Stereoselectivity in the intermolecular Pauson-Khand reaction of electron-deficient terminal alkynes

A family of terminal alkyne dicobalthexacarbonyl complexes bearing groups with a range of electron-withdrawing abilities has been synthesized. After submitting these complexes to the intermolecular Pauson-Khand reaction with norbornadiene, electron-deficient substrates afforded up to 26% of the unexpected endo-cyclopentenone.     

Marcinkiewicz–Zygmund inequalities

We study a generalization of the classical Marcinkiewicz–Zygmund inequalities. We relate this problem to the sampling sequences in the Paley–Wiener space and by using this analogy we give sharp necessary and sufficient computable conditions for a family of points to satisfy the Marcinkiewicz–Zygmund inequalities.     

Simultaneous spectrophotometric determination of Zinc(II) and Nickel(II) with 1-(2-pyridylazo)-2-naphthol

A new procedure for the simultaneous spectrophotometric determination of Zn(II) and Ni(II) with 1-(2-pyridylazo)-2-naphthol as chromogenic reagent has been developed. It is based on resolution of the mixed spectrum over the wavelength range 530–590 nm by applying a least-squares fitting program to standard spectra from each component. The spectra are recorded at an apparent pH of 5.0, provided by an acetate buffer in a mixed 6040 v/v 2-ethoxyethanol/water medium. The relative standard deviations for determination of 13.7 g of Ni and 31.0 g of Zn were 1.9 and 1.0%, respectively (8 replications). The method has been applied to the analysis of iron surfaces coated with a Zn-Ni alloy. The results are compared with those obtained by atomic-absorption spectrophotometry.     

New synthetic “tricks” using old reagents. A mild method for the conversion of RCONHR′ to RCONHR″

N-Alkyl-N-nitrosoamides, RCON(NO)R′, react with primary aliphatic amines (NH₂R″), in refluxing dichloromethane or at room temperature, to give RCONHR″ in 65–98% yields.     

Substituted m-phenylene bridges as strong ferromagnetic couplers for Cu(II)-bridge-Cu(II) magnetic interactions: new perspectives

Based on a combined theoretical-experimental study, we propose that substituted m-phenylene ligands (m-N-Phi-N) can act as tuneable strong ferromagnetic couplers connecting Cu(II) ions; a new complex presenting that bridge with J close to +15 cm(-1) has been suggested and synthesized.     

Electrostatics of phosphatidic acid monolayers: Insights from computer simulations

In this paper, we argue that many of the fascinating electrostatic effects that take place in amphiphilic systems are strongly related to the particular organization of the oxygen atoms within each individual molecule. In particular, we focus on two effects: charge inversion and dielectric overscreening. For that purpose, we present molecular dynamics simulations of phosphatidic acid (DMPA²⁻) in the presence of divalent counterions. Our results show that the many oxygens present in DMPA²⁻ cooperatively create strong binding sites for counterions, which in some cases lead to charge inversion. We also present an analysis of the role of interfacial water and relate our analysis to the phenomenon of dielectric overscreening. Several experimental implications are discussed in the conclusions.     

Theoretical study of alkyl-pi and aryl-pi interactions. Reconciling theory and experiment

Quantum mechanical and quantum mechanical/molecular mechanical calculations in conjunction with continuum solvation models have been used to analyze CH-pi interactions in model systems of aryl- and alkyl-aromatic interactions, as well as in a model folding system designed to study those interactions. High level calculations reproduced accurately the interaction of CH-pi interactions in both alkyl- and aryl-based model systems. Dispersion effects dominate the interaction, but the electrostatics term is also relevant for aryl CH-pi interactions. Theoretical calculations were also used to examine the influence of CH-pi interactions in determining the conformational flexibility of folding models. Finally, a critical comparison of the results obtained from high level calculations on model systems and the experimental data derived for folding models in apolar solvents was carried out, which allowed us to reconcile the apparent discrepancy found between both data.     

Use of diode-array detectors for the simultaneous spectrophotometric determination of calcium and magnesium by flow injection

A method for the simultaneous flow injection spectrophotometric determination of calcium and magnesium with Arsenazo III based on the use of diode-array detector and merging zones is described. The method is applicable to the resolution of mixtures in which the chromogenic reagent has a high absorbance and its spectrum strongly overlaps those of its complexes. In resolving the mixtures, the excess reagent is considered as another component. Quantitation is based on the normal absorbance and first-derivative absorbance spectra. The method is applied to 0.2–1.5 μg ml^?1 Ca and 0.1–1.0 μg ml^?1 Mg. The analysis rate is 50 h^?1.     

Simultaneous determination of two components by spectrofluorimetric techniques

A procedure is described for the resolution of two-component mixtures by use of data from variable-angle fluorescence spectrometry. The experimental data are treated by multilinear regression. The method is applied to the resolution of mixtures of fluorescein and dichlorofluorescein at the ng ml^?1 level; mean errors are < 4%.