Linear and Bent Cp*2Si: Reversible Phase Transition of a Key Molecule

The solid-state structure of decamethylsilicocene Cp*₂Si with a bent and a linear molecule in the same unit cell was so far considered an exception in relation to the structures of its all-bent heavier analogues Cp*₂E with E=Ge, Sn, Pb. Here, we present the solution to this conundrum by reporting a low-temperature phase, where all three symmetrically independent molecules are present in a bent formation. This reversible enantiotropic phase transition occurs in the temperature range between 80 K and 130 K and provides a rationale for the unexpected linear molecule based in entropy beyond hand-waving explanations such as electronic reasons or packing effects.     

Cation-specific interactions with carboxylate in amino acid and acetate aqueous solutions: X-ray absorption and ab initio calculations

Relative interaction strengths between cations (X = Li (+), Na (+), K (+), NH 4 (+)) and anionic carboxylate groups of acetate and glycine in aqueous solution are determined. These model systems mimic ion pairing of biologically relevant cations with negatively charged groups at protein surfaces. With oxygen 1s X-ray absorption spectroscopy, we can distinguish between spectral contributions from H 2O and carboxylate, which allows us to probe the electronic structure changes of the atomic site of the carboxylate group being closest to the countercation. From the intensity variations of the COO (-) aq O 1s X-ray absorption peak, which quantitatively correlate with the change in the local partial density of states from the carboxylic site, interactions are found to decrease in the sequence Na (+) > Li (+) > K (+) > NH 4 (+). This ordering, as well as the observed bidental nature of the -COO (-) aq and X (+) aq interaction, is supported by combined ab initio and molecular dynamics calculations.     

Accelerator mass spectrometry of small biological samples

Accelerator mass spectrometry (AMS) is an ultra-sensitive technique for isotopic ratio measurements. In the biomedical field, AMS can be used to measure femtomolar concentrations of labeled drugs in body fluids, with direct applications in early drug development such as Microdosing. Likewise, the regenerative properties of cells which are of fundamental significance in stem-cell research can be determined with an accuracy of a few years by AMS analysis of human DNA. However, AMS nominally requires about 1 mg of carbon per sample which is not always available when dealing with specific body substances such as localized, organ-specific DNA samples. Consequently, it is of analytical interest to develop methods for the routine analysis of small samples in the range of a few tens of microg. We have used a 5 MV Pelletron tandem accelerator to study small biological samples using AMS. Different methods are presented and compared. A (12)C-carrier sample preparation method is described which is potentially more sensitive and less susceptible to contamination than the standard procedures.     

Homogenization of the Maxwell Equations: Case I. Linear Theory

The Maxwell equations in a heterogeneous medium are studied. Nguetseng's method of two-scale convergence is applied to homogenize and prove corrector results for the Maxwell equations with inhomogeneous initial conditions. Compactness results, of two-scale type, needed for the homogenization of the Maxwell equations are proved.     

Influencing the Self-Sorting Behavior of [2.2]Paracyclophane-Based Ligands by Introducing Isostructural Binding Motifs

Two isostructural ligands with either nitrile ( L^nit ) or isonitrile ( L^iso ) moieties directly connected to a [2.2]paracyclophane backbone with pseudo-meta substitution pattern have been synthesized. The ligand itself ( L^nit ) or its precursors ( L^iso ) were resolved by HPLC on a chiral stationary phase and the absolute configuration of the isolated enantiomers was assigned by XRD analysis and/or by comparison of quantum-chemical simulated and experimental electronic circular dichroism (ECD) spectra. Surprisingly, the resulting metallosupramolecular aggregates formed in solution upon coordination of [(dppp)Pd(OTf)₂] differ in their composition: whereas L^nit forms dinuclear complexes, L^iso exclusively forms trinuclear ones. Furthermore, they also differ in their chiral self-sorting behavior as (rac)- L^iso undergoes exclusive social self-sorting leading to a heterochiral assembly, whereas (rac)- L^iso shows a twofold preference for the formation of homochiral complexes in a narcissistic self-sorting manner as proven by ESI mass spectrometry and NMR spectroscopy. Interestingly, upon crystallization, these discrete aggregates undergo structural transformation to coordination polymers, as evidenced by single-crystal X-ray diffraction.     

