Cross relaxation studied by β NMR

The dipole-dipole interaction between polarized β active probes implanted into a metal and the surrounding host nuclei may lead to a transfer of polarization from the probe to the host. As energy must be conserved in this cross-relaxation (CR) process, resonance-like dips show up at certainB values, if the polarization of the probes is measured as a function of magnetic inductionB. The probe isotope¹²B has been studied in several metallic stoppers. For¹²B in the fec crystals Al, Cu the three possible cubic implantation sites could easily be discriminated by means of the CR technique. The temperature dependence of the dips yielded information on the diffusion of the probes. In the case of¹²B, in Cu above 400 K a further CR structure was found due to trapping of the probe at another site. In the bcc metal VCR spectra belonging to two different¹²B trapping sites could be separately registered using a special radiofrequency technique. Both, static and dynamical properties of the CR could be quantitatively explained by theory.     

Lithium-bis(silyl)amide und Tris(silyl)amine Synthese und Kristallstrukturen

Lithium Bis(silyl)amides and Tris(silyl)amines Synthesis and Crystal Structures Lithiated di-tert-butylfluorosilylamine reacts with difluorosilanes by substitution ( 1, 2 ). The siloxy-( 3, 4 ) and tert-butyloxy-( 5 )-silylamines are formed in reaction of 1 and 2 with LiOR (R = SiMe₃, CMe₃). The lithium derivatives of 3 and 4 are dimers forming an (LiFSiN)₂-eight-membered ring ( 6, 7a ). Using 12 crown-4 the amide and the coordinated lithium are forming free ions ( 7 c ). The lithium derivative of 5 ( 8 ) crystallizes as a dimeric LiF-adduct of an iminosilane, forming a LiF-four-membered ring. In thf 7 reacts with Me₃SiCl by a fluorine/chlorine exchange and 9 is obtained. In 9 lithium is coordinated with nitrogen, oxygen and two thf molecules, forming an (SiNOLi)-four-membered ring. 6 and 7 react with fluorosilanes to give tris(silyl)amines 10 – 12 .     

NMR and cross relaxation of substitutional and interstitial12B in vanadium

Cross-relaxation resonances of the polarization of¹²B nuclei in single-crystal Vanadium were observed at various values of the external B field. From the comparison with calculated relaxation spectra the electric field gradients at the neighbouring host nuclei were determined and the locations of¹²B were found to be the substitutional and the octahedral interstitial sites.     

The equilibrium unfolding of MerP characterized by multivariate analysis of 2D NMR data

A general problem when analysing NMR spectra that reflect variations in the environment of target molecules is that different resonances are affected to various extents. Often a few resonances that display the largest frequency changes are selected as probes to reflect the examined variation, especially in the case, where the NMR spectra contain numerous resonances. Such a selection is dependent on more or less intuitive judgements and relying on the observed spectral variation being primarily caused by changes in the NMR sample. Second, recording changes observed for a few (albeit significant) resonances is inevitably accompanied by not using all available information in the analysis. Likewise, the commonly used chemical shift mapping (CSM) [Biochemistry 39 (2000) 26, Biochemistry 39 (2000) 12595] constitutes a loss of information since the total variation in the data is not retained in the projection into this single variable. Here, we describe a method for subjecting 2D NMR time-domain data to multivariate analysis and illustrate it with an analysis of multiple NMR experiments recorded at various folding conditions for the protein MerP. The calculated principal components provide an unbiased model of variations in the NMR spectra and they can consequently be processed as NMR data, and all the changes as reflected in the principal components are thereby made available for visual inspection in one single NMR spectrum. This approach is much less laborious than consideration of large numbers of individual spectra, and it greatly increases the interpretative power of the analysis.     

Pair distribution function and 71Ga NMR study of aqueous Ga3+ complexes

The atomic structures, and thereby the coordination chemistry, of metal ions in aqueous solution represent a cornerstone of chemistry, since they provide first steps in rationalizing generally observed chemical information. However, accurate structural information about metal ion solution species is often surprisingly scarce. Here, the atomic structures of Ga³⁺ ion complexes were determined directly in aqueous solutions across a wide range of pH, counter anions and concentrations by X-ray pair distribution function analysis and ⁷¹Ga NMR. At low pH (<2) octahedrally coordinated gallium dominates as either monomers with a high degree of solvent ordering or as Ga-dimers. At slightly higher pH (pH ≈ 2–3) a polyoxogallate structure is identified as either Ga₃₀ or Ga₃₂ in contradiction with the previously proposed Ga₁₃ Keggin structures. At neutral and slightly higher pH nanosized GaOOH particles form, whereas for pH > 12 tetrahedrally coordinated gallium ions surrounded by ordered solvent are observed. The effects of varying either the concentration or counter anion were minimal. The present study provides the first comprehensive structural exploration of the aqueous chemistry of Ga³⁺ ions with atomic resolution, which is relevant for both semiconductor fabrication and medical applications.

With changing pH four different structural regions in Ga³⁺ aqueous solutions are observed. In contrast the effects of different anions and concentrations are minimal.     

In Situ Total X‐Ray Scattering Study of WO3 Nanoparticle Formation under Hydrothermal Conditions

Pair distribution function analysis of in situ total scattering data recorded during formation of WO₃ nanocrystals under hydrothermal conditions reveal that a complex precursor structure exists in solution. The WO₆ polyhedra of the precursor cluster undergo reorientation before forming the nanocrystal. This reorientation is the critical element in the formation of different hexagonal polymporphs of WO₃.     

Implantation site of boron in heavily doped silicon: A β-NMR study

Using β-NMR with¹²B as nuclear probes the temperature dependence of the lattice-site occupation of boron implanted into heavily doped silicon is studied. In p-type material the unperturbed substitutional fraction of¹²B increases from 10% at 300 K to ≃40 % at 950 K. In n-type material this fraction starting from 20% at 300 K approaches the saturation value of ≃80 % at 600 K already. This behaviour suggests that the site of implanted boron in silicon is controlled by the Fermi level.     

Cross relaxation of β active nuclei: Principles and applications to12B in fcc metals

The cross-relaxation technique was used to determine the implantation sites of¹²B in Al and Cu crystals. From the temperature dependence of the cross-relaxation spectra information on the diffusion of the probes was obtained.