Ein neues Oxogermanat: Li8GeO6 = Li8O2[GeO4]. (Mit einer Bemerkung über Li8SiO6 und Li4GeO4)

A New Oxogermanate: Li₈GeO₆ ? Li₈O[GeO₄] Transparent colourless single crystals of Li₈GeO₆(P6₃cm, a = 550.09(8), c = 1072.2(3) pm, Z = 2; 4-circle-diffractometer Siemens AED 2, MoKα; 326 I_o(hkl), R = 2.4%, R_w = 2.0%), have been prepared. As by-product we always got colourless isometric single crystals of Li₄GeO₄. For the first time we could grow single crystals of Li₈SiO₆ of suitable size and quality. Our structure refinement confirms the assumed structure model [2]: Li₈GeO₆ and Li₈SiO₆ are isotypic with Li₈CoO₆[3] (Li₈SiO₆: a = 542.43(8), c = 1062.6(2) pm, Z = 2; 4-circle-diffractometer Siemens AED 2, MoK_α; 306 I_o(hkl), R = 3.6%, R_w= 3.0%). The known crystal structure of Li₄GeO₄ [4] is confirmed and refined (Cmcm, a = 776.6(2), b = 735.7(3), c = 604.9(2) pm, Z = 4; 4-circle-diffractometer Siemens AED 2, MoKα, 298 I_o(hkl), R = 1.9%, R_w = 1.4%). The Madelung Part of Lattice Energy, MAPLE, and Effective coordination-Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, are calculated.     

Zur Kenntnis ternärer Oxide des Bleis: Über Na2PbO2

On Ternary Oxides of Lead. The Na₆PbO₅ Na₆PbO₅ (pale yellow) crystallizes orthorhombic in Cmcm with a=10.6₈, b=5.70₉, c=10.9₉, å; Z=1. Parameters were refined by least-squares (479 hkO–hk7, MO–K_α) R=0.112₄, R′=0.116, (parameters see text). Isolated PbO₅ groups of pseudotetragonal pyramids lend Pb⁴⁺ Coordination Number (CN) 5. Along [100] and [010] the pyramids are equivalent orientated, but along [001] alternately twisted by 180°. Na+ occupies holes between O^2?, leading to CN 6 (Na″+) and Na″′⁺ and CN 4 (Na′⁺). The MADELUNG part of lattice energy (MAPLE) is calculated and discussed.     

A nonadiabatic lower bound calculation of H 2 + and D 2 +

Accurate nonadiabatic lower and upper bounds for groundstate energies of H ₂ ⁺ and D ₂ ⁺ are calculated with the linearized method of variance minimization. The results in a.u. are –0.59713906₃<E ₀(H ₂ ⁺ )<–0.59713899₄ –0.59878877₅<E ₀(D ₂ ⁺ )<–0.59877873₈ i.e. the values are determined with an absolute error smaller than 0.02 cm^–1 for H ₂ ⁺ and 0.01 cm^–1 for D ₂ ⁺ .     

Sr2InF7-Sr2[InF7] oder Sr2F[InF6]? [1]

The new compound Sr₂InF₇ single crystals has been prepared from a melt using single crystals, the strucutre of Sr₂InF₇ is isotypic with K₂NbF₇ [2]. Details of the structure are discussed, using the Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN.     

Enhancing the effectiveness of virtual screening by fusing nearest neighbor lists: a comparison of similarity coefficients

This paper evaluates the effectiveness of various similarity coefficients for 2D similarity searching when multiple bioactive target structures are available. Similarity searches using several different activity classes within the MDL Drug Data Report and the Dictionary of Natural Products databases are performed using BCI 2D fingerprints. Using data fusion techniques to combine the resulting nearest neighbor lists we obtain group recall results which, in many cases, are a considerable improvement on standard average recall values obtained for individual structures. It is shown that the degree of improvement can be related to the structural diversity of the activity class that is searched for, the best results being found for the most diverse groups. The group recall of active compounds using subsets of the class is also investigated: for highly self-similar activity classes, the group recall improvement saturates well before the full activity class size is reached. A rough correlation is found between the relative improvement using the group recall and the square of the number of unique compounds available in all of the merged lists. The Tanimoto coefficient is found unambiguously to be the best coefficient to use for the recovery of active compounds using multiple targets. Furthermore, when using the Tanimoto coefficient, the "MAX" fusion rule is found to be more effective than the "SUM" rule for the combination of similarity searches from multiple targets. The use of group recall can lead to improved enrichment in database searches and virtual screening.     

Neue Alkalioxoarsenate(V): NaLi2[AsO4] — ein neuer Formeltyp [1]

New Alkali Oxoarsenates(V): NaLi₂[AsO₄] — A New Type of Formula [1] . By heating of well ground mixtures of the binary oxides As₂O₃, Na₂O, and Li₂O₂, molar ratio As:Na:Li = 1.0:1.0:2.0, in a well closed Ni tube (650°C, 21 d) colourless single crystals of NaLi₂[AsO₄] were obtained for the first time. The new orthoarsenate(V) crystallizes orthorhombic (space group P mn2₁-C, No. 31) with Z = 2. The structure determination showed that it is isostructural to β_II-Li₃[VO₄] and that means the Li₃[PO₄]-type. The lattice constants a = 702.9(2) pm, b = 520.5(1) pm, c = 505.4(2) pm were taken from Guinier-Simon powder data. The structure was determined by four-circle diffractometer data [Philips PW 1 100, AgKα , 679 independent out of 2 373 I_o(hkl), R = 3.03%, R_w = 2.29%; parameter see text]. The Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, these calculated via Mean Fictive Ionic Radii, MEFIR, are calculated and discussed.     

