Local atomic and electronic structure in the LiVPO4(F,O) tavorite-type materials from solid-state NMR combined with DFT calculations

⁷Li, ³¹P, and ¹⁹F solid-state nuclear magnetic resonance (NMR) spectroscopy was used to investigate the local arrangement of oxygen and fluorine in LiVPO₄F_1-yO_y materials, interesting as positive electrode materials for Li-ion batteries. From the evolution of the 1D spectra versus y, 2D ⁷Li radiofrequency-driven recoupling (RFDR) experiments combined, and a tentative signal assignment based on density functional theory (DFT) calculations, it appears that F and O are not randomly dispersed on the bridging X position between two X–VO₄–X octahedra (X = O or F) but tend to segregate at a local scale. Using DFT calculations, we analyzed the impact of the different local environments on the local electronic structure. Depending on the nature of the VO₄X₂ environments, vanadium ions are either in the +III or in the +IV oxidation state and can exhibit different distributions of their unpaired electron(s) on the d orbitals. Based on those different local electronic structures and on the computed Fermi contact shifts, we discuss the impact on the spin transfer mechanism on adjacent nuclei and propose tentative signal assignments. The O/F clustering tendency is discussed in relation with the formation of short V^IVO vanadyl bonds with a very specific electronic structure and possible cooperative effect along the chain.     

Synthesis and characterization of TiO2/Rh3+ nanoparticulate sols, xerogels and cryogels for photocatalytic applications

Nanoparticulate TiO₂/Rh³⁺ sols have been synthesized by the colloidal sol?Cgel route. The combination of the data measured with optical techniques such as laser diffraction, dynamic light scattering and multiple light scattering with a near-infrared light allows us to follow up the evolution of the peptization process and to establish the effect of the presence of Rh³⁺ on it. It is observed that the presence of rhodium ions retards the peptization step (t₂) and decreases both the average particle size of the nanoparticles and the viscosity of the nanoparticulate sols. In addition, when Rh³⁺ is present the isoelectric point shifts up to higher pH, which suggests that chemical adsorption of the rhodium (III) cations onto the surface of the TiO₂ nanoparticles is produced. The xerogels and cryogels obtained from the sols are constituted by anatase as major phase and traces of brookite. The phase transition is observed at lower temperatures for the xerogels containing rhodium (III) and at higher temperatures in the case of the cryogels. Finally, photocatalytic activity is higher in the case of the TiO₂/Rh³⁺ sols due to the rhodium (III) effect on the electronic transitions from the valence band to the conduction band.     

Thermal study of l-alanine, l-threonine, and taurine crystals related to hydrogen bonding

In this study l-alanine, l-threonine, and taurine crystals were characterized through dilatometric technique and thermogravimetric and differential thermal analysis. The dilatometric analysis shows that the thermal expansion of the crystals is correlated with the strengths of local hydrogen bonding in the amino acid structures at room temperature. Thermogravimetric analysis and differential thermal analysis of the l-alanine, taurine, and l-threonine crystals have been performed at high temperatures. No clear correlation between the hydrogen bonding strengths and endothermic peak positions was observed.     

Preparation and evaluation of carbon-supported catalysts for ethanol oxidation

Supported PtSnIr/C, PtSn/C, and IrSn/C catalysts with potential application in a direct alcohol fuel cell were prepared by chemical reduction employing Pechini methodology. The catalyst particles were characterized by high-resolution transmission electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy (XPS). Linear sweep voltammetry (LV), chronoamperometry, and electrochemical impedance spectroscopy (EIS) measurements were performed by using a glassy carbon working electrode covered with the catalyst in a 1 M ethanol?+?0.5 M H₂SO₄ solution at 60 °C. It was demonstrated through XPS that PtSnIr/C and IrSn/C contain both IrO₂ and SnO₂. LV and chronoamperometry show a better catalytic behavior for ethanol oxidation on PtSnIr/C in the low-potential region and the improvement is attributed to the presence of both Sn and Ir oxides. The EIS accurately established that PtSnIr/C improved ethanol oxidation at lower potentials than PtSn/C.     

Determination of seven selected antipsychotic drugs in human plasma using microextraction in packed sorbent and gas chromatography–tandem mass spectrometry

A method using microextraction by packed sorbent (MEPS) and gas chromatography–tandem mass spectrometry (GC-MS/MS) is described for the determination of seven antipsychotic drugs in human plasma. The studied compounds were chlorpromazine (CPZ), haloperidol (HAL), cyamemazine, quetiapine, clozapine, olanzapine (OLZ), and levomepromazine; promazine, protriptyline, and deuterated CPZ were used as internal standards. The validation parameters included selectivity, linearity and limits of detection and quantitation, intra- and interday precision and trueness, recovery, and stability and were studied according to internationally accepted guidelines. The method was found to be linear between the lower limit of quantitation and 1000 ng/mL, except for OLZ and HAL (200 ng/mL), with determination coefficients higher than 0.99 for all analytes, and extraction efficiencies ranged from 62 to 92 %. Intra- and interday precision ranged from 0.24 to 10.67 %, while trueness was within a ±15 % interval from the nominal concentration for all analytes at all studied levels. MEPS has shown to be a rapid procedure for the determination of the selected antipsychotic drugs in human plasma, allowing reducing the handling time and the costs of analysis. Furthermore, GC-MS/MS has demonstrated to be a powerful tool for the simultaneous quantitation of the studied compounds, enabling obtaining adequate selectivity and sensitivity using a sample volume of as low as 0.25 mL.     

Site-specific C content by quantitative isotopic C Nuclear Magnetic Resonance spectrometry: A pilot inter-laboratory study

Isotopic ¹³C NMR spectrometry, which is able to measure intra-molecular ¹³C composition, is of emerging demand because of the new information provided by the ¹³C site-specific content of a given molecule. A systematic evaluation of instrumental behaviour is of importance to envisage isotopic ¹³C NMR as a routine tool. This paper describes the first collaborative study of intra-molecular ¹³C composition by NMR. The main goals of the ring test were to establish intra- and inter-variability of the spectrometer response. Eight instruments with different configuration were retained for the exercise on the basis of a qualification test. Reproducibility at the natural abundance of isotopic ¹³C NMR was then assessed on vanillin from three different origins associated with specific δ¹³C_i profiles. The standard deviation was, on average, between 0.9 and 1.2‰ for intra-variability. The highest standard deviation for inter-variability was 2.1‰. This is significantly higher than the internal precision but could be considered good in respect of a first ring test on a new analytical method. The standard deviation of δ¹³C_i in vanillin was not homogeneous over the eight carbons, with no trend either for the carbon position or for the configuration of the spectrometer. However, since the repeatability for each instrument was satisfactory, correction factors for each carbon in vanillin could be calculated to harmonize the results.