Synthesis of Fullerene Derivatives for Incorporation in Sol-Gel Glasses

We report on the synthesis of fullerene derivatives containing a trialkoxysilane moiety that exhibit enhanced solubility in solvents commonly used for sol-gel processing. These derivatives have been successfully embedded in glassy bulk samples and thin films that show very promising optical limiting properties.     

Experimental and Theoretical DFT Investigations in the [2,3]-Wittig-Type Rearrangement of Propargyl/Allyl-Oxy-Pyrazolones

Here we report the synthesis of interesting 3-alkyl-4-hydroxy-1-aryl-4-(propa-1,2-dienyl)1H-pyrazol-5(4H)-ones and 9-alkyl-7-aryl-1-oxa-7,8-diazaspiro[4.4]nona-3,8-dien-6-ones, starting from 1,2-diaza-1,3-dienes (DDs) and propargyl alcohol. The reaction proceeds through a sequence Michael-type nucleophilic attack/cyclization/[2,3]-Wittig rearrangement. In the same way, the reaction between the aforementioned DDs and allyl alcohol furnished 4-allyl-4-hydroxy-3-alkyl-1-aryl-1H-pyrazol-5(4H)-ones. A DFT study was also carried out, in order to have decisive clarifications about the mechanism.     

σ−π Contributions in metal-co bond: A theoretical study of RhCO and PdCO

The electronic structure of linear RhC0 and PdCO molecules has been studied by means of effective core potential calculations including configuration interaction. The study of the ⁴Δ and ²Σ⁺ states of RhCO and the ³Σ⁺ and ¹Σ⁺ states of PdCO has shown that their low-spin states are bound and their high-spin states are repulsive. The binding energies of RhCO and PdCO are 4.0 and 7.2 kcal/mol, for the equilibrium bond lengths of 3.87 and 4.03 au, respectively. The interaction between the diffuse 5s metal orbital and the carbon lone pair produces a repulsion in the α-space, as depicted by the negative region along the metal-carbon axis in the electron density difference contour maps. The bonding in low-spin states of RhCO and PdCO is due to the metal-CO π “backdonation”. The present investigation on RhCO and PdCO, preliminary results on ScCO, as well as published results on FeCO, NiCO, CuCO and PtCO allows us to make a general conjecture that a low occupancy of the outer s atomic orbital of the transition metal atom in the complex favours the bonding interaction with the CO moiety. The analogy with properties of LiCO and NaCO systems is pointed out.     

The lift computation for an oscillating flat plate in incompressible potential flow

The initial aim of this work was the estimation of the lift acting on a flat plate performing small oscillations in a plane uniform stream by means of a simplified model based on one or at the most two lumped vortices, and the assessment of its results by comparison to those that were exact. The model was found to work well up to a reduced frequency of about 1 or 2, above which the results diverged from those that were correct. In order to improve the model, its behaviour at very high frequencies was then investigated, discovering: (i) that if the number of lumped vortices is greater than one the possibility to impose all boundary conditions is subject to certain geometrical constraints; (ii) that the asymptotical behaviour is not the right one. A straightforward extension of this conclusion to the exact case of a continuous, vorticity distribution simulating the motion of the plate and to the classical equation describing it leads apparently to an incorrect result. The reason for the discrepancy is found in the singularity displayed by the integral equation which cannot be reproduced by the discrete model. It this therefore concluded that the latter can be trusted at low and middle frequencies but its extension to higher ones is fundamentally uncorrect.

