Electrochemical and photoelectrochemical characterization of CuFeO2 single crystal

CuFeO₂ single crystal, synthesized by the flux method, is a narrow band gap semiconductor crystallizing in the delafossite structure with a direct optical transition of 1.63 eV. The oxide exhibits a good chemical stability; the semi-logarithmic plot gave an exchange current density of 0.60 μA cm⁻² in KCl (0.5 M) electrolyte. CuFeO₂ shows p-type conductivity; the origin of acceptors Cu²⁺ results from oxygen insertion in the layered lattice where most of excess holes are trapped in surface-polaron states. The electrochemical study is confined in the (a,b) plane and reversible oxygen intercalation is evidenced from the intensity potential characteristics. The detailed photoelectrochemical studies have been reported for the first time on the single crystal. The photocurrent is ascribed to the transfer Cu⁺:3d→3d. The capacitance measurement (C⁻²–V) shows a linear behavior from which a flat band potential of +0.54 V_SCE and a density N _A of 1.60 × 10¹⁸ cm⁻³ were determined. The valence band, located at 5.33 eV below vacuum, is made up of Cu-3d orbital typical of delafossite oxides. The Nyquist plot shows a semicircle attributed to a capacitive behavior with a low density of surface states within the gap. The centre is localized below the real axis with an angle of 16.2° ascribed to a constant phase element (CPE), a single barrier of the junction CuFeO₂/electrolyte and one relaxation time of the electrical equivalent circuit.

The magnetic concentration gradient force—Is it real?

There are suggestions in the electrochemical literature that a body force F _∇c acts when an electrolyte with a non-uniform concentration c of paramagnetic ions is subject to a uniform magnetic field. We demonstrate, experimentally and theoretically, that no such magnetic body force exists, to first order. A second-order correction associated with the demagnetizing field does lead to a very small concentration-dependent body force, which is not expected to produce any observable effect in electrochemistry. Contribution to special issue on Magnetic field effects in Electrochemistry.

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Luminescent Properties of Hexanuclear Molybdenum(II) Chloride Clusters Containing Thiolate Ligands

The emission spectra of a series of molybdenum(II) chloride clusters containing thiolate ligands, [Mo₆(μ₃-Cl)₈(SR)₆]²⁻ (R = Et, Bu, Ph, Bn), have been recorded. These complexes all show a broad emission at ~700 nm after excitation at 337, 400 and 410 nm. Determination of the excited state lifetimes and quantum yields of these complexes in acetonitrile reveals that (Bu₄N)₂[Mo₆(μ₃-Cl)₈(SPh)₆] displays the longest excited state lifetime of this series (26 μs at 296 K).

Mass transfer effects on the electropolymerization current efficiency of 3-methylthiophene in the magnetic field

3-Methylthiophene was chosen as a representative conducting polymer precursor, whose electropolymerization proceeds through the coupling of cation radicals. The density of the solution that contains electrogenerated 3-methylthienyl radicals and early oligomers is higher than the density of the surrounding solution, and at low (<0.2 M) monomer concentrations the diffusion layer falls, compromising the electropolymerization current efficiency. Applying a homogeneous magnetic field (3.0 T) perpendicular to the electrode surface (θ=0°) produces concentration gradient paramagnetic forces (F _∇C) that hold the diffusion layer in contact with the electrode. This gives time for more oligomers to get oxidized and for more cation radicals to couple so that the current efficiency increases from 0.028 to 0.037 at 0.05 M of monomer concentration, and from 0.051 to 0.071 at 0.1 M. With the magnetic field parallel to the electrode surface (θ=90°) Lorenz forces causing magnetohydrodynamic convection, in combination with F _∇C forces keeping the flow pattern in contact with the electrode, increase the current efficiency even more, to 0.048 at 0.05 M of monomer concentration, and to 0.076 at 0.1 M. At higher monomer concentrations (>0.2 M), the rate of radical coupling is evidently fast enough so that, even in the absence of a magnetic field, no natural convection effects are observed and the current efficiency (0.7–0.8) is not affected by the magnetic field.

