On the characterization of local aromatic properties in benzenoid hydrocarbons

A generalization of the Kekulé index for composite valence structures provides a quantitative value corresponding to Clar's assessment of benzenoid character of aromatic ring systems. The idea involved is further specialized for the individual local rings in the molecules. Comparison with limited experimental data supports the validity of this approach.     

On the evaluation of the characteristic polynomial via symmetric function theory

The use of power sum symmetric functions leads to Newton's identities, which relate the traces of various powers ofA, the adjacency matrix of a graph, and the coefficients of the characteristic polynomials. While it is possible to solve Newton's identities and generate the coefficients by recursion or, alternatively, to derive them by sequential manipulations (yielding the explicit formulas), we show how the results can be expressed using a combinatorial approach and relate the evaluation of the coefficients to selected Young diagrams.     

Spectrophotometric determination of ethanol by an enzymatic method with 2, 2'-azino-di(3-ethylbenzthiazo- line-6-sulfonate)

The method is based on the oxidation of ethanol in the presence of alcohol oxidase, followed by oxidation of 2,2'-azino-di(3-ethylbenzthiazoline-6-sulfonate) in the presence of peroxidase.     

Critical behaviour of the compressible magnet with exchange anisotropy

Then-component magnet with exchange anisotropy on a compressible lattice, with isotropic elastic properties, is studied. The renormalization group method is applied ind =4 — dimensions. The fixed points and the stability regions are explored to the order ², and the analysis is concentrated upon the casen<4—2 +O( ²). Investigation of the fixed points reveals various crossover phenomena which are not present in the corresponding rigid model. Renormalization of the anisotropy crossover exponent is demonstrated. It is shown that macroscopic instabilities, leading to the first order phase transition, may appear.     

Raman scattering in SnSe

Zone center optical phonons in SnSe were investigated using Raman scattering data obtained here and previously reported far infrared reflectivity measurements. The values of the ratio of the pairs of interlayer force constants were calculated te be between 3·3 and 4·5. Thus it can be concluded that SnS is a layer like semiconductor but less layer like than GeS, GeSe or SnS.     

Selective pervaporation of water through a nonselective microporous titania membrane by a dynamically induced molecular sieving mechanism

Pervaporation experiments were performed on microporous titania membranes using several binary liquids containing 2-20 wt % water. The membrane was nonselective in the separation of water from alcohols and p-dioxane but showed a remarkably high selectivity in the separation of water from ethylene glycol/water mixtures with < or =15 mol % water. The absence of selectivity under most conditions is explained by the large pore size (0.9 nm) of microporous titania. The high selectivity for water in the separation from ethylene glycol can be explained by the formation of a hydrogen-bonded network of ethylene glycol in the micropores, which blocks transport of ethylene glycol, while water can still permeate through. These networks are disrupted by water at higher concentrations, leading to full loss of membrane selectivity.     

Intermolecular potential energy surface and spectra of He-HCl with generalization to other rare gas-hydrogen halide complexes

A two-dimensional (rigid monomer) intermolecular potential energy surface (PES) of the He-HCl complex has been obtained from ab initio calculations utilizing the symmetry-adapted perturbation theory (SAPT) and an spdfg basis set including midbond functions. The bond length in HCl was chosen to be equal to the expectation value in the ground vibrational state of isolated HCl. The rigid-monomer potential should be a very good approximation to the complete (three-dimensional) potential for H-Cl distances corresponding to the lowest vibrational levels of the monomer since the He-HCl interaction energy was found to be only weakly dependent on the HCl bond length in this region, at least as compared to systems such as Ar-HF. The calculated points were fitted using an analytic function with ab initio computed asymptotic coefficients. As expected, the complex is loosely bound, with the dispersion energy providing the majority of the attraction. Our SAPT PES agrees with the semiempirical PES of Willey et al. [J. Chem. Phys. 96, 898 (1992)], in finding that, atypically for rare gas-hydrogen halide complexes including the lighter halide atoms, the global minimum is on the Cl side (with intermonomer separation 3.35 A and depth of 32.8 cm(-1)), rather than on the H side, where there is only a local minimum (3.85 A, 30.8 cm(-1)). The ordering of the minima was confirmed by single-point calculations in larger basis sets and complete basis set extrapolations, and also using higher levels of theory. We show that the opposite findings in the recent calculations of Zhang and Shi [J. Mol. Struct: THEOCHEM 589, 89 (2002)] are due to the lack of midbond functions in their basis set. Despite the closeness in depth of the two linear minima, the existence of a relatively high barrier between them invalidates the assumption of isotropy, a feature of some literature potentials. The trends concerning the locations of minima within the family of rare gas-hydrogen halide complexes are rationalized in terms of the physical components of the intermolecular forces and related to monomer properties. The accuracy of the SAPT PES was tested by performing calculations of rovibrational levels. The transition frequencies obtained were found to be in excellent agreement (to within 0.02 cm(-1)) with the measurements of Lovejoy and Nesbitt [J. Chem. Phys. 93, 5387 (1990)]. The SAPT PES predicts a dissociation energy for the complex of 7.74 cm(-1) which is probably more accurate than the experimental value of 10.1+/-1.2 cm(-1). Our analysis of the ground-state rovibrational wave function shows that the He-HCl configuration is favored over the He-ClH configuration despite the ordering of minima. This is due to the greater volume of the well in the former case. We have also determined positions and widths of three low-lying resonance states through scattering calculations. These predictions are expected to be more accurate than values derived from experiment.     

