Theory of the interface between a classical plasma and a hard wall

The interfacial density profile of a classical one-component plasma confined by a hard wall is studied in planar and spherical geometries. The approach adapts to interfacial problems a modified hypernetted-chain approximation developed by Lado and by Rosenfeld and Ashcroft for the bulk structure of simple liquids. The specific new aim is to embody self-consistently into the theory a “contact theorem”, fixing the plasma density at the wall through an equilibrium condition which involves the electrical potential drop across the interface and the bulk pressure. The theory is brought into fully quantitative contact with computer simulation data for a plasma confined in a spherical cavity of large but finite radius. It is also shown that the interfacial potential at the point of zero charge is accurately reproduced by suitably combining the contact theorem with relevant bulk properties in a simple, approximate representation of the interfacial charge density profile.     

Freezing of a modulated liquid: The superionic-to-normal transition of strontium chloride

The anionic component in SrCl₂ near melting is treated as a modulated liquid in the periodic potential of the sublattice of cations. With decreasing temperature from the melting point, we find a diffuse disorder—order transition. The calculated behaviours of the order parameter and of thermodynamic properties approximate those observed for SrCl₂ across its superionic transition.     

Phenol production in benzene/air plasmas at atmospheric pressure. Role of radical and ionic routes

Benzene can be efficiently converted into phenol when it is treated by either corona or dielectric barrier discharge (DBD) plasmas operating at atmospheric pressure in air or mixtures of N(2) and O(2). Phenol produced by corona discharge in an atmospheric pressure chemical ionization source (APCI) has been detected as the corresponding radical cation C(6)H(5)OH(+*) at m/z 94 by an ion trap mass spectrometer. On the other hand, phenol has been observed also as neutral product by gas chromatography-mass spectrometry analysis (GC-MS) after treatment in a DBD plasma. Experiments aimed at shading light on the elementary processes responsible for benzene oxidation were carried out (i) by changing the composition of the gas in the corona discharge source; (ii) by using isotopically labeled reagents; and (iii) by investigating some relevant ion-molecule reactions (i.e. C(6)H(6)(+*) + O(2), C(6)H(5)(+) + O(2)) via selected guided ion beam measurements and with the help of ab initio calculations. The results of our approach show that ionic mechanisms do not play a significant role in phenol production, which can be better explained by radical reactions resulting in oxygen addition to the benzene ring followed by 1,2 H transfer.     

Characterization of Al(III) complexes with hematein in artistic alum logwood inks

The complexes between Al(III) and hematein, the main coloring matter in alum logwood inks, were characterized by Raman and ²⁷Al NMR (nuclear magnetic resonance) spectroscopies. Raman spectra of the crystallized complexes and of the compounds applied on a paper substrate are presented and assigned based on published data for the parent compounds. These Raman spectra show that the coordination of the hematein to the Al(III) ions takes place in both cases through the carbonyl and hydroxyl groups in the molecule, and that the aromatic rings are also involved in the interaction. The Raman spectra of the pure hematein–Al(III) complexes were found to be consistent with those recorded for a logwood ink prepared following a late 19th century recipe, using logwood chips instead of pure hematein, and applied on a paper substrate. These spectra can be used as references for the noninvasive identification of the compounds in works of art. ²⁷Al solid‐state NMR showed that the coordination of the Al(III) atoms in the crystallized powder is predominantly octahedral, while when applied on a paper substrate the colorant is present mainly as a tetrahedral complex, with an octahedral coordination also present in a smaller proportion. The fact that the predominant coordinations for the complexes in the crystallized material and for the ones present on the paper substrate are different is relevant for the study of the lightfastness and thermal stability of works of art bearing these media. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic characterization of a thiophene‐based active material for resistive organic nonvolatile memories

A combined theoretical and experimental Raman study is presented on a diphenyl bithiophene molecule known as a good candidate for the development of organic nonvolatile memory devices. Spectroscopic markers suitable to distinguish the different stable conformers of the molecule have been predicted and detected. The combined analysis of theoretical and experimental Raman spectra recorded in solution indicates that at room temperature a dynamical equilibrium, characterized by interconversion between the two more stable conformers (namely trans and cis), takes place and that the more populated species is the cis form. Referring to the solid phase instead, Raman spectra of single‐crystal samples show the presence of the only trans conformer, as confirmed by X‐ray measurements. Finally, Raman spectra of thin films, as those used for the memory device, were collected; samples just deposited from solution and after few hours from the deposition were analyzed. Following the evolution of selective spectroscopic Raman markers, an isomerization process from the abundant cis (as‐deposited) to the totally trans (after few hours) conformer in the solid phase was detected. These results open the way to the identification of the molecular isomers present in the thin film of the memory cell and finally of the active molecular species involved in the switching mechanism of the operating device. Copyright © 2009 John Wiley & Sons, Ltd.     

