113Cd Solid‐State NMR for Probing the Coordination Sphere in Metal–Organic Frameworks

Spectroscopic techniques are a powerful tool for structure determination, especially if single‐crystal material is unavailable. ¹¹³Cd solid‐state NMR is easy to measure and is a highly sensitive probe because the coordination number, the nature of coordinating groups, and the geometry around the metal ion is reflected by the isotropic chemical shift and the chemical‐shift anisotropy. Here, a detailed investigation of a series of 27 cadmium coordination polymers by ¹¹³Cd solid‐state NMR is reported. The results obtained demonstrate that ¹¹³Cd NMR is a very sensitive tool to characterize the cadmium environment, also in non‐single‐crystal materials. Furthermore, this method allows the observation of guest‐induced phase transitions supporting understanding of the structural flexibility of coordination frameworks.     

Increased torque transducer sensitivity via oversampling

The sensitivity of a commercial torque transducer under oscillatory shear conditions is extended by about a factor 3–5 into the small torque region. This increased sensitivity is a result of a straightforward data treatment in the time domain termed “on the fly oversampling”. This method is enabled by the emergence of modern ADC-cards. The underlying ideas of oversampling together with a first experimental verification of this method are described in detail. Received: 22 August 2000 Accepted: 14 November 2000     

Simultaneous Determination of 12 Sulfonylurea Herbicides in Drinking Water after SPE by LC-DAD

A liquid chromatographic method (LC) with diode array detection (DAD) for the routine screening and quantification of highly applicated polar herbicides in drinking water samples was developed. The investigated herbicides consisted of 12 sulfonylurea herbicides (amidosulfuron, flazasulfuron, foramsulfuron, iodosulfuron-methyl Na, mesosulfuron-methyl, metsulfuron-methyl, nicosulfuron, prosulfuron, thifensulfuron-methyl, triasulfuron and tritosulfuron) together with 6 polar pesticides of relevance (atrazine, desethylatrazine, desisopropylatrazine, chlortoluron, diuron, fluoxypyr). The herbicides were extracted and concentrated by off-line solid-phase extraction and subsequently eluates were analyzed by LC-DAD. Recoveries obtained from fortified water samples at 100 ng L^?1 were in the range of 84–107% with RSD’s <20%. The limit of detection varied from 2 to 16 ng L^?1.     

Extraction properties of 25,27-bis(carbonylmethoxy)calix[4]arenes towards Sr2+: competitive extraction and extraction in a synthetic groundwater

The Sr²⁺ extraction properties of some 25,27-bis(carbonylmethoxy)calix[4]arenes, in which ester (–CH₂CO₂Et), keto (–CH₂COCH₃) and acid functionalities (–CH₂CO₂H) have been attached to the lower rim, are reported. Strontium ion extraction experiments were performed in a chloroform/water system, and the extraction performance analyzed by radiotracing using the short-lived radio nuclide ⁸⁵Sr. Effects of pH value, ligand to metal ratio, temperature, extraction time and strontium species on the extraction behavior were examined. It was observed that the 25,27-bis(carboxymethoxy)calix[4]arene derivatives are potent extracting agents towards the strontium ion, showing remarkable extraction performance in competition with a series of organic and inorganic impurities and in a synthetic groundwater. The crystal structure of 25,27-bis(acetonyloxy)calix[4]arene is also reported.     

A portable Halbach magnet that can be opened and closed without force: the NMR-CUFF

Portable equipment for nuclear magnetic resonance (NMR) is becoming increasingly attractive for use in a variety of applications. One of the main scientific challenges in making NMR portable is the design of light-weight magnets that possess a strong and homogeneous field. Existing NMR magnets can provide such magnetic fields, but only for small samples or in small regions, or are rather heavy. Here we show a simple yet elegant concept for a Halbach-type permanent magnet ring, which can be opened and closed with minimal mechanical force. An analytical solution for an ideal Halbach magnet shows that the magnetic forces cancel if the structure is opened at an angle of 35.3° relative to its poles. A first prototype weighed only 3.1 kg, and provided a flux density of 0.57 T with a homogeneity better than 200 ppm over a spherical volume of 5mm in diameter without shimming. The force needed to close it was found to be about 20 N. As a demonstration, intact plants were imaged and water (xylem) flow measured. Magnets of this type (NMR-CUFF = Cut-open, Uniform, Force Free) are ideal for portable use and are eminently suited to investigate small or slender objects that are part of a larger or immobile whole, such as branches on a tree, growing fruit on a plant, or non-metallic tubing in industrial installations. This new concept in permanent-magnet design enables the construction of openable, yet strong and homogeneous magnets, which aside from use in NMR or MRI could also be of interest for applications in accelerators, motors, or magnetic bearings.     

Behavior of quantum correlations under local noise

We characterize the behavior of quantum correlations under the influence of local noisy channels. Intuition suggests that such noise should be detrimental for quantumness. When considering qubit systems, we show for which channels this is indeed the case: The amount of quantum correlations can only decrease under the action of unital channels. However, nonunital channels (e.g., such as dissipation) can create quantum correlations for some initially classical states. Furthermore, for higher-dimensional systems even unital channels may increase the amount of quantum correlations. Thus, counterintuitively, local decoherence can generate quantum correlations.