Why copper is intrinsically more selective than silver in alkene epoxidation: ethylene oxidation on Cu(111) versus Ag(111)

The heterogeneously catalyzed epoxidation of alkenes is experimentally challenging, theoretically interesting, and technologically important. Although large-scale ethylene epoxidation is universally carried out with Ag catalysts, recent laboratory studies on single crystal surfaces show that Cu is intrinsically much more selective than Ag in the epoxidation of a variety of terminal alkenes. The reasons for this striking difference between Ag and Cu have been investigated by means of density functional theory. It is found that the fundamental cause is the inversion in the ordering of activation barriers for the competing pathways to epoxide formation versus acetaldehyde formation (the latter being the first step on the route to combustion). On Cu, epoxide formation is less activated than aldehyde formation; the opposite is true on Ag. This behavior is associated with a late transition state to epoxidation on Cu (i.e., product-like) compared to an early (reactant-like) transition state to epoxidation on Ag.     

Hydrophobic Molecular Similarity from MST Fractional Contributions to the Octanol/water Partition Coefficient

Summary The use of a recently proposed hydrophobic similarity index for the alignment of molecules and the prediction of their differences in biological activity is described. The hydrophobic similarity index exploits atomic contributions to the octanol/water transfer free energy, which are evaluated by means of the fractional partitioning scheme developed within the framework of the Miertus-Scrocco-Tomasi continuum model. Those contributions are used to define global and local measures of hydrophobic similarity. The suitability of this computational strategy is examined for two series of compounds (ACAT inhibitors and 5-HT₃ receptor agonists), which are aligned to maximize the global hydrophobic similarity using a Monte Carlo-simulated protocol. Indeed, the concept of local hydrophobic similarity is used to explore structure–activity relationships in a series of COX-2 inhibitors. Inspection of the 3D distribution of hydrophobic/hydrophilic contributions in the aligned molecules is valuable to identify regions of very similar hydrophobicity, which can define pharmacophoric recognition patterns. Moreover, low similar regions permit to identify structural elements that modulate the differences in activity between molecules. Finally, the quantitative relationships found between the pharmacological activity and the hydrophobic similarity index points out that not only the global hydrophobicity, but its 3D distribution, is important to gain insight into the activity of molecules. J.M.M. and S.P. have contributed equally to this study.     

Probing the reactivity of oxomanganese-salen complexes: an electrospray tandem mass spectrometric study of highly reactive intermediates

Electrospray ionization in combination with tandem mass spectrometric techniques has been employed to study the formation of oxomanganese-salen complexes upon oxidation of [Mn(III)(salen)]+ cations as well as the properties and reactions of the oxidized species in the gas phase. Two species could be characterized as the principal oxidation products: the oxomanganese(v) complex, [Mn=O(salen)]+, which is the actual oxygen-transfer agent in epoxidation reactions, and the dinuclear, mu-oxo bridged [L(salen)Mn-O-Mn(salen)L]2+ with two terminal ligands L; the latter acts as a reservoir species. The effects of various substituents in the 5- and 5'-positions, respectively, of the salen ligand on the reactivity of the epoxidation catalyst were determined quantitatively from CID (collision-induced dissociation) experiments and B3LYP density functional calculations. Accordingly, the effect of axial donor ligands on the reactivity of the epoxidation catalyst was studied. Electron-withdrawing substitutents on the salen ligand and additional axial ligands decrease the stability and thus enhance the reactivity of the Mn=O moiety, while electron-donating salen substituents have a strong stabilizing effect.     

ESR. Spectra and Structures of Radical Anions in the Dibenzo[a,e]cyclooctene Series

ESR. studies are reported for the radical anions of 5,6-didehydro- and 5,6,11,12-tetradehydro-dibenzo[a,e]cyclooctene (III and IV, resp.), in addition to that of dibenzo[a,e]cyclooctene (II) itself, the spectrum of which has been reexamined. Comparison of the proton and ¹³C coupling constants for II·?, III·? and IV·? indicates that the three radical anions do not differ greatly in their electronic and molecular structures. This statement implies that II·? should also be substantially planar, i.e., the tub-shaped eight-membered ring in II is expected to flatten on passing from the neutral molecule to its radical anion. Support for postulating such a change in geometry, analogous to that encountered with the parent cyclooctatetraene (I), is provided by INDO calculations.     