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Sommario Lo scopo iniziale di questo lavoro era il calcolo approssimato della portanza agente su una lamina piana soggetta a piccole oscillazioni in una corrente bidimensionale uniforme per mezzo di un modello semplificato basato su uno o al più due vortici concentrati, e il confronto dei risultati con quelli esatti. Il modello risulta funzionare bene per frequenze ridotte inferiori ad 1 o 2, sopra le quali, tuttavia, i risultati si allontanano da quelli corretti. Per migliorarlo si è allora studiato il suo comportamento alle frequenze molto alte, scoprendo che: (i) la possibilità di imporre tutte le condizioni al contorno quando il numero dei vortici concentrati è superiore a uno è soggetta a certe limitazioni sulla configurazione geometrica; (ii) che il comportamento asindotico non è quello corretto. Un'estensione automatica di questa conclusione al caso esatto in cui il moto della lamina è simulato da una distribuzione continua di vorticità e alla classica equazione che lo descrive sembra condurre ad un risultato errato. La ragione di questa discrepanza viene individuata nella singolarità contenuta nell'equazione integrale, che non può essere riprodotta dal modello discreto. Se ne conclude perciò che esso è utilizzabile alle basse e medie frequenze ma che una sua estensione alle alte è fondamentalmente errata.

     

Problemi e convergenze variazionali su domini non limitati

Summary We introduce a class of second order elliptic operators from H ₀ ¹ () to his dual space H^–1(), where is an open set in Rⁿ that we allow to be unbounded. We prove that such operators are continuously invertible and that the constant majoryzing the norm of their inverses depends only on the parameters of the class. We prove moreover that if T H^–1() is given then the set of the L^–1T, where L belongs to the mentioned class is relatively compact in L²(). Next we study the relationships between several kinds of convergence (one of them is the G-convergence) and we study in what cases the spectrum function is semicontinuous or continuous on certain subsets of our class of operators.     

Chiral Diselenides from Benzylamines: Catalysts in the Diethylzinc Addition to Aldehydes

A series of new chiral diselenides with a N-atom in the side chain was prepared by a short synthetic sequence (Scheme 1). Only 1 mol-% of these diselenides catalyzed very effectively the diethylzinc addition to various aromatic and α,β-unsaturated aldehydes yielding the secondary alcohols in up to 98% ee (Scheme 2 and Tables 1 and 2). An asymmetric amplification was observed with these catalysts. Detailed NMR studies were performed to characterize the catalytically active species.     

Assessment of cooperativity in self-assembly

A method has been proposed to assess cooperativity in self-assembly processes. The method is based on a clear distinction between intermolecular and intramolecular processes which are compared with the corresponding reference reactions. It has been applied to two classical cases, namely the self-assembly of helicates and of porphyrin ladders, by using data previously published by the groups of Lehn and Anderson, respectively. Contrarily to the conclusions of the authors, pointing out self-assembly processes driven by positive cooperativity, the method here presented indicates in both cases the absence of cooperative effects. The methods previously used to assess cooperativity, in particular Scatchard plot and/or Hill plot, are criticized as being inappropriate for self-assembly, because they are pertinent to a specific case only, namely the intermolecular binding of a monovalent ligand L to a multivalent receptor M, a case very different from self-assembly which involves both inter- and intramolecular interactions. The present method underscores the fact that positive cooperativity in artificial self-assembling systems is probably much more rare than it was previously thought.     

Solvation of tetraalkylammonium chlorides in acetonitrile-water mixtures: mass spectrometry and molecular dynamics simulations.

The solvation of tetramethylammonium chloride (Me4NCl) and tetra-n-butylammonium chloride (Bu4NCl) in water-acetonitrile mixtures was investigated by mass spectrometry of clusters isolated from the solution. As far as the positive ions are concerned, clusters composed of alkylammonium ions and acetonitrile molecules only were observed, even for mixtures with high water content. In contrast, for the negative ions, clusters composed of chloride with both water and/or acetonitrile molecules were observed. For the smaller system (Me4NCl) we performed quantum chemical calculations and molecular dynamics simulations. It was found that even though water is present in the solvation shell of Me4N+, only acetonitrile has a strong electrostatic interaction with the cation. Water molecules around Me4N+ form hydrogen bonds with other water molecules, and they interact with Me4N+ mainly via dispersive interactions. These results indicate that Me4N+ behaves like a hydrophobic solute. On the other hand, the interaction of Cl- with water and acetonitrile is of comparable strength and, in both cases, the electrostatic interaction dominates. Herein we demonstrate experimentally and theoretically that positive and negative ions give rise to characteristic solvation structures in mixed solvents: even a relatively small organic cation, such as Me4N+, exhibits a hydrophobic-like solvation shell.