Accroissement de la reactivite des phosphorines en tant que dienes et philodienes par complexation du phosphore

The 2-Phenyl-4,5-dimethylphosphorin P-W(CO)₅ complex reacts easily as a dienophile with 2,3-dimethylbutadiene through its 1,6-positions and as a diene with N-phenylmaleimide, dimethyl acetylenedicarboxylate and cyclopentadiene, through its 1,4-positions.     

Analysis of90Sr in environmental and biological samples by extraction chromatography using a crown ether

The use of crown ethers is a simple and rapid method for the separation and determination of strontium in various samples. After sample dissolution oxalate precipitation was carried out to remove K. Chromatographic separation of strontium from most inactive and radioactive interference was done with a crown ether. The chemical recovery was determined gravimetrically. Radiostrontium was then determined by liquid scintillation counting.     

Experimental observation of light-induced solitary waves of analyte bands in capillary electrophoresis.

The phenomenon of electrophoresis in free solution has been studied theoretically down to the molecular level for decades. In addition, intermolecular photo-induced proton transfer reactions, which occur in a wide class of molecules (phenols and aminoarenes) as well as proteins (green fluorescent protein), were also studied extensively. However, the study of the effect of light-induced electrophoretic mobility changes of the analytes in electrophoresis was begun only recently. In the present work, capillary zone electrophoresis was chosen as the environment to measure the magnitude of these electrophoretic mobility shifts induced by light. Background electrolytes (running electrolytes) with high refractive indices were developed, allowing the capillary to work like an optical fiber. The experimental conditions for obtaining stable coupling and guided laser light along the liquid core are discussed. Experimental evidence of band compression is observed, leading to a solitary wave behavior of the analyte band (2-naphthol). These solitary waves result from competition between thermal diffusion (dispersion mechanism) and a nonlinear (band compression) effect due to the combined electrophoresis phenomenon and absorption of guided light by the molecules of the band (which are subjected to a "reversible intermolecular proton transfer reaction" as one of their decay routes). The possibilities of applying this effect to different methods and techniques are also discussed.     

Photoinduced electron transfer at liquid|liquid interfaces. Part VII. Correlation between self-organisation and structure of water-soluble photoactive species

The self-organisation of a variety of dyes at the water|1,2-dichloroethane interface was studied by admittance measurements, photocurrent–potential curves and light polarisation anisotropy of the photocurrent. The heterogeneous photo-oxidation of ferrocene was studied at interfaces sensitised by Sn(IV) meso-tetra-(4-carboxyphenyl) porphyrin dichloride (SnTPPC), chlorin e-6, protoporphyrin IX (protoIX) and Fe(III) protoporphyrin IX chloride (Fe-protoIX). Cyclic voltammograms and capacitance voltage curves exhibit different features associated with the self-assembly of the dye species at the liquid|liquid boundary. In the case of SnTPPC, the capacitance curves displayed the characteristic responses commonly associated with the specific adsorption of ionic species. On the other hand, chlorin e-6, protoIX and Fe-protoIX show rather complex behaviour suggesting not only changes in the excess charge but also in the dielectric permittivity of the interface. Differences in the photocurrent efficiency were also observed under the same experimental conditions. The relative magnitude of the photocurrent responses were rationalised in terms of the phenomenological electron transfer rate constant, the photon capture cross-sections and the lifetime of the triplet state as obtained from nanosecond flash photolysis. Finally, the average molecular orientation of the adsorbed photoactive species was estimated from the photocurrent dependence on the angle of light polarisation in total internal reflection. The results show a clear correlation between the orientation of the transition dipole and the distribution of the peripheral carboxyl groups responsible for the hydrophilic nature of the dyes.     

Structure‐Dependent Photoinduced Electron Transfer in Fullerodendrimers with Light‐Harvesting Oligophenylenevinylene Terminals