Imaging atomic structure in metal nanoparticles using high-resolution cryo-TEM.

It has been shown, by imaging gold (200) planes, that it is possible to achieve better than 0.20-nm structural resolution in cryo-transmission electron microscopy (cryo-TEM). This has been done using commercially available cryo equipment and using a 300-kV field emission gun (FEG) TEM. The images of 15-nm gold particles embedded in amorphous frozen water clearly show the (111) planes (separated by 0.235 nm) in gold. Fourier transform demonstrates the presence of (200) planes in the image, proving a resolution of better than 0.20 nm. The experimental results are supported by image simulations using the multislice method. These simulations suggest that it should be possible to achieve the same resolution even in smaller particles and particles of lighter elements. The crucial experimental problem to overcome is keeping the thickness of the amorphous film low and to work at low electron dose conditions.     

Assessing an imaging ammonium sensor using time correlated pixel-by-pixel calibration

In this study, a ratiometric imaging ammonium sensor (optode) and a novel calibration technique for imaging optodes were developed, tested and thoroughly evaluated. The overall sensing scheme is based on ammonium coextraction together with a solvent sensitive dye in a hydrogel–ether emulsion. Quantification of ammonium during 10 days of experiments was made through an image ratio (excitation:emission/excitation:emission; 572 nm:592 nm/520 nm:572 nm ) coupled to the calibration technique i.e. time correlated pixel-by-pixel calibration (TCPC). Sensor performance was validated according to International Union of Pure and Applied Chemistry (IUPAC) recommendations for within-laboratory requirements of reproducibility, and compared with the more frequently used approach of image pixel calibration. Using the TCPC protocol; a limit of detection (LOD) of 1.2 ± 0.2 × 10⁻⁶ M was calculated. The preliminary estimate of precision at 200 × 10⁻⁶ M, using relative standard deviation (R.S.D.) as a measure, was 5.2% over the 10-day period. The within-batch repeatability was 2.3% (R.S.D.) at 200 × 10⁻⁶ M. A linear fit of the data compared the drift over time for the various calibration techniques. Without time correlation, the ratiometric pixel-by-pixel calibrated signal drifted significantly over time. The drift was more or less completely removed using the TCPC protocol. The image resolution, experimentally determined at the distance applied (3 × 10⁻¹ m) by the 10–90% edge-response approach, was 210 × 10⁻⁶ m. Imaging concentrations and diffusive transport of ammonium in soil during dissolution of a fertilizer stick was given as an example of a versatile sensor application.     

Synthesis of the marine alkaloid caulersin

A three-step synthesis of caulersin (3) from indole-2-acetic acid methyl ester and indole-2-carbonyl chloride is described. As the spectral data of the synthetic sample differed from those reported for the natural product, the structure was determined by X-ray crystallography.     

Capture of CO2 by a Cationic Nickel(I) Complex in the Gas Phase and Characterization of the Bound,Activated CO2 Molecule by Cryogenic Ion Vibrational Predissociation Spectroscopy

We describe a systematic method for the preparation and spectroscopic characterization of a CO₂ molecule coordinated to an activated bisphenoidal nickel(I) compound containing a tetraazamacrocyclic ligand in the gas phase. The resulting complex was then structurally characterized by using mass‐selected vibrational predissociation spectroscopy. The results indicate that a highly distorted CO₂ molecule is bound to the metal center in an η²‐C,O coordination mode, thus establishing an efficient and rational method for the preparation of metal‐activated CO₂ for further studies using ion chemistry techniques.