are significant for the two gases and the particle size distribution of the aerosol. Vaporization of the aerosol is enhanced when helium is used, which is attributed to a better energy-transfer from the plasma to the central channel of the ICP and a higher diffusion rate of the vaporized material. This minimizes elemental fractionation caused by sequential evaporation and reduces diffusion losses in the ICP. The sensitivity change with carrier gas flow variation is dependent on m/z of the analyte ion and the chemical properties of the element. Elements with high vaporization temperatures reach a maximum at lower gas flow rates than easily vaporized elements. The sensitivity change is furthermore dependent on m/z of the analyte ion, due to the mass dependence of the ion kinetic energies. The mass response curve of the ICPMS is thus not only a result of space charge effects in the ion optics but is also affected by radial diffusion of analyte ions and the mismatch between their kinetic energy after expansion in the vacuum interface and the ion optic settings.     

Does ionized diacetylene have a positive proton affinity?

Singly and doubly charged C₄H₃^+/2+ ions generated upon electron ionization (EI) of the neutral precursors 1,3-butadiene, benzene, and exo-methylene cyclopropane, respectively, are examined by sector-field mass-spectrometry. Charge stripping of the mass-selected monocations affords the corresponding dications and charge exchange of the C₄H₃²⁺ dications allows for the reverse redox process. Refined analysis and additional MS/MS studies suggest that the monocations are mixtures of isomeric ions formed upon ionization, whereas only a single type of dication seems to be formed. As an average of energy-resolved measurements, a vertical ionization energy of IE_v(C₄H₃⁺)=16.5±0.4 eV is derived. In addition to the experimental work, density functional theory is used for a computational exploration of the mono- and dicationic species. The best theoretical estimates are IE_a(C₄H₃⁺)=16.33 eV and IE_v(C₄H₃⁺)=16.49 eV for the most stable isomer H₂C=C---CCH⁺. Combination of the experimental and theoretical findings leads to the conclusion that the diacetylene cation C₄H₂⁺ has indeed a positive proton affinity of PA(C₄H₂⁺)=1.50±0.42 eV.     

Sensitivity improvement in laser ablation inductively coupled plasma mass spectrometry achieved using a methane/argon and methanol/water/argon mixed gas plasma

The influence of the addition of carbon using methane or methanol/water to an inductively coupled plasma (ICP) via the carrier gas flow on the sensitivity in laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was studied. During the ablation of SRM NIST 610 with simultaneous addition of CH(4) (0.6-1.4 ml min(-1)), a sensitivity enhancement of more than one order of magnitude for selected analytes (e.g. (75)As(+)) was observed. In addition to the sensitivity enhancement for As, Te, I and Se, also all other measured elements showed a significantly enhanced sensitivity (minimum by a factor of 2). Potential mechanisms for the observed intensity enhancement include charge transfer reactions, a change in the ICP shape and a temperature increase in the plasma. Furthermore, the aspiration of a methanol-water mixture into a cooled spray chamber and the simultaneous addition to the laser ablated aerosol was investigated. This type of mixing leads to a sensitivity enhancement up to a factor of 20. To prevent clogging of the sampler cone and skimmer cone by carbon deposition, a fast cleaning procedure for the interface is tested during running ICP, which allows the application of such a set-up for specific applications.     

Laser ablation particle beam glow discharge time of flight mass spectrometry for the analysis of halogenated polymers and inorganic solid material

A laser ablation particle beam pulsed glow discharge mass spectrometer (LA-PB-GD-TOFMS) was designed and used for fundamental studies. The instrument consists of a three stage aerodynamic lens system, a hollow cathode pulsed glow discharge and a time-of-flight mass spectrometer. The particle beam interface was constructed to provide an efficient particle transfer into the hollow cathode. Calculations showed that particles between 1 and 3000 nm in diameter are able to pass through this interface.     