Oligophenylenevinylene (OPV)‐terminated phenylenevinylene dendrons G1 – G4 with one, two, four, and eight “side‐arms”, respectively, were prepared and attached to C₆₀ by a 1,3‐dipolar cycloaddition of azomethine ylides generated in situ from dendritic aldehydes and N‐methylglycine. The relative electronic absorption of the OPV moiety increases progressively along the fullerodendrimer family C₆₀G1 – C₆₀G4 , reaching a 99:1 ratio for C₆₀G4 (antenna effect). UV/Vis and near‐IR luminescence and transient absorption spectroscopy was used to elucidate photoinduced energy and electron transfer in C₆₀G1 – C₆₀G4 as a function of OPV moiety size and solvent polarity (toluene, dichloromethane, benzonitrile), taking into account the fact that the free‐energy change for electron transfer is the same along the series owing to the invariability of the donor–acceptor couple. Regardless of solvent, all the fullerodendrimers exhibit ultrafast OPV→C₆₀ singlet energy transfer. In CH₂Cl₂, the OPV→C₆₀ electron transfer from the lowest fullerene singlet level (¹C₆₀*) is slightly exergonic (ΔG_CS≈0.07 eV), but is observed, to an increasing extent, only in the largest systems C₆₀G2 – C₆₀G4 with lower activation barriers for electron transfer. This effect has been related to a decrease of the reorganization energy upon enlargement of the molecular architecture. Structural factors are also at the origin of an unprecedented OPV→C₆₀ electron transfer observed for C₆₀G3 and C₆₀G4 in apolar toluene, whereas in benzonitrile, electron transfer occurs in all cases. Monitoring of the lowest fullerene triplet state by sensitized singlet oxygen luminescence and transient absorption spectroscopy shows that this level is populated through intersystem crossing and is not involved in photoinduced electron transfer.     

Stability of tin etiopurpurin

Formulations of the tin etiopurpurin (SnET2) have been observed to undergo a spectral change on storage in dimethylformamide solution. This results in an alteration in the action spectrum, with enhanced photodynamic activity at lower wavelengths and decreased activity at 660 nm. On the basis of structural considerations, a reduced analog of SnET2 was prepared with an absorbance maximum at 640 nm. Formation of this product, termed SnET2H(2) relieves steric strain inherent in the parent molecule. Nuclear magnetic resonance, spectral and photodynamic data are consistent with the formation of SnET2H(2) during SnET2 storage. Slight modifications in the original synthesis are also reported, resulting in improved yields of intermediate products.     

Rotational band crossings in the actinides

In the actinides bothi _13/2 protons andj _15/2 neutrons are close to the Fermi surface. At rapid rotation these high-j particles will unpair and align their orbital angular momentum along the axis of rotation giving rise tos-bands that cross the ground-state band. Coulomb excitation of the odd nuclei ₂₃₇ ⁹³ Np (established up to the 45/2⁺ state) and ₂₃₅ ⁹² U (established up to the 51/2^? state) provides specific information about these band crossings: From the saturating alignment of the odd high-j particle in both nuclei at intermediate rotational frequencies we find the aligned angular momentum of thei _13/2 protons-band to be 6.6? while the corresponding value for thej _15/2 neutrons-band is 5.5?. At more rapid rotation above ?ω=0.18 MeV we observe additional alignment in²³⁵U. This is ascribed to the interaction of the protons-band. From the gradual onset of the additional alignment we deduce that forZ=92 the protons-band interacts strongly with the ground-state band and from a comparison of the actual amount of alignment with the full value of 6.6? we estimate the crossing to occur around ?gw _c ^p =0.25 MeV.     

Corps euclidiens,unités exceptionnelles et courbes elliptiques

Lenstra [Invent. Math.36 (1977), 237–254] has proved that a number field possessing sequences of integers whose differences are units, having a big enough length with respect to the discriminant of the field, is Euclidean. In this paper, we use this result to construct some new Euclidean fields; suitable sequences are obtained by using points of finite order on some elliptic curves.     

Structure activity relationship by NMR and by computer: a comparative study

There has recently been considerable interest in using NMR spectroscopy to identify ligand binding sites of macromolecules. In particular, a modular approach has been put forward by Fesik et al. (Shuker, S. B.; Hajduk, P. J.; Meadows, R. P.; Fesik, S. W. Science 1996, 274, 1531-1534) in which small ligands that bind to a particular target are identified in a first round of screening and subsequently linked together to form ligands of higher affinity. Similar strategies have also been proposed for in silico drug design, where the binding sites of small chemical groups are identified, and complete ligands are subsequently assembled from different groups that have favorable interactions with the macromolecular target. In this paper, we compare experimental and computational results on a selected target (FKBP12). The binding sites of three small ligands ((2S)1-acetylprolinemethylester, 1-formylpiperidine, 1-piperidinecarboxamide) in FKBP12 were identified independently by NMR and by computational methods. The subsequent comparison of the experimental and computational data showed that the computational method identified and ranked favorably ligand positions that satisfy the experimental NOE constraints.