Molar and Partial Molar Excess Volumes of Benzene in Methyl Ester Solutions at 25°C

Molar and partial molar excess volumes of mixtures of benzene with several methyl esters (from methanoate to decanoate) were determined, over the whole concentration range, at 25°C and atmospheric pressure from experimental densities and correlated by a suitable equation. The applicability of the Flory and Priggogine–Flory–Pattersort models for predicting molar excess volumes is tested. The calculated values with Flory and Priggogine–Flory–Patterson are similar and agree poorly with the experimental data.     

Direct Electrochemical Sensing and Detection of Natural Antioxidants and Antioxidant Capacity in Vitro Systems

This review highlights the role of electrochemical approaches in the sensing of antioxidants and their antioxidant capacity with especial attention to the analytical possibilities of electrochemistry in the direct evaluation of antioxidant capacity exhibited by food and biological samples due to the termed dietary, natural or biological antioxidants (mainly polyphenols, and vitamins C and E). The analytical potency of the electrochemistry is comprehensively stated and the selected results found in the literature are summarized and discussed critically. The main electrochemical approaches used have been cyclic voltammetry (CV) and flow injection analysis with amperometric detection (FIA‐ED). In addition, miniaturization is going to break new frontiers in the evaluation of antioxidant activity.     

Electron impact and field desorption mass spectrometry of some macrocyclic-acyclic polyether-ester compounds

The electron impact mass spectrometric behaviour of a series of macrocyclic-acyclic polyether-ester compounds containing an isoxazole, benzene or pyridine ring is discussed in detail with the aid of exact mass measurements and metastable ions (linked scans B/E and B²/E). The nature of unusual [M+H]⁺ species, present in the crown moieties only, is elucidated by means of low electron energy measurements and labelling experiments. The field desorption spectra show only intense peaks corresponding to [M+H]⁺ species and rule out the presence of dimeric moieties.     

Binding of fluoride and carbonate by open chain polyammonium cations

The formation of open chain polyammonium cation-fluoride and -carbonate complexes was studied by potentiometric and calorimetric techniques at t=25 °C. Several species of H_iAL (A=amine; L=F⁻, CO₃²⁻) are formed in both systems with a mean stability (ζ=|z_anion×z_cation|) and for fluoride and carbonate, respectively. The comparison with analogous systems (chloride and acetate for fluoride and hydrogenphosphate, sulfate and malonate for carbonate) showed that fluoride and carbonate form the most stable species with open chain polyammonium cations, among low molecular weight anions. The N-alkyl substitution does not play negligible role in the stability of these complexes, the species formed by substituted amines being more stable.     

The energetics and structural properties of diazomethyl (HCNN) and cyanomidyl (HNCN) radicals and their related cations and anions from ab initio calculations

The molecular structures and energetics of diazomethyl (HCNN) and cyanomidyl (HNCN) radicals and their related cations (HCNN(+),HNCN(+)) and anions (HCNN(-),HNCN(-)) are reported at a high level of accuracy. The singles and doubles coupled-cluster method including a perturbational correction for connected triple excitations with systematic sequences of correlation consistent basis sets have been employed. Extrapolation to the complete basis set limit has been used with accurate treatments of core-valence correlation effects in order to accurately predict molecular properties, ionization potentials, electron affinities as well as C-H and N-H bond dissociation energies. For all the species studied, harmonic vibrational frequencies have also been evaluated in order to obtain zero-point corrections to ionization potentials, electron affinities, and dissociation energies.     

Addressing the scope of the azide-nitrile cycloaddition in glycoconjugate chemistry. The assembly of C-glycoclusters on a calix[4]arene scaffold through tetrazole spacers

New glycoclusters constituted of ribosylmethyl, galactosylmethyl, and glucosylmethyl fragments assembled on a calix[4]arene platform by means of propoxytetrazole spacers have been prepared by coupling the corresponding sugar azides with p-toluenesulfonyl cyanide, and then reacting 1-glycosylmethyl-5-sulfonyl-tetrazole derivatives thus formed with a calix[4]arene tetrol.     

