Flow injection determination of aluminium by spectrofluorimetric detection after complexation with N-o-vanillidine-2-amino-p-cresol: the application to natural waters

An on-line flow injection spectrofluorimetric method for the direct determination of aluminium in water samples is described. The method is based on the reaction of aluminium with N-o-vanillidine-2-amino-p-cresol (OVAC) in acidic medium at pH 4.0 to form a water-soluble complex. The excitation and emission wavelengths were 423.0 and 553.0 nm, respectively, at which the OVAC-Al complex gave the maximum fluorescence intensity at pH 4.0 in a 50% methanol-50% water medium at 50 °C. An interference from fluoride ions was minimised by the addition of Be²⁺. Other ions were found not to interfere at the concentrations likely to be found in natural waters. The proposed methods were validated in terms of linearity, repeatability, detection limit, accuracy and selectivity. Under these conditions, the calibration was linear up to 1000 μg L⁻¹ (r = 0.999). The limit of detection (3σ) for the determination of Al(III) was 0.057 μg L⁻¹ and the precision for multiple determinations of 3 ng mL⁻¹ Al(III) prepared in ultra-pure water was found to be 0.62% (n = 10).The Schiff base ligand could be used to determine ultra-trace aluminium from natural waters. Analysis of environmental certified reference materials showed good agreement with the certified values. The procedure was found to be equally applicable to both freshwater and saline solutions, including seawater.     

Synthesis, magnetism, and high-frequency EPR spectroscopy of a family of mixed-valent cuboctahedral Mn13 complexes with 1,8-naphthalenedicarboxylate ligands

Four mixed-valent (Mn(IV)Mn(III)(6)Mn(II)(6)) tridecanuclear Mn clusters [Mn(13)O(8)(OH)(6)(ndc)(6)] (1), [Mn(13)O(8)(OEt)(5)(OH)(ndc)(6)] (2), [Mn(13)O(8)(O(2)CPh)(12)(OEt)(6)] (3), and [Mn(13)O(8)(OMe)(6)(ndc)(6)] (4) are reported, where ndcH(2) is 1,8-naphthalenedicarboxylic acid. This is the first use of the latter in Mn chemistry. Complexes 1-3 are essentially isostructural and possess a central core composed of three layers. The middle layer consists of a Mn(II)(6) hexagon containing a central Mn(IV) atom, and above and below this are Mn(III)(3) triangular units. These core Mn atoms are held together by a combination of O(2-), RO(-), or HO(-) bridging groups. The overall metal topology is an unusual one, with the overall geometry being a metal-centered cuboctahedron (heptaparallelohedron). Variable-temperature, solid-state dc, and ac magnetization studies were carried out on complexes 1-4 in the 5.0-300 K range. Compound 1 was found to possess an S = 9/2 ground-state spin, whereas 2, 3, and 4 have an S = 11/2 ground state. Fitting of the magnetization (M) versus field (H) and temperature (T) data by matrix diagonalization and including only axial zero-field splitting, D, gave D = -0.14 cm(-1) for 1. High-frequency EPR studies were carried out on single crystals of 1.xDMF, and these confirmed D to be very small, that is, 1 is essentially isotropic. The combined work demonstrates the ligating ability of 1,8-naphthalenedicarboxylate, notwithstanding its robust organic backbone and the restricted parallel disposition of its two carboxylate moieties, and its usefulness in the synthesis of new polynuclear Mn(x) clusters. The work also demonstrates a sensitivity of the ground-state spin in this Mn(13) family of complexes to relatively small structural perturbations, while the high-frequency EPR study demonstrated the magnetically isotropic nature of the Mn(13) core.     

Stein fillability and the realization of contact manifolds

There is an intrinsic notion of what it means for a contact manifold to be the smooth boundary of a Stein manifold. The same concept has another more extrinsic formulation, which is often used as a convenient working hypothesis. We give a simple proof that the two are equivalent. Moreover it is shown that, even though a border always exists, its germ is not unique; nevertheless the germ of the Dolbeault cohomology of any border is unique. We also point out that any Stein fillable compact contact -manifold has a geometric realization in via an embedding, or in via an immersion.

     

Structural investigation of the promotional effect of hydrogen during the selective catalytic reduction of NOx with hydrocarbons over Ag/Al2O3 catalysts

In situ EXAFS has been used to examine the hydrogen effect on the selective catalytic reduction of NOx over silver/alumina catalysts. For all SCR conditions used, with or without co-reductant (H2 or CO), the catalyst structure remained the same. Significant changes in the catalyst were only found under reducing conditions. The enhanced activity found in the presence of hydrogen is thought to be due to a chemical effect and not the result of a change in the structure of the active site.     

Ferrimagnetically ordered nanosized polyoxomolybdate-based cluster spheres

The self-assembly of molybdate building units in the presence of Fe(II) and V(IV) produces the first mixed-spin heterometal keplerate-type clusters displaying ferrimagnetic interactions.     

The forced vibrational response of a rectangular parallelepiped with rigid-lubricated boundaries

The Green function for a rectangular parallelepiped with rigid-lubricated boundaries is developed by a normal mode approach, the free vibration solutions being used. Explicit solutions are presented for a concentrated impulse, which serves as a model for an acoustic emission stress wave, and for a concentrated step force. Numerical results for short times show good agreement with the infinite space solution. Analogous solutions are developed for the inverse boundary conditions.     

Thermally induced nuclear dipolar polarization in powders

Photon spin symmetry conversion of methyl groups undergoing tunnelling rotation in powder samples at low temperature is accompanied by induced proton dipolar polarization.