Kinetic solvent effects on proton and hydrogen atom transfers from phenols. Similarities and differences

Bimolecular rate constants for proton transfer from six phenols to the anthracene radical anion have been determined in up to eight solvents using electrochemical techniques. Effects of hydrogen bonding on measured rate constants were explored over as wide a range of phenolic hydrogen-bond donor (HBD) and solvent hydrogen-bond acceptor (HBA) activities as practical. The phenols' values ranged from 0.261 (2-MeO-phenol) to 0.728 (3,5-Cl(2)-phenol), and the solvents' values from 0.44 (MeCN) to 1.00 (HMPA), where and are Abraham's parameters describing relative HBD and HBA activities (J. Chem. Soc., Perkin Trans. 2 1989, 699; 1990, 521). Rate constants for H-atom transfer (HAT) in HBA solvents, k(S), are extremely well correlated via log k(S) = log k(0) - 8.3 , where k(0) is the rate constant in a non-HBA solvent (Snelgrove et al. J. Am. Chem. Soc. 2001, 123, 469). The same equation describes the general features of proton transfers (k(S) decreases as increases, slopes of plots of log k(S) against increase as increases). However, in some solvents, k(S) values deviate systematically from the least-squares log k(S) versus correlation line (e.g., in THF and MeCN, k(S) is always smaller and larger, respectively, than "expected"). These deviations are attributed to variations in the solvents' anion solvating abilities (THF and MeCN are poor and good anion solvators, respectively). Values of log k(S) for proton transfer, but not for HAT, give better correlations with Taft et al.'s (J. Org. Chem. 1983, 48, 2877) beta scale of solvent HBA activities than with . The beta scale, therefore, does not solely reflect solvents' HBA activities but also contains contributions from anion solvation.     

XPS core level line shapes in metallic compounds: A probe for the nature of the electrons at the Fermi level

In several metallic compounds the X-ray excited photoemission spectra show different asymmetric lineshapes for the core signals of different constituting elements. With a simple model calculation we show that the asymmetry is directly related to the density of states on the constituent atoms at the Fermi level. We provide experimental evidence showing that the line shape asymmetry can be used to determine the character of electrons at the Fermi level.     

Five-component food-grade microemulsions: structural characterization by SANS

In this paper we present the structural characterization of a five-component food-grade microemulsion containing Tween 80, R(+)-limonene, ethanol, glycerol, and water. Our main approach to investigating the microstructure of dense microemulsions, and how it can be influenced by the various components, was to employ small-angle neutron scattering and the new evaluation technique for dense, interacting systems, the Generalized Indirect Fourier Transformation. We started our investigation with the impact of glycerol and ethanol on Tween 80 micelles in water. We found that glycerol increases the aggregation number and withdraws the hydrating agents from the headgroup region of the surfactant, resulting in a higher packing density of molecules in a micelle at slightly increasing size. The same trend holds when the micelles are oil swollen and/or ethanol is present. Ethanol, on the other hand, redistributes mainly between water and the interface-headgroup region of the surfactant. Part of it replaces surfactant molecules in the micelles, which increases the available interface and results in a higher number of micelles with shrinking size. The same trend holds when the micelles are oil swollen and/or glycerol is present in the aqueous phase. We also investigated samples along the dilution of a mixture of surfactant and oil phase (R(+)-limonene and ethanol), which can be diluted with aqueous phase (mixture of water and glycerol) without the occurrence of phase separation. In some samples of this dilution most probably bicontinuous structures are present. To elucidate this point, we also employed dynamic light scattering, viscosity, and conductivity measurements.     

Defect ordering in early transition-metal sulfides with the NaCl-type structure

The defect rock-salt type materials Zr_1?xS, Sc_1?xS, and Lu_1?xS have been shown to exhibit a complex variety of ordered superstructures. These superstructures are reviewed within the context of Landau's theory of symmetry and phase transitions, and it is concluded that there is a significant similarity in the transitions when they are considered from the point of view of reciprocal space.     

Kinetics of Alloxan-Induced Inhibition on δ-Aminolevulinate Dehydratase Activity in Mouse Liver Homogenates

This study evaluated the effects of alloxan on the kinetics properties of the δ-aminolevulinate dehydratase (δ-ALA-D) using mouse liver homogenates. δ-ALA-D is an important sulfhydryl enzyme that catalyses the second step in heme biosynthesis and is commonly diminished in experimental and human diabetes. Despite the known effects of alloxan in models of experimental diabetes, there are no data in the literature demonstrating the effects of alloxan on the kinetics properties of the δ-ALA-D. The results showed that alloxan (1.25–20 μM) caused a concentration-dependent inhibition of hepatic δ-ALA-D activity. The inhibition constant (K _i ) for alloxan-induced inhibition on δ-ALA-D was 3.64 μM. The alloxan (5 μM) caused a decrease in V _max (65.8%) and in K _m (53.1%), which is suggestive of an uncompetitive inhibition of enzyme. In addition, dithiothreitol (700 and 1,000 μM) completely prevented the δ-ALA-D activity inhibition induced by 10 and 20 μM alloxan. Similar protection was obtained in the presence of 2,000 μM glutathione. Therefore, this work showed that the inhibition of hepatic δ-ALA-D activity can be obtained in vitro at low micromolar levels of alloxan, and can also be prevented by reducing agents. Moreover, these results may help to understand the abnormalities in heme pathway found in models of experimental diabetes in vivo.     

The Physicochemical Behavior of Phytosterol Ethoxylates

In this work the physicochemical behavior of a series of phytosterol ethoxylates in water is presented. The influence of the length of the polyoxyethylene chain is studied. The surfactant solutions have been examined by means of birefringent microscopy, surface tension, self-diffusion 1H NMR, dynamic and static light scattering, and rheology. The surfactants with a hydrophilic chain of 10 oxyethylene units or more gave a micellar region. The CMC values were generally very low and a reverse relationship between the CMC value and the polyoxyethylene chain length was obtained. The time required to reach equilibrium surface tension was very long, more than 150 min. For the hydrophobic surfactants large lamellar regions appeared while for the more hydrophilic surfactants cubic and hexagonal structures were present which remained stable up to temperatures of 100 degrees C. In the micellar region prolate aggregates were formed which showed "ghostlike" behavior, consisting of cross-linked micelles with very fast relaxation times. Copyright 1999 Academic Press.