Synthesis and x-ray chracterization of the dianions trihydridodecacarbonyltrirhenate(2—)and trihydrido-μ3-oxoenneacarbonyltrirhenate(2—)

The synthesis of the new anions [H₃Re₃(CO)₁₀]^2? and [H₃Re₃(μ₃-O)(CO)₉]^2?, obtained from [H₄Re4(CO)₁₅]^2?, and their characterization by IR, NMR and X-ray analyses are reported; both species contain a triangular metal atom cluster and the former, which is electron deficient, contains a ReRe double bond. ca]To whom correspondence should be addressed.     

Study of a monomer-monomer reaction system with short-range repulsive interactions

The critical behavior of an interacting two species catalytic surface reaction model is studied by means of Monte Carlo simulations and a mean-field approach. The model has two parameters, namely the relative adsorption rate of species p _A and a short-range repulsive interaction r between the same type of adsorbed species. The system exhibits an stationary reactive phase and two symmetrically equivalent absorbing phases. These latter phases are unique and correspond to surfaces saturated by a single type of reacting species. For , the system exhibits a second-order phase transition that belongs to the directed percolation (DP) universality class. However, in the absence of repulsive interaction (r = 0), a bicritical point is found at p _A = 1/2 whose critical behavior is compatible with dynamical mean-field exponents. Our findings indicate that the bicritical point belongs to the Voter Model universality class, whose upper critical dimension is d _c = 2. In addition, we propose a method to study the crossover from MF to DP behavior based on the estimation of the crossover time T _c . We find that T _c diverges according to a power-law as where is the crossover exponent. For strong repulsion, a new transient effect appears associated with the onset of almost inactive chessboad patterns.Received: 8 July 2004, Published online: 31 August 2004PACS: 02.50.-r Probability theory, stochastic processes, and statistics - 05.40.-a Fluctuation phenomena, random processes, noise, and Brownian motion - 82.45.Jn Surface structure, reactivity and catalysisR.A. Monetti: Permanent address: Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas, (INIFTA), CONICET, UNLP. Suc. 4, C. C. 16, (1900) La Plata, Argentina     

Relationships between a microscopic parameter and the stochastic equations for interface's evolution of two growth models

The relationship between a microscopic parameter p, that is related to the probability of choosing a mechanism of deposition, and the stochastic equation for the interface's evolution is studied for two different models. It is found that in one model, that is similar to ballistic deposition, the corresponding stochastic equation can be represented by a Kardar-Parisi-Zhang (KPZ) equation where both λ and ν depend on p in the following way: ν(p) = νp and λ(p) = λp ^3/2. Furthermore, in the other studied model, which is similar to random deposition with relaxation, the stochastic equation can be represented by an Edwards-Wilkinson (EW) equation where ν depends on p according to ν(p) = νp ². It is expected that these results will help to find a framework for the development of stochastic equations starting from microscopic details of growth models. Received 26 August 2002 / Received in final form 20 November 2002 Published online 6 March 2003 RID="a" ID="a"e-mail: ealbano@inifta.unlp.edu.ar     

Synthetic Methods for the Preparation of Conformationally Restricted Analogues of Nicotine

In the context of naturally occurring nitrogen heterocycles, nicotine is a chiral alkaloid present in tobacco plants, which can target and stimulate nicotinic acetylcholine receptors (nAChRs), a class of ligand-gated ion channels commonly located throughout the human brain. Due to its well-known toxicity for humans, there is considerable interest in the development of synthetic analogues; in particular, conformationally restricted analogues of nicotine have emerged as promising drug molecules for selective nAChR-targeting ligands. In the present mini-review, we will describe the synthesis of the conformationally restricted analogues of nicotine involving one or more catalytic processes. In particular, we will follow a systematic approach as a function of the heteroarene structure, considering: (a) 2,3-annulated tricyclic derivatives; (b) 3,4-annulated tricyclic derivatives; (c) tetracyclic derivatives; and (d) other polycyclic derivatives. For each of them we will also consider, when carried out, biological studies on their activity for specific nAChR subunits.     

Characterization of Polyethylene Nascent Powders Synthesized with TpTiCl2(OR) Catalysts

Summary: Different kinds of polyethylene and ethylene-1-hexene copolymers were synthesized with TpTiCl₂(OR) (Tp = hydrotris(pyrazolyl)borate; R = Et, i-Pr, n-Bu) catalysts with and without H₂. The polymers were characterized by ¹³C NMR, capillary viscosimetry or GPC, and DSC. The homopolymers showed properties characteristic of ultra-high molecular weight polyethylenes (UHMWPE) with linear structure and high density polyethylenes (HDPE) with molecular weights in the range of commercial grades under hydrogen atmosphere. The copolymers showed a 1-hexene incorporation up to 6 mol-%. Important differences in the thermal properties were observed between the first DSC (nascent powders) and the second DSC heatings (melt-crystallized samples), which evidenced the molecular weights influence on the melt-crystallized samples.     