Electronic properties of the surface of perylene tetracarboxylic acid dianhydride film upon deposition of the ultrathin conjugated layers of Pyronine B

Ultrathin conjugated layers of Pyronine B were thermally deposited in UHV on the surface of perylene tetracarboxylic acid dianhydride (PTCDA) film. The structure of unoccupied electron states located 5-20 eV above the Fermi level (E_F) and the surface potential were monitored during the Pyronine B overlayer deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Electronic work function of the PTCDA surface changed from 4.9 ± 0.1 eV, during the Pyronine B deposition due to the change of the contents of the surface layer, until it reached a stable value 4.6 ± 0.1 eV at the Pyronine B film thickness 8-10 nm. The interface dipole corresponds to electron transfer from the Pyronine B overlayer to the PTCDA surface and the polarization in the Pyronine B overlayer was found confined within approximately 1 nm near the interfaces. The edges of major bands of density of unoccupied electronic states (DOUS) of PTCDA substrate and of the Pyronine B overlayer were unaffected by the process of the interface formation. The major TCS spectral features of the Pyronine B film corresponding to the DOUS band edges were identified and the assignment of the π^*, σ^*(C-C) and σ^*(CC) character was suggested.     

An interionic force law for HgCl2 from first-principles molecular calculations

Mercury dichloride is an ionic compound solidifying into a unique layer structure of rod-like monomers, so that unusual structural and physical properties can be expected for its liquid state. We propose a set of pseudoclassical interionic potentials, including three-body forces and electronic-polarization terms, patterned on the results of relativistic first-principles calculations on the molecular monomer, dimer and trimer. The proposed force law will allow structural studies of the condensed phases by molecular-dynamics simulations, with the main aims of exploring the nature of the short-range and intermediate-range order in the melt and the process of ionization at high pressure and temperature.     

Insulator-conductor transition in liquid aluminium trichloride

We report molecular-dynamics simulations of ionic structure and diffusion in a pseudoclassical model of AlCl₃ over a wide region of the pressure-temperature (p,T) plane. The model melts from a layer crystal into a molecular liquid at low (p,T) and into a dissociated ionic liquid at high (p,T), in accord with x-ray diffraction studies of the AlCl₃ material. We show that pressure drives a broad liquid-liquid transition from a molecular insulator to an ionic conductor and trace the transition line in the (p,T) plane. This line has a negative dp/dT slope and appears to branch out from an anomaly in the melting curve.     

Applications of dynamic combinatorial chemistry for the determination of effective molarity

A new strategy for determining thermodynamic effective molarities (EM) for macrocylisation reactions using dynamic combinatorial chemistry under dilute conditions is presented. At low concentrations, below the critical value, Dynamic Libraries (DLs) of bifunctional building blocks contain only cyclic species, so it is not possible to quantify the equilibria between linear and cyclic species. However, addition of a monofunctional chain stopper can be used to promote the formation of linear oligomers allowing measurement of EM for all cyclic species present in the DL. The effectiveness of this approach was demonstrated for DLs generated from mixtures of 1,3-diimine calix[4]arenes, linear diaminoalkanes and monoaminoalkanes. For macrocycles deriving from one bifunctional calixarene and one diamine, there is an alternating pattern of EM values with the number of methylene units in the diamine: odd numbers give significantly higher EMs than even numbers. For odd numbers of methylene units, the alkyl chain can adopt an extended all anti conformation, whereas for even numbers of methylene units, gauche conformations are required for cyclisation, and the associated strain reduces EM. The value of EM for the five-carbon linker indicates that this macrocycle is a strainless ring.     

Distinct effect of xenobiotics on the metal‐binding properties of protein molecules

The X‐ray standing‐wave method was applied to study the elemental composition and molecular organization of ordered protein films of alkaline phosphatase exposed to different xenobiotics (drug compounds, lead). Binding of metal ions from triply distilled water to protein molecules has been experimentally observed. Definite differences in the arrangement of impurity metal ions in the films have been established. The considerable enhancement of protein–metal interactions is attributed to partial rearrangement of the protein native structure, induced by xenobiotics.