Fourier transform Raman and DFT study of three annulated oligothiophenes with different molecular shapes.

Herein, we study the conjugation properties of three different thienoacenes, each of which has three or four fused thiophene rings, by means of Fourier transform Raman spectroscopy. The B3LYP/6-31G** vibrational analysis of all of the collected spectroscopic data evidences that the selective enhancement of a limited number of Raman scatterings is related to the occurrence in the three thienoacenes of a vibronic coupling between the lowest unoccupied frontier molecular orbital (LUMO) and some Raman-active skeletal nu(C==C) stretching modes of 1600-1300 cm(-1).     

Desirability functions as response in a d‐optimal design for evaluating the extraction and purification steps of six tranquillizers and an anti‐adrenergic by liquid chromatography‐tandem mass spectrometry

Internal standards can be added at different stages of an analytical procedure. When they are added at the beginning of a multiresidue method and their behavior is not exactly the same as that of the analytes, the intended correction for small variations within the analytical process could not be achieved. Because of this, in the present work, the use of d ‐optimal designs together with desirability functions is proposed to state the experimental response under study. The overall desirability function used relates two analytical criteria: to assess a similar chemical behavior of each analyte in relation to its internal standard and to avoid a significant reduction of the absolute peak area of the internal standards. This strategy has been applied to the analysis of the effect of four factors related to the extraction and purification steps of six tranquillizers and a β‐blocker from pig muscle analyzed by liquid chromatography–tandem mass spectrometry. The effect of those factors has been evaluated by means of an ad hoc d ‐optimal design consisting of only 11 experiments. The resulting levels of the four factors that enable to achieve the greatest overall desirability have also been compared with those obtained when either the standardized or absolute peak area has been considered as response. Differences in both the significant factors and their optimum levels have been observed. It is noticeable that the experimental effort necessary to study the effect of the factors has been reduced by more than 50% thanks to the d ‐optimal design. Copyright © 2015 John Wiley & Sons, Ltd.     

Diastereoselective hydrogenation of α,β-unsaturated but-2-enamides to access the chiral 3-(p-tolyl) butanoic acids

An alternative methodology for the synthesis of chiral 3-(p-tolyl) butanoic acids is presented. This was accomplished through the diastereoselective hydrogenation reaction of different chiral N-3-(p-tolyl) but-2-enamides, using Pd/C in EtOH, to produce the corresponding chiral N-3-(p-tolyl) butanamides with high chemical yields and moderate diastereomeric ratios. Removal of the chiral auxiliary from N-3-(p-tolyl) butanamides gave the respective enantiomerically pure acids.     

Additive Differential Pulse Voltammetry for the Study of Slow Charge Transfer Processes at Spherical Electrodes

The application of additive differential pulse voltammetry to the study of the kinetics of a charge transfer process is studied. A simple analytical solution is presented, valid for spherical electrodes of any size and for electrode processes of any reversibility. From this solution, valuable diagnostic criteria for the elucidation of the electrochemical reversibility are established based on the variation of the ADPV signal with the duration of the potential pulses, the electrode radius and the pulse height. Working curves for the determination of the kinetic parameters are also given. The value of the ADPV technique is experimentally demonstrated by studying the kinetics of the reduction of 3‐nitrophenolate^? and europium³⁺ at mercury hemispherical microelectrodes.     

Optimization of a solid-phase extraction procedure in the fluorimetric determination of sulfonamides in milk using the second-order advantage of PARAFAC and D-optimal design