Electron free energy levels in oxidic solutions: relating oxidation potentials in aqueous and non-aqueous systems

We provide background to the problem of describing the state of redox couples in different types of solvent media ranging from acidic aqueous solutions to high temperature molten silicates, pointing out the essential similarity between these solvent media in Lewis acid–base terms. We review the adaptation of the Gurney proton energy level diagram approach to the case of electron transfer processes. Using data from various spectroscopic and analytical chemistry sources, we review the construction of electron free energy level diagrams for redox couples in aqueous and non-aqueous systems using, as a common reference, the potential of the oxygen gas (1 atm)/oxide ion couple in the solution of interest. We emphasize the anomalous effect of “oxide ion activity” (mean ionic activity of alkali oxide) on the state of equilibrium and interpret this in terms of oxide ion transfers that accompany electron transfers. After showing the essential agreement between recent direct electrochemical assessments of the energy levels and those deduced in our original analysis of oxidic melts of different glass formers, we provide an interpretation of the apparent “oxide ion transfer” in terms of the differential medium polarization by the two redox species involved in the equilibrium. We anticipate the extension of these ideas to redox chemistry in the currently burgeoning field of “ionic liquids” in its recent ambient temperature liquid incarnation. Dedicated to the 85th birthday of John O’M. Bockris.

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The Physical and Electronic Structure of M<Subscript>2</Subscript> Quadruply Bonded Complexes: A Density Functional Theory Study

The electronic structures and the physical properties (vertical excitation energies, vibrational stretching frequencies, and bond lengths) of a variety of M–M quadruply bonded (M = Mo, W) complexes are investigated using density functional theory (DFT). By utilizing a variety of pure and hybrid exchange-correlation (XC) functionals and a number basis sets, we are able to recommend a theoretical methodology for most efficiently probing the electronic structures of homoleptic M₂(O₂CR)₄ and bridged M₂(O₂C-X-CO₂)M₂ (R = organic group, typically H; X = conjugated organic group) complexes.

Core-shell structured electrode materials for lithium ion batteries

Recent progress in studies of several types of core-shell structured electrode materials, including TiO₂/C, Si/C, Si/SiO _x , LiCoO₂/C, and LiFePO₄/C nanocomposites, including details of their preparation and their electrochemical performance is briefly reviewed. Results clearly show that the coating shell can effectively prevent the aggregation of the nanosized cores, which are the electrochemically active materials. In addition, the diffusion coefficients of lithium ions can be increased, and the reversibility of lithium intercalation and deintercalation is improved. As a result, the cycling behavior is greatly improved. The reviewed results suggest that core-shell nanocomposites are a good starting point for further development of new promising electrode materials.

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A comparative study of hydrothermal and sol–gel methods in the synthesis of MnO<Subscript>2</Subscript> nanostructures

In this work, MnO₂ nanostructure powders with different crystalline phases have been successfully prepared by hydrothermal and sol–gel methods. The obtained products were characterized by XRD and SEM techniques and their crystallographic phases and morphological properties compared. Results showed that α-MnO₂, β-MnO₂, and δ-MnO₂ nanorods were synthesized by hydrothermal method and γ-MnO₂ polymorph was obtained by sol–gel method.

A fluorous tag-bound fluorescence derivatization reagent, F-trap pyrene, for reagent peak-free HPLC analysis of aliphatic amines

We have developed a novel pre-column fluorescence derivatization reagent for amines, F-trap pyrene. This reagent comprises a fluorescent pyrene moiety, an amine-reactive Marshall linker, and a fluorophilic perfluoroalkyl group known as fluorous tag. When the reagent reacts with aliphatic amines and amino acids to give fluorescent derivatives, the fluorous tag in the reagent is eliminated simultaneously. Therefore, excess unreacted reagents in the derivatization reaction solution still have the fluorous tag and could be removed by fluorous solid-phase extraction selectively before high-performance liquid chromatography (HPLC) analysis. By using this reagent, 13 kinds of aliphatic amine (C₂–C₁₆) derivatives can be separated within 40 min by reversed-phase HPLC with gradient elution. In this chromatogram, unreacted reagents peak at around 28 min, greatly decrease after fluorous solid-phase extraction, and do not interfere with the quantification of each amine. The detection limits (S/N = 3) for examined aliphatic amines are 3.6–25 fmol per 20 μL injection. We have also applied this reagent successfully to the amino acid analysis.

Three crystal structures reported as lacking centers that should be refined as centrosymmetric

The compounds {[Fe(phen)₃]²⁺(TCNQ–TCNQ)²⁻) · 2(CH₃OH)} (FIWPRD), {[Fe(C₅H₅)(C₅H₄CH₂NMe₃)]⁺)(TCNQ⁻) (IKONOL), and {[Cu(1,4,5,12-tetraazacyclo-pentadecane)]²⁺(TCNQ⁻)₂} (AVOJEA) were reported in the non-centrosymmetric space groups Cc (#9), Pna2₁ (#33), and P1 (#1). Examination of the several sets of atomic coordinates shows that the space groups are more likely to be C2/c (#15), Pnma (#63), and (#2), respectively. Confirmation of the centrosymmetric models requires access to the diffraction intensities; unfortunately these are not in the public domain.

A novel Ce<Superscript>III</Superscript>-cyclam type complex and its redox chemistry in aqueous solutions

A new Ce^III complex was synthesized by mixing the ligand “dioxocyclam” with Ce^III ions at pH 8.0 and its redox properties were investigated.

Modeling of solvent viscosity effects on the electroreduction of Pt(II) aquachlorocomplexes

Solvent dynamics effects on the electroreduction of [PtCl₃(H₂O)]⁻ at a mercury electrode are explored in the framework of Sumi–Marcus model using an efficient computational scheme. According to results of density functional calculations, the second electron transfer step may be regarded as rate controlling. The nonmonotonous influence of solvent viscosity on the reaction rate is predicted and explained in terms of the saddle point avoidance. The results of model calculations are employed to interpret experimental data reported earlier in the literature.

Core-shell effects of functionalized oxide nanoparticles inside long-range meso-ordered spray-dried silica spheres

Core-shell and homogeneous distributions of functionalized cerium oxide nanoparticles within spray-dried mesostructured silica spheres are achieved by modification of synthesis parameters such as the templating agent and nanoparticle capping functions. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

NEMCA effect: why are the work function changes of the gas exposed catalyst-electrode surface one-to-one related to the changes in the catalyst working electrode potential?

In the present work, an important point concerning the NEMCA effect is addressed. We analyse the reasons why the changes in the work function of the gas exposed catalyst-electrode surface are one to one related to the changes in the catalyst working electrode potential E with respect to a reference electrode. It is concluded that this is due to the unique properties of the catalyst/solid electrolyte interface: the structure of the double layer in this region is very different from that in liquid electrolytes, being the potential difference at this interface mainly determined by the specific adsorption of the mobile species in the solid electrolyte.

Influence of structural defects on the electrocatalytic activity of platinum

Structural defects play major role in catalysis and electrocatalysis. Nanocrystalline (or nanostructured) materials composed of nanometer-sized crystallites joined via grain boundaries have been recognized for their specific structure and properties, differentiating them from single crystals, coarsely grained materials or nanometer-sized supported single-grained particles (Gleiter, Nanostruct Mater 1:1–19, 1992). In this paper, we use Pt electrodes, prepared by electrodeposition on glassy carbon and gold supports, as model nanocrystalline materials to explore the influence of grain boundaries and other structural defects on electrocatalysis of CO and methanol oxidation. We build on the recently established correlations between the nanostructure (lattice parameter, grain size, and microstrains) of electrodeposited Pt and the deposition potential (Plyasova et al., Electrochim. Acta 51:4447–4488, 2006) and use the latter to obtain materials with variable density of grain boundary regions. The activity of electrodeposited Pt in the oxidation of methanol and adsorbed CO exceeds greatly that for Pt(111), polycrystalline Pt, or single-grained Pt particles. It is proposed that active sites in nanostructured Pt are located at the emergence of grain boundaries at the surface. For methanol electrooxidation, the electrodes with optimal nanostructure exhibit relatively high rates of the “direct” oxidation pathway and of the oxidation of strongly adsorbed poisoning intermediate (CO_ads), but not-too-high methanol dehydrogenation rate constant. These electrodes exhibit an initial current increase during potentiostatic methanol oxidation explained by the CO_ads oxidation rate constant exceeding the methanol decomposition rate constant.