Ranking ranges in cross-efficiency evaluations

The existence of alternate optima for the DEA weights may reduce the usefulness of the cross-efficiency evaluation, since the ranking provided depends on the choice of weights that the different DMUs make. In this paper, we develop a procedure to carry out the cross-efficiency evaluation without the need to make any specific choice of DEA weights. The proposed procedure takes into consideration all the possible choices of weights that all the DMUs can make, and yields for each unit a range for its possible rankings instead of a single ranking. This range is determined by the best and the worst rankings that would result in the best and the worst scenarios of each unit across all the DEA weights of all the DMUs. This approach might identify good/bad performers, as those that rank at the top/bottom irrespective of the weights that are chosen, or units that outperform others in all the scenarios. In addition, it may be used to analyze the stability of the ranking provided by the standard cross-efficiency evaluation.     

Synthesis and characterization of new tetraalkylaminophosphonium chlorides

New tetraalkylaminophosphonium chlorides were readily prepared by four-fold condensation of commercially available [P(CH₂OH)₄]Cl with a range of fifteen aryl based primary amines, in EtOH, at ambient temperature. All compounds have been characterized by FT–IR spectroscopy and elemental analysis. Solution ³¹P{¹H} NMR studies of these chloride salts reveal their instability with respect to various P^III/P^V species. The structures of three examples have been determined by single crystal X-ray diffraction and confirm the pseudotetrahedral arrangement around the P^V center. Extensive N–H···Cl hydrogen bonding is observed.     

Practical Study into Enhancing IgG Immobilisation on Screen-Printed Electrodes for Environmental Applications

This work is focused on the influence of pH and applied potential for the immobilisation of rabbit IgG on a carbon screen-printed electrode. The orientation of IgG molecule at the surface is fundamental for activity and reproducibility of the immunosensor. As the electrode potential is increased a particular order may be brought to the immobilised antibodies, i.e. the antibodies adapt a more favourable arrangement on the surface to facilitate better binding. The response increased when changing the pH from basic to acidic medium and the reverse trend was observed for the limit of detection (LOD). When a potential was applied to the electrode, the response generally decreased and the LOD increased in the order acidic > basic > neutral pH. The LODs obtained from antibodies immobilised at acidic pH and + 100 mV were better than the LODs obtained at other conditions.     

Insensitivity of leptogenesis with flavor effects to low energy leptonic CP violation

If the baryon asymmetry of the Universe is produced by leptogenesis, CP violation is required in the lepton sector. In the seesaw extension of the standard model with three hierarchical right-handed neutrinos, we show that the baryon asymmetry is insensitive to the Pontecorvo-Maki-Nagakawa-Sakata phases: thermal leptogenesis can work for any value of the observable phases. This result was well known when there were no flavor effects in leptogenesis; we show that it remains true when flavor effects are included.     

Liquid chromatography with ultraviolet absorbance detection for the analysis of tetracycline residues in honey

The separation of tetracyclines (TCs) using reversed-phase liquid chromatography (LC) is proposed. The use of an amide-based stationary phase prevents the interaction of tetracyclines with the residual silanol groups and thus avoids the appearance of tailed peaks. Detection was based on using an UV spectrophotometer and gradient elution with acetonitrile-oxalic acid as mobile phase permitted good separation of all the peaks. Specificity was demonstrated by the retention characteristics, UV spectra and peak purity index. Linearity, precision, recovery and sensitivity were satisfactory. The procedure was applied to the analysis of tetracycline residues (tetracycline, oxytetracycline (OTC), chlortetracycline (CTC), doxycycline (DC), minocycline (MINO) and methacycline (MTC)) in honey of different types. Extraction involved using a mild acidic solvent containing EDTA to release protein-bound or sugar-bound tetracyclines. For the clean-up step, solid phase extraction using phenyl cartridges was applied. Detection limits in the honey using the proposed procedure are between 15 and 30 ng g(-1), depending on the tetracycline.