Magnetic correlations in disordered MncZn1−cTe alloys

The spin correlations between Mn ions in Zn_1?cMn_cTe alloys have been measured as a function of temperature and concentration. The spin correlations follow the same sign sequence as for a type III antiferromagnet out to seven shells of neighbours. Complete long range magnetic order was not observed for any c or T.     

Parallel implementation of a mesh-based density functional electronic structure code

We describe the implementation of the mesh-based first-principles density functional code DMol on nCUBE 2 parallel computers. The numerical mesh nature of DMol makes it naturally suited for a massively parallel computational environment. Our parallelization strategy consists of a domain decomposition of mesh points. This evenly distributes mesh points to all available processors and leads to a substantial computational speedup with limited communication overhead and good node balancing. To achieve better performance and circumvent memory storage limitations, the torus wrap method is used to distribute both the Hamiltonian and overlap matrices, and a parallel matrix diagonalization routine is employed to calculate eigenvalues and eigenvectors. Benchmark calculations on a 128-node nCUBE 2 are presented. © 1995 by John Wiley & Sons, Inc.     

Origin of adduct ions in the electron-capture mass spectrum of tetracyanoethylene

The high-pressure electron-capture (HPEC) mass spectrum of tetracyanoethylene (TCNE) is dominated by unexpected hydrogen atom and hydrocarbon radical adduct ions when methane is used as the buffer gas. The origin of these unexpected ions was investigated by three separate mass spectrometric experiments: the electron-capture (EC) rate constant of TCNE was determined and integrated into a previously developed kinetic model of HPEC ion source events; electron impact mass spectra of TCNE were obtained following exposure of the ion source surfaces to irradiated methane and irradiated carbon dioxide; and TCNE was determined by gas chromatographic introduction into the HPEC ion source with multiple ion monitoring. All of these experiments suggest that the reactions leading to the major adduct ions observed in the HPEC mass spectrum of TCNE are initiated by alteration of TCNE on the walls of the ion source rather than in the gas phase.     

A general strategy for the NMR observation of half-integer quadrupolar nuclei in dilute environments.

A general strategy for the observation of low gamma half-integer quadrupolar nuclides in biological systems is presented. The methodology combines low-temperature (4-100 K) techniques with cross-polarization (CP) experiments while employing a so-called Carr-Purcell-Meiboom-Gill spin-echo sequence (CPMG). This combined approach is termed CP/QCPMG. Also discussed are data processing issues that are unique to the induced signals that result from the QCPMG pulse sequence. Central to this strategy is the development of a stable low-temperature (4 to 250 K) NMR double-resonance probe. The probe is robust enough to handle multiple contact experiments and long acquisition periods with 1H decoupling. This approach is illustrated with low-temperature solid-state 67Zn and 25Mg NMR CP/QCPMG experiments on model compounds. The conclusion reached is that the strategy affords sufficient sensitivity to examine Zn2+ and/or Mg2+ binding sites in metalloproteins.     

Diastereomeric Xe chemical shifts in tethered cryptophane cages

Cryptophane cages serve as host molecules to a Xe atom. Functionalization of cryptophane-A has permitted the development of Xe as a biosensor. Synthetic routes used to prepare cryptophanes result in racemic mixtures of the chiral cages. In the preparation of a tethered cryptophane-A cage for biosensor applications, some achiral and chiral substituents such as left-handed amino acids have been used. When the substituent is achiral, the NMR signal of the Xe atom in the functionalized cage in solution is a single isotropic peak, since the Xe shielding tensor components in the R and L cages differ by no more than the signs of the off-diagonal elements. Chiral substituents can split the cage-encapsulated Xe NMR signal into one or more sets of doublets, depending on the number of asymmetric centers in the substituent. We carry out quantum mechanical calculations of Xe nuclear magnetic shielding for the Xe atom at the same strategic position within an L cryptophane-A cage, under the influence of chiral potentials that represent r or l substituents outside the cage. Calculations of the Xe shielding response in the Lr and Ll diastereomeric pairs permit the prediction of the relative order of the Xe chemical shifts in solutions containing the Rl and Ll diastereomers. Where the substituent itself possesses two chiral centers, comparison of the calculated isotropic shielding responses in the Llr, Lrl, Rll, and Lrr systems, respectively, permits the prediction of the Xe spectrum of diastereomeric systems in solutions containing Llr, Rlr, Lll, and Rll systems. Assignment of the peaks observed in the experimental Xe NMR spectra is therefore possible, without having to undertake the difficult synthetic route that produces a single optically pure enantiomer.     

Rotating stall in axial compressors

The phenomenon known as rotating stall is described. Basically, it involves a nonuniform pattern of flow, steadily rotating relative to both the fixed and the rotating blades of axial-flow compressors. Attempts to analyze the phenomenon by means of small-perturbation theories are reviewed. It is shown that the work of several investigators can be included in a single formulation and that they differ only in the type of blade-characteristic relations assumed. The present author's analyses based on linearized airfoil and channel relations are presented in detail. The various theories coincide in the qualitative conclusion that self-induced rotating-stall patterns can occur in stalled, rigid blade rows. A comparison with available experimental results is inconclusive, so far as a choice between the several theories is concerned.

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Zusammenfassung Die als «umlaufende Abreißströmung» bekannte Erscheinung wird beschrieben, welche durch ein Strömungsbild gekennzeichnet ist, das sowohl bezüglich des Stators wie auch des Rotors in einem axialen Verdichter stetig umläuft. Eine Übersicht der theoretischen Ansätze zur Beschreibung dieser Erscheinung im Rahmen einer Störungsrechnung ist gegeben. Es kann gezeigt werden, dass die meisten Ansätze einheitlich behandelt werden können und sich nur in der angenommenen Schaufelcharakteristik unterscheiden. Berechnungen des Verfassers auf Grund von linearisierten «Auftriebskurven» und «Kanalbeziehungen» sind ausführlich dargestellt. Alle Theorien liefern das gleiche qualitative Ergebnis eines selbstinduzierten, umlaufenden Strömungsbildes in einem starren Gitter mit abgerissener Strömung. Ein Vergleich mit Versuchen ergibt vorläufig keine zwingenden Schlüsse betreffend die beste Wahl zwischen den Theorien.

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This research was partially supported by the United States Air-Force under Contract No. AF 33 (038)-21406, monitored by the Office of Scientific Research, Air Research and Development Command.