Carbo‐biphenyls and Carbo‐terphenyls: Oligo(phenylene ethynylene) Ring Carbo‐mers

Ring carbo‐mers of oligo(phenylene ethynylene)s (OPEn, n=0–2), made of C₂‐catenated C₁₈ carbo‐benzene rings, have been synthesized and characterized by NMR and UV‐vis spectroscopy, crystallography and voltammetry. Analyses of crystal and DFT‐optimized structures show that the C₁₈ rings preserve their individual aromatic character according to structural and magnetic criteria (NICS indices). Carbo‐terphenyls (n=2) are reversibly reduced at ca. ?0.42 V/SCE, i.e. 0.41 V more readily than the corresponding carbo‐benzene (?0.83 V/SCE), thus revealing efficient inter‐ring π‐conjugation. An accurