Resolution of benzo[a]pyrene in complex mixtures of other polycyclic aromatic hydrocarbons. Comparison of two spectrofluorimetric methods applied to water samples

Two spectrofluorimetric methods, second-derivative constant-energy synchronous luminescence (SDCESL) and constant-wavelength synchronous luminescence (CWSL) in combination with multiple linear regression (MLR), for the quantification of benzo[a]pyrene (BaP) at sub-ng mL-1 levels, in the presence of benzo[b]fluoranthene (BbFt), benzo[k]fluoranthene (BkFt), benzo[ghi]perylene (BghiP) and indeno[1,2,3-cd]pyrene (IP), were developed and compared in detail. SDCESL presents lower limits of detection and quantification than CWSL/MLR and also gives more exact and precise results for levels close to the quantification limit. For BaP, SDCESL achieved quantification limits of 0.019 ng mL-1 in river waters and 0.007 ng mL-1 in drinking waters. This work offers a sensitive, precise, accurate, rapid, simple and economic methodology for monitoring BaP in waters for public consumption, meeting all the requirements of the EC Directive 98/83/CE that fixes the maximum admissible limit for this polycyclic aromatic hydrocarbon in drinking waters at of 0.010 ng mL-1.     

Liquid-Phase Selective Oxidation of Propane on Silica-Supported Nafion Catalysts

The liquid-phase partial oxidation of propane in the presence of the Fen+/H2O2 Fenton system at 70℃ and 1.4 atm on silica supported Nafion catalysts has been investigated. The reaction proceeds via a radical reaction path the efficiency of which is improved by silica-supported Nafion catalysts. Because of the direct relationship between reaction rate and concentration of sulphonic acid sites of Nafion catalysts, it is inferred that the active phase enahnces the kinetics of propane conversion by promoting the rate of active radicals generation.     

"Chiral perturbation factor" approach reveals importance of entropy term in stereocontrol of the 2,4-pentanediol-tethered reaction

The stereocontrol mechanism of the 2,4-pentanediol (PD)-tethered reaction was studied in detail using a reaction system consisting of phenyl and rhodium carbenoid moieties. Different tethers were examined to analyze the effects of the methyl groups on the PD tether. Among the reactions with these tethers, the PD tether achieves an unmeasurably high stereoselectivity in a diastereomeric ratio of >500. Another tether showing a high but measurable stereoselectivity in a ratio of 41 is mostly controlled by the entropy term. To clarify the role of the methyl groups on the chiral tethers, which are the origin of the stereocontrol, the "chiral perturbation factor" is introduced. This parameter is defined as the rate of a chiral reaction relative to that of an achiral reference reaction. By analyzing the temperature dependence of the chiral perturbation factors for different chiral-tethered reactions, high potentials of the PD-tethered reaction in its stereocontrol are concluded to be due to the entropy term.     

Adsorption and thermal condensation mechanisms of amino acids on oxide supports. 1. Glycine on silica

Glycine was adsorbed on the surface of a well-defined silica from aqueous solutions of variable concentrations and pHs. The adsorbed molecules were characterized using middle-IR and UV-vis-NIR spectroscopies. Except at the lowest pH (2.0), they were predominantly present on the surface as zwitterions. Two successive deposition mechanisms were evidenced with increasing glycine concentration. At low concentrations, glycine is specifically adsorbed on silica surface sites, probably through its NH3+ moiety. The pH dependence suggests that these sites may be silanolate groups (approximately equal to Si-O-). At higher concentrations, specific adsorption sites are saturated and surface-induced precipitation of beta-glycine is observed. The thermal reactivity of adsorbed/deposited glycine was then investigated by thermogravimetric analysis, in situ diffuse reflectance IR spectroscopy, and thermoprogrammed desorption coupled with mass spectrometry. Adsorbed glycine molecules react to form peptide bonds at a temperature considerably lower than that for bulk crystalline alpha-glycine. The main reaction product is the cyclic dimer diketopiperazine, with no evidence of the linear dimer. The activation mechanism is not diffusionally limited; the formation of "surface acyls", previously proposed for related systems, has not been evidenced here. These findings are of relevance for the evaluation of prebiotic peptide synthesis scenarios.     

