Influence of surface composition on hydrogen peroxide decomposition catalyzed by Co(II) aminopyridine-supported compounds on amorphous silica gel

Cobalt compounds supported on 2-, 3-, and 4-aminopyridine-modified silica surfaces, named Sil2Co, Sil3Co, and Sil4Co, respectively, were used to catalyze the decomposition of hydrogen peroxide on ethanolic solutions at 293, 298, and 303 K. The calculated k values (x10(-4) s(-1)) for Sil2Co, Sil3Co, and Sil4Co are 0.65, 1.24, and 4.78 (293 K); 1.23, 1.87, and 6.33 (298 K); and 1.80, 2.80, and 10.30 (303 K), respectively. All obtained results evidence that such decomposition is a first-order reaction. Zinc-, nickel-, and copper-supported compounds were also tested, but exhibited a very low catalytic activity. By using the k values at 298 and 303 K, and employing the equation ln (k1/k2) = E(a)/R(1/T2-1/T1), the activation energy values for the considered reaction were Sil2Co = 57.20, Sil3Co = 60.60, and Sil4Co = 73.10 kJ mol(-1), respectively. The low values calculated for E(a) are in agreement with a free-radical mechanism.     

Axial peaking of ultra-cold neutrons in a guide tube

A simple focussing device in the form of a cone has been used to produce a localised flux increase of ultra-cold neutrons.     

Highly symmetric networks derived from cubane-related octametallic complexes of a new oxyanion of carbon, C4O7(4-), each molecule attached to eight neighbors by 24 equivalent hydrogen bonds

Aqueous reaction mixtures containing dihydroxyfumaric acid, M2+ (M = Zn, Co, Fe, and Mg) and acetate ion at room temperature give crystals, suitable for single-crystal X-ray diffraction studies, of composition [(MII)8(C4O7)4(H2O)12].24H2O, in which the C4O74- component is the alkoxide-tricarboxylate tetra-anionic species -OC(CO2-)3, a new oxyanion of carbon. All four compounds crystallize in the body-centered space group I3m with cell dimensions a approximately 15.0 A. Discrete electrically neutral [(MII)8(C4O7)4(H2O)12] molecules are present, with a cubane-like core consisting of four trigonal prismatic metal centers and four mu3-alkoxo bridges from separate -OC(CO2-)3 ligands. The octametallic molecule is highly symmetrical, with four fac tri-aqua octahedral metal centers tetrahedrally appended to the central cubane-based [(MII)8(C4O7)4 ]8- unit. The molecules assemble in the crystal in a body-centered cubic fashion, each being attached to its eight neighbors by 24 equivalent hydrogen bonds-an extreme example of self-complementarity.     

Influence of amide linkages on acidity determinations of humic substances Testing with model-mixtures

A set of experiments, using model compounds, was carried out in order to evaluate the influence of amide linkages on acidity measurements of humic substances (HS). Three benzene-carboxylic acids (2,4-dihydroxybenzoic acid; 3,5-dihydroxybenzoic acid, and phthalic acid) and two peptides (dl-alanyl-dl-alanine, and glycil-l-leucine) were employed to simulate the major acidic functional groups present in humic material. The acidity of the samples was measured, using the Schnitzer and Gupta methods, for each compound separately, as well as for selected mixtures with different compound combinations. General results showed that, in the absence of peptides, phenolic groups with high pK_a values are not detected in the barium hydroxide reaction and the data so obtained do not represent the real concentrations of the acidic groups. For the mixtures containing peptides, the presence of amide linkages distorts the results relative to the total acidity (TA) mainly because under the extreme conditions of the reaction with Ba(OH)₂ hydrolysis of the peptide occurs, consuming extra hydroxyl groups and increasing artificially the phenolic content. Such a condition is minimized by the reduction of the reaction time.     

Novel pulsed switched power supply for a fast field cycling NMR spectrometer

In this paper, we outline the operating principles of a pulsed switched power supply for a fast field-cycling nuclear magnetic resonance spectrometer. The power supply uses a variant of a four-quadrant chopper with a duty cycle that defines the average output current. With this topology only two semiconductors are necessary to drive hundreds of amperes with an output power of several kilowatts. The output current ripple has a well-defined shape that can be reduced to acceptable values by a careful design of the semiconductors' controlling circuits and drivers. A power supply prototype was tested with a home build air-core magnet operating with fields between 0 and 0.21 T. The system is computer controlled using pulse generator and data acquisition PC cards, and specific user-friendly home-developed software. A comparative proton relaxometry study in two well-known liquid crystal compounds 5CB and MBBA was performed to check the reproducibility of the T1 measurements.