High field magnetization and57Fe hyperfine field in amorphous RuxFe80−xB20 alloys

Magnetism in amorphous alloys of Ru_xFe_80–xB₂₀, 0 × 22 has been investigated using high field magnetization and Mössbauer spectroscopy. The average Fe hyperfine field in Ru₁₈Fe₆₂B₂₀ shows an anomalous rise at low temperatures. In Ru rich alloys a low field component appears in the hyperfine field distribution and the alloys do not saturate magnetically in 80 kG fields. It is inferred that their magnetic structure is non collinear and that antiferromagnetic exchange in the system increases with Ru concentration.     

Magnetic and119Sn Mössbauer studies on a new Heusler alloy Co2ScSn

A new alloy, having the formula, Co₂ScSn, has been prepared. This is found to have the cubic Heusler L2₁ type structure (a = 6.19A). Magnetization measurements reveal a large drop in magnetization around 270K. Low field ac susceptibility also shows a transition at 268K which we take to be the Curie temperature of the alloy. The magnetic moment at 5K is found to be 0.55 _B per cobalt atom.¹¹⁹Sn Mössbauer measurements at 8K show a hyperfine split spectrum with a hyperfine field of about 40 kOe. Attempts to prepare other Co₂RSn alloys particularly with R=Lu, were not successful.     

P[N(i-Bu)CH2CH2]3N: a versatile ligand for the Pd-catalyzed amination of aryl chlorides

[reaction: see text] Palladium-catalyzed amination reactions of aryl chlorides with amines proceeded in the presence of the bicyclic triaminophosphine P[N(i-Bu)CH(2)CH(2)](3)N to afford the corresponding arylamines in good to excellent yields. Electron-poor, electron-neutral, and electron-rich aryl chlorides all participated with equal ease.     

15N-NMR Spectra of Azoles with Two Heteroatoms

The ¹⁵N-NMR spectra of azoles, with natural isotope abundance, have been measured under different experimental conditions, and chemical shifts are reported for imidazoles, pyrazoles, oxazoles, isoxazoles, thiazoles, and isothiazoles. General trends of substituent effects in this heterocyclic series are discussed based on the data of 67 substituted azoles, dihydro- and tetrahydroazoles. ¹⁵N, ¹H spin-coupling constants have been determined from spectra obtained by [¹H] → ¹⁵N polarizationtransfer experiments, i.e. an application of INEPT and DEPT pulse sequences. Two-bond and three-bond coupling constants are fully assigned and are discussed in terms of the specific pathways in azoles. The potential of structural applications of the new data is illustrated for isomeric nitro-imidazoles and highly-substituted pyrazoles, and in the case of ring-chain tautomerism of 2-substituted tetrahydrooxazoles.     

Cucurbituril anchored silica gel

A cucurbit[6]uril anchored silica gel is synthesized via reaction of perallyloxycucurbit[6]uril and mercaptopropyl functionalized silica gel and fully characterized by various spectroscopic methods; the amount of accessible host molecules attached on silica surface is quantified by fluorescence spectroscopy using FITC-spermine as a guest molecule.     

Triphenylphosphonium perchlorate as an efficient catalyst for mono- and bis-intramolecular imino Diels-Alder reactions: synthesis of tetrahydrochromanoquinolines

Triphenylphosphonium perchlorate (TPP) is found to be an efficient catalyst for the mono- and bis-intramolecular imino Diels-Alder (IMIDA) reaction of aldimines derived from aromatic amines and O-allyl derivatives of salicylaldehydes to afford the corresponding tetrahydrochromano[4,3-b]quinolines in excellent yields and short reaction times under mild conditions.     

Spectrophotometric studies on oxidation of some primary alcohols by 2,2′-bipyridinium chromate. A kinetic study

Some primary alcohols were oxidized by 2,2-bipyridinium chromate (BPC) in the presence of oxalic acid and TsOH giving aldehyde as major product. The reactions were carried out in 80% MeCN-DMF (v/v) medium under varied experimental conditions. The rate depends on the first power of the concentration of BPC and fractional power on the concentrations of alcohol, oxalic acid and TsOH.     

