Some applications of silica aerogels

Silica aerogels are very highly divided materials which are synthesised through the association of a chemical step, the so-called sol–gel chemistry, with a physical step which is a particular way of drying the wet gel, namely under supercritical conditions with respect to the liquid phase filling its porosity. This drying process preserves the texture of the dry material: in practice it strongly reduces the pore collapse. The resulting hyperporous solids that have bulk densities of the same magnitude as air develop new and very interesting physical and even chemical properties. Owing to their poor chemical reactivity, very large surface areas (of the order of 1,000 m ²/g), unusual porous volumes (greater than 95%), morphologies (monoliths or powders), optical properties (transparent, opaque or translucent), and very low thermal conductivity, they find high added-value applications in the physics of high-energy particles (Cherenkov emitters), transparent and superinsulating double windows, life and space science as well.     

Cu K-edge EXAFS characterisation of copper(I) arenethiolate complexes in both the solid and liquid state: detection of Cu-Cu coordination

Herein we describe a structural characterisation with EXAFS of copper(I) arenethiolate complexes in both the solid and liquid state. Previously noted difficulties in the detection of the Cu-Cu interaction have been attributed to anti-phase behaviour of different Cu-Cu neighbour contributions. A data analysis procedure solely based on EXAFS parameters is presented which resolves these problems. A careful analysis of the individual coordination shells and the use of different k-weightings during the data analysis are shown to be an absolute necessity to obtain reliable results. During R-space fitting, the difference file technique is used to separate, examine and compare the individual contributions. Using this technique their statistical significance and correctness can be determined. Anti-phase behaviour can be detected and accounted for in this way. An additional mixed organocopper aggregate [Cu4(SAr)2(Mes)2] with different Cu sites is analysed, which proves the value of the analysis procedure described above. Moreover, this newly developed EXAFS data analysis procedure is applicable to any other EXAFS spectrum obtained. The structural analysis of these organocopper complexes with EXAFS provides information about their actual structure and dynamic behaviour in solution. The technique can now be used to obtain insights into the reactivity of these complexes and the way in which they form catalytic reaction intermediates.     

Time-resolved optical Kerr-effect spectroscopy of low-frequency dynamics in Di-L-alanine,poly-L-alanine,and lysozyme in solution

The low-frequency spectra of peptides and proteins in solution have been investigated with optical heterodyne-detected Raman-induced Kerr-effect spectroscopy. Spectra were obtained for di-l-alanine ALA(2) and poly-l-alanine (PLA) in dichloroacetic acid solution. The conformational dependence of those spectra at low frequency has been analyzed. ALA(2) displays a band centered at 50 cm-1, whereas the alpha-helical PLA shows two shoulders at 60 and 140 cm-1. The similarity of the spectral features observed in PLA to those in water can be explained by analogous acoustic translational modes in the hydrogen network of the PLA alpha-helix. The mostly alpha-helical protein lysozyme in aqueous solution has also been investigated and showed significantly more structure with modes at 10, 35, 73, 106, and 164 cm-1.     

A polycationic dendrimer as noncovalent support for anionic organometallic complexes

Polycationic dendrimer 1 forms a well-defined, stoichiometric assembly with eight anionic metal complexes; this assembly is successfully applied as a Lewis acidic catalyst which performs comparably to the unsupported metal complex.     

Infrared spectroscopy of neutral C7H7 isomers: benzyl and tropyl

The gas-phase infrared absorption spectra of neutral benzyl and tropyl, isomers of formula C7H7, have been measured in the 400-1800 cm-1 spectral region. In addition, a quantum chemical calculation has been performed to model the infrared spectra. For the benzyl radical, the theory shows satisfactory overlap with the experiment, although vibrations involving the CH2 group might be anharmonic. The tropyl radical, which is subject to the Jahn-Teller effect, seems well modeled for the out-of-plane vibrational modes, but less so for the in-plane vibrational modes.     

A fast,sensitive determination of doxorubicin in rat plasma by solid-phase extraction and reversed-phase ion-pair chromatography

A very selective method is described for the determination of doxorubicin in rat plasma. Doxorubicin is extracted from the plasma on a pretreated octadecyl silane column and eluted with phosphate buffer pH 2.6/methanol (25/75, v/v) containing sodium 1-heptanesulfonate as ion-pairing agent. The extraction procedure is suitable for samples which contain doxorubicin encapsulated in liposomes if Triton X-100 is added. A portion of the evaporated eluate is used for high-performance reversed-phase chromatography with the same eluent and a fluorescence detector. Daunorubicin is used as internal standard. Extraction of doxorubicin from plasma is quantitative. The calibration graph is linear for 0.2-100 μg l^?1 doxorubicin with a limit of detection of 0.2 μg l^?1 for 0.5 ml of plasma. The relative standard error of estimate of the calibration was typically 3%.     

Spontaneously formed trans-anethol/water/alcohol emulsions: mechanism of formation and stability

We studied the spontaneous emulsification and droplet growth mechanism in trans-anethol/water/ethanol solutions, also known as the beverage ouzo, using dynamic light scattering spectroscopy. This simple ternary mixture is a generic example of a system that forms microemulsions spontaneously when brought into the two-phase region. The volume fraction of the dispersed phase was found to profoundly affect the growth rates of the droplets, which is a new finding that has not been predicted by the Lifshitz-Slyozov-Wagner theory. Time-dependent measurements show that the droplet growth is governed by Ostwald ripening (OR), and no coalescence was observed. Furthermore, the OR rates increase with increasing oil concentration at low alcohol content. We attribute this behavior to enhanced droplet-droplet interactions. At high ethanol concentrations, we found that the measured rates decreased as the oil concentration increased. The OR growth mechanism completely correlates with changes in droplet size. The kinetics of droplet growth shows that the ripening has a saturation limit at a droplet radius of about 1.5 mum. Thus, formed emulsions remain stable for months.     

Covalently bonded platinum(II) complexes of alpha-amino acids and peptides as a potential tool for protein labeling

Arylplatinum(II) complexes have been covalently bonded to the N and C termini and to the alpha-carbon of various amino acid derivatives. These organometallic-functionalized amino acid compounds can be converted into the corresponding free amino acids under both basic and acidic conditions; this demonstrates the excellent stability properties of these biomolecules. Due to the NMR activity displayed by the 195Pt nucleus (natural abundance 33.8%, I = 1/2) these compounds are functional bio-markers. Furthermore, the ability of the arylplatinum functional group to bind SO2 gas, selectively and reversibly as indicated by changes in the spectroscopic properties (1H, 13C, 195Pt NMR and UV spectra) of these compounds, allows for the potential use of these complexes as in vitro biosensors.     

Ferrocenyltrithiocarbonates : I. Direct access from α-ferrocenylcarbinols by a SNi mechanism. Absolute x-ray structure determination of (R)-ferrocenylmethylmethane S-methyl-trithiocarbonate

¹⁰³Rh Chemical shifts of a variety of mono- and di-nuclear rhodium carbonyl complexes are reported together with the modifications to the probe and decoupler unit of a JEOL PS-100 PFT spectrometer which enable these ¹⁰³Rh-decoupled ¹³C NMR measurements to be made. These data are discussed in conjunction with ¹³C NMR data on other rhodium carbonyls.