ESR, electrochemical and reactivity studies of antitrypanosomal palladium thiosemicarbazone complexes

Cyclic voltammetry (CV) and electron spin resonance (ESR) techniques were used in the investigation of novel palladium complexes with bioactive thiosemicarbazones derived from 5-nitrofurane or 5-nitrofurylacroleine. Sixteen palladium complexes grouped in two series of the formula [PdCl(2)HL] or [PdL(2)] were studied. ESR spectra of the free radicals obtained by electrolytic reduction were characterized and analyzed. The ESR spectra showed two different hyperfine patterns. The stoichiometry of the complexes does not seem to affect significantly the hyperfine constants however we observed great differences between 5-nitrofurane and 5-nitrofurylacroleine derivatives. The scavenger properties of this family of compounds were lower than Trolox.     

Helicates of chiragen-type ligands and their aptitude for chiral self-recognition

Two (-)-5,6-pinene-bipyridine moieties connected by a para-xylylene bridge (so-called chiragen-type ligands), (-)-L1, undergo self-assembly upon reaction with equimolar amounts of CuI to form enantiopure circular hexanuclear P-helicates. If both enantiomers of L1 are used, mixtures of P and M hexanuclear helicates are exclusively obtained through a complete chiral recognition; that is, no mixing of the (+) and (-) ligands, respectively, occurs upon complexation. This was proven by a) NMR spectroscopy where identical spectra to those for complexes with the enantiomerically pure ligands were obtained and b) circular dichroism (CD) spectroscopy. The reaction is completely changed by the use of the corresponding meso-L1. Instead of well-defined species, oligomeric mixtures are observed, a result demonstrating the crucial role played by ligand chirality in self-assembly processes. Structural variations on the chiral ligand L1, such as a meta-xylylene bridge instead of a para-xylylene one (in L4) or four pinene groups instead of two (in L5 and L6), favor nondiscrete coordination assembly.     

Pattern formation driven by cross-diffusion in a 2D domain

In this work we investigate the process of pattern formation in a two dimensional domain for a reaction–diffusion system with nonlinear diffusion terms and the competitive Lotka–Volterra kinetics. The linear stability analysis shows that cross-diffusion, through Turing bifurcation, is the key mechanism for the formation of spatial patterns. We show that the bifurcation can be regular, degenerate non-resonant and resonant. We use multiple scales expansions to derive the amplitude equations appropriate for each case and show that the system supports patterns like rolls, squares, mixed-mode patterns, supersquares, and hexagonal patterns.     

Coordination chemistry of tetradentate N-donor ligands containing two pyrazolyl-pyridine units separated by a 1,8-naphthyl spacer: dodecanuclear and tetranuclear coordination cages and cyclic helicates

The tetradentate ligand L(naph) contains two N-donor bidentate pyrazolyl-pyridine units connected to a 1,8-naphthyl core via methylene spacers; L45 and L56 are chiral ligands with a structure similar to that of L(naph) but bearing pinene groups fused to either C4 and C5 or C5 and C6 of the terminal pyridyl rings. The complexes [Cu(L(naph))](OTf) and [Ag(L(naph))](BF4) have unremarkable mononuclear structures, with Cu(I) being four-coordinate and Ag(I) being two-coordinate with two additional weak interactions (i.e., "2 + 2" coordinate). In contrast, [Cu4(L(naph))4][BF4]4 is a cyclic tetranuclear helicate with a tetrafluoroborate anion in the central cavity, formed by an anion-templating effect; electrospray mass spectrometry (ESMS) spectra show the presence of other cyclic oligomers in solution. The chiral ligands show comparable behavior, with [Cu(L45)](BF4) and [Ag(L45)](ClO4) having similar mononuclear crystal structures and with the ligands being tetradentate chelates. In contrast, [Ag4(L56)4](BF4)4 is a cyclic tetranuclear helicate in which both diastereomers of the complex are present in the crystal; the two diastereomers have similar gross geometries but are significantly different in detail. Despite their different crystal structures, [Ag(L45)](ClO4) and [Ag4(L56)4](BF4)4 behave similarly in solution according to ESMS studies, with a range of cyclic oligomers (up to Ag9L9) forming. With transition-metal dications Co(II), Cu(II), and Cd(II), L(naph) generates a series of unusual dodecanuclear coordination cages [M12(L(naph))18]X24 (X- = ClO4- or BF4-) in which the 12 metal ions occupy the vertices of a truncated tetrahedron and a bridging ligand spans each of the 18 edges. The central cavity of each cage can accommodate four counterions, and each cage molecule is chiral, with all 12 metal trischelates being homochiral; the crystals are racemic. Extensive aromatic stacking between ligands around the periphery of the cages appears to be a significant factor in their assembly. The chiral analogue L45 forms the simpler tetranuclear, tetrahedral coordination cage [Zn4(L45)6](ClO4)(8), with one anion in the central cavity; the steric bulk of the pinene chiral auxiliaries prevents the formation of a dodecanuclear cage, although trace amounts of [Zn12(L45)18](ClO4)24 can be detected in solution by ESMS. Formation of [Zn4(L45)6](ClO4)8 is diastereoselective, with the chirality of the pinene groups controlling the chirality of the tetranuclear cage.     

