Thermogravimetric characterisation of poly(methyl methacrylate) photopolymerised by colloidal cadmium sulphide

The thermal stability of poly(methyl methacrylate) (PMMA) photopolymerised using colloidal cadmium sulphide as the photoinitiator was studied by thermogravimetry (TG) and differential TG (DTG).The thermal stability of the CdS initiated PMMA was greater than that of conventional radically polymerised PMMA and approached that of anionically prepared PMMA. The DTG curve of the CdS initiated PMMA was a composite of four peaks, three of which correspond to the three peaks observed in the DTG curve of standard radically prepared PMMA. It is suggested that the additional peak arises from a new mode of depolymerisation initiation, that is, from chain end unsaturation introduced into the polymer chain during polymerisation initiation with the colloidal CdS.     

Hydrogen recombination on metals: vibrational excitation of desorbed molecules

The vibrational excitation of hydrogen molecules formed by recombination of hydrogen atoms on different metal surfaces has been studied. A recently developed experimental technique was used to determine vibrational state populations of the molecules up to v=7. Excitation of high vibrational levels is observed for molecules formed on metal surfaces which strongly chemisorb hydrogen. Metals with weaker chemisorption lead to vibrational excitation of lower levels, predominantly up to v=3. Knowledge of the vibrational state of desorbed molecules gives information on the energy of adsorbed hydrogen atoms and on the dynamics of surface reactions. This information is complementary to that obtained from studies of hydrogen reactions with metal clusters.     

A multichannel electron energy loss spectrometer for low-temperature condensed films

We describe a wide-gap multichannel cylindrical deflection electron energy analyzer suitable for measuring the weak signals characteristic of electronically inelastic electron energy loss spectra. The analyzer has nearly ideal fringing field termination, and its resolution and energy dispersion were characterized as a function of energy by solving numerically the equation of motion of electrons in an ideal cylindrical electric field. The numerical results for the radial location of the electrons at the detector as a function of the entrance location, angle, and energy are closely approximated by a second order polynomial, and match closely with those observed. The detection efficiency of the analyzer is 100-150 times better than that of an equivalent single-channel instrument, but limited energy transmission of the zoom lens system used in our case reduced it by a factor of about 2. The performance of the new instrument was demonstrated by measuring the (3)E(1u) electronic spectrum of benzene in only 2 min and the spectrum of endo-benzotricyclo[4.2.1.0(2.5)]nonane.     

Mass spectrometric determination of the ionisation cross-sections of BaO, Ba, BaF2 and BaI2 by electron impact

A Nier-type mass spectrometer coupled with a Knudsen cell was used for the ionisation cross-section measurements of Ba, BaO, BaF₂ and BaI₂, using appropriate internal standards with known ionisation cross-sections. The vapour pressure ratio of standard to investigated compound, needed for calculation was obtained via the chemical analysis of the effusate. The use of this approach eliminated several sources of uncertainties. The problems concerning the preferential loss of fragment ions was also discussed and partly eliminated. The results obtained were compared with predictions, usually used in the field of high temperature mass spectrometry and a more recent approach using a model for ionisation cross-sections of high temperature molecules (MCM).     

Kinetics of Cu<Superscript>2+</Superscript> binding to the poly(acrylic acid) hydrogel

Isothermal kinetics of copper (ion) binding to poly(acrylic acid) (PAA) hydrogel at 20, 25, 35 and 45°C was investigated. Isothermal conversions and kinetic curves of Cu²⁺ binding to the PAA hydrogel were determined. It was found that the well-known kinetic models of Peppas cannot be applied to describing the entire process of Cu²⁺ binding. The new method for the determination of the kinetic model of the Cu²⁺ binding process, as well as the activation energy density distribution functions of PAA hydrogel interaction with Cu²⁺, were established. It was found that Cu²⁺ diffusion to the active centers (with E _a = 9 kJ/mol) has a dominant influence on the kinetics of the process at temperatures T ≥ 30°C, but at T ≥ 30°C and for the degree of bound Cu²⁺ α ≥ 0.2, the interaction of Cu²⁺ from the adsorption center with E _a = 26 kJ/mol is dominant. The text was submitted by the authors in English.     

Influence of iron ions on the structural properties of some inorganic glasses

The effects of iron on the structural properties of Zn-borosilicate glass and Pb-metaphosphate glass were studied using X-ray diffraction,⁵⁷Fe Mössbauer spectroscopy and IR spectroscopy. Zn-borosilicate glass was prepared with varying amounts of Fe₂O₃ (up to 30% wt.). It was found that the chemical form of added iron (-FeOOH, -Fe₂O₃ or Fe₃O₄) affects the Fe³⁺/Fe²⁺ ratio, as well as the distribution of iron ions at different coordination sites. At high concentration of iron the crystallization of zinc ferrite in the glass matrix takes place. X-ray diffraction and⁵⁷Fe Mössbauer spectroscopy showed that the amount of zinc ferrite in Zn-borosilicate glass decreases with the following order of addition: -FeOOH-Fe₂O₃Fe₃O₄. In Pb-metaphosphate glass doped with high concentration of -Fe₂O₃, the crystallization of Fe₃(PO₄)₂ is pronounced. The assignments of IR band positions and the corresponding interpretation are given. The importance of this study for the technology of vitrification of high-level radioactive wastes is emphasized.     

