Justification of the kinematic assumptions for thin-walled rods with shallow profile

In this Note we present a justification of the kinematic assumptions for thin-walled rods with shallow profile. These assumptions are fundamental to writing the one-dimensional equilibrium equations for such structures. The obtained kinematics are different from the Vlassov case, which is only valid for strongly curved profiles. They are also different from the that classically used in shell theory. The justification given in this Note is based on an asymptotic approach. To cite this article: L. Grillet et al., C. R. Mecanique 333 (2005).     

Probing the Diastereoselectivity of Staudinger Reactions Catalyzed by N‐Heterocyclic Carbenes

The reaction of ethylphenylketene with 1,3‐dimesitylimidazol‐2‐ylidene (IMes) or 1,3‐dimesitylimidazolin‐2‐ylidene (SIMes) afforded the corresponding azolium enolates in high yields. The two zwitterions were fully characterized by various analytical techniques. Their thermal stabilities were monitored by thermogravimetric analysis and the molecular structure of SIMes ? EtPhC?C?O was determined by means of X‐ray crystallography. A mechanism was proposed to account for the trans‐diastereoselectivity observed in the [2+2] cycloaddition of ketenes and N‐protected imines catalyzed by N‐heterocyclic carbenes and an extensive catalytic screening was performed to test its validity. The steric bulk of the NHC catalyst markedly affected the cis/trans ratio of the model β‐lactam product. The nature of the solvent used to carry out the Staudinger reaction also significantly influenced its diastereoselectivity. Conversely, the nature of the substituent on the N‐sulfonated imine reagent and the reaction temperature were less critical parameters.     

A Bistable Microelectromechanical System Actuated by Spin-Crossover Molecules

We report on a bistable MEMS device actuated by spin-crossover molecules. The device consists of a freestanding silicon microcantilever with an integrated piezoresistive detection system, which was coated with a 140 nm thick film of the [Fe(HB(tz)₃)₂] (tz=1,2,4-triazol-1-yl) molecular spin-crossover complex. Switching from the low-spin to the high-spin state of the ferrous ions at 338 K led to a reversible upward bending of the cantilever in agreement with the change in the lattice parameters of the complex. The strong mechanical coupling was also evidenced by the decrease of approximately 66 Hz in the resonance frequency in the high-spin state as well as by the drop in the quality factor around the spin transition.     

On the crucial importance of the pH for the formation and self-stabilization of protein microgels and strands

Stable suspensions of protein microgels are formed by heating salt-free β-lactoglobulin solutions at concentrations up to about C = 50 g·L(-1) if the pH is set within a narrow range between 5.75 and 6.1. The internal protein concentration of these spherical particles is about 150 g·L(-1) and the average hydrodynamic radius decreases with increasing pH from 200 to 75 nm. The formation of the microgels leads to an increase of the pH, which is a necessary condition to obtain stable suspensions. The spontaneous increase of the pH during microgel formation leads to an increase of their surface charge density and inhibits secondary aggregation. This self-stabilization mechanism is not sufficient if the initial pH is below 5.75 in which case secondary aggregation leads to precipitation. Microgels are no longer formed above a critical initial pH, but instead short, curved protein strands are obtained with a hydrodynamic radius of about 15-20 nm.     

Shear-induced disruption of dense nanoemulsion gels

The structural evolution and rheology of dense nanoemulsion gels, which have been formed by creating strong attractions between slippery nanodroplets, are explored as a function of steady shear rate using rheological small-angle neutron scattering (rheo-SANS). For applied stresses above the yield stress of the gel, the network yields, fracturing into aggregates that break and reform as they tumble and interact in the shear flow. The average aggregate size decreases with increasing shear rate; meanwhile, droplet rearrangements within the clusters, allowed by the slippery nature of the attractive interaction, increase the local density within the aggregates. At the highest shear rates, all clusters disaggregate completely into individual droplets.     

