Determination of self-diffusion coefficients in mixtures with benchtop 13C NMR spectroscopy via polarization transfer.

Nuclear magnetic resonance (NMR) is an established method to determine self-diffusion coefficients in liquids with high precision. The development of benchtop NMR spectrometers makes the method accessible to a wider community. In most cases, 1H NMR spectroscopy is used to determine self-diffusion coefficients due to its high sensitivity. However, especially when using benchtop NMR spectrometers for the investigation of complex mixtures, the signals in 1H NMR spectra can overlap, hindering the precise determination of self-diffusion coefficients. In 13C NMR spectroscopy, the signals of different compounds are generally well resolved. However, the sensitivity of 13C NMR is significantly lower than that of 1H NMR spectroscopy leading to very long measurement times, which makes diffusion coefficient measurements based on 13C NMR practically infeasible with benchtop NMR spectrometers. To circumvent this problem, we have combined two known pulse sequences, one for polarization transfer from 1H to the 13C nuclei (PENDANT) and one for the measurement of diffusion coefficients (PFG). The new method (PENPFG) was used to measure the self-diffusion coefficients of three pure solvents (acetonitrile, ethanol and 1-propanol) as well as in all their binary mixtures and the ternary mixture at various compositions. For comparison, also measurements of the same systems were carried out with a standard PFG-NMR routine on a high-field NMR instrument. The results are in good agreement and show that PENPFG is a useful tool for the measurement of the absolute value of the self-diffusion coefficients in complex liquid mixtures with benchtop NMR spectrometers.

Investigating cooperative effects in small cobalt and cobalt-nickel alloy clusters with attached ethanol.

Small cationic cobalt, and cobalt-nickel alloy clusters with ethanol attached are generated in a pulsed molecular beam experiment using a laser ablation source. While the metal center is successively varied with respect to size and composition, a full-size study of these transition metal clusters is possible. The clusters are investigated via IR photodissociation spectroscopy in the region of OH- and CH-stretching vibrations. The results are compared with theoretical data obtained from DFT calculations. Both frequency shifts and structural changes according to cluster size and composition are identified and discussed in detail, also with respect to cooperative effects. Trimeric metal clusters with an uneven number of nickel atoms show evidence for C-O cleavage of the ethanol molecule. This result is elucidated by further calculations concerning the reactivity, charge and energetic distributions.

Structural Investigation of Small Nickel-Ethanol Clusters Using Vibrational Spectroscopy in a Molecular Beam.

The structural identification of small nickel clusters with ethanol can help to understand fundamental steps for heterogenous catalysis. We investigate the rows [Nix (EtOH)1 ]+ with x=1-4, and [Ni2 (EtOH)y ]+ with y=1-3 via IR photodissociation spectroscopy in a molecular beam experiment. Analyzing the CH- and OH-stretching frequencies and comparing these experimental results with density functional theory (DFT) calculations on the PW91/6-311+G(d,p) level leads to the identification of intact motifs for all clusters and hints for C-O cleavage of the ethanol in two particular cases. Furthermore, we analyze the effects of frequency shifts with the increasing clusters sizes using the results of natural bond orbitals (NBO) analyses and an energy decomposition method.

Investigation of Radial Shaft Seal Swelling Using a Special Tribometer and Magnetic Resonance Imaging.

Compatibility between the rubber material of radial shaft seals and the lubricants to be sealed is an important requirement that customers demand of their lubricant suppliers. Among other effects that may result from incompatibility, the penetration of lubricant components into the rubber (swelling) can impair the seal's functionality due to changes in its geometry and mechanical behavior. Typically, the penetration of a lubricant into an elastomer is evaluated after an immersion test using volumetric, gravimetric, and extraction measurements. Due to the small changes that need to be detected, such methods may not be sufficient to obtain meaningful results. In this contribution, we use magnetic resonance imaging (MRI) to investigate swelling on special tribometer samples as well as a radial shaft seal that were previously used in component tests. Several combinations of rubbers and lubricants that have proven to be compatible were tested in addition to combinations with expected incompatibilities in real applications. The results indicate that MRI measurements can be used to quantify the penetration depth and potentially also the velocity with which the lubricant diffuses into the rubber, thereby yielding detailed insights into the swelling process of the seal.

Palladium(II)-Mediated Assembly of a M2L2 Macrocycle and M3L6 Cage from a Cyclopeptide-Derived Ligand.

A cyclic tetrapeptide L comprising two l-proline and two 3-amino-5-(pyridin-4-yl)benzoic acid subunits assembles into a dimetallic metallamacrocycle Pd2L2 and a trimetallic coordination cage Pd3L6 in the presence of suitable palladium(II) precursors. Pd2L2 recognizes organic anions in aqueous media, forming a particularly stable complex with sodium 2,6-naphthalenedisulfonate, in which two dianionic guest molecules reside in the cavity surrounded by the cyclopeptide ligands.