Disorder effects in Mn12–acetate at 83 K

The structure of hexadeca‐μ‐acetato‐tetra­aqua­dodeca‐μ₃‐oxo‐dodecamanganese bis(acetic acid) tetrahydrate, [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄]·2CH₃COOH·4H₂O, known as Mn₁₂–acetate, has been determined at 83 (2) K by X‐ray diffraction methods. The fourfold (S₄) molecular symmetry is disrupted by a strong hydrogen‐bonding interaction with the disordered acetic acid mol­ecule of solvation, which displaces one of the acetate ligands in the cluster. Up to six Mn₁₂ isomers are potentially present in the crystal lattice, which differ in the number and arrangement of hydrogen‐bonded acetic acid mol­ecules. These results considerably improve the structural information available on this molecular nanomagnet, which was first synthesized and characterized by Lis [Acta Cryst. (1980), B 36 , 2042–2046].     

Core–shell nanogels by RAFT crosslinking polymerization: Synthesis and characterization

A synthetic methodology is described for the preparation of core–shell nanogels by reversible addition‐fragmentation chain transfer. Well‐defined macro chain transfer agents (macro‐CTA's) were prepared in a first step using monomers that yield sensitive polymers. In the second step, a crosslinker alone or with the addition of a functionalized comonomer were used to form a crosslinked core. The ratio of crosslinker to macro‐CTA is crucial to yield nanogels. Furthermore, the polymerization time has an impact in the architecture of the nanomaterial obtained: it evolves from a core‐crosslinked star to a core–shell nanogel. Controlling the molecular weight of the macro‐CTA and the type of comonomer in the core forming step, core–shell nanogels with hydrodynamic diameters from 22 to 168 nm and a core that represents from 35 to 77% of the size, were prepared containing functional groups in the core which could be used as catalytic scaffolds. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012.     

Effects of allylic and homoallylic substituents on the ring closing metathesis reaction used to synthesise simplified eleuthesides

During the course of our synthetic studies towards simplified eleuthesides, we have found that p-methoxyphenyl (PMP) protected allylic alcohols are compatible with the RCM reaction and can give better yields than the corresponding free allylic alcohols.     

Electrochemically Generated Nanoparticles of Halogen‐Bridged Mixed‐Valence Binuclear Metal Complex Chains

Spherical nanoparticles composed of MMX chains can be made by a polymerization strategy driven by electrochemical processes. In particular, the [Pt₂(MeCS₂)₄I₂] (MMI₂) dimetal subunit is employed as a monomer for the formation of [Pt₂(MeCS₂)₄I]_n spherical nanostructures on surfaces. We have paid particular attention to elucidating the general mechanism of the deposition process on the basis of in situ electrochemical measurements. The reduction of MMI₂ to give the electrodeposition of nanostructures agrees well with formation of the reduced [MMI₂]^? species followed by a disproportionation mechanism mediated by iodide anions. The chemical composition of the particles was determined by energy‐dispersive X‐ray spectroscopy (EDX) and X‐ray photoelectron spectroscopy (XPS) to reveal the MMI₂ polymer.     

Electronic and Structural Properties of Highly Aluminum Ion Doped TiO2 Nanoparticles: A Combined Experimental and Theoretical Study

This study presents the experimental and theoretical study of highly internally Al‐doped TiO₂ nanoparticles. Two synthesis methods were used and detailed characterization was performed. There were differences in the doping and the crystallinity, but the nanoparticles synthesized with the different methods share common features. Anatase to rutile transformation occurred at higher temperatures with Al doping. X‐ray photoelectron spectroscopy showed the generation of oxygen vacancies, which is an interesting feature in photocatalysis. In turn, the band‐gap energy and the valence band did not change appreciably. Periodic density functional calculations were performed to model the experimentally doped structures, the formation of the oxygen vacancies, and the band gap. Calculation of the density of states confirmed the experimental band‐gap energies. The theoretical results confirmed the presence of Ti⁴⁺ and Al³⁺. The charge density study and electron localization function analysis indicated that the inclusion of Al in the anatase structure resulted in a strengthening of the Ti?O bonds around the vacancy.