Synthesis and dopamine receptor binding affinity of 4H-thiochromenoapomorphines

Abstract  The synthesis of 4H-thiochromene derivatives of apomorphines, a novel class of isoquinoline alkaloid-related compounds, has been achieved by different O-dealkylation methods applied on previously published heteroring-fused aporphinoids. Detailed DFT study has been presented regarding the mechanism of the L-selectride-mediated multiple O-dealkylation of a seven-ring aporphine analogue. Dopamine-binding tests confirmed the essential function of 11-hydroxy moiety of the aporphine skeleton and revealed a remarkable D₁ over D₂ specificity for the derivative having the 4H-thiochromene ring system attached to positions 2 and 3 of the aporphine backbone. Graphical Abstract  

CAL3JHH: a Java program to calculate the vicinal coupling constants (<Superscript>3</Superscript><Emphasis Type="Italic">J</Emphasis><Subscript>H,H</Subscript>) of organic molecules

Here, we present a free web-accessible application, developed in the JAVA programming language for the calculation of vicinal coupling constant (³ J _H,H) of organic molecules with the H–C_sp3–C_sp3–H fragment. This JAVA applet is oriented to assist chemists in structural and conformational analyses, allowing the user to calculate the averaged ³ J _H,H values among conformers, according to its Boltzmann populations. Thus, the CAL3JHH program uses the Haasnoot–Leeuw–Altona equation, and, by reading the molecule geometry from a protein data bank (PDB) file format or from multiple pdb files, automatically detects all the coupled hydrogens, evaluating the data needed for this equation. Moreover, a “Graphical viewer” menu allows the display of the results on the 3D molecule structure, as well as the plotting of the Newman projection for the couplings. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

Molecular and crystal structure of ethylene acetal of <Emphasis Type="Italic">endo</Emphasis>-<Emphasis Type="Italic">endo</Emphasis>-3-trityloxymethylbicyclo[3.3.1]nonane-2-on-7-ol

Molecular structure of ethylene acetal of 3-trityloxymethylbicyclo[3.3.1]nonane-2-on-7-ol (3) was determined by X-ray investigation. It was revealed that 3 has endo-endo-configuration and adopt chair-boat-conformation. The distortion of rings was evaluated by calculation of the Zefirov–Palyulin and Cremer–Pople puckering parameters. Failure of esterification of 3 was rationalized in terms of steric hindrance and intramolecular hydrogen bonding.

Sol–gel transitions of sodium montmorillonite dispersions by cationic end-capped poly(ethylene oxides) (surface modification of bentonites, IV)

Poly(ethylene oxides), PEO, end-capped with hexadecyl dimethylammonium, HDDMA⁺, and trimethylammonium hexyldimethylammonium, THA²⁺, changed distinctly the sol–gel transition of sodium montmorillonite dispersions. In the presence of HDDMA⁺-PEO 1,500 and 4,000, domains of sol with increased salt tolerance (c _k=550–1,000 mmol NaCl/l) were found at high polymer and low montmorillonite contents. The corresponding PEO of higher molar mass (20,000 and 35,000) led to extended fields of flocs. THA²⁺-PEO 1,500 formed attractive gels at polymer concentrations >2–5 g/l and montmorillonite contents >0.5%. These gels showed very high yield values. THA²⁺-PEO of higher molar mass acted as stabilizing agents. The salt tolerance was highest (300–750 mmol/l) in the presence of THA²⁺-PEO 20,000. The observed sol–gel diagrams reveal the interplay between polymer end-group fixation on the clay mineral particles, polymer conformation, and colloidal stabilization and destabilization mechanisms.

In defence of the correlation coefficient

The relationship between calibrated range, residual standard deviation and correlation coefficient r is discussed. It is shown that in typical chemical calibration applications with appropriately distributed calibration points (particularly, with range comparable to mean and with approximately even or with ‘successive dilution’ spacing), the linear correlation coefficient has valid application as a routine criterion for acceptable fit if used with due care.