Single-shot experiments for the acquisition of coherence-transfer functions in real time.

Simple pulse sequences are introduced that make it possible to acquire experimental Hartmann-Hahn transfer functions for arbitrary multiple-pulse sequences in a single shot. With this approach it is possible to study the detailed dependence of coherence-transfer functions on experimental parameters in real time.     

Offset dependence of homonuclear hartmann-hahn transfer based on residual dipolar couplings in solution state NMR

The offset dependence of homonuclear Hartmann-Hahn transfer is markedly different if a given multiple-pulse sequence is applied to dipolar coupled spins or to isotropically coupled spins. The different offset dependence of the transfer efficiency is studied by numerical simulations of a multiple-pulse sequence and by the analytical treatment of a simple model experiment.     

Lattice sum calculations for 1/rp interactions via multipole expansions and Euler summation

A method is developed here for doing multiple calculations of lattice sums when the lattice structure is kept fixed, while the molecular orientations or the molecules within the unit cells are altered. The approach involves a two‐step process. In the first step, a multipole expansion is factored in such a way as to separate the geometry from the multipole moments. This factorization produces a formula for generating geometry constants that uniquely define the lattice structure. A direct calculation of these geometry constants, for all but the very smallest of crystals, is computationally impractical. In the second step, an Euler summation method is introduced that allows for efficient calculation of the geometry constants. This method has a worst case computational complexity of O(( log N)²/N), where N is the number of unit cells. If the lattice sum is rapidly converging, then the computational complexity can be significantly less than N. Once the geometry constants have been calculated, calculating a lattice sum for a given molecule becomes computationally very fast. Millions of different molecular orientations or molecules can quickly be evaluated for the given lattice structure. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 208–215, 2001     

(Meta)stabiles CaC2

(Meta)stable CaC₂ One out of four modifications of CaC₂ is the so‐called metastable Calcium Carbide, CaC₂‐III, which was synthesized as pure material. It forms by heating monoclinic CaC₂‐II (C2/c) above 150 °C and remains stable after cooling down to room temperature. The structure was refined from X‐ray powder patterns (C2/m, Z = 4, a = 722.6(1) pm, b = 385.26(7) pm, c = 737.6(1) pm, β = 107.345(2)°). After grinding CaC₂‐III transforms back into CaC₂‐II. Heating CaC₂‐III induces a reversible phase transition into the cubic modification (CaC₂‐IV) at 460 °C. Differences between the three different structures of CaC₂ I–III, being stable at ambient conditions are also shown by ¹³C‐MAS‐NMR measurements, especially the presence of two distinct types of carbon atoms in the structure of the title compound.     

Struktur und elektrochemische Untersuchung von Nb3Cl8

Structure and Electrochemical Study of Nb₃Cl₈ The compound Nb₃Cl₈ was synthesized from NbCl₅ and niobium metal in a sealed quartz ampoule at 700 °C. Single crystals, obtained from LiCl melt were used for X‐ray structure determination (space group P 3 m1, Z = 2, lattice parameters a = b = 672.95(7) pm, c = 1223.2(2) pm (at 100 K), R1 = 0.029, wR2 = 0.064 for all independent reflections). Electrical resistivity measurements are reported. Electrochemical intercalation of lithium into the structure of Nb₃Cl₈ was studied.     

SIAM,a Novel NMR Experiment for the Determination of Homonuclear Coupling Constants

The simultaneous acquisition of in-phase and antiphase multiplets with high sensitivity and minimum overlap (see section of 2D spectra on the right) is possible in a novel NMR experiment. Based on this method, homonuclear coupling constants such as the ³J(H^N,H^α) couplings in peptides and proteins can be determined quantitatively without isotope labeling.     

Über die Koexistenz von tetragonalem und monoklinem CaC2: Strukturelle und spektroskopische Untersuchungen an Erdalkalimetallacetyliden,MC2 (M = Ca,Sr, Ba)

On the Coexistence of Tetragonal and Monoclinic CaC₂: Structural and Spectroscopic Studies on Alkaline Earth Metal Acetylides, MC₂ (M = Ca, Sr, Ba) The alkaline earth acetylides CaC₂, SrC₂ and BaC₂ can be considered to occur in three polymorphic structures each. The monoclinic low-temperature form, the tetragonal form, and the cubic high-temperature form. No deviation from axial symmetry is obtained for the C₂^2– ions in the tetragonal structure determinations, as confirmed by X-ray single-crystal structures and ¹³C MAS NMR studies. The CaC₂ samples prepared by us were always a mixture of monoclinic and tetragonal phase. Their Raman spectra exhibited two distinct C₂ streching vibrations. Problems arising from the coexistence of these two phases for the interpretation of ¹³C MAS NMR spectra are discussed.     

On-Line Coupling of Separation Techniques to NMR

The hyphenation of chromatographic separation techniques with NMR spectroscopy is one of the most powerful and time-saving methods for the separation and structural elucidation of unknown compounds and molecular compositions of mixtures. Most of the routinely used NMR flow-cells have detection volumes between 40–180 μL for conventional separations with analytical columns, and the newest designs employ detection volumes in the order of 200 nL for capillary separations. The low flow rates used in capillary chromatography permit the use of deuterated solvents. Unequivocal structural assignment of unknown chromatographic peaks is possible by two-dimensional stopped-flow capillary HPLC-NMR experiments.