The Oxidative Mechanism in Electrophilic C?H Activation: The Case of CH2F2 and CH2Cl2

The H^.‐atom transfer (HAT) reaction is investigated in the gas phase, starting from two different entrance channels, O₂^.+/CH₂X₂ and CH₂X₂^.+/O₂ (X=F, Cl), that correspond to a step of hydride transfer and to HAT, respectively. Analysis of the spin and charge along the reaction pathway shows that HAT occurs through the same reacting configuration, irrespective of whether the reactants are formed within the complex or are free isolated species.     

Chirality of products in acid-catalysed rearrangement of α-pinene

Protonation of (1R)-(+)-a-pinene using gaseous H₃ ⁺ ions led to a racemic mixture of camphene and limonene, to partial and no racemization, in acidic solutions and in solid acids, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interaction between pentaethylene glycol n-octyl ether and low-molecular-weight poly(acrylic acid)

The interaction between pentaethylene glycol n-octyl ether (C8E5) and low-molecular-weight poly(acrylic acid) (PAA, M(w)=2000) in aqueous solution has been investigated by various experimental techniques at constant polymer concentration (0.1% w/w) with varying surfactant molality. Spectrofluorimetry, using pyrene as molecular probe, shows (i) the formation of surfactant-polymer aggregates at a surfactant molality (T(1)) lower than the critical micelle concentration (cmc) of C8E5 in water and (ii) the formation of free micelles at a surfactant molality (T(2)) slightly higher than the cmc. Fluorescence quenching measurements indicate that the presence of PAA induces a lowering of the C8E5 aggregation number. Calorimetry confirms spectrofluorimetric evidence; in addition, it shows the presence of weak interactions below T(1) between monomeric surfactant molecules and the polymer chains. Tensiometry shows that, above T(1), only a low fraction of surfactant molecules interact with the polymer and that free micelle formation occurs before polymer saturation. The peculiarities of the interaction between surfactants and low-molecular-weight polymers have been discussed.     

Radiation-Induced Polymerization of Vinylidene Chloride in Bulk and Included in Thiourea Crystals

Vinylidene chloride (VDC) or 1,1-dichloroethylene was polymerized with γ radiation in bulk or as inclusion complex in thiourea crystals (inclusion polymerization). The resulting poly(vinylidenechloride) (PVDC) samples obtained from the two different polymerization techniques were characterized by FT-IR and electronic absorption spectroscopies, by ozonolysis and by thermal analysis (TGA, DTG and DTA). It was found that two selective secondary reactions occur in the two PVDC samples, respectively obtained from bulk polymerization or from inclusion polymerization. In the former case, the main reaction is only a crosslinking reaction, while in the latter case, with the PVDC included into the thiourea channels, the crosslinking reaction is fully inhibited and instead a dehydrohalogenation reaction takes place producing the polyene structures. The presence of polyene structures in the PVDC synthesized by the inclusion polymerization was demonstrated by electronic absorption spectroscopy and by ozonolysis experiments. The presence of polyene segments in the PVDC causes a reduction in the thermal stability of the polymer, lowers its melting point and reduces its crystallinity.     

Synthesis and Study of the Thermal and Chiro-Optical Properties of Polyacetylenes with Bulky Side Groups: Poly(1-ethynyl-4-biphenyl), Poly(1-ethynyl-4- phenoxybenzene) and Poly(1-ethynyl-4-pentylbenzene)