Density fluctuations in amorphous and semicrystalline polymers

Summary The small-angle scattering of amorphous and semicrystalline polymers contains an intensity component due to density fluctuations within the crystalline and amorphous domains.For amorphous polymers, the density fluctuations aboveT _g correspond to the theoretical value for a fluid system in thermodynamic equilibrium. BelowT _g , a temperature dependence proportional to T is observed over a range of about 50°. At lower temperatures, a linear relationship with a smaller slope has been found which extrapolates to a non-zero value at 0 °K. This value corresponds to the frozen-in disorder, the slope at low temperatures is related to thermal vibrations and can be evaluated in terms of photon-phonon scattering.Semicrystalline polymers show a temperature dependence of the density fluctuation similar to that of the amorphous polymers. At constant temperature the density fluctuations vary linearly with crystallinity.Natural rubber shows an increase of the density fluctuations with increasing cross-linking densities from which information on the density changes in the vicinity of a cross-link and on the statistics of the distribution of cross-linking can be obtained.

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Zusammenfassung Die Kleinwinkelstreuung amorpher und teilkristalliner Polymere besitzt eine Intensitätskomponente, die von Dichtefluktuationen innerhalb der kristallinen und amorphen Bezirke herrührt. Für amorphe Polymere entspricht die Dichtefluktuation oberhalb vonT _g dem theoretischen Wert für ein fluides System im thermodynamischen Gleichgewicht. UnterhalbT _g wird eine Temperaturabhängigkeit proportional zuT über einen Bereich von etwa 50° beobachtet. Bei tieferen Temperaturen wird eine lineare Beziehung mit einer geringeren Steigung gefunden, welche zu einem endlichen Wert bei 0 °K extrapoliert werden kann. Dieser Wert bezieht sich auf die eingefrorene Fehlordnung, die Steigung bei tiefen Temperaturen ist auf thermische Schwingungen zurückzuführen und kann als Photon-Phonon-Streuung ausgewertet werden.Teilkristalline Polymere zeigen eine Temperaturabhängigkeit der Dichtefluktuation, die der von amorphen Polymeren ähnlich ist. Bei konstanter Temperatur ändert sich die Dichtefluktuation linear mit der Kristallinität.Naturkautschuk zeigt eine mit der Vernetzungsdichte ansteigende Dichtefluktuation, aus der man Information über die Dichteänderung in der Umgebung eines Netzpunktes und die Statistik der Netzpunktverteilung erhalten kann.

     

A novel method for the filterless preconcentration of iron

A novel method of analysis of iron by filterless preconcentration is presented. This is the first example of efficient preconcentration of a refractory transition metal where coprecipitation and columns were omitted. The method applies a manifold of flow injection analysis (FIA) to iron species that are preconcentrated on the inner walls of a tubular reactor. It was found that the adsorption of iron species to the walls was particularly pronounced in reactors of nylon material and enrichment factors of 30-35 could be attained, as dependent on the length of the reactor and on the time of preconcentration. In the preconcentration step of the FIA accessory, the optimum efficacy was obtained when the acidity of the samples was adjusted by HCl to pH = 2.5 whereas the ammonia preconcentration buffer should be kept alkaline at pH = 9.8. After being preconcentrated on the tubular reactor, the iron species were eluted by hydrochloric acid and analysed by flame atomic absorption spectrometry (FAAS). An unprecedented low limit of detection (LOD, 3sigma) of 1.8 microg L(-1) was thus obtained for the analysis of iron by FAAS, and the calibration line was linear up to 100 microg L(-1). A high sampling frequency of 40 per hour was obtained and the protocol allowed analysis of small amounts of iron in drinking water, in digested samples and in saline water. The major influence of interferences originated from ligands that are known to complex strongly to iron species. The method thus developed was uncomplicated in operation and it exhibited an excellent long-term stability and a high selectivity. Further, it was environmentally safe owing to production of non-toxic waste disposals. Equally high enrichment factors (EF) were obtained for Fe(ii) and Fe(iii), which is explained by the very low solubility product of both species.     

Application of the method of images on electrostatic phenomena in aqueous Al2O3 and ZrO2 suspensions

A new solution for the Poisson equation for the diffuse part of the double layer around spherical particles will be presented. The numerical results are compared with the solution of the well-known DLVO theory. The range of the diffuse layer differs considerably in the two theories. Also, the inconsistent representation of the surface and diffuse layer charge in the DLVO theory do not occur in the new theory. Experimental zeta potential measurements were used to determine the charge of colloidal Al2O3 and ZrO2 particles. It is shown that the calculated charge can be interpreted as a superposition of independent H+ and OH- adsorption isotherms. The corresponding Langmuir adsorption isotherms are taken to model the zeta potential dependence on pH. In the vicinity of the isoelectric point the model fits well with the experimental data, but at higher ion concentrations considerable deviations occur. The deviations are discussed. Furthermore, the numerical results for the run of the potential in the diffuse part of the double layer were used to determine the electrostatic interaction potential between the particles in correlation with the zeta potential measurements. The corresponding total interaction potentials, including the van der Waals attraction, were taken to calculate the coagulation half-life for a suspension with a particle loading of 2 vol%. It is shown that stability against coagulation is maintained for Al2O3 particles in the pH region between 3.3 and 7 and for ZrO2 only around pH 5. Stability against flocculation can be achieved in the pH regime between 4.5 and 7 for Al2O3, while the examined ZrO2 particles are not stable against flocculation in aqueous suspensions.