A Paramagnetic Diniobium Complex with a Very Short Nb-Nb Distance: Evidence for a Pseudo Nb-Nb Triple Bond?

In spite of the short Nb-Nb distance (2.268 ?) and the presumable existence of an Nb identical withNb bond, the paddle-wheel-shaped diniobium(II) complex 1 is paramagnetic. Theoretical calculations indicate that the presence of LiCl moieties on the intermetallic axis lowers the Nb-Nb bond order and is responsible for the observed paramagnetism.     

The Inverse Problem of the Calculus of Variations: Separable Systems

This paper deals with the inverse problem of the calculus of variations for systems of second-order ordinary differential equations. The case of the problem which Douglas, in his classification of pairs of such equations, called the separated case is generalized to arbitrary dimension. After identifying the conditions which should specify such a case for n equations in a coordinate-free way, two proofs of its variationality are presented. The first one follows the line of approach introduced by some of the authors in previous work, and is close in spirit, though being coordinate independent, to the Riquier analysis applied by Douglas for n = 2. The second proof is more direct and leads to the discovery that belonging to the separated case has an intrinsic meaning for the given second-order differential equations: the system is separable in the sense that it can be decoupled into n pairs of first-order equations.     

6Li and 7li solid-state NMR spectroscopy of nitrogen ceramic phases

Two nitrogen ceramic phases, the oxynitride LiSiON and the nitride LiSi2N3, have been studied by 6Li and 7Li NMR. Magic angle spinning (MAS) NMR experiments have been carried out at two magnetic field strengths (7.05 and 14.1 T). The spectra give evidence of the relative effects of the quadrupolar and chemical shift interactions. The electric field gradient tensor of both phases has been determined accurately by iterative fitting of the 6Li and 7Li MAS NMR line shapes at the two magnetic field strengths. Due to the fact that for 7Li the quadrupolar interaction is much larger than the chemical shift interaction, it is shown that neither the small chemical shift anisotropy nor the relative orientation of the two interaction tensors can be determined accurately by 7Li MAS NMR. For 6Li, the two interactions are comparable and the value of these parameters obtained from the fits of the 6Li experimental MAS line shapes are therefore much more reliable.     

Practical Methods for Solid-State NMR Distance Measurements on Large Biomolecules: Constant-Time Rotational Resonance

Simple modifications of the rotational resonance experiment substantially reduce the total experimental time needed to measure weak homonuclear dipolar couplings, a critical factor for achieving routine internuclear distance measurements in large biomolecular systems. These modifications also address several problems cited in the literature. Here we introduce a constant-time rotational resonance experiment that eliminates the need for control spectra to correct for effects from variable RF heating, particularly critical for accurate long-distance measurements. This reduces the total number of experiments needed by as much as a factor of 2. Other improvements incorporated include achieving selective inversion with a delay rather than a weak pulse (P. R. Costa et al., J. Am. Chem. Soc. 119, 10487-10493, 1997), which we observe results in the elimination of oscillations in peak intensities for short mixing time points. This reduces the total experiment time in two ways. First, there is no longer a need to average different "zero"-time points (S. O. Smith et al., Biochemistry 33, 6334-6341, 1994) to correct for intensity variations. Second, short-mixing-time lineshape differences observed in large membrane-bound proteins only appear with the weak-pulse inversion and not when using the delay inversion. Consistent lineshapes between short and long mixing times permit the use of a single spectrum for subtraction of natural abundance background signals from all labeled-protein time points. Elimination of these effects improves the accuracy and efficiency of rotational resonance internuclear distance measurements.     

The divergence theorem and the Laplacian in Minkowski space

Let (X, B) be a Minkowski space (finite-dimensional Banach space) with unit ball B. Using a Minkowski definition of unit normal to a hypersurface, a Minkowski analogue of Euclidean divergence is defined. We show that the divergence theorem holds. Using the Minkowski divergence, a Minkowski Laplacian is defined. We prove that this Laplacian is a second-order, constant-coefficient, elliptic, differential operator. Furthermore, the symbol of this Laplacian is computed and used to associate a natural Euclidean structure with (X, B).Supported, in part, by NSERC Operating Grant #4066.     

Hysteretic magnetic moment of YBa2Cu3O7?δ: An angular study

The hysteretic magnetization of an YBa₂Cu₃O_7−δ crystal containing a high density of Y₂BaCuO_x precipitates was investigated by two-axis, quasistatic, de magnetometry. As the eff-axis applied magnetic fieldH was increased and the magnetization magnitude, decreased, its orientation approached a direction near the crystallinec axis. The bulk persistent currentsJ _p predominately flowed parallel to the Cu−O planes for a wide range of temperature and applied field. Even forH almost perpendicular toc, where the deduced irreversibility line maximized as commonly observed, the resulting magnetization was observed to be within 15° of [001]. Managed by Lockheed Martin Energy Research Corp, under contract AC05-96OR22464 with the U. S. Dept. of Energy.     

Magnetization study of polycrystalline YBa2AlxCu(3−x)O7−δ (x = 0.05, 0.10 and 0.20) compounds

Superconductivity in polycrystalline YBa₂Al_xCu_(3−x)O_7−δ materials was characterized by dynamic AC and quasistatic DC magnetometry. Intragranular persistent current density and low-loss intergranular critical current density were deduced using DC and AC techniques, respectively. Addition of aluminum produced modest increases in the intragranular persistent current for x < 0.2, but drastically reduced the intergranular critical current density for x = 0.2. The critical temperature T_c for superconductivity decreased only 4% for Al content up to x = 0.2.     

Chromatographic analysis of phospholipid and cholesterol oxidation

Capillary thin layer and gas chromatographic methods for analysis of the extent of oxidation in phosphatidyl choline/cholesterol samples are described. Examples of systems suitable for qualitative and quantitative analysis, based on use of unmodified samples or of their derivatives, are illustrated. A method for concurrent quantitative determination of phospholipid and sterol without preseparation is introduced and is based on extension of a previous lipid trans-methylation technique.     

Study of the R meson produced in 13 GeV/cπ+p interactions

The missing mass spectrum opposite the proton in a 750 000 picture exposure, 13 GeV/cπ⁺p bubble chamber experiment, is investigated in two and four pion channels for structures observed or denied by boson spectrometers at the same energy in the reaction π^?p→X^?p.In the four pion final state the R⁺ enhancement is observed clearly and the possibility of multi-component mass structure is considered. Treating the phenomenon as one object, cross sections and branching ratios are derived for intermediate quasi two and three body decay modes (ωπ, ??, A₂π, ?ππ). The signals in the two pion state and for the neutral state produced opposite Δ⁺⁺(1231) are also presented and an elasticity is calculated.