Konstanten der potentiellen Energie,mittlere Schwingungs-amplituden,Bastiansen—Morino-Schrumpfeffekt und thermodynamische Funktionen einiger isotoper Arten des Chlorcyans

Zusammenfassung Mit Hilfe der gruppentheoretischen Methode werden aus den Grundschwingungsfrequenzen isotoper Arten des Chlorcyans, das eine lineare asymmetrische Struktur der Symmetriepunktgruppe C _v besitzt, vier Bindungskraftkonstanten ermittelt. Aus den Symmetriekoordinaten werden von den gebundenen und nichtgebundenen Atompaaren die mittleren Amplitudenquadrate die verallgemeinerten mittleren Amplitudenquadrate (mittlere Parallelamplitudenquadrate, mittlere Senkrechtamplitudenquadrate und mitlere gemischte Produkte) und die mittleren Schwingungsamplituden für die Temperaturen 298°K, 500°K und 1000°K berechnet. Außer diesen Molekülkonstanten werden für alle vier isotopen Arten des Moleküls die molaren thermodynamischen Funktionen für den Temperaturbereich von 200°K bis 2000°K berechnet. Dieser Rechnung wird das Modell eines harmonischen Oszillators und starren Rotators zugrunde gelegt. Die Natur der beiden charakteristischen Bindungen wird an Hand der erhaltenen Ergebnisse kurz besprochen.

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Potential energy constants, mean amplitudes of vibration, bastiansen—Morino shrinkage effect and thermodynamic functions in some isotopic species of cyanogen chloride

On the basis of the group theoretical method four valence force constants have been evaluated by employing the fundamental frequencies of isotopic species of the cyanogen chloride molecule possessing a linear asymmetrical structure with the symmetry point group C_v. Mean-square amplitude quantities, generalized mean-square amplitude quantities (mean-square parallel amplitudes, mean-square perpendicular amplitudes and mean cross products), and mean amplitudes of vibration for the bonded and nonbonded atom pairs have been computed at the temperatures 298°K, 500°K, and 1000°K by employing the symmetry coordinates. In addition to these molecular constants, the molar thermodynamic functions for all the four isotopic species of the molecule have also been calculated for the temperature range 200–2000°K on the assumption of a rigid rotator, harmonic oscillator model. A brief discussion of the results is given with respect to the nature of the two characteristic bonds.

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Diese Arbeit wurde vom U.S. Army Research Office-Durham unter dem Zeichen Grant DA-ARO-D-31-124-G 864 unterstützt.     

Structural and photoluminescence characteristics of molecular beam epitaxy-grown vertically aligned In0.33Ga0.67As/GaAs quantum dots

In this paper, we present the results of structural and photoluminescence (PL) studies on vertically aligned, 20-period In_0.33Ga_0.67As/GaAs quantum dot stacks, grown by molecular beam epitaxy (MBE). Two different In_0.33Ga_0.67As/GaAs quantum dot stacks were compared. In one case, the In_0.33Ga_0.67As layer thickness was chosen to be just above its transition thickness, and in the other case, the In_0.33Ga_0.67As layer thickness was chosen to be 30% larger than its transition thickness. The 2D–3D growth mode transition time was determined using reflection high-energy electron diffraction (RHEED). Structural studies were done on these samples using high-resolution X-ray diffraction (HRXRD) and cross-sectional transmission electron microscopy (XTEM). A careful analysis showed that the satellite peaks recorded in X-ray rocking curve show two types of periodicities in one sample. We attribute this additional periodicity to the presence of an aligned vertical stack of quantum dots. We also show that the additional periodicity is not significant in a sample in which the quantum dots are not prominently formed. By analyzing the X-ray rocking curve in conjunction with RHEED and PL, we have estimated the structural parameters of the quantum dot stack. These parameters agree well with those obtained from XTEM measurements.     

Locating concentrators in centralized computer networks

Topological design of centralized computer communication networks is a complex problem that is generally solved in two phases. The first phase of the design process involves dividing network nodes (terminals or clusters of terminals) into groups, and selecting a concentrator location for each group so that all the nodes in a group are assigned to the same concentrator. The next phase determines topology of links that connect network nodes to concentrators and concentrators to each other and to the central computer. The design problem studied in this paper contains some aspects of both phases. In this problem locations of concentrators, assignments of user nodes to concentrators and the topology of the links connecting concentrators to the central computer are jointly determined. The proposed design method is built around the well known sweep heuristic which is used to partition the node space into sectors. Each of these sectors contain a backbone path connecting concentrators to the central computer.