Synthesis of fused or not β-lactam-biaryl hybrids by free radical aryl-aryl coupling of 2-azetidinone-tethered haloarenes

cis and trans-Aryl-2-azetidinone-tethered haloarenes can be stereoselectively prepared using the ketene-imine cyclization. These β-lactam-tethered haloarenes were used for the regiocontrolled preparation of β-lactam-biaryl hybrids including fused tetracyclic biaryl-2-azetidinones as well as C4-dearylated not fused biphenyl‐2-azetidinones via aryl-aryl radical cyclization and/or rearrangement. Alternatively, trans-dibenzocarbacephems could be stereoselectively prepared, both in racemic and enantiopure form, through the Staudinger reaction between phenanthridine and activated ketenes.     

Application of horizontal staircase electrophoresis in agarose minigels to the random intergenic spacer analysis of clinical samples

The random intergenic spacer analysis is a recently developed technique for the study of microbial populations. The bacterial intergenic spacer (ITS) is located between 16S rRNA and 23S rRNA genes and presents different length and sequence among bacterial species. Therefore, the amplicons can be separated by electrophoresis commonly performed at low voltage during several hours. Although this technique is especially useful for unculturable microorganisms, it has not been applied before to clinical sample analysis. As these samples have a limited number of bacterial species, the size of the gels may be reduced to facilitate their handling and to reduce the running time. To obtain maximum separation among the ITS bands, we analysed in this work different electrophoretical conditions including staircase electrophoresis, a technique based on the application of several voltage steps. The results obtained showed a different behaviour of the electrical resistance during the performance of submarine horizontal and vertical staircase electrophoresis. In the first case the resistance decreased during most of the running time whereas in the second case it increased. Here, we show that the performance of horizontal staircase electrophoresis reduces the running time more than 80% with respect to conventional electrophoresis at low voltages. This procedure was applied to the separation of ITS bands from bacterial DNA present in a tissue from a vocal cord biopsy. The sequencing of these bands allowed their identification. This new procedure may be very useful in the rapid diagnosis of bacteria present in human, animal and plant tissues.     

Heck reaction catalyzed by Pd/C, in a triphasic-organic/Aliquat 336/aqueous-solvent system

The rate of the Pd/C catalyzed Heck coupling of Ar-I with CH(2)=CH-R is accelerated tenfold by the presence of Aliquat 336 (A336), a well known phase transfer catalyst, and an ionic liquid. Both when conducted in A336 as solvent, and in an isooctane/A336/aqueous triphasic mixture, the Heck reaction of aryl iodides with electron deficient olefins, catalyzed by Pd/C, proceeds with high yields and selectivity. When KOH is used instead of Et(3)N, selective formation of the biphenyl rather than the Heck product, is observed. Aryl bromides react more sluggishly, and only the more activated ones undergo the Heck reaction. In the absence of the olefin, aryl halides possessing an electron withdrawing group are reduced to the corresponding Ar-H.     

Degradation and stabilization of high molecular weight polyethylene in solution

The changes in the molar mass of high molecular weight polyethylene, in o-dichlorobenzene, with time are examined using viscometry. The results show that the molar mass decreases with time and that the observed decay can be satisfactorily explained by a simple degradation model. Commercial antioxidants can inhibit the degradation process if the appropriate concentration is chosen. Thus, at very low concentrations a polynuclear phenol shows good antioxidant behavior but this decreases with an increase in concentration due to direct oxidation of the inhibitor. Conversely, zinc dimethyldithiocarbamate shows the best stabilizing activity which increases with concentration. The results are explained on the basis of the established mechanistic action of these compounds and show that viscometry is not an accurate method for measuring the molar mass of high molecular weight polyethylene samples unless the appropriate conditions are chosen. Even with the best inhibitor, some degradation is still observed.