The effect of calcination conditions on the superconducting properties of Y−Ba−Cu−O powders

Calcination conditions of the precursor powders, i.e. temperature, type of atmosphere and duration, were determined with a view to obtain superconducting powders with the most advantageous physico-chemical properties. Investigated were powders in the Y?Ba?Cu?O system prepared by the sol-gel method. Thermogravimetric examinations of the powders have revealed that the decomposition kinetics of BaCO₃ determines the formation rate of the superconducting YBa₂Cu₃O_7?x (‘123’) phase. It follows from the decomposition kinetics of BaCO₃ that the process is the most intensive in argon, whereas in static air and oxygen it is the slowest. The phase composition analysis (XRD) and low-temperature magnetic susceptibility measurements of the calcinated powders, confirm the above mentioned changes in the decomposition kinetics. The reaction of barium carbonate can be completed if the calcination process is conducted at the temperature of 850°C for 25 h, yielding easily sinterable powders for obtaining single-phase superconducting bulk samples with advantageous functional parameters.     

Sorption of trace concentrations of gallium(III) and iodide ions on tin(II) hydroxide

Summary The results of sorption measurements for gallium (III) and iodide ion traces onto tin(II) hydroxide from aqueous heterogeneous systems are given. Since only tagged sorbate was used (⁶⁷Ga and 1311), in the determination of sorption by radioactive tracer method, it was necessary to establish the pH interval of tin(II) hydroxide in/stability. This was done by a turbidimetric method and the pH region of instability between 3 and 9 was found.The sorption results indicate a difference in sorption ability of gallium(III) and iodide as it appears from the dependence of the amount sorbed on contact time between heterogeneous reactants (sorbentsorbate). The amounts sorbed increase with increasing contact times at higher pH values for Ga(III) but remain unaffected for iodide sorption. Thus the sorption of iodide ions seems to be restricted to the solid surface while the sorption results for gallium(III) indicate the diffusion of sorbate into sorbent as a possible parallel mechanism. Those processes could be taken as compatible with the porous-double-layer mechanism which has been proposed in literature for aqueous heterogeneous oxide systems.

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Zusammenfassung In der Arbeit werden Ergebnisse von Sorptionsmessungen für Ga³⁺- und j-Spuren an Zinn(II)-Hydroxid aus wäßrigen heterogenen Systemen berichtet. Zur analytischen Erfassung wurden radioaktives⁶⁷Ga und 131 J verwendet. Die Stabilität von Zinn(II)-Hydroxid wurde durch Trübungsmessungen untersucht. Im pH-Bereich von 3–9 ist das Zinn(II)-Hydroxid als kolloide Suspension instabil.Ga³⁺ und J^– verhalten sich bei der Adsorption unterschiedlich. Die adsorbierte Menge steigt mit zunehmender Kontaktzeit bei hohen pH-Werten von Gallium, bleibt jedoch für Jod unverändert. Die Unterschiede werden gedeutet durch die Annahme, daß Jod nur an der festen Oberfläche adsorbiert wird, während die Galliumionen auch in das Sorbens diffundieren können. Man könnte die Ergebnisse als verträglich mit einem porösen Doppelschichtmechanismus ansehen; ein solcher Mechanismus ist in der Literatur für wäßrige heterogene Oxidsysteme vorgeschlagen worden.

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With 4 figures     

Combinatorial and noncommutative list-processing and associated graphs

Recently the authors have proposed a list-processing approach to the modeling of algebraic quantum field theory methods in quantum mechanics in which the noncommutative algebra of quantum-mechanical operators is emulated by lists. The processing produces reordered sequences of elements of a ring with a unit commutator and generates dynamic structures which, for some initial arrangements, correspond to partially ordered graphs characterized by recurrence relations and combinatorial identities. Likewise, in another list-processing application to physical problems, a simulation of Feynman diagrams hinged on predominantly combinatorial aspects and demanded explicit generation of certain combinatorial objects. This motivated an investigation into the combinatorial nature of noncommutative list-processing and of recursive algorithms for explicit construction of combinatorial lists, which we now present. The emphasis is also placed on the consideration of associated graphs and the graph-theoretic origin of the appearance of recurrence relations in the reordering theorems of the noncommutative algebra.