Odd-Even Effect on Luminescence Properties of Europium Aliphatic Dicarboxylate Complexes

The odd–even effect in luminescent [Eu₂(L)₃(H₂O)_x]⋅y(H₂O) complexes with aliphatic dicarboxylate ligands (L: OXA, MAL, SUC, GLU, ADP, PIM, SUB, AZL, SEB, UND, and DOD, where x=2–6 and y=0–4), prepared by the precipitation method, was observed for the first time in lanthanide compounds. The final dehydration temperatures of the Eu³⁺ complexes show a zigzag pattern as a function of the carbon chain length of the dicarboxylate ligands, leading to the so-called odd-even effect. The FTIR data confirm the ligand–metal coordination via the mixed mode of bridge–chelate coordination, except for the Eu³⁺-oxalate complex. XRD results indicate that the highly crystalline materials belong to the monoclinic system. The odd–even effect on the 4 f–4 f luminescence intensity parameters (Ω₂ and Ω₄) is explained by using an extension of the dynamic coupling mechanism, herein named the ghost-atom model. In this method, the long-range polarizabilities ( ) were simulated by a ghost atom located at the middle of each ligand chain. The values of were estimated using the localized molecular orbital approach. The emission intrinsic quantum yield ( ) of the Eu³⁺ complexes also presented an the odd-even effect, successfully explained in terms of the zigzag behavior shown by the Ω₂ and Ω₄ intensity parameters. Luminescence quenching due to water molecules in the first coordination sphere is also discussed and rationalized.     

[Ga3+8Sm3+2, Ga3+8Tb3+2] Metallacrowns are Highly Promising Ratiometric Luminescent Molecular Nanothermometers Operating at Physiologically Relevant Temperatures

Nanothermometry is the study of temperature at the submicron scale with a broad range of potential applications, such as cellular studies or electronics. Molecular luminescent-based nanothermometers offer a non-contact means to record these temperatures with high spatial resolution and thermal sensitivity. A luminescent-based molecular thermometer comprised of visible-emitting Ga³⁺/Tb³⁺ and Ga³⁺/Sm³⁺ metallacrowns (MCs) achieved remarkable relative thermal sensitivity associated with very low temperature uncertainty of S_r=1.9 % K⁻¹ and δT<0.045 K, respectively, at 328 K, as an aqueous suspension of polystyrene nanobeads loaded with the corresponding MCs. To date, they are the ratiometric molecular nanothermometers offering the highest level of sensitivity in the physiologically relevant temperature range.     

Heterostructured organic interfaces probed by resonant photoemission

The application of the resonant photoemission spectroscopy (RPES) to various organic molecule based systems is reviewed. The chemical specificity and the possibility to conduct experiments in the energy domain that provides a time scale for charge dynamics, make the RPES a powerful tool to study organic heterojunctions and in particular to probe the charge transfer processes at organic interfaces. We briefly discuss the models used in RPES data analysis to extract the time scale of the excited charge delocalisation and the spatial correlation of core, valence occupied and unoccupied molecular states. As an example we report on 3,4,9,10-perylene tetracarboxylic acid dianhydride (PTCDA) on (1 × 2) Au(1 1 0) surface where organic layer metallicity is directly evidenced in RPES experiments. A particular attention is dedicated to bio-mimetic model molecules whose electronic structure at interfaces is the fundamental key for the design of real devices. In the last section we consider recent experiments that could open the way to new fields of applications regarding biological molecules and single molecule systems where RPES could elucidate the link between the quantum and the meso-scopic properties of such systems.     

Emission spectra investigation of fs induced NPs probed by the ns laser pulse of a fs/ns DP-LIBS orthogonal configuration

A dual-pulse fs/ns laser induced breakdown spectroscopy configuration, where an initial 250 fs ablating pulsed laser followed by a delayed ns laser beam placed at a fixed distance, orthogonally with the expanding plasma plume, has been used in air on a Al₆₅Cu₂₃Fe₁₂ quasicrystal. The obtained emission data were acquired with a set-up arrangement providing space detections, with a resolution up to 15 μm, of the ns laser pulse generated signals. Assuming the fulfillment of local thermodynamic equilibrium conditions, the role played by the time lag between the two laser beams on the induced plasma excitation temperatures and electronic densities, as well as a space resolved process survey, has been followed. The spatial and time resolved spectra show, almost, steady values of the determined elementary plasma features with the development of nanoparticles occurring during the fs laser pulsed ablation process. The ns laser probe of the dual-pulse LIBS configuration here presented confirms that the nanoparticles induced can be largely widespread in both space and time whose compositions, overall, could retain the starting target stoichiometry. It is shown that these nanoparticles formation can actually take place at different times following the initial ultra-short laser beam incidence and that, especially at long inter-pulse delays (>100 μs), modest compositional changes can be observed.