Heterotopic helicand for designing heterometallic helicates

A new diazine tetratopic helicand, H4L, is obtained from 3-formylsalicylic acid and hydrazine. The reaction between H4L and cobalt(II) perchlorate, iron(III) perchlorate, and sodium carbonate leads to triple-stranded tetranuclear anionic helicates, [L3Co(II)2Fe(III)2]2-, which are connected through Na ions, resulting in chiral coordination polymers, [L3Na2Fe2Co2(H2O)4(EtOH)2].3H2O.     

Mathematical discretization of size-exclusion chromatograms applied to commercial corn maltodextrins

Discretization of a size-exclusion chromatography (SEC) chromatogram is shown here to be an important calculation for characterizing the distribution of a polydisperse polymer, especially when the polydispersity is large. Commercial poly-glucose maltodextrins are known to have such a polydispersity. A mathematical discretization method with Gaussian peaks centered on each individual degree of polymerization is proposed and is performed on the entire SEC chromatogram for three different grades of corn maltodextrins. Because SEC and high-performance anion exchange chromatography with pulsed amperometric detection (HPAEC-PAD) are based on different separation mechanisms, they can be considered orthogonal techniques, and HPAEC-PAD was therefore used to validate the SEC discretization procedure. Because this validation proved satisfactory for all commercially available oligomers, the discretization is extended to all of their SEC chromatograms. Comparing the number-average molar weight and the weight-average molar weight before and after the mathematical discretization verifies that such a mathematical treatment does not denaturate the chromatogram. This approach tentatively leads to a more exhaustive characterization of a broadly polydisperse sample, such as maltodextrins, than was previously available, as it (i) gets rid of the apparent, chemically irrelevant, continuous molar weight distribution obtained by raw SEC and (ii) addresses the current detection and quantitation limits of the HPAEC-PAD technique without any sample treatment.     

Effect of the density of the C(18) surface coverage on the adsorption mechanism of a cationic compound and on the silanol activity of the stationary phase in reversed phase liquid chromatography

RPLC columns with different surface coverages (a C(1) endcapped column with a bonding density of 3.92 micromol/m(2) and four C(18)-bonded, endcapped columns, with octadecyl chain densities of 0.42, 1.01, 2.03, and 3.15 micromol/m(2)) were used to investigate the effects of the density of the surface coverage of RPLC columns on the adsorption mechanism of a cationic compound, amitriptyline chloride, and on the silanol activity of these columns. The mobile phases used were acetonitrile-water (30/70, v/v) solutions, buffered at either pH 2.7 or pH 6.9. At pH 2.7, the residual silanol groups are not ionized. At pH 6.9, some of these groups are ionized and these surface anions can strongly interact with the cationic compound. The adsorption isotherms were measured by frontal analysis (FA) at pH 2.7 and by frontal analysis by characteristic points (FACP) at pH 6.9, because the very high retention observed at neutral pH made FA measurements excessively long and poorly accurate. The adsorption energy distributions (AEDs) were calculated when possible, according to the expectation-maximization (EM) algorithm. A bimodal and a trimodal energy distribution were found for all the columns at pH 2.7 and 6.9, respectively. The third site measured at pH 6.9 was attributed to the strong ion-exchange interactions between the ionized silanol groups and the amitriptylinium cation. The contribution of the ionized silanol groups to the overall retention is maximum for the phases with intermediary bonding densities (1.01 and 2.03 micromol/m(2)). The peak tailing is most pronounced for the lowest (C(1) column) and the highest (3.15 micromol/m(2)) surface coverages.     

Adsorption mechanism in reversed-phase liquid chromatography. Effect of the surface coverage of a monomeric C18-silica stationary phase

The effect of the bonding density of the octadecyl chains onto the same silica on the adsorption and retention properties of low molecular weight compounds (phenol, caffeine, and sodium 2-naphthalene sulfonate) was investigated. The same mobile phase (methanol:water, 20:80, v/v) and temperature (T = 298 K) were applied and two duplicate columns (A and B) from each batch of packing material (neat silica, simply endcapped or C1 phase, 0.42, 1.01, 2.03, and 3.15 micromol/m2 of C18 alkyl chains) were tested. Adsorption data of the three compounds were acquired by frontal analysis (FA) and the adsorption energy distributions (AEDs) were calculated using the expectation-maximization method. Results confirmed earlier findings in linear chromatography of a retention maximum at an intermediate bonding density. From a general point of view, the saturation capacity of the adsorbent tends to decrease with increasing bonding density, due to the vanishing space intercalated between the C18 bonded chains and to the decrease of the specific surface area of the stationary phase. The equilibrium constants are maximum for an intermediary bonding density (approximately 2 micromol/m2). An enthalpy-entropy compensation was found for the thermodynamic parameters of the isotherm data. Weak equilibrium constants (small deltaH) and high saturation capacities (large deltaS) were observed at low bonding densities, higher equilibrium constants and lower saturation capacities at high bonding densities, the combinations leading to similar apparent retention in RPLC. The use of a low surface coverage column is recommended for preparative purposes.     

