Isotope exchange in ionised CO2/CO mixtures: the role of asymmetrical C2O3+ ions

A hitherto unknown, atmospherically relevant, isotope-exchange reaction was studied in ionised gaseous mixtures containing carbon dioxide and monoxide. The mechanism of the O exchange, proceeding over a double-minimum potential-energy surface, was positively established by mass spectrometric and theoretical methods that also allowed the identification and characterisation of the C2O3+ intermediate. The increase of internal energy displaces the observed reactivity towards an endothermic reaction path that involves only CO2 and represents an indirect route to the dissociation of carbon dioxide.     

Fourier spectral and wavelet solvers for the incompressible Navier–Stokes equations with volume-penalization: Convergence of a dipole–wall collision

In this study, we use volume-penalization to mimic the presence of obstacles in a flow or a domain with no-slip boundaries. This allows in principle the use of fast Fourier spectral methods and coherent vortex simulation techniques (based on wavelet decomposition of the flow variables) to compute turbulent wall-bounded flow or flows around solid obstacles by simply adding one term in the equation. Convergence checks are reported using a recently revived, and unexpectedly difficult dipole–wall collision as a benchmark computation. Several quantities, like the vorticity isolines, truncation error, kinetic energy and enstrophy are inspected for a collision of a dipole with a no-slip wall and compared with available benchmark data obtained with a standard Chebyshev pseudospectral method. We quantify the possible deteriorating effects of the Gibbs phenomenon present in the Fourier based schemes due to continuity restrictions of the penalized Navier–Stokes equations on the wall. It is found that Gibbs oscillations have a negligible effect on the flow evolution allowing higher-order recovery of the accuracy on a Fourier basis by means of postprocessing. An advantage of coherent vortex simulations, on the other hand, is that the degrees of freedom of the flow computation can strongly be reduced. In this study, we quantify the possible reduction of degrees of freedom while keeping the accuracy. For an optimal convergence scenario the penalization parameter has to scale with the number of Fourier and wavelet modes. In addition, an implicit treatment of the Darcy drag term in the penalized Navier–Stokes equations is beneficial since this allows one to set the time step independent from the penalization parameter without additional computational or memory requirements.     

Ultrahigh vacuum deposition of L-cysteine on Au(110) studied by high-resolution X-ray photoemission: from early stages of adsorption to molecular organization

We report on a high-resolution X-ray photoemission spectroscopy study on molecular-thick layers of L-cysteine deposited under ultrahigh vacuum conditions on Au(110). The analysis of core level shifts allowed us to distinguish unambiguously the states of the first-layer molecules from those of molecules belonging to the second layer. The first-layer molecules strongly interact with the metal through their sulfur headgroup. The multipeaked structure of the N 1s, O 1s, and C 1s core levels is interpreted in terms of different molecular moieties. The neutral acidic fraction (HSCH2CH(NH2)COOH) is abundant at low coverage likely associated with isolated molecules or dimers. The zwitterionic phase (HSCH2CH(NH3+)COO-) is largely dominant as the coverage approaches the monolayer limit and is related to the formation of ordered self-assembled molecular structures indicated by electron diffraction patterns. The occurrence of a small amount of cationic molecules (HSCH2CH(NH3+)COOH) is also discussed. The second-layer molecules mainly display zwitterionic character and are weakly adsorbed. Mild annealing up to 100 degrees C leads to the desorption of the second-layer molecules leaving electronic states of the first layer unaltered.     

Photoaddition of thienocoumarin derivatives to DNA: stoichiometry and kinetics of binding

Photoreaction of the 6,9-dimethyl-4-methoxymethyl-2H-thieno[3,2-g]-1-benzopyran-2-one (compound I) and 4-acetoxymethyl-6,9-dimethyl-2H-thieno[3,2-g]-1-benzopyran-2-one (compound II) to DNA was studied. The quantitative evaluation of the photobound molecules was performed by means of inductively coupled plasma atomic emission spectrometry (ICP-AES), exploiting the presence of the sulphur atom inside the tricyclic chromophore. The concurrent estimation of the phosphorus atom, present exclusively in the macromolecule, allowed possible intercalation sites to be identified and their involvement in the photoaddition reaction to be determined. The development of a kinetic model made it possible to discriminate and evaluate the single kinetic events that constitute the overall photoaddition process of I and II to DNA.     