Solid-state photodimerization of 4-aryl-1,4-dihydropyridines studied by C CPMAS NMR spectroscopy

¹³C CPMAS NMR spectroscopy has been applied to monitor the solid-state reaction of two different photodimerizing 4-phenyl-1,4-dihydropyridines yielding a cage dimer in one case and an anti-dimer in the other case. The spectra of the reacting monomers exhibit a magnetical inequivalence of chemically equivalent CO and C2/4 carbon atoms caused by a rotation of the pseudoaxially oriented 4-phenyl substituent out off the plane through N1, C3, C8 which could be determined by X-ray crystal structure analyses of the centrosymmetrically arranged monomers. The ¹³C CPMAS NMR monitoring of the cage dimer formation proves that the reaction takes place in two steps via a syn-dimer for which a non-symmetrical structure was derived from the spectrum. The non-symmetrical structure was confirmed by X-ray crystal structure analysis of one structurally related derivative. A centrosymmetric structure for both the finally formed cage dimer and the anti-dimer of the other monitored photoreaction was proved by their spectra with one set of signals for each half of the dimers, respectively. Thus, conformational properties of the molecules as well as the symmetry of the products can be directly derived from the ¹³C CPMAS NMR spectra.     

Phenomenological studies on structure and elemental composition of nanosecond and femtosecond laser-generated aerosols with implications on laser ablation inductively coupled plasma mass spectrometry

In laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), the properties of laser-generated aerosols, such as size and composition, are crucial for matrix-independent quantification. In this study, the aerosol particle morphology and elemental composition generated by two state-of-the-art laser systems (ArF excimer nanosecond-UV laser and Ti:sapphire femtosecond-IR laser) were investigated by electron microscopic techniques. Electrostatic sampling of the aerosols directly onto transmission electron microscopy (TEM) grids allowed us to study the morphology and elemental composition of the aerosols using TEM and TEM–EDX (energy dispersive X-ray spectroscopy) analyses, respectively. The results of the electron microscopic studies were finally compared to the LA-ICPMS signals of the main matrix components. The investigations were carried out for non-conducting materials (glass and zircon), metallic samples (steel and brass) and semiconductors (sulfides). The studies confirm that ns-LA-generated aerosols dominantly consist of nanoparticle agglomerates while conducting samples additionally contain larger spherical particles (diameter typically 50 to 500 nm). In contrast to ns-laser ablation, fs-LA-generated aerosols consist of a mixture of spherical particles and nanoparticle agglomerates for all investigated samples. Surprisingly, the differences in elemental composition between nanoparticle agglomerates and spherical particles produced with fs-LA were much more pronounced than in the case of ns-LA, especially for zircon (Si/Zr fractionation) and brass (Cu/Zn fractionation). These observations indicate different ablation and particle formation mechanisms for ns- and fs-LA. The particle growth mechanism for ns-LA is most likely a gas-to-particle conversion followed by agglomeration and additional hydrodynamic sputtering for conducting samples. On the other hand, phase explosion is assumed to be responsible for the mixture of large spherical particles and nanoparticle agglomerates as found for fs-LA-generated aerosols. Based on these mechanisms, the overall temporal elemental fractionation effects in ns-LA-ICPMS seem to occur mainly during the ablation. This effect was not observed for fs-LA-ICPMS despite the element separation into different particle fractions, which, on the other hand, could induce severe ICP-induced fractionation.     

A Raman study of the structural properties of YBa2(Cu1 − xFex)3Oy

The effects of Fe-substitution of YBa₂Cu₃O_y have been investigated by means of Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. A series of samples of YBa₂(Cu_{1 − x}Fe_x)₃O_y with different dopant concentration (0 x 0.15) has been prepared in two batches, the second set having undergone twice the heat and mechanical treatment used to produce the first batch. Considerable improvement in the superconducting transition temperature, T_c, is obtained upon reprocessing. A phase transformation from orthorhombic to tetragonal symmetry is observed for x=0.05 from the X-ray measurements in agreement with previous work. Using a micro-Raman technique, all five A_g vibrational modes have been measured and their dependence on Fe-concentration is analyzed. There are indications that iron substitutes for copper at both sites and that the structure is a mixture of orthorhombic and tetragonal microdomains for all x.