Solvent effect on the tautomers' stabilities of protonated N,N‐dimethylnitrosamine: The role of hydrogen bonds network

DFT calculations have been applied in order to study the free energies of the structures corresponding to the three different protonation sites of N,N‐dimethylnitrosamine (DMNA). The solvent effect has been taken into account through the study of clusters consisting of protonated DMNA and up to four explicit water molecules, either in the absence or in the presence of a continuum (CPCM) solvation model. Addition of water molecules has been done by a careful screening procedure through which all important hydrogen bonds are likely to be considered. Protonation of DMNA makes all their lone pairs no longer available for hydrogen bond formation with water molecules, such that hydrogen bonds have been observed, for almost all structures, only between water molecules and between one water molecule and the protonated DMNA, in this latter case intermediated by the proton. The stabilities of the solvated structures are governed not only by the number of hydrogen bonds but also by the positions of the water molecules involved in these bonds, as well as by which of them donate or accept H atoms. Our results indicate that oxygen protonation is the most favorable one, regardless of the presence of water molecules. In vacuum protonation at the N‐amino ( 2a ) is approximately as favorable as protonation at the N nitroso ( 2c ). However, in water the former protonation is by far the less favorable one. Our best estimates for the ΔG values in bulk solvent are: ΔG( 2a ) ≈ 17.9, ΔG( 1c ) ≈ 4.3, and ΔG( 2c ) ≈ 4.9 kcal/mol.     

Site-specific cross-linking of nucleic acids using the abasic site

[formula: see text] An efficient site-specific cross-linking reaction between two carbohydrate residues present in two complementary DNA sequences is described. One oligodeoxynucleotide, 5'd(GGCTGA*CTGCG)3', carries an amino nucleophile tethered to the 2'-hydroxyl of an adenosine residue (A*). The target electrophile is an abasic site generated in the complementary sequence, 5'd(CGCAGDCAGCC)3' (D represents the deoxyribose). The cross-linking reaction was carried out by a reductive amination reaction in > 95% yield.     

Die Natur des Wassers in synthetischen Hydrogelen. III. Dilatometrie, spezifische Leitf?higkeit und dielektrische Relaxation von Poly (2,3-dihydroxypropylmethacrylat)

Ohne Zusammenfassung     

Quasi-equilibrium volume changes of polyaniline films upon redox switching. Formal potential distribution and configurational modeling

The quasi-equilibrium electrochemomechanical behavior of relatively thick polyaniline films in sulfuric acid is investigated through experimental measurements and theoretical modeling. The leucoemeraldine (LE)-emeraldine (EM) conversion, or redox switching, is studied. The dependence of film volume and electrochemical charge is determined as a function of applied potential. It is observed that the film volume follows the charge, showing an expansion during the second half of the LE-EM oxidation. The model postulates the existence of a stable intermediate, protoemeraldine (PE), with a formal potential distribution for the PE-EM reaction. The volume change is modeled statistically considering contributions from mixing, polymer deformation, and electrostatic charge. The model shows very good agreement with the experiments, indicating that, in the conditions studied, the deformation contribution dominates the volume changes as a result of the conformational modifications undergone by the polymer in the PE-EM oxidation.     

Real space trajectory of the ultrafast magnetization dynamics in ferromagnetic metals

We have measured the real space trajectory of the ultrafast magnetization dynamics in ferromagnetic metals induced by femtosecond optical pulses. Our approach allows the observation of the initial change of the modulus and orientation of the magnetization, occurring within a few hundreds of femtoseconds, as well as its subsequent precession and damping around the effective field. The role of the magnetocrystalline anisotropy shows up in the magnetization reorientation occurring during the electron-lattice relaxation. In addition, we propose a model which takes into account the initial demagnetization in the Bloch formalism describing the magnetization dynamics.     

Gravitational energy-momentum density in teleparallel gravity

In the context of a gauge theory for the translation group, a conserved energy-momentum gauge current for the gravitational field is obtained. It is a true spacetime and gauge tensor, and transforms covariantly under global Lorentz transformations. By rewriting the gauge gravitational field equation in a purely spacetime form, it becomes the teleparallel equivalent of Einstein's equation, and the gauge current reduces to the Moller's canonical energy-momentum density of the gravitational field.     

Selecting the optimum number of partial least squares components for the calibration of attenuated total reflectance-mid-infrared spectra of undesigned kerosene samples

Selecting the correct dimensionality is critical for obtaining partial least squares (PLS) regression models with good predictive ability. Although calibration and validation sets are best established using experimental designs, industrial laboratories cannot afford such an approach. Typically, samples are collected in an (formally) undesigned way, spread over time and their measurements are included in routine measurement processes. This makes it hard to evaluate PLS model dimensionality. In this paper, classical criteria (leave-one-out cross-validation and adjusted Wold's criterion) are compared to recently proposed alternatives (smoothed PLS-PoLiSh and a randomization test) to seek out the optimum dimensionality of PLS models. Kerosene (jet fuel) samples were measured by attenuated total reflectance-mid-IR spectrometry and their spectra where used to predict eight important properties determined using reference methods that are time-consuming and prone to analytical errors. The alternative methods were shown to give reliable dimensionality predictions when compared to external validation. By contrast, the simpler methods seemed to be largely affected by the largest changes in the modeling capabilities of the first components.     

Evaluation of Spectroscopic and Morphological Properties of PMMA Modified with ILs

Summary: In recent years, an important class of new compounds, ionic liquids (ILs), has been investigated as additives for polymers. In this work, we carried out the synthesis of 4 different ILs: 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM.BF₄), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM.PF₆), and the chiral ILs acetyl-(−)menthol-1-buthylimidazole tetrafluoroborate ([amebim]BF₄) and acetyl-(−)menthol-1-buthylimidazole hexafluorophosphate ([amebim]PF₆). These ILs were added to poly(methyl methacrylate) (PMMA) in 10, 20 and 30% ratios by weight and the ILs, PMMA and the obtained materials (PMMA:ILs) were characterized by infrared, visible spectroscopy and atomic force microscope. The ILs added to PMMA did not provide significant changes in the infrared spectra, compared to pure PMMA. However, the absorption spectra in the visible region of the PMMA:ILs materials showed differences, compared to the spectrum of pure PMMA.     

On linewidth of phonons associated to a disorder mechanism

An expression for the linewidth of phonons associated to a disorder mechanism in crystals is deduced. The linewidth of these ‘noise’ phonons is a function of the correlation time describing the statistical behavior of the disorder mechanism. A new view point to the application of the fluctuation-dissipation theorem for order-disorder crystals is presented. The relationship between the behavior of the linewidth of these phonons and phase-transitions mechanisms is discussed.