Detection of radical intermediates in the photo-oxidation of alkanols by ceric ammonium nitrate-CAN. an EPR study

The photolysis of acetonitrile solutions of alkanols in the presence of ceric ammonium nitrate-CAN, at different temperatures, leads to the formation of alkyl, peroxyl and nitroxyl radicals identified by EPR spectroscopy. The involvement of free alkoxy and alkylperoxy radicals as intermediates in the reaction path will be discussed.     

Chitosan membrane: tool for chromium (III) recovery from aqueous solutions

In this study four kinds of Chitosan membranes were prepared. The permeability of the membranes was then investigated by permeation of K+ in these membranes. The performances of the prepared membranes for recovery of chromium (III) from aqueous solution were evaluated. The effects of operation conditions on permeation characteristics were determined, and permeation mechanism was discussed. The stability of the membranes was also studied.     

Selective n,n-dibenzylation of primary aliphatic amines with dibenzyl carbonate in the presence of phosphonium salts

In the presence of catalytic amounts of tetraalkylphosphonium salts and under solventless conditions, primary aliphatic amines (RNH(2): R = PhCH(2), Ph(CH(2))(2), n-decyl, and 1-naphthylmethyl) are efficiently N-benzylated to the corresponding RN(CH(2)Ph)(2), using dibenzyl carbonate as the benzylating reagent. Compared to the reaction run without salt, where the competitive formation of the benzyl carbamate is favored, the phosphonium salt promotes high selectivity toward the benzylated amine and an increase of the reaction rate as well. However, in a single case explored for an amino acidic compound, namely 4-(aminomethyl)benzoic acid [4-(NH(2)CH(2))C(6)H(4)CO(2)H], both N,N-dibenzylation and esterification of the acid group were observed. Analysis of the IR vibrational modes of benzylamine in the presence of tetrabutylphosphonium bromide supports the hypothesis that this enhanced selectivity may be due to an acid-base interaction between the salt and the amine, which increases the steric bulk of the amine and favors attack of the nucleophile on the less hindered alkyl terminus of dibenzyl carbonate.     

Evidence of an electron-transfer mechanism in the peroxynitrite-mediated oxidation of 4-alkylphenols and tyrosine

The mechanism of interaction of the peroxynitrite with some 4-alkylphenols and tyrosine was mainly studied by means of ESR spectroscopy and product analysis. The radical intermediates, detected as spin adducts to the 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO), were identified as carbon-centered radicals to the benzene ring. The reaction seems to proceed via an electron transfer process (ET), most likely mediated by a NOx derivative, leading to the intermediacy of a phenoxyl-type radical as proved by the detection of the corresponding Pummerer-type ketone. No evidence of the formation of hydroxyl radicals, due to the homolytic cleavage of the peroxynitrite at physiological pH was obtained, even though DEPMPO hydroxyl spin adducts were detected: the latter most likely arises from the direct attack of the spin trap by the oxidant species. The possible involvement of HCO(3)(-)/CO(2), i.e., the formation of the nitrosoperoxycarbonate, ONOOCO(2)(*)(-), was also investigated.     

Near-Curie magnetic anomaly at the Ni/C interface observed by electron holography

We analyze with electron holography carried out in a transmission electron microscope the near-Curie behavior of magnetism at the edge of a Nickel thin film coated with Carbon. In-situ experiments with finely controlled variations of the sample temperature reveal an anomaly in the ferromagnetic to paramagnetic phase transition when the film temperature is a few degrees above the nominal Curie point. We interpret the anomaly as a strain-induced spin reorientation transition triggered by the differential thermal expansion of Carbon and Nickel. We present a model that quantitatively reproduces the main features of the observed signal. The model is developed in terms of an anisotropic, temperature dependent exchange coupling between the Nickel moments at the Carbon interface that favors their vertical alignment at low temperatures.     

