Modulation of the Physicochemical Properties of Heteropolynuclear Assemblies Containing Lanthanide Ions and 2,2′-oxydiacetate

Summary: This work focuses on the polynuclear complexes formed by a lanthanide ion (Ln), a bivalent cation (M) and oxydiacetate (oda) as bridging ligand, with general formula [Ln₂M₃(oda)₆(H₂O)₆] · xH₂O. The complexes can adopt more than one polymorphic structure, being the hexagonal (P6/mcc) and cubic (Fd3c) the most noticeable, as determined by x-ray powder diffraction. The influence of Ln and M on the solid-state structure is discussed. In addition, potentiometric studies are performed to support the existence of polynuclear species in solution.     

Comparison of halogen bonding and van der Waals and π–π interactions in 4,5‐dibromo‐2‐hexyloxyphenol

The title compound, C₁₂H₁₆BrO₂, is an interesting case of a simple organic molecule making use of five different types of intra‐ and intermolecular interactions (viz. conventional and nonconventional hydrogen bonds, and π–π, Br...Br and Br...O contacts), all of them relevant in the molecular and crystal structure geometry. The molecules are strictly planar, with an intramolecular O—H...O hydrogen bond, and associate into two‐dimensional structures parallel to (01) through two different types of halogen bonding. The planar structures, in turn, stack parallel to each other interlinked by C—H...π and π–π contacts. Also discussed are the relevant structural features leading to the rather low melting point of the compound.     

Proton-induced luminescence of mono- and dinuclear rhenium(I) tricarbonyl complexes containing 4-pyridinealdazine

New mono- and dinuclear rhenium(I) tricarbonyls, of formulas [Re(bpy)(CO)3(PCA)]+ (1), [(bpy)(CO)3Re(I)(PCA)Re(I)(CO)3(bpy)]2+ (2), and [(bpy)(CO)3Re(I)(PCA)Ru(II)(NH3)5]3+ (3) (bpy = 2,2'-bipyridine, PCA = 4-pyridinecarboxaldehydeazine), have been synthesized as PF6- salts and characterized by spectroscopic, electrochemical, and photophysical techniques. These species do not emit at room temperature in CH(3)CN; however, in aqueous solutions, a decrease in pH induces luminescence in all of them, due to protonation of one of the N atoms of the -C=N-N=C- chain of PCA, as indicated by the pKa values of the ground states, obtained by absorption measurements, which are ca. 3 orders of magnitude lower than the pKa value of the pyridine N of PCA in complex 1. On the other hand, the values of pKa* of the excited states, obtained by emission measurements, of complexes 1 and 2 are similar (pKa* = 2.7 +/- 0.1 at I = 0.1 M) and higher than those of the corresponding ground states. At low values of pH, chemical decomposition takes place rapidly in complex 3, but not in 1 and 2, supporting the possible use of these latter species as luminescent sensors of pH. The heterodinuclear complex, of formula [(bpy)(CO)3Re(I)(PCA)Ru(III)(NH3)5]4+, was obtained by bromine oxidation of the [Re(I), Ru(II)] precursor in CH3CN solution; from spectral and electrochemical measurements, the recombination charge-transfer reaction [Re(II), Ru(II) ] --> [Re(I), Ru(III)], which occurs after photoexcitation, is predicted to lie in the Marcus inverted region.     

Zeeman effect on the nuclear quadrupole resonance of 81Br in single crystals of methyl 4-bromobenzoate, 4,4′-dibromodiphenyl-ether and 4,4′-dibromodiphenylsulphide at 77 K

The Zeeman effect on the N.Q.R. of methyl 4-bromobenzoate, 4,4′-dibromodiphenylether, 4,4′-dibromodiphenylsulphide has been studied at 77 K on single crystals, using the ‘geometric method’. The apparatus used in the measurements, which allows one to maintain any temperature between room temperature and 77 K, is described, and the possible significance of the slight variations in the molecular arrangement found in the present measurements and in those performed at room temperature is discussed.     

Ion-interaction chromatography of neutral molecules

Summary The chromatographic behavior of neutral molecules in ion-interaction chromatography (IIC) is investigated theoretically. The physical and chemical modification of the stationary phase in the presence of ion interaction Reagent (IIR) in the eluent, and adsorption competition between test analytes and IIR for inner layer sites are shown theoretically to change the partition coefficient of neutral molecules. The most reliable, literature experimental results, concerming retention behaviour of neutral molecules in IIC, were used to test the new theory. The wide variability among them was elucidated on the basis of the exhaustive retention model developed. Retention equations were compared to those which can be obtained, if the change of the analyte is zero, from the most important retention model in IIC.     

On the antioxidant activity of eumelanin biopigments: a quantitative comparison between free radical scavenging and redox properties

Abstract

The antioxidant activity of eumelanin, a ubiquitous pigment in flora and fauna, constitutes one of its most fascinating physicochemical properties. To shed light on free radical scavenging vs redox facets of such activity, we applied hydrogen atom transfer- and electron transfer-based assays to pristine Sepia ink, eumelanin from Sepia ink, chemically controlled eumelanins and their precursor building blocks. Our work contributes to the rational use of the antioxidant properties of eumelanin for health, cosmetics and environmental applications.     

Tricyclic heteroaromatic systems. 1,2,4-triazolo[1,5-a]quinoxaline

The parent 1,2,4-triazolo[1,5-a]quinoxaline was prepared by a few-step reaction from ethyl N¹-(2-nitrophenyl)-N³-ethoxalyloxamidrazonate, which afforded the key intermediate diethyl 1-(2-nitrophenyl)-1,2,4-triazole-3,5-dicarboxylate in satisfactory yield. The latter was cyclized, hydrolyzed, decarboxylated, reduced and dehydrogenated to yield the title compound.     

Extended Thermodynamic Approach to Ion Interaction Chromatography: Influence of Organic Modifier Concentration

Summary The influence of organic modifier and ion interaction reagent (IIR) concentrations on analyte retention in ion interaction chromatography (IIC) was investigated via a bivariate approach. The system examined is as follows: ODS-Hypersils was used as stationary phase; the eluent was a variable mixture of a phosphate buffer pH 2.1 and methanol ranging 0%–40% (v/v). The organic modifier decreases both the analyte and the IIR free energy of adsorption hence their interaction is lower when the organic modifier concentration is higher. Retention equations describe how these two factors interact with each another. The estimated equilibrium constant for the IIR adsorption corresponds to G° = –17.3 KJ mol^–1. This value is reliable because it makes sense physically. The chi-squared (k _calc – k _exp)² is 1.11E + 00 and 2.85E-02, for octopamine and 2-naphthalenesulfonate, respectively. Retention equations were compared with those obtained from the outstanding retention models in IIC. For the modelled system, the extended thermodynamic approach is able to explain experimental evidence that cannot be rationalised by existing retention models: it proves superior for fitting experimental data and is also able to predict when the purely electrostatic approach may work properly. In the absence of IIR the present retention equation reduces, as expected, to the relationship that describes the influence of organic modifier on retention behaviour in reversed-phase liquid chromatography.     

Monitoring of volcanic sulphur dioxide emissions using differential absorption lidar (DIAL), differential optical absorption spectroscopy (DOAS), and correlation spectroscopy (COSPEC)

2 fluxes of the order of 1000, 100, and 10 tonnes/day were measured for Mt. Etna, Stromboli, and Vulcano, respectively. Received: 17 April 1998