(e, 3e) test on e-e correlations in helium

The angular variations of the five-fold differential cross section obtained by using different wave functions of helium are compared with experimental data. It is found that in the coplanar geometry two kinematical arrangements, (i) equal energy sharing between the two ejected electrons with one of them ejected along the momentum transfer direction and the other along varying direction and (ii) the Bethe ridge condition with fixed sum of ejected electron energies and varying angle between them, are very sensitive to e-e correlations contained in the target wave function. This comparison has been used to show that open-shell class of wave functions better incorporate e-e correlations than the closed-shell class.     

Modélisation de structures électromagnétiques périodiques – matériaux bianisotropes avec mémoire

In this Note we propose a rigorous justification of the limit constitutive law of a periodic bi-anisotropic electromagnetic structure with memory. This study is based on the periodic unfolding method, introduced by D. Cioranescu, A. Damlamian and G. Griso, and is applied on the time domain and on the frequency domain. To cite this article: A. Bossavit et al., C. R. Acad. Sci. Paris, Ser. I 338 (2004).     

Influence of the degree of coverage of C18-bonded stationary phases on the mass transfer mechanism and its kinetics

The influence of the degree of coverage of a silica surface with bonded C18 alkyl chains on the mass transfer mechanism in RPLC was investigated. Five packing materials were used, prepared with the same batch of silica particles (5 microm diameter, 90 A average pore size): one column was packed with the silica derivatized by trimethylchlorosilane (TMS) (C1, 3.92 micromol/m2), and the other four with the silica first derivatized with octadecyl-dimethyl-chlorosilane (C18, 0.42, 1.01, 2.03, and 3.15 micromol/m2), and then endcapped with TMS. A solution of methanol and water (25/75, v/v) was used as the mobile phase. The experimental HETP curves were acquired for each column by measuring the first moment and the second central moment of phenol and correcting them for the influence of the temperature increase due to the heat generated by the friction of the stream against the bed. The different kinetic parameters of the mass transfer in these packed chromatographic columns were identified (longitudinal diffusion, eddy diffusion, film mass transfer, and transparticle mass transfer) and quantified by fitting the experimental data to a new general HETP equation recently derived [F. Gritti, G. Guiochon, Anal. Chem., in press (AC-060203R).]. The agreement was excellent and allowed the comparison of the kinetic parameters among the six columns used. The highest column efficiency measured at conventional or fast flow rates (>0.5 ml/min) is obtained for the most retentive column, which has a surface coverage of 2.03 micromol/m2. The smallest HETP measured is as low as 10 microm, only twice the average particle diameter dp, due to the large contribution of surface diffusion (90%) to the particle effective diffusivity. However, no significant difference was observed between the efficiencies of the columns packed with C1 and C18 derivatized silica.     

Characterization and kinetic performance of 2.1 × 100 mm production columns packed with new 1.6 μm superficially porous particles

The overall kinetic performance of three production columns (2.1 mm × 100 mm format) packed with 1.6 μm superficially porous CORTECS‐C₁₈+ particles was assessed on a low‐dispersive I‐class ACQUITY instrument. The values of their minimum intrinsic reduced plate heights (h_min = 1.42, 1.57, and 1.75) were measured at room temperature (295 K) for a small molecule (naphthalene) with an acetonitrile/water eluent mixture (75:25, v/v). These narrow‐bore columns provide an average intrinsic efficiency of 395 000 plates per meter. The gradient separation of 14 small molecules shows that these columns have a peak capacity about 25% larger than similar ones packed with fully porous BEH‐C₁₈ particles (1.7 μm) or shorter (50 mm) columns packed with smaller core–shell particles (1.3 μm) operated under very high pressure (>1000 bar) for steep gradient elution (analysis time 80 s). In contrast, because their permeabilities are lower than those of columns packed with larger core–shell particles, their peak capacities are 25% smaller than those of narrow‐bore columns packed with standard 2.7 μm core–shell particles.     

Liquid-junction potential effects in measurements of sodium ion activity in unbuffered aqueous solutions

Measurements of pNa with the glass-membrane ion-selective electrode in unbuffered aqueous solutions require care in the constitution of the measuring cell, because of varialbe liquid-junction potentials at the salt bridge. Of the bridge electrolytes tested, with calomel or silver/silver chloride reference electrodes, 3 M ammonium chloride in agar-agar gel was most satisfactory. Calibration graphs were of almost theoretical slope in the range 10^?3?5 × 10^?1 M sodium ion.     

Synthese de N-(tetrazol-5-yl) azetidin-2-ones

The title compounds were obtained by addition of a glycine enolate to a N-(tetrazol-5-yl) imine followed by cyclisation. This route provided both the $\text{cis}$ and the $\text{trans}$ substituted azetidinones. The $\text{cis}$ compounds possessed improved antibiotic activity as compared to the $\text{trans}$ counterparts.     

Theoretical thermochemistry of some LiXHn and BeXHn compounds

Summary In this theoretical work, we consider the geometrical, electronic and energetic properties of some lithium and beryllium derivatives. The standard heats of formation of these compounds have been calculated at the MP4=SDTQ/6-31+G(2df, p)//MP2=FULL/6-31G(d, p) level. The values obtained at this level of the theory are also compared with the heats of formation deduced from a composite procedure in which it is assumed that some corrections can be treated separately and combined in an additive manner. We find that the values determined with the complete 6-31+G(2df, p) basis set are the more accurate.     

A chromatographic estimate of the degree of surface heterogeneity of RPLC packing materials. III. Endcapped amido-embedded reversed phase

The difference in adsorption behavior between a conventional monomeric endcapped C18 stationary phase (3.43 micromol/m2) and an endcapped polymeric RP-Amide phase (3.31 micromol/m2) was investigated. The adsorption isotherms of four compounds (phenol, caffeine, sodium 2-naphthalene sulfonate, and propranololium chloride) were measured by frontal analysis (FA) and the degree of heterogeneity of each phase for each solute was characterized by their adsorption energy distributions (AED), derived using the Expectation-Maximization method. The results show that only certain analytes (phenol and 2-naphthalene sulfonate) are sensitive to the presence of the polar embedded amide groups within the RP phase. Their binding constants on the amide-bonded phase are significantly higher than on conventional RPLC phases. Furthermore, an additional type of adsorption sites was observed for these two compounds. However, these sites having a low density, their presence does not affect much the retention factors of the two analytes. On the other hand, the adsorption behavior of the other two analytes (caffeine and propranololium chloride) is almost unaffected by the presence of the amide group in the bonded layer. Strong selective interactions may explain these observations. For example, hydrogen-bond interactions between an analyte (e.g., phenol or naphthalene sulfonate) and the carbonyl group (acceptor) or the nitrogen (donor) of the amido-embedded group may take place. No such interactions may take place with either caffeine or the cation propranololium chloride. This study confirms the hypothesis that analytes have ready access to locations deep inside the bonded layer, where the amide groups are present.     

Synthèse et étude de modèles de NADH à noyaux accolés en série pyrrolo[2,3-b] et [3,2-b]pyridines

Pyrrolo[2,3-b] and [3,2-b]pyridines derivatives, precursors for annealated NADH models have been prepared. Several methods for building a pyridinic annealates ring have been used starting from 2- or 3-aminopyrrole. The corresponding dihydropyridine structure leads to highly reactive NADH models compounds and are very much more stable than common models.