Accelerated Projected Gradient Method for Linear Inverse Problems with Sparsity Constraints

Regularization of ill-posed linear inverse problems via ? ₁ penalization has been proposed for cases where the solution is known to be (almost) sparse. One way to obtain the minimizer of such an ? ₁ penalized functional is via an iterative soft-thresholding algorithm. We propose an alternative implementation to ? ₁-constraints, using a gradient method, with projection on ? ₁-balls. The corresponding algorithm uses again iterative soft-thresholding, now with a variable thresholding parameter. We also propose accelerated versions of this iterative method, using ingredients of the (linear) steepest descent method. We prove convergence in norm for one of these projected gradient methods, without and with acceleration.     

Mass spectrometry in the characterization of ambers. II. Free succinic acid in fossil resins of different origin

Baltic amber contains high levels of succinic acid, most of which is part of the polymer framework, and only recently has the presence of microcrystals of free succinic acid been documented by scanning electron microscopy and X‐ray diffraction analysis. The determination of succinic acid can be used for an easy identification of Baltic amber among other fossil resins and to distinguish it from imitations, widely diffused on the market, produced with natural or modified recent or sub‐fossil resins, such as copals. We report here a simple method, based on negative ion electrospray ionization mass spectrometry, for evaluating the free succinic acid content in water/methanol extracts of ambers from different origin, using a sample of about 20 mg. The limit of quantification is better than 1 ppm and, when applied to a set of amber samples of different origin, it was able to distinguish those of Baltic origin (showing free succinic acid levels in the range 50–400 ppm) from the others (for which succinic acid was undetectable, i.e. at levels lower than 1 ppm). Copyright © 2009 John Wiley & Sons, Ltd.     

Heterocyclic azallyl badicals

Methyl-thlyl radical selectively abstracts allylic hydrogens from oxazolyn and thiazolin derivatives, yielding nitrogen centered azallyl radicals.     

An iterative algorithm for sparse and constrained recovery with applications to divergence-free current reconstructions in magneto-encephalography

We propose an iterative algorithm for the minimization of a ? ₁-norm penalized least squares functional, under additional linear constraints. The algorithm is fully explicit: it uses only matrix multiplications with the three matrices present in the problem (in the linear constraint, in the data misfit part and in the penalty term of the functional). None of the three matrices must be invertible. Convergence is proven in a finite-dimensional setting. We apply the algorithm to a synthetic problem in magneto-encephalography where it is used for the reconstruction of divergence-free current densities subject to a sparsity promoting penalty on the wavelet coefficients of the current densities. We discuss the effects of imposing zero divergence and of imposing joint sparsity (of the vector components of the current density) on the current density reconstruction.     

Synthesizing Interpenetrated Triazine-based Covalent Organic Frameworks from CO2

Crystalline triazine-based covalent organic frameworks (COFs) are aromatic nitrogen-rich porous materials. COFs typically show high thermal/chemical stability, and are promising for energy applications, but often require harsh synthesis conditions and suffer from low crystallinity. In this work, we propose an environmentally friendly route for the synthesis of crystalline COFs from CO₂ molecules as a precursor. The mass ratio of CO₂ conversion into COFs formula unit reaches 46.3 %. The synthesis consists of two steps; preparation of 1,4-piperazinedicarboxaldehyde from CO₂ and piperazine, and condensation of the dicarboxaldehyde and melamine to construct the framework. The CO₂-derived COF has a 3-fold interpenetrated structure of 2D layers determined by powder X-ray diffraction, high-resolution transmission electron microscopy, and select-area electron diffraction. The structure shows a high Brunauer–Emmett–Teller surface area of 945 m² g⁻¹ and high stability against strong acid (6 M HCl), base (6 M NaOH), and boiling water over 24 hours. Post-modification of the framework with oxone has been demonstrated to modulate hydrophilicity, and it exhibits proton conductivity of 2.5×10⁻² S cm⁻¹ at 85 °C, 95 % of relative humidity.