The monosubstituted acetylenes 1-ethynyl-4-biphenyl, 1-ethynyl-4-phenoxybenzene and 1-ethynyl-4-pentylbenzene were polymerized to the corresponding polyacetylenes with bulky side substituents: PEBIP, PEPHENO and PEPB, respectively, using a Rh(I) complex as polymerization catalyst. The FT-IR and UV-VIS spectra of PEBIP, PEPHENO and PEPB were studied using the corresponding spectra of polyphenylacetylene (PPA), polytolylacetylene (PETOL) and poly(1-ethynyl-naphthalene) PEN as reference. Based on the spectral analysis, high cis-trasoid structures were assigned to PEBIP, PEPHENO and PEPB. The specific optical rotation of PEBIP, PEPHENO and PEPB was measured in the chiral solvent α (-)pinene and, only for PEPB, also in α (+)pinene. It was found that even at low concentration PEBIP, PEPHENO and PEPB cause changes in the specific optical rotation of the chiral medium where they have been dissolved. This phenomenon has not been observed when toluene or atactic polystyrene are dissolved in the same chiral solvent in similar concentration. This suggests that the polyacetylenes with bulky side groups are able to rearrange in a helical configuration (prevalently right-handed or vice-versa, depending on the type of chiral medium used) once hosted in a chiral medium giving a contribution to the chirality of the solvent where they have been dissolved. The thermal stability of PEBIP, PEPHENO and PEPB was studied under N₂ flow by thermal analysis (TGA, DTG and DTA) in comparison to reference monosubstituted polyacetylenes PPA, PETOL and PEN. The results show that the thermal stability of monosubstituted polyacetylenes is greatly improved by the presence of bulky substitutents as theoretically expected, and similarly, also the char-forming tendency increases with the bulkiness of the side groups.     

Mixed micellar aggregates of nonionic surfactants with short hydrophobic tails

Mixed micelle formation by aqueous mixtures of pentaethylene glycol monohexyl ether (C₆E₅) and diethylene glycol monohexyl ether (C₆E₂) has been investigated by determining the temperature-composition phase diagrams and measuring the intradiffusion coefficients. In binary aqueous solutions, C₆E₅ shows a high tendency to micellize, while C₆E₂, because of its too short hydrophilic head, presents a macroscopic phase separation in a water-rich and a surfactant-rich phases. The temperature-composition phase diagrams obtained for the ternary water-C₆E₅---C₆E₂ mixtures present a micellar region in which both surfactants co-aggregate. The dimensions of this micellar region depend on the relative ratio between the two surfactants molalities. By analysing the micelle intradiffusion coefficient, measured through the PGSE-NMR technique, the hydrodynamic radius of the aggregates has been estimated at 25 °C. It is similar to that of triethylene glycol monohexyl ether (C₆E₃) in binary aqueous solution. The experimental evidences suggest that in mixed ethoxylated surfactants the micellar behaviour is mainly determined by the mean number of ethoxylic units per surfactant molecule, independently of their distribution.     

The role of chemometrics in single and sequential extraction assays: a review. Part II. Cluster analysis, multiple linear regression, mixture resolution, experimental design and other techniques

Single and sequential extraction procedures are used for studying element mobility and availability in solid matrices, like soils, sediments, sludge, and airborne particulate matter. In the first part of this review we reported an overview on these procedures and described the applications of chemometric uni- and bivariate techniques and of multivariate pattern recognition techniques based on variable reduction to the experimental results obtained. The second part of the review deals with the use of chemometrics not only for the visualization and interpretation of data, but also for the investigation of the effects of experimental conditions on the response, the optimization of their values and the calculation of element fractionation. We will describe the principles of the multivariate chemometric techniques considered, the aims for which they were applied and the key findings obtained. The following topics will be critically addressed: pattern recognition by cluster analysis (CA), linear discriminant analysis (LDA) and other less common techniques; modelling by multiple linear regression (MLR); investigation of spatial distribution of variables by geostatistics; calculation of fractionation patterns by a mixture resolution method (Chemometric Identification of Substrates and Element Distributions, CISED); optimization and characterization of extraction procedures by experimental design; other multivariate techniques less commonly applied.     

Calorimetric adaptation of the inhibited autoxidation method to determine the activity of individual antioxidants and natural extracts

Journal of Thermal Analysis and Calorimetry - This work aims to determine the antioxidants activity of individual antioxidants and natural extracts by isothermal calorimetry. The proposed...