Experimental evidence of the influence of the surface chemistry of the packing material on the column pressure drop in reverse-phase liquid chromatography

The permeabilities of six columns packed with different packing materials (neat silica, C(1) endcapped silica at 3.92 micro mol/m(2), C(18) bonded and endcapped silica with 0.42, 1.01, 2.03, and 3.15 micro mol/m(2) of C(18) bonded chains) were measured. All these materials derive from the same batch of spherical particles, 5 micro m in diameter. The columns have the same tube inner diameter (phi=0.460+/-0.003 cm) and length (L=15.000+/-0.003 cm). The experimental conditions were the same, flow-rate (F(v)=1.000+/-0.003 mL/min) and temperature (295 K). Nevertheless, it was found that the column permeability decreases significantly, by about 25%, from the neat silica column to the one packed with the highest density of C(18)-bonded silica (3.15 micro mol/m(2)). The results measured on two duplicate columns were very reproducible. Accurate (+/-0.5 %) measurements of the hold-volumes with concentrated and dilute solutions of NO(3)(-) showed that the columns had all nearly the same external porosity. The result cannot be explained by the error made on the volume of the column tube either as it was measured accurately for all the columns. The residual explanation is that the interstitial velocity distribution between the packed particles depends on the chemical nature of the external surface of these particles.     

Intrachain electron transfer in conducting oligomers and polymers: the mixed valence approach

Organic mixed valence compounds consisting of bisdiarylamino charge-bearing units with an oligothiophene bridge and oligothiophene radical cations have been compared using molecular modeling. The study has been performed with oligomers of 1 to 22 thiophene units. These two series of molecules have several properties in common, and intramolecular Single Electron Transfer (SET) in both series can be described within the same theoretical framework. Conducting oligomer radical cations and slightly doped conducting polymers appear as special cases of the vast ensemble of organic mixed valence compounds. Short oligomers are class III, whereas longer oligomers and conducting polymers are class II. Therefore, doped conducting polymers cannot be correctly modeled using oligomers with a short conjugation length. Experimental evidence extracted from the literature confirms these findings. Single electron transfer theories can thus be used when studying interchain and intrachain electron transfer in slightly doped conducting polymers and in materials consisting of short oligomers. This makes it possible to extract from the UV-vis-near-IR spectra the electron-transfer constant rate along or between the pi-conjugated chain. The main differences among inorganic, organic, and conducting oligomer or polymer mixed valence compounds lies in the H(ab) and lambda values associated with these different series. Inorganic mixed valence compounds have small H(ab) and lambda values; organic mixed valence compounds have large H(ab) and lambda values, whereas conducting oligomers and polymers have large H(ab) but small lambda values. This induces charge delocalization to occur for systems larger than those of inorganic and nitrogen-centered organic mixed valence compounds.     

Determination of the surface energy of kaolinite and serpentine using PACHA formalism-comparison with immersion experiments

Clays play an important role in a wide variety of industrial processes. Indeed, they present interesting surface properties. For these reasons, we study the surface energy of models of clays by solid state calculations using electronegativities equalization. In this article, we focus on kaolinite and serpentine, two clays characterized by a simple structure of the TO type. We describe the clays and their structures and we develop a simple model from solid state calculations used to determine the surface energy. The results are in agreement with a recent interpretation of the immersion of kaolinite in water. This article must be related to some others focusing on solid surface energy, especially some treating talc and chlorites, and montmorillonites saturated by alkaline cations, to be published.     

Groupes Moyennables, Dimension Topologique Moyenne et sous-décalages

Let G be an infinite countable residually finite amenable group. In this paper we construct a continuous action of G on a compact metrisable space X such that the dynamical system (X, G) cannot be embedded in the G-shift on [0,1] ^G . This result generalizes a construction due to E. Lindenstrauss and B. Weiss (Mean topological dimension, Israel J. Math. 115 (2000), 1–24) for .     

Fast production of highly concentrated reactive [F] fluoride for aliphatic and aromatic nucleophilic radiolabelling

The use of a polymeric solid support loaded with a long alkyl chain quaternary ammonium allows the rapid and efficient recovery of cyclotron produced [¹⁸F]F⁻ from [¹⁸O]water to a low water content organic solution compatible with fast nucleophilic labelling of most precursors for PET radiopharmaceuticals in high yield.