Spectral and temporal features of multiple spontaneous NMR-maser emissions

When a system composed of dissolved laser-polarized xenon with negative spin temperature is put inside a high field NMR magnet, a series of spontaneous maser emissions can be observed. We report here their spectral and temporal features using a processing model derived from the solution of the Bloch equations in the presence of radiation damping. We show, in particular, that by combining Fourier transformation and squared modulus, a parameter allowing the characterization of the burst of transverse magnetization can be determined. This parameter is shown to be correlated with the radiated energy. Moreover, this processing clearly reveals features which can probably be assigned to effects resulting from distant dipolar fields. Finally, the analysis of the experimental data reveals an unexpected behavior of the ¹²⁹Xe transverse self-relaxation.     

Control of Duplex Formation and Columnar Self‐Assembly with Heterogeneous Amide/Urea Macrocycles

The perfect blend : A new class of self‐assembling cyclooligomers with mixed urea/amide backbone is described (see figure). A high level of hierarchical and directional control is achieved: depending on the level of backbone preorganization, columnar or tubular arrangements with either parallel or antiparallel growing modes can be selected.

     

Raman Spectroscopic Study of the Phase Transition of Amorphous to Crystalline β‐Carbonic Acid

What's the matter? The laboratory Raman spectra for carbonic acid (H₂CO₃), both for the β‐polymorph and its amorphous state, are required to detect carbonic acid on the surface of the pole caps of Mars in 2009, when the Mars Microbeam Raman Spectrometer lands on the planet. The picture shows a martian crater with ice of unknown composition, possibly containing carbonic acid (image adapted from DLR, with permission from ESA, DLR, and FU Berlin –‐G. Neukum).

     

Signal improvement in elemental analyzer/continuous flow isotope ratio mass spectrometry for samples with low sulfur contents using a pre-concentration technique for on-line concentration adjustment

Elemental analyzer/continuous flow isotope ratio mass spectrometry (EA/CF-IRMS) has become a standard procedure for the determination of delta(34)S values. Common procedures are, however, frequently less than satisfactory for organic as well as for mineral samples with very low concentrations of sulfur (<2000 ppm). Here we present a method which employs cold trapping of SO(2) to adjust the gas concentration for subsequent isotope signature determination. Analytical accuracy is comparable with common EA/CF-IRMS analysis without trapping, showing a precision of better than +/-0.4 per thousand in delta(34)S (1 SD). The virtual absence of memory effects was established by analyzing adjacent samples exhibiting a large difference in delta(34)S and by prolonged freezing of the carrier gas, yielding virtually no S concentration peak. The method was tested using less than 15% (6 microg) of the S required for a conventional isotope analysis at comparable signal intensity. Even smaller samples can be analyzed with high precision. This facilitates the on-line delta(34)S determination in small biological and mineral samples, minimizing matrix effects in various materials including sandstone, soil, and plant samples.     

A computational study of the nondissociative mechanisms that interchange apical and equatorial atoms in square pyramidal molecules

The lowest energy transition state for the nondissociative apical/equatorial atom exchange mechanism for three square pyramidal AEX5 molecular species was calculated (CCSD(T)/pVTZ; B3LYP/pVTZ, aug-cc-pV5Z) to have a hemidirected geometry with C(s) symmetry for BrF5, IF5, and XeF5+. In contrast, holodirected C2v-symmetric transition states for this process were located for the AEX5 square pyramidal molecules ClF5, ICl5, and IBr5. Imaginary frequencies were calculated and examined in a visual/dynamic fashion to gain insight into these fluxional processes. Although both mechanisms exchange one apical for one equatorial atom in each cycle of motion, processes that pass through C2v transition states have characteristic features of the well-known Berry pseudorotation and Lever mechanisms while those which pass through transition states of C(s) symmetry have features that are a mixture of Berry, Lever, and turnstile-like character. Two periodic trends are observed: as the atomic number on the central atom increases (same terminal atoms), the barrier for apical/equatorial exchange and the value of the imaginary frequency both decrease. Similarly, as the atomic number of the terminal atoms increase (same central atom), the barrier for apical/equatorial exchange decreases, as does the computed imaginary frequency.     

Structure and mobility of lipid membranes on a thermoplastic substrate

Supported lipid membranes constitute one of the most important model systems for cell membranes. The properties of lipid membranes supported by the hydrophobic solid polymer cyclic olefin copolymer (COC) were investigated. Lipid layers consisting of varying amounts of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP, cationic) and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC, neutral) prepared by vesicle fusion and solvent exchange were compared. All lipid mixtures coated the COC surface homogeneously forming a fluid membrane as verified by fluorescence microscopy and fluorescence recovery after photobleaching (FRAP). The exact structure of the supported membranes was determined by synchrotron reflectivity experiments using a microfluidic chamber. The X-ray data are in agreement with a compressed (head-to-head distance = 29 angstroms) and less densely packed bilayer.     

Suzuki-Miyaura, alpha-ketone arylation and dehalogenation reactions catalyzed by a versatile N-heterocyclic carbene-palladacycle complex

[reaction: see text] The activity of the complex (IPr)PdCl(eta2-N,C-C12H7NMe2), 1 [IPr = (N,N'-bis(2,6-diisopropylphenyl)imidazol)-2-ylidene], in the Suzuki-Miyaura cross-coupling reaction involving unactivated aryl chlorides and triflates with arylboronic acids at room temperature in technical grade 2-propanol is described. These conditions allow for the synthesis of di- and tri-ortho-substituted biaryls in very short reaction times. This complex also displays very high activity for alpha-ketone arylation and dehalogenation reactions of activated and unactivated aryl chlorides.     

Two New Retigerane‐Type Sesterterpenoids from the Lichen Leprocaulon microscopicum

Two new sesterterpenes, 1 and 2 , have been isolated from the lichen Leprocaulon microscopicum. In addition to classic chromatographic methods, a liquid‐liquid chromatography technique, namely centrifugal partition chromatography (CPC) was applied for the purification of compound 2 . The structures were determined by analyses of mass spectrometry and 1D‐ and 2D‐NMR data. The relative configuration of the isolated compounds was assigned on the basis of 2D‐NOESY experiments. The two compounds possess a rare pentacyclic carbon skeleton typical for lichen metabolism, and quite unusual in the vegetal kingdom.