A new method to analyze copolymer based superplasticizer traces in cement leachates

Enhancing the flowing properties of fresh concrete is a crucial step for cement based materials users. This is done by adding polymeric admixtures. Such additives have enabled to improve final mechanicals properties and the development of new materials like high performance or self compacting concrete. Like this, the superplasticizers are used in almost cement based materials, in particular for concrete structures that can have a potential interaction with drinking water. It is then essential to have suitable detection techniques to assess whether these organic compounds are dissolved in water after a leaching process or not. The main constituent of the last generation superplasticizer is a PolyCarboxylate-Ester copolymer (PCE), in addition this organic admixture contains polyethylene oxide (free PEO) which constitutes a synthesis residue. Numerous analytical methods are available to characterize superplasticizer content. Although these techniques work well, they do not bring suitable detection threshold to analyze superplasticizer traces in solution with high mineral content such as leachates of hardened cement based materials formulated with superplasticizers. Moreover those techniques do not enable to distinguish free PEO from PCE in the superplasticizer.Here we discuss two highly sensitive analytical methods based on mass spectrometry suitable to perform a rapid detection of superplasticizer compounds traces in CEM I cement paste leachates: MALDI-TOF mass spectrometry, is used to determine the free PEO content in the leachate. However, industrial copolymers (such as PCE) are characterized by high molecular weight and polymolecular index. These two parameters lead to limitation concerning analysis of copolymers by MALDI-TOFMS. In this study, we demonstrate how pyrolysis and a Thermally assisted Hydrolysis/Methylation coupled with a triple-quadrupole mass spectrometer, provides good results for the detection of PCE copolymer traces in CEM I cement paste leachates.     

A tetrathiafulvalene-functionalized schiff base macrocycle: synthesis, electrochemical, and photophysical properties

Highly selective formation of 2+2 macrocycle 1 from 2,5-bis(3-formyl-2-hydroxyphenyl)-1,3,4-oxadiazole and a diamine-functionalized tetrathiafulvalene derivative is reported. Its electronic properties have been studied experimentally by the combination of electrochemistry and UV-vis-NIR spectroscopy. Particularly, its largely extended π-conjugation renders this novel macrocycle simultaneously a good multielectron donor and a strong chromophore, which is rationalized on the basis of density functional theory.     

Theoretical and experimental studies on the carbon-nanotube surface oxidation by nitric acid: interplay between functionalization and vacancy enlargement

The nitric acid oxidation of multiwalled carbon nanotubes leading to surface carboxylic groups has been investigated both experimentally and theoretically. The experimental results show that such a reaction involves the initial rapid formation of carbonyl groups, which are then transformed into phenol or carboxylic groups. At room temperature, this reaction takes place on the most reactive carbon atoms. At higher temperatures a different mechanism would operate, as evidenced by the difference in activation energies. Experimental data can be partially related to first-principles calculations, showing a multistep functionalization mechanism. The theoretical aspects of the present article have led us to propose the most efficient pathway leading to carboxylic acid functional groups on the surface. Starting from mono-vacancies, it ends up with the synergistic formation of dangling -COOH groups and the enlargement of the vacancies.     

On the elucidation of the mechanism of Vinca alkaloid fluorination in superacidic medium

Detailed investigations on one of the key steps of the superacidic fluorination of Vinca alkaloids that is the origin of C20' activation are reported. While two different pathways can be envisioned for the emergence of the transient secondary carbocationic intermediate, isotopic labeling experiments unambiguously revealed the involvement of a 1,2-hydride shift mechanism.     

Ultrafast dynamics of isolated fluorenone

The ultrafast dynamics of isolated 9-fluorenone was studied by femtosecond time-resolved photoionization and photoelectron spectroscopy. The molecule was excited around 264-266 nm into the S(6) state. The experimental results indicate that the excitation is followed by a multistep deactivation. A time constant of 50 fs or less corresponds to a fast redistribution of energy within the initially excited manifold of states, i.e., a motion away from the Franck-Condon region. Internal conversion to the S(1) state then proceeds within 0.4 ps. The S(1) state is long-lived, and only a lower bound of 20 ps can be derived. In addition, we computed excited state energies and oscillator strengths by TD-DFT theory, supporting the interpretation of the experimental data.