Mössbauer emission spectroscopy of pure and doped MgO:57Co

Mössbauer emission studies on ⁵⁷Co embedded in pure and doped MgO have been carried out. The appearance of anomalous ⁵⁷Fe+³ in the Mössbauer spectra is related to the concentration and ionization degree of the electron acceptor levels existing in the energy gap. The temperature dependence of isomer shift of both ferrous and ferric lines agrees well with the Debye model.     

Mechanism of 1,3-migration in allylperoxyl radicals: computational evidence for the formation of a loosely bound radical-dioxygen complex

The three pathways postulated for 1,3-migration of the peroxyl group in the allylperoxyl radical (1a), a key reaction involved in the spontaneous autoxidation of unsaturated lipids of biological importance, have been investigated by means of quantum mechanical electronic structure calculations. According to the barrier heights calculated from RCCSD(T)/6-311+G(3df,2p) energies with optimized molecular geometries and harmonic vibrational frequencies determined at the UMP2/6-311+G(3df,2p) level, the allylperoxyl rearrangement proceeds by fragmentation of 1a through a transition structure (TS1) with a calculated DeltaH++(298 K) of 21.7 kcal/mol to give an allyl radical-triplet dioxygen loosely bound complex (CX). In a subsequent step, the triplet dioxygen moiety of CX recombines at either end of the allyl radical moiety to convert the complex to the rearranged peroxyl radical (1a') or to revert to the starting peroxyl radical 1a. CX shows an electron charge transfer of 0.026 e in the direction allyl --> O(2). The dominant attractive interactions holding in association the allyl radical-triplet dioxygen pair in CX are due chiefly to dispersion forces. The DeltaH(298 K) for dissociation of CX in its isolated partners, allyl radical and triplet dioxygen, is predicted to be at least 1 kcal/mol. The formation of CX prevents the diffusion of its partners and maintains the stereocontrol along the fragmentation-recombination processes. The concerted 1,3-migration in allylperoxyl radical is predicted to take place through a five-membered ring peroxide transition structure (TS2) showing two long C-O bonds. The DeltaH++(298 K) calculated for this pathway is less favorable than the fragmentation-recombination pathway by 1.9 kcal/mol. The cyclization of 1a to give a dioxolanyl radical intermediate (2a) is found to proceed through a five-membered ring transition structure (TS3) with a calculated DeltaH++(298 K) of 33.9 kcal/mol. Thus, the sequence of ring closure 1a --> 2a and ring opening 2a --> 1a' is unlikely to play any significant role in allylperoxyl rearrangement 1a --> 1a'. In the three pathways investigated, the energy of the transition structure is predicted to be somewhat lower in either heptane or aqueous solution than in the gas phase. Although the energy lowering calculated for TS1 is smaller than the calculated for TS2 and TS3, it is very unlikely that the solvent effects may reverse the predicted preference of the fragmentation-recombination pathway over the concerted and stepwise ring closure-ring opening mechanisms.     

Porosity Variations in Saline Media Induced by Temperature Gradients: Experimental Evidences and Modelling

Porosity variations in saline media containing humidity are induced by temperature gradients. A temperature imposed on a porous salt sample prepared with some brine and closed to mass transfer leads to significant variations of porosity in few weeks. Modelling of the experiments permits to understand the processes involved.     

A detailed workforce planning model including non-linear dependence of capacity on the size of the staff and cash management

This paper introduces an original planning model which integrates production, human resources and cash management decisions, taking into account the consequences that decisions in one area may have on other areas and allowing all these areas to be coordinated. The most relevant characteristics of the planning problem are: (1) production capacity is a non-linear function of the size of the staff; (2) firing costs may depend on the worker who is fired; (3) working time is managed under a working time account (WTA) scheme, so positive balances must be paid to workers who leave the company; (4) there is a learning period for hired workers; and (5) cash management is included. A mixed integer linear program is designed to solve the problem. Despite the size and complexity of the model, it can be solved in a reasonable time. A numerical example, the main results of a computational experiment and a sensibility analysis illustrate the performance and benefits of the model.     

Photochemistry of 3-substituted bicyclo[3.1.0]hex-3-en-2-ones. Regioselective synthesis of ortho-substituted phenols by Pauson-Khand reaction

[reaction: see text] 3-Substituted bicyclo[3.1.0]hex-3-en-2-ones 3, easily obtained by Pauson-Khand reaction between terminal alkynes and cyclopropene, have been quantitatively converted into ortho-substituted phenols 4 by irradiation with UV light (350 nm). The kinetics and mechanism of this photochemical process have been studied by means of FT-IR and semiempirical (AM1 3x3 CI) calculations.     

Isolation and analysis of polycyclic aromatic hydrocarbons from natural water using accelerated solvent extraction followed by gas chromatography-mass spectrometry

An innovative analytical procedure for the analysis of polycyclic aromatic hydrocarbons (PAHs) from large-volume water samples is presented. It involves sample preparation, sampling and the elution process in an automated continuous procedure involving the ASE technique. Prior to sampling, a XAD-2 resin column is prepared on the basis of a commercial accelerated solvent extraction (ASE) cartridge so that the resin bed is permanently fixed. Then, the XAD column inside the ASE cartridge is cleaned and conditioned. The sampling procedure involves conventional filtration with subsequent isolation of dissolved PAHs on an XAD-2 resin contained in the ASE cartridge. After sampling, the XAD-2 resin content inside the cartridge is eluted by ASE without any further sample preparation and subsequently reused. In order to validate the procedure, the PAHs were isolated from water samples from the Lake Maggiore (North of Italy) using both XAD-2 resin adsorption and hexane liquid-liquid extraction according to the International Standard Methodology ISO 17993. The mean percentages of deviation between concentrations obtained by both methodologies range from 6% for benzo(a)pyrene to 15% for fluoranthene and benzo(b,k)fluoranthene. Compared to the traditional techniques, this procedure offers numerous practical advantages: easy to perform, fast, savings in solvent volume and in time, all steps are fully automated thus avoiding any XAD-2 resin manipulation during and between steps and moreover, low detection limits were provided (0.001 ng l⁻¹ for chrysene, benzo(b,k)fluoranthene, benzo(a)pyrene, dibenz(a,h)anthracene, benzo(g,h,i)perylene and indeno(1,2,3-cd)pyrene, and 0.01 ng l⁻¹ for acenaphthylene and fluoranthene).This procedure was developed in the frame of a project aimed at evaluating the diffuse input of organic contaminants in the Lake Maggiore.     

Toward a total synthesis of amphidinolide X and Y. The tetrahydrofuran-containing fragment C12-C21

The key THF derivative (9a) for an enantioselective synthesis of amphidinolide X/Y was obtained from 1a via a selenoetherification reaction. In fact, among the cyclization methods investigated, the highest yield and stereocontrol were achieved at -78 degrees C with PhSeCl/EtiPr2N from diols 1a (anti-Z) and 1b (anti-E) and with PhSeCl/ZnBr2 from diols 1c (syn-Z) and 1d (syn-E). Also, surprisingly, use of protecting groups (on the allylic OH) was detrimental in the cases studied. The diverse THF-tetrasubstituted stereoisomers will provide a series of amphidinolide X/Y analogues. [structure: see text]