A new route towards nanoporous TiO2 as powders or thin films from the thermal treatment of titanium-based hybrid materials

Calcination of cyclopentadienyltitanium-based organic-inorganic hybrid materials at 450-500 °C led to the formation of anatase titanium dioxide as white powders consisting of a porous network of aggregated nanoparticles, the nanoporosity detected being related to the inter-particle space. Depending on the calcination temperatures, the surface area of the titanium dioxide particles varied from 65 to 158 m(2) g(-1).     

Photophysics of a series of efficient fluorescent pH probes for dual-emission-wavelength measurements in aqueous solutions

This paper evaluates the 5-aryl-2-pyridyloxazole backbone to engineer donor-acceptor fluorescent pH probes after one- or two-photon absorption. Parent fluorophores, as well as derivatives that can be used to label biomolecules, can be easily obtained in good yields. These molecules exhibit a large one-photon absorption in the near-UV range, and a strong fluorescence emission that covers the whole visible domain. The 5-aryl-2-pyridyloxazole derivatives also possess significant cross sections for two-photon absorption. Upon pyridine protonation, large shifts were observed in the absorption spectra after one- and two-photon excitation, as well as in the emission spectra. This feature was used to measure the pK(a) of the investigated compounds that range between 2 and 8. In most of the investigated derivatives, the pK(a) increased upon light excitation and protonation exchanges took place during the lifetime of the excited state, as shown by phase-modulation fluorometry analysis. Several 5-aryl-2-pyridyloxazole derivatives are suggested as efficient probes to reliably measure the pH of aqueous solutions by means of ratiometric methods that are dependent on fluorescence emission.     

Effect of boron oxide addition on the Nd environment in a Nd-rich soda-lime aluminoborosilicate glass

The environment of Nd³⁺ ions has been studied using optical absorption spectroscopy and EXAFS at the Nd L₃-edge, in a series of soda lime aluminoborosilicate glasses with increasing B₂O₃ content. The proportion of BO₄ units has been determined by ¹¹B MAS NMR in an equivalent glass series with La³⁺ ions replacing the majority of Nd³⁺ ions, and complementary information has been obtained by measuring the Nd³⁺ decay fluorescence times in these latter glasses. In these glasses with low Al₂O₃ content, the R′ ratio, with R′ = [Na₂O^exc] / [B₂O₃] and [Na₂O^exc] = [Na₂O] − [Al₂O₃] − [ZrO₂], plays a key role in controlling the structural organization and crystallization resistance, in a similar way as the R ratio in the Dell and Bray model of sodium borosilicate glasses. At R′ > 0.5, the Nd³⁺ ions are located in a mixed silicate-borate environment and, by slow cooling of the melt, they tend to crystallize within a silicate apatite phase close to the Ca₂Nd₈(SiO₄)₆O₂ composition. At R′ < 0.5, the structural results are compatible with Nd³⁺ ions located in a borate-type environment (not excluding Si neighbors), and, by slow cooling of the melt, they segregate with Ca²⁺ ions within a Si-depleted separated borosilicate phase.     

13C NMR studies of 2,3-dihydrofurans. Restricted rotation of bulky aryl rings in cis-2-aryl-3-methyl-2,3-dihydrofurans

¹³C NMR spectra of some 2,3-dihydrofurans were measured. Restricted rotation of the bulky aromatic groups of cis-2-aryl-3-methyl-2,3-dihydrofurans is observed when these groups have substituents in both ortho positions. Barriers to rotation were deduced from the variations in line widths of the ¹³C NMR signals. Exchange rates are highly affected by the buttressing effect of meta-methyl groups.     

Is the allylpalladium structure altered between solid and solutions?

Recent EXAFS measurements on [(Ph(2)PCH(2)CH(2)PPh(2))Pd(H(2)CCHCMe(2))]O(3)SCF(3) (Tromp et al. J. Am. Chem. Soc. 2002, 124, 14814) were interpreted as evidence that, when the complex is dissolved in THF, the allyl ligand adopts an eta(2) structure with a dangling allyl CH(2) substituent. DFT calculations of the Pd complex using H(2)P-CH(2)CH(2)-PH(2) as a model for Ph(2)P-CH(2)CH(2)-PPh(2) (dppe), in the absence or the presence of the triflate counteranion, and modeling the THF solvent by explicit Me(2)O molecules or by a continuum model give always a conventional eta(3)-H(2)CCHCMe(2) structure with equal Pd-C bonds to the terminal carbon centers of the allyl. QM/MM calculations using the dppe ligand also fail to support an eta(2)-allyl structure as a global minimum. The EXAFS parameter space is shown to have multiple minima. These have very similar overall EXAFS, but have very different structural parameters. The minimum that was the basis for the previous structural conclusion gives a slightly better fit but has unrealistic Debye-Waller factors and threshold energies.     

Les systèmes fer-oxygène-fluor et fer-soufre-fluor

The action of fluorine and hydrogen fluoride on the oxides and sulfides of iron, as well as that of hydrogen sulfide on iron oxides, has been investigated. In addition, a study has been made of the solid-state reactions between the oxides and fluorides of iron on the one hand and iron fluorides and sulfides on the other. Two oxyfluorides with a rutile- and spinel-type structure have been identified in the iron-oxygen-fluorine system. In contrast, no fluorosulfides have been found. The results have been interpreted on the basis of a comparison between Fe-O, Fe-S and Fe-F bonds.     

1,3-Dipolar cycloaddition reactions of nitrones with vinylacetic olefins in the racemic series and synthesis of δ-lactams via isoxazolidines

A study of the cycloaddition behavior of a series of esters and nitriles α-chloro- and α-hydroxyvinyl-acetic dipolarophiles with C-aryl-N-alkylnitrones has been carried out. Regiospecific cycloadditions are observed; the reactions lead to a mixture of 5-substituted isoxazolidines either erythro or threo, wherever the nitrone is involved. We report the synthesis of some δ-lactams in which isoxazolidines are used as latent synthons.     

Short and efficient synthesis of a stock material of dihydroxyacetone phosphate from glycidol

As enzymatic syntheses are expensive for a large-scale preparation of DHAP, a precursor leading to DHAP was synthesized in three steps starting from (±) glycidol; the stable benzylated stock material afforded by hydrogenolysis DHAP in high purity, which may be used directly without purification in enzymatic aldol synthesis.