Design of a new mimochrome with unique topology

Peptide-based metalloprotein models represent useful systems to help understand how metalloproteins can support different functions, by the use of similar metal ion cofactors. In order to shed light on the role of the protein matrix in modulating the heme properties, we developed new models: mimochromes. They are pseudo-C(2) symmetric systems, composed of two helical peptides covalently linked to the deuteroporphyrin. The use of C(2) symmetry is particularly advantageous, because it simplifies the design, synthesis and characterization. However, it leaves the problem of possible diastereomeric forms. In the cobalt complex of the first derivative, mimochrome I, Lambda and Delta isomers were indeed experimentally observed. All the insights derived from the Co(III)-mimochrome I structure were used to obtain a re-designed molecule, mimochrome IV. The spectroscopic characterization of the iron and cobalt derivatives suggested the presence of the Lambda isomer as unique species. The NMR solution structure of the diamagnetic Co(III)-mimochrome IV confirmed the ability of the molecule to adopt a unique topology, and revealed the peptide chains to be in helical conformation, as designed. The insertion of intramolecular, inter-chain interactions was successful in favoring the formation of one of the two possible diastereomers. The stereochemically stable structure of mimochrome IV provides an attractive model for modulating the redox potential of the heme, by simple changing the peptide chain composition around the heme.     

A Deep Eutectic Solvent‐based Amperometric Sensor for the Detection of Low Oxygen Contents in Gaseous Atmospheres

The possibility of adopting deep eutectic solvents (DESs) instead of room temperature ionic liquids (RTILs) in membrane‐free electrochemical gas probes was estimated by first evaluating the performance of ethaline as electrochemical medium. This very easily prepared DES was chosen as prototype since it displays high conductivity and fairly modest viscosity, comparable with those of RTILs usually adopted in electrochemical measurements. Its electrostability window at Au, Pt and GC electrodes was first detected, together with diffusion coefficients displayed in this medium by ferrocene in the range 2.0–26.5 °C, it being adopted as prototype analyte in view of its well known electrochemical behavior and high enough solubility in ethaline. These diffusion coefficients were then used to infer viscosity values of ethaline at all temperatures considered, by exploiting the Stokes‐Einstein equation. Even though ferrocene diffusion coefficients turned out to be remarkably lower than those displayed in usual aprotic solvents, they were fairly higher than those usually found in electrochemical measurements conducted in RTILs, thus pointing out that the use of DESs as solvents adhering to electrode surfaces for assembling electroanalytical gas sensors could be advantageous. On these bases, a conveniently assembled DES‐based probe was tested for the electrochemical detection of low oxygen contents in cooled atmospheres. The quite satisfactory results found indicated that the drawback affecting DESs, consisting in the low values of diffusion coefficients displayed by dissolved analytes, can be overcome by using thin enough DES layers and resorting to a high sensitive detection approach such as amperometry under flow conditions. In fact, good sensitivities were found at all temperatures considered (2.0–26.5 °C), accompanied by a low detection limit (ca. 0.1 % v/v).     

Parameters affecting the determination of mercury by anodic stripping voltammetry using a gold electrode

The electrochemical determination of aqueous Hg(II) by anodic stripping voltammetry (ASV) at a solid gold electrode is described. The aim of this work is to optimise all factors that can influence this determination. Potential wave forms (linear sweep, differential pulse, square wave), potential scan parameters, deposition time, deposition potential and surface cleaning procedures were examined for their effect on the mercury peak shape and intensity. Five supporting electrolytes were tested. The best responses were obtained with square wave potential wave form and diluted HCl as supporting electrolyte. Electrochemical and mechanical surface cleaning, aimed at removing the amount of mercury deposited onto the gold surface, were necessary for obtaining a good performance of the electrode. Response linearity, repeatability, accuracy and detection limit were also evaluated.     

Random Packing of Small Blocks: Pressure Effects,Orientational Correlations and Application to Polymer‐Based Composites

Packing is a complex phenomenon of prominence in many natural and industrial processes (liquid crystals, granular materials, infiltration, melting, flow, sintering, segregation, sedimentation, compaction, etc.). A variety of computational methods is available in particular for spheroid particles. Our aim is to apply the principle of the random sequential addition algorithm but with small blocks of varying size and orientation. Here the main purpose is to reproduce the observed arrangement of graphitic assemblies in polymeric matrices. Random packing is improved by applying an external pressure implemented with a drifted diffusive motion of the fillers. Attention is also paid to the emergence of structural and orientational order. Interestingly, mixtures of fillers of irregular shapes can be dealt with efficiently using the proposed algorithm.