The objective of this work is to optimize a solid-phase extraction procedure for the simultaneous determination of sulfadiazine, sulfamerazine, and sulfamethazine in milk by fluorimetric detection. For this task, an alternative strategy is employed, which allows one to reduce noticeably the number of experiments without losing the quality of the estimations. It consists of the use of a D-optimal design together with PARAFAC decomposition for the calculation of the response in the experimental design. Effects of amount of cartridge sorbent, kind of milk, volume of conditioning solutions, kind of wash and elution, and kind of mixture of sulfonamides have been evaluated, for maximizing sulfonamide mean recovery and minimizing its standard deviation. Since milk without sulfonamides may give some matrix effect over the fluorescence signal, its behavior has also been studied. Optimal conditions have been selected where the ratio between sulfonamide recovery and milk without sulfonamides was the highest, which are 500 mg of cartridge sorbent, acid wash, and elution and 3 mL of conditioning solutions. The type of milk and mixture of sulfonamides not significant. This makes the procedure suitable for the combined determination of sulfadiazine, sulfamerazine, and sulfamethazine in any kind of milk. Finally, an experimental procedure is proposed, obtaining a sulfonamide mean recovery equal to 68.5% with values of standard deviation between 7 and 8 μg kg⁻¹.     

Hydrodynamic Fluctuations in Laminar Fluid Flow. I. Fluctuating Orr-Sommerfeld Equation

In recent years it has become evident that fluctuating hydrodynamics predicts that fluctuations in nonequilibrium states are always spatially long ranged. In this paper we consider the application of fluctuating hydrodynamics to laminar fluid flow, using plane Couette flow as a representative example. Specifically, fluctuating hydrodynamics yields a stochastic Orr-Sommerfeld equation for the wall-normal velocity fluctuations, where spontaneous thermal noise acts as a random source.This stochastic equation needs to be solved subject to appropriate boundary conditions. We show how an exact solution can be obtained from an expansion in terms of the eigenfunctions of the Orr-Sommerfeld hydrodynamic operator. We demonstrate the presence of a flow-induced enhancement of the wall-normal velocity fluctuations and a resulting flow-induced energy amplification and provide a quantitative analysis how these quantities depend on wave number and Reynolds number.     

Synthesis and conformational analysis of azacyclophanes from l-tyrosine

The synthesis of a new azacyclophane formed by two l-tyrosine units joined by two methylene bridges is presented. The structural and conformational characteristics are briefly discussed. Spectroscopic and theoretical data reveal a syn structure with two intramolecular hydrogen bonds.     

Thermodynamics of the Dimer Formation of 2I,3I-O-(o-Xylylene)-per-O-Me-γ-cyclodextrin: Fluorescence, Molecular Mechanics and Molecular Dynamics

The thermodynamics of the dimer formation of 2^I,3^I-O-(o-xylylene)-per-O-Me-γ-cyclodextrin (XmγCD) in aqueous solution was studied by fluorescence techniques, Molecular Mechanics and Molecular Dynamics. Lifetime averages á t ñ \left\langle \tau \right\rangle , obtained from fluorescence decay profiles upon excitation of the xylylene appended group, were used as the property sensitive to the association process. The dimerization equilibrium constants (K _D) were obtained from non-linear regression analysis of the plots of á t ñ \left\langle \tau \right\rangle against [XmγCD] at several temperatures and they were compared with the values obtained for the counterparts Xmα- and XmβCDs. The van’t Hoff plot allows us to obtain the ΔH and ΔS showing that the dimerization process was also entropically disfavoured. Molecular Mechanics as well as Molecular Dynamics calculations in the presence of water were also employed to study the conformational behaviour of isolated XmγCDs, the possible structure of the dimers formed and the driving forces involved in such association processes. Results indicate that those conformations where Xy moiety does not block the cavity entrance are favoured. Dimers are preferably formed by head-to-head CD approaching. However, the formation of stable head-to-tail is not dismissed.     

Elastic scattering of neutrons on natPb at forward angles

Angular distributions were measured for neutron elastically scattered by ^natPb at 2.9, 3.0, 3.1, 3.2 and 3.4MeV at very forward angles. The mono-energetic neutron flux was produced at the tandem accelerator facility of the National Institute of Nuclear Research (ININ), Mexico, using the Associated Particle Technique (APT) in the D(d, n)³He reaction. Optical model predictions with best potential systematic show a fairly good agreement with our data, and also the need for a better understanding of the neutron scattering process at very small angles from the theoretical point of view.