Photoluminescence studies of vacancies and vacancy-impurity complexes in annealed GaAs

Photoluminescence measurements made at various depths below the surface of annealed GaAs single crystals are compared with vacancy distribution profiles obtained from electrical measurements. Results on undoped n-GaAs indicate that isolated Ga or As vacancies form non radiative centers. A broad-band emission at 1.20 eV, arising from V_Ga-donor complexes, is observed in spectra taken from n-type samples doped with Si, Sn or Te. The intensity of the 1.20eV band varies with depth and reaches its maximum value in the region where Ga vacancies are dominant. These results show the consistency between photoluminescence and electrical measurements. A band at 1.37eV has previously been assigned to V_As-acceptor complexes. This band was observed in this study only when the samples had been annealed in ampoules prepared from quartz containing traces of Cu. It is concluded that the 1.37eV band is due to Cu contamination rather than V_As-acceptor complexes.     

The calculations of cell dimensions of stoichiometric phases and binary and ternary solid solutions with the A15 structure

Equations that successfully reproduce the unit cell dimensions of stoichiometric (i) transition metal-transition metal (T-T) and (ii) transition metal-non-transition metal (T-B) phases with the β-Wolfram or A15 structure in terms of the diameters of the component atoms for CN 12 [J. Less-Common Metals, 81, 161 (1981)], are hear applied to binary and ternary solid solutions with the A15 structure, to discover how well and under what conditions they reproduce the observed lattice parameter variation. That in the main, they perform satisfactorily gives further credence to the model on which they are based, which shows how the various interatomic contacts within the interpenetrating icosahedra and CN 14 polyhedra of the structure, combine together in controlling the cell dimensions of the phases. The model also indicates that it is the contacts between the transition metal atoms that are paramount in controlling a. This fact leads to a specific prediction for the lattice parameter variation in transition-metal-rich solid solutions of T-B phases, which is essentially substantiated for phases of niobium.     

Hartree-Fock calculations of surface-symmetry properties with finite-range forces

Complete Hartree-Fock calculations on asymmetric semi-infinite nuclear matter have been performed with a family of finite-range Gogny-type forces. For one of these forces all the computed values of the droplet-model symmetry coefficients J, L, M and Q are in good agreement with recent mass-formula fits.     

Symmetry rules for predicting the course of chemical reactions

Rules are given for a reaction of any molecularity which show whether it is allowed or forbidden by orbital symmetry properties. The rules are based on a rigorous treatment, but correspond to simple concepts in practice. A rough MO sequence for the reactants is all that is needed.

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Zusammenfassung Für Reaktionen beliebiger Molekularität werden Regeln dafür angegeben, ob die Reaktion aus Symmetriegründen erlaubt oder verboten ist. Diese Regeln werden streng hergeleitet, sind jedoch in der Praxis einfach anwendbar. Benötigt wird lediglich die Reihenfolge der MO der Reaktionen.

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Résumé On donne des règles de sélection pour une réaction de molécularité arbitraire selon les propriétés de symétrie orbitale. Les règles sont basées sur un traitement rigoureux mais correspondent en practique à des concepts simples. On a seulement besoin de connaître l'ordre des orbitales moléculaires des réactants.

     

New inhalation anaesthetics: III. Fluorinated aliphatic ethers

A range of fluorinated methyl ethyl ethers, diethyl ethers and methyl butyl ethers has been synthesised as potential inhalation anaesthetics. Seven compounds produced anaesthesia with minimal side effects in mice, and their biological testing will be reported in more detail elsewhere [1].     

Enzyme-catalyzed synthesis of furanosyl nucleotides

A bacterial alpha-d-glucopyranosyl-1-phosphate thymidylyltransferase was found to couple four hexofuranosyl-1-phosphates, as well as a pentofuranosyl-1-phosphate, with deoxythymidine 5'-triphosphate, providing access to furanosyl nucleotides. The enzymatic reaction mixtures were analyzed by electrospray ionization mass spectrometry and NMR spectroscopy to determine the anomeric stereochemistry of furanosyl nucleotide products. This is the first demonstration of a nucleotidylyltransferase discriminating between diastereomeric mixtures of sugar-1-phosphates to produce stereopure, biologically relevant furanosyl nucleotides.     

Magnetoresistance anisotropy of a one-dimensional superconducting niobium strip

We investigated confinement effects on the resistive anisotropy of a superconducting niobium strip with a rectangular cross section. When its transverse dimensions are comparable to the superconducting coherence length, the angle dependent magnetoresistances at a fixed temperature can be scaled as R(theta,H) = R(H/Hctheta) where Hctheta =Hc0(cos2theta + gamma(-2)sin2theta)(-1/2) is the angular dependent critical field, gamma is the width to thickness ratio, and Hc0 is the critical field in the thickness direction at theta=0 degrees . The results can be understood in terms of the anisotropic diamagnetic energy for a given field in a one-dimensional superconductor.     

Development of a practical Buchwald-Hartwig amine arylation protocol using a conveniently prepared (NHC)Pd(R-allyl)Cl catalyst

Continuing efforts to establish a more general "user-friendly" protocol for the palladium-catalysed arylation of amines (Buchwald-Hartwig reaction) are described herein. Significant advances have been made through the use of the versatile (SIPr)Pd(methallyl)Cl complex in conjunction with the reliable base lithium hexamethyldisilazide (LHMDS).