Column high-performance liquid chromatographic determination of norfloxacin and its main impurities in pharmaceuticals

A gradient reversed-phase column high-performance liquid chromatographic method was developed for the detection and quantification of norfloxacin and its major impurities in norfloxacin-containing pharmaceuticals. Chromatographic separations were performed under the following experimental conditions: column, Zorbax SB RP-18 (5 microm, 250 x 4.6 mm); injection volume, 20 microL; mobile phase, 0.05 M NaH2PO4 (pH 2.5)-acetonitrile (87 + 13) for 16 min and (58 + 42) for 9 min (stepwise gradient); and flow rate, 1.3 mL/min. All analyses were performed at 25 degrees C, and the eluate was monitored at 275 nm using a diode array detector. Linearity (correlation coefficient = 0.999), recovery (99.3-101.8%), relative standard deviation (0.2-0.7%), and quantitation limit (0.12-0.47 microg/mL) were evaluated and found to be satisfactory. The method is simple, rapid, and convenient for purity control of norfloxacin in both raw materials and dosage forms.     

Artificial cytochrome b: computer modeling and evaluation of redox potentials.

We generated atomic coordinates of an artificial protein that was recently synthesized to model the central part of the native cytochrome b (Cb) subunit consisting of a four-helix bundle with two hemes. Since no X-ray structure is available, the structural elements of the artificial Cb were assembled from scratch using all known chemical and structural information available and avoiding strain as much as possible. Molecular dynamics (MD) simulations applied to this model protein exhibited root-mean-square deviations as small as those obtained from MD simulations starting with the crystal structure of the native Cb subunit. This demonstrates that the modeled structure of the artificial Cb is relatively rigid and strain-free. The model structure of the artificial Cb was used to determine the redox potentials of the two hemes by calculating the electrostatic energies from the solution of the linearized Poisson-Boltzmann equation (LPBE). The calculated redox potentials agree within 20 meV with the experimentally measured values. The dependence of the redox potentials of the hemes on the protein environment was analyzed. Accordingly, the total shift in the redox potentials is mainly due to the low dielectric medium of the protein, the protein backbone charges, and the salt bridges formed between the arginines and the propionic acid groups of the hemes. The difference in the shift of the redox potentials is due to the interactions with the hydrophilic side chains and the salt bridges formed with the propionic acids of the hemes. For comparison and to test the computational procedure, the redox potentials of the two hemes in the native Cb from the cytochrome bc(1) (Cbc(1)) complex were also calculated. Also in this case the computed redox potentials agree well with experiments.     

Association of copper(II) isonicotinamide moieties via different anionic bridging ligands: Two paths of ferromagnetic interaction in the azide coordination compound

Three isonicotinamide (isn) copper(II) complexes with different bridging ligands, azide, thiocyanate and sulfate, have been prepared. The molecular structure of [Cu₂(μ-1,1-N₃)₂(μ-1,3-N₃)₂(isn)₂]_n (1) is composed of binuclear species, Cu₂(μ-1,1-N₃)₂(isn)₂, inter-connected by additional four azide bridges in the end-to-end mode (1,3). This gives a CuN₄N square-pyramidal coordination sphere around each copper(II) ion. A trans mononuclear octahedral coordination sphere CuN₄S₂ is present in [Cu(μ-N,S-NCS)₂(isn)₂]_n (2), with thiocyanato ligands serving as bridges between the adjacent Cu(isn)₂ moieties. The third anionic ligand, i.e. sulfate, in {[Cu(μ-O,O’-SO₄)(H₂O)(isn)₂]·2H₂O}_n (3) completes the CuO₂N₂O square-pyramidal coordination sphere, and thus enables bridging between the mononuclear Cu(H₂O)(isn)₂ moieties. The ligands that bridge the principal building blocks, i.e. binuclear in 1 and mononuclear in 2 and 3, connect the axial ligands with the equatorial positions of the copper(II) coordination spheres in all three cases. A ferromagnetic interaction FM is found for 1, while 2 and 3 are paramagnetic. Therefore, the key structural difference between 1 on one hand, and 2 and 3 on the other, is found in the anionic ligand, serving in 1 also as the intra-binuclear bridge, showing the main path (J₁) for the FM interaction. Additionally, the inter-binuclear pathway in 1 gives another contribution (J₂) to the whole FM interaction seen herein (J₁ = 18.5 cm^–1, J₂ = 4.9 cm^–1).     

Supramolecular Architecture via Self-Complementary Amide Hydrogen Bonding in [Ag(pia)<Subscript>2</Subscript>](NO<Subscript>3</Subscript>)·H<Subscript>2</Subscript>O: Synthesis,Thermal and Structural Studies

Abstract  

The synthesis, thermal and spectral characterization and crystal structure of silver(I) complex with picolinamide, [Ag(C₆H₆N₂O)₂](NO₃)·H₂O, are reported. The silver(I) atom is chelated by two picolinamide (pia) ligands in approximately square planar geometry. The distortion within the coordination environment is mainly imposed by formation of the chelate rings, but it is also observed in two longer (Ag–O) and two shorter (Ag–N) bond lengths. The compound crystallizes in the triclinic space group P[`1]P\bar{1} with a = 7.1265(2) ?, b = 8.9157(4) ?, c = 12.9527(4) ?, α = 83.934(3)°, β = 86.094(2)°, γ = 67.023(3)° and Z = 2. Cationic complexes are linked through amide–amide hydrogen bonds of ‘head-to-head’ R ₂²(8) motif leading to infinite chains, while nitrate anions and H₂O molecules act only as a cross-link between such four symmetry related cationic chains via hydrogen bonds forming 2D supramolecular double sheets. Therefore, the ‘head-to-head’ amide interactions in [Ag(C₆H₆N₂O)₂](NO₃)·H₂O are robust enough to accommodate the usually disruptive NO₃ ⁻ anion and H₂O molecule and could be regarded as a tool for controlling the assembly of this silver complex.