Above-threshold ionization of hydrogen and hydrogen-like ions by X-ray pulses

This paper adresses the problem of above-threshold ionization (ATI) of hydrogen interacting with an intense X-ray electromagnetic field. Two approaches have been used. In the first approach, we calculate generalized differential and total cross sections based on second-order perturbation theory for the electron interaction with a monochromatic plane wave, with the A ² and A · P contributions from the nonrelativistic Hamiltonian (including retardation) treated exactly. In the second approach, we solve the time-dependent Schrödinger equation (TDSE) for a pulsed plane wave using a spectral approach with a basis of oneelectron orbitals, calculated with L ²-integrable B-spline functions for the radial coordinate and spherical harmonics Y _lm for the angular part. Retardation effects are included up to O(1/c), they induce extra terms forcing the resolution of the TDSE in a three dimensional space. Relativistic effects [of O (1/c ²)] are fully neglected. The isoelectronic series of hydrogen is explored in the range Z = 1 ? 5 in both TDSE and perturbative approaches. Photoelectron angular distributions are obtained for photon energies of 1 keV and 3 keV for hydrogen, and photon energy of 25 keV for the hydrogenic ion B⁴⁺. Perturbative and TDSE calculations are compared.     

Observations of resonant modes formation in microwave generated magnetized plasmas

Ion sources have a significant number of applications in accelerator facilities and in industrial applications. In particular, the electron cyclotron resonance ion sources (ECRIS) are nowadays the most effective devices that can feed particle accelerators in a continuous and reliable way, providing high current beams of low and medium charge state ions and lower, but still remarkable, beam current for highly charged ions. In recent years several experiments have shown that the current, the charge states and even the beam shape change by slightly varying the microwave frequency (the so-called frequency tuning effect – FTE). The theoretical explanation of these results is based on the difference in the electromagnetic field pattern over the resonance surface, i.e. that region where the electrons resonantly interact with the incoming wave. In order to be consistent with the experiments, this model requires that standing waves are formed also in presence of a dense plasma. The proof was sought by means of a series of measurements performed with a network analyzer and with a plasma reactor operating at 2.45 GHz, according to the principles of the microwave discharge ion sources (MDIS). The measurements have been carried out with the aim to achieve the electromagnetic characterization of the plasma chamber in terms of possible excited resonant modes with and without plasma, and they reported that resonant modes are excited inside the cavity even in presence of a dense plasma. It was observed that the plasma dynamics strongly depends on the structure of the standing waves that are generated. The measurement of the eigen-frequencies' shifts were carried out for several values of pressure and RF power, thus linking the shift with the plasma density measured by a Langmuir probe. The changes in plasma shape, density and electron temperature have been also monitored for different operating conditions. A strong variation of plasma properties has been observed as a consequence of the introduction of the Langmuir probe inside the resonant cavity, thus demonstrating that the standing wave can be strongly perturbed even by means of relatively small metallic electrodes. The measurements reported hereinafter are relevant also for ECRIS, because they confirm the validity of the theoretical model that describes the frequency tuning.