Simple DPPH.‐Based Electrochemical Assay for the Evaluation of the Antioxidant Capacity: a Thorough Comparison with Spectrophotometric Assays and Evaluation with Real‐World Samples

An assay based on the electrochemical detection of 2,2‐diphenyl‐1‐picrylhydrazyl radical (DPPH^.) for the evaluation of the total antioxidant capacity (TAC) was optimized. The assay is interchangeable with the classic spectrophotometric tests for TAC based on the same radical. In addition, it can be used for the analysis of dilute samples with low antioxidant capacities. A good linear correlation (R²=0.97) was obtained between the results obtained with the proposed electrochemical assay and the Trolox Equivalent Antioxidant Capacity test based on ABTS radical. The assay was successfully used to evaluate the antioxidant capacity of two red wines obtained by six different maceration‐fermentation techniques.     

Effect of reducing atmosphere on the structure of ceria-supported nano-pt catalysts

Bulk platinum has been known to be chemically inert, but shows remarkably high activity as a catalyst when finely dispersed as nano particles (10 nm) in ceramic substrates. In order to understand this mechanism, we have studied the effect of a reducing environment on the structure, morphology, and distribution of these Pt nano particles as well as of oxide support in gadolinium-doped ceria catalysts by transmission electron microscopy (TEM) and x-ray photoelectron spectroscopy (XPS) methods. In fresh catalysts prepared by calcinations, the Pt nano particles were found to be crystalline with many of them twinned and distributed randomly in the microstructure of crystalline ceria support. The size of the Pt nano particles ranged between 20 and 50 nm. A special feature found in these catalysts is that no constituting Pt particle is partially or fully embedded in the ceria grain. Upon reduction with hydrogen gas at high temperatures, both Pt and ceria crystals showed evidence of crystalline defects. The Pt nano particles in reduced catalysts appeared similar in size and shape to those observed in the fresh catalysts, and they resided on the surface of the ceria crystal. The results are discussed in relation to their expected catalytic activity in autothermal reforming of iso-octane.     

TMSCH2Li-LiDMAE: a new nonnucleophilic reagent for C-2 lithiation of halopyridines

A new superbasic reagent has been discovered by combining TMSCH₂Li and LiDMAE in hexane. This reagent was found highly efficient for the C-2 lithiation of sensitive chloro- and fluoropyridines. The metallation occurred chemo- and regioselectively at 0 °C avoiding the nucleophilic addition or substrate degradation commonly obtained with other alkyllithiums even at lower temperatures.     

Generalized analytical expressions for the burning velocity in a combustion model with non-constant transport coefficients and several specific heats

We derive new expressions to estimate the burning velocity of a laminar gas flame in a simplified combustion model based on a one-step single reaction with transport coefficients (mass and heat) depending on temperature, and species with different specific heats. These new expressions generalize the bounds and approximations previously derived by Williams, von Karman, Zeldovich and Frank-Kamenetskii, Benguria and Depassier, and the matching asymptotic expansion method in a two zone model. The comparison of the flame speed predicted by these new analytical expressions with that numerically simulated by the full combustion model for a large variety of cases allows us to determine their range of validity. The upper bound based on the Benguria and Depassier method provides very good approximations for the actual propagation speed of combustion flames, being substantially better than the asymptotic method used in the recent papers.     

A Ready Synthesis of Some N-Substituted 3-Alkoxycarbonyl-2-Pyrrolinones

Abstract

The synthetic methods for the preparation of pyrrolopyrimidines have been recently reviewed and since that report an increasing number of papers have dealt with the chemistry of these heterocyclic systems for their structural resemblance to purines, natural occurrence and biological significance. Good examples of this are k4 the pyrrolo [2,3-d] pyrimidine nucleosides Q and Q. Among others, 3-alkoxycarbonyl-2-pyrrolinones 2 can be useful intermediates for the preparation of pyrrolo [2,3-d] pyrimidines? However, it appears from the literature that for the preparation of compounds which belong to class g few methods have been described, all preparing the 2-pyrrolinone system by cyclization of a suit able intermediate? The direct functionalization of the carbon 3 of 2-pyrrolinones has not been exploited yet, althogh few examples of related systems have 6 been realized. On the other hand, it has bFen shown that the aldol condensation of 2-pyrrolinone with aromatic aldehydes proceeds at position 3 when a strong electron 7 withdrawing group is attached to the lactamic nitrogen. In this case, the acidity of the hydrogens a to the carbony1 is enhanced, although yields are lmered by competitive condensation at the N-protecting group, if this 8 is an acetyl moiety.     

Type-II matrices and combinatorial structures

Type-II matrices are a class of matrices used by Jones in his work on spin models. In this paper we show that type-II matrices arise naturally in connection with some interesting combinatorial and geometric structures.     

Nucleation of CaCO3 polymorphs from a colloidal alcoholic solution of Ca(OH)2 nanocrystals exposed to low humidity conditions

A study of the stability of calcium carbonate polymorphs formed as a result of the carbonation process from an alcoholic colloidal solution of nanocrystals of Ca(OH)₂ in low relative humidity (RH) conditions (33% and 54% RH) is presented in this research. The crystalline behavior, the time dependence of nucleation and the phases’ transformations as a result of exposure to low humidity conditions are evaluated. The carbonation process is slow, starting with the nucleation of amorphous calcium carbonate, associated to an amorphization process that affects both the portlandite (Ca(OH)₂) and the initial unstable CaCO₃ polymorphs. The excess of alcohol in the solution decreases the surface tension and the nucleation is accelerated by the fast evaporation of the solvent, which avoids the particles to diffuse to their lowest energy sites, giving smaller particles with lower crystallinity as RH decreases.     

Microwave assisted synthesis of 10b-aza-10c-borapyrene

The synthesis of 10b-aza-10c-borapyrene, an analog of the polycyclic aromatic hydrocarbon pyrene with an internal boron–nitrogen bond, has been improved. We have dramatically reduced the reaction times and ease of workup for the synthesis of this molecule by using a microwave reactor and carefully controlling reaction conditions.