Heteronuclear polarization transfer by symmetry-based recoupling sequences in solid-state NMR

We demonstrate a new set of methods for transferring spin polarization between different nuclear isotopes in magic-angle-spinning solid-state NMR. The technique employs symmetry-based recoupling sequences on one irradiation channel and a simple sequence of between one and three strong radiofrequency pulses on the second channel. A phase shift of the recoupling sequences is applied at the same time as a pi/2 pulse on the second channel. The trajectory of the transferred polarization may be used to estimate heteronuclear distances. The method is particularly attractive for nuclei with low gyromagnetic ratios or for those experiencing strong anisotropic spin interactions, where conventional Hartmann-Hahn cross-polarization is difficult to apply. We demonstrate the method on 1H-13C, 1H-15N and 19F-109Ag systems.     

Anomalous temperature dependence of the dephasing time in mesoscopic kondo wires

We present measurements of the magnetoconductance of long and narrow quasi-one-dimensional gold wires containing magnetic iron impurities in a temperature range extending from 15 mK to 4.2 K. The dephasing rate extracted from the weak antilocalization shows a pronounced plateau in a temperature region of 300-800 mK, associated with the phase breaking due to the Kondo effect. Below the Kondo temperature, the dephasing rate decreases linearly with temperature, in contradiction with standard Fermi-liquid theory. Our data suggest that the formation of a spin glass due to the interactions between the magnetic moments is responsible for the observed anomalous temperature dependence.     

Reaktionen von Zinnchloriden mit Polysulfiden. Die Kristallstrukturen von (PPh4)2[SnCl2(S6)2], (PPh4)2[Sn4Cl4S5(S3)O] und (PPh4)2[SnCl6] · S8 · 2CH3CN

Reaction of Tin Chlorides with Polysulfides. Crystal Structures of (PPh₄)₂[SnCl₂(S₆)₂], (PPh₄)₂[Sn₄Cl₄S₅(S₃)O], and (PPh₄)₂[SnCl₆] · S₈ · 2CH₃CN . The reaction of PPh₄[SnCl₃] with Na₂S₄ in acetonitrile in the presence of small amounts of water yields (PPh₄)₂[Sn₄Cl₄S₅(S₃)O] and minor amounts of (PPh₄)₂[SnCl₂(S₆)₂], PPh₄Cl · 2S₈ and (PPh₄)₂[SnCl₆]. SnCl₄ is partially reduced by (PPh₄)₂S_x, PPh₄[SnCl₃] and (PPh₄)₂[SnCl₆] · S₈ · 2CH₃CN being produced. According to the X-ray crystal structure determination the [Sn₄Cl₄S₅(S₃)O]^2?-ion consists of an O atom that is coordinated by four Sn atoms which in turn are liked with one another by five single S atoms and one S₃ group. In the [SnCl₂(S₆)₂]^2?-ion the Sn atom is octahedrally coordinated by two Cl atoms in trans arrangement and by two chelating S₆ groups. Octahedral [SnCl₆]^2? ions and S₈ molecules in the crown conformation are present in (PPh₄)₄[SnCl₆] · S₈ · 2CH₃CN.     

Analysis of free-edge effects by boundary finite element method

The scaled boundary finite element method is a novel semi-analytical analysis technique that combines the advantages of the finite element method and the boundary element method. Only a part of the boundary of the considered domain has to be discretized but nevertheless the method is solely finite element based. The governing equations are solved in the so-called scaling direction analytically, whereas a finite element approximation of the solution is performed in the circumferential directions, which form the boundary of the considered domain. Thus, the numerical effort can be reduced considerably when handling stress concentration problems such as e.g. the free-edge effect in laminated plates. In order to analyze the free-edge effect in a semi-infinite half plane, some kinematic coupling equations have to be introduced, that not only couple the degrees of freedom on the boundary, but also within the non-discretized domain. The implementation of kinematic coupling equations within the method is presented. Finally, the efficiency of the new approach is shown in some benchmark examples. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Assignment of phycocyanobilin in HMPT using triple resonance experiments

A complete assignment of all resonances of a small organic molecule is a prerequisite for a structure determination using NMR spectroscopy. This is conventionally obtained using a well‐established strategy based on COSY, HMQC and HMBC spectra. In case of phycocyanobilin (PCB) in HMPT this strategy was unsuccessful due to the symmetry of the molecule and extreme signal overlap. Since ¹³C and ¹⁵N labeled material was available, an alternative strategy for resonance assignment was used. Triple resonance experiments derived from experiments conventionally performed for proteins are sensitive and easy to analyze. Their application led to a complete and unambiguous assignment using three types of experiments. Copyright © 2011 John Wiley & Sons, Ltd.