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     

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.     

Eisenanalyse

Ohne Zusammenfassung     

The Singularities of the Distance Function Near Convex Boundary Points

In an open bounded set Ω, we consider the distance function from ∂Ω associated to a Riemannian metric with C ^1,1 coefficients. Assuming that Ω is convex near a boundary point x ₀, we show that the distance function is differentiable at x ₀ if and only if there exists the tangent space to ∂Ω at x ₀. Furthermore, if the distance function is not differentiable at x ₀ then there exists a Lipschitz continuous curve, with initial point at x ₀, such that the distance function is not differentiable along such a curve.      

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.     

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.     

Ultrastructural characterization of the spermatozoa of Aethalion reticulatum Linnaeus 1767 (Hemiptera: Auchenorrhyncha: Aethalionidae)

The Hemiptera order is currently divided into four suborders. Among them the Auchenorrhyncha suborder is considered to be paraphyletic. Morphology of insect spermatozoa has provided promising characteristics that can be used for phylogenetic inference. In this study, Aethalion reticulatum (Aethalionidae) spermatozoa were examined by light and electron microscopy. The head of the spermatozoa is composed of an acrosome and a nucleus. The nucleus is linear and filled with compact chromatin but has electron-lucid spaces. The centriole adjunct initiates parallel to the nucleus and terminates in the region anterior to the mitochondrial derivatives. Flagella consist of an axoneme, two mitochondrial derivatives and two accessory bodies and the axoneme has the typical 9 + 9 + 2 microtubule pattern. The mitochondrial derivatives are symmetric. The accessory bodies are long and are shaped like a half moon when viewed from a cross-section. The presence of accessory bodies differs from other species of Cicadomorpha previously studied. Spermatozoa morphology of other Auchenorrhyncha families can reveal synapomorphies and contribute to systematics of the suborder.     

Polymer mixtures in confined geometries: Model systems to explore phase transitions

While binary (A,B) symmetric polymer mixtures ind = 3 dimensions have an unmixing critical point that belongs to the 3d Ising universality class and crosses over to mean field behavior for very long chains, the critical behavior of mixtures confined into thin film geometry falls in the 2d Ising class irrespective of chain length. The critical temperature always scales linearly with chain length, except for strictly two-dimensional chains confined to a plane, for whichT _c ∝N ^5/8 (this unusual exponent describes the fractal contact line between segregated chains in dense melts in two spatial dimensions,d = 2). When the walls of the thin film are not neutral, but preferentially attract one species, complex phase diagrams occur due to the interplay between capillary condensation and wetting phenomena. For ‘competing walls’ (one wall prefers A, the other prefers B) particularly interesting interface localization-delocalization transitions occur, while analogous phenomena in wedges are related to the ‘filling transition’.