Modification of MCM-48-, SBA-15-, MCF-, and MSU-type mesoporous silicas with transition metal oxides using the molecular designed dispersion method

Four various mesoporous silicas (MCM-48, SBA-15, MCF, and MSU) were modified by the molecular designed dispersion method using Fe(acac)3, Cr(acac)3, and Cu(acac)2 complexes. The deposition was performed at the same concentration of the metal acetylacetonate (acac) complex in a toluene solution. All as-synthesized samples were investigated by diffuse reflectance infrared Fourier transform spectroscopy, Fourier transform infrared photoacoustic spectroscopy, and thermogravimetric analysis. The calcined materials were studied with respect to their textural properties (Brunauer-Emmett-Teller adsorption isotherm) and chemical composition (electron microprobe analysis). It allowed elucidation of the mechanism of interaction between the acac complex and the silanol groups. For the MCM-48, SBA-15, and MCF materials, the formation of hydrogen bonding was found for the chromium- and copper-modified samples, whereas the Fe-containing materials showed the ligand exchange mechanism. The strong interaction of the MSU support and the different acetylacetonate complexes, resulting in a loss of at least one acac ligand, was observed. The mesoporous silicas modified with transition metal oxides were studied by UV-vis-DR spectroscopy. The different metal dispersions were found for the samples containing various transition metal oxides.     

Approximation of the Distribution of the Supremum of a Centered Random Walk. Application to the Local Score

Let (X _n ) _{n 0} be a real random walk starting at 0, with centered increments bounded by a constant K. The main result of this study is: |P(S _n n x)–P( sup_{0 u 1} B _u x)| C(n,K) n/n, where x 0, ² is the variance of the increments, S _n is the supremum at time n of the random walk, (B _u ,u 0) is a standard linear Brownian motion and C(n,K) is an explicit constant. We also prove that in the previous inequality S _n can be replaced by the local score and sup₀ u 1 B _u by sup₀ u 1|B _u |.     

Intense-laser-field ionization of molecular hydrogen in the tunneling regime and its effect on the vibrational excitation of H2+

H2 molecules were ionized by Ti:sapphire (45 fs, 800 nm) and Nd-doped yttrium aluminum garnet lasers (6 ns, 1064 nm). The relative populations of the vibrational levels of the H+2 ions were determined and found to be concentrated in the lowest vibrational levels. Tunneling ionization calculations with exact field-modified potential curves reproduce the experimental results. The reason for the departure from conventional Franck-Condon-like distributions is the rapid variation of the ionization rate with internuclear distance.     

Short-time dynamics through conical intersections in macrosystems

The short-time dynamics through a conical intersection of a macrosystem with a vast number of nuclear degrees of freedom (modes) is investigated. For convenience, the macrosystem is decomposed into a system carrying a few modes and a "bath." By transforming the bath modes to new ones, it is shown that only three effective bath modes contribute to the conical intersection. They govern--together with the system's modes--the short-time dynamics of the macrosystem. The remaining bath modes do not directly couple the electronic states and become relevant at longer times. An extensive numerical example is presented.     

Electrochemical properties and structures of surface-fluorinated graphite for the lithium ion secondary battery

Surface modification of graphite powder has been performed by elemental fluorine and radiofrequency (rf) plasma fluorination. Both methods give rise to an enlargement of the surface areas of graphite samples and a change of the pore volume distribution. The capacities of surface-fluorinated graphite samples are higher than those of original samples and even more than the theoretical capacity of graphite, 372 mAh g⁻¹, without any reduction of the first colombic efficiencies. The increments of the capacities are ∼5, 10, and 15% for graphite samples with average particle diameters of 7, 25 and 40 μm, respectively.     

Co-tunneling of the charge through a 2-D electron island

A double barrier Single Electron Transistor is realized in two dimensions by confining the 2-D electron gas of a GaAs/GaAlAs heterojunction to a small island by means of Schottky gates. Two gates provide adjustable tunnel barriers and a central gate controls the electron number in the island. The island has small single-particle energy level spacing and forms a metallic island. Periodic conductance oscillations characteristic of Coulomb blockade are observed when the central gate voltage is varied. The ability to vary the tunnel conductance allows us to study the basic physics of the Coulomb blockade: our results show that the quantum charge fluctuation mechanism which limits the tunneling blockade at low temperature is of second order in tunnel barrier transparencies in agreement with the charge Macroscopic Quantum Tunneling (q-MQT) or co-tunneling model.     

Facile and selective metalation of 7-dehydro-cholesterol (pro-vitamin D3)

Subsequent treatment of 7-dehydrocholesterol with trimethylsilylmethylpotassium in tetrahydrofuran, dry ice, mineral acid and diazomethane gives a regioisomerically pure, homoconjugated ester as a diastereomeric mixture, the minor component of which turns out to be thermodynamically more stable than a conjugated isomer.     

Optimalisation de certains traitements numériques avec une forme quadratique

When processing some given signals it is very often necessary to filter and then square this signal. When the signal is continuous these two operations are performed separately by analogous means; equivalent digital procedures are possible and have been already proposed. The authors considered whether this treatment could not be done with better results in one single digital operation, with respect to two specific problems : power spectrum estimation, and finite time variance estimation (particularly useful to specify oscillators stability). For this second problem they found some advantage to use this more general treatment, while for the first this approach does not bring any advantage. They proposed an optimal choice for this new approach by using a least mean squares method, where the choice of a window gauge is made according to the required accuracy.     

Propargylic derivatives. IX. Propargyl-pyrrolidinols and propargyl-perhydro-azepinols and their esters

The authors describe the synthesis of 3-propargyl-pyrrolidin-3-ols and 4-propargylperhydro-azepin-4-ols, and their esters. The antitussive, analgetic, and local anesthetic activities as well as the respiration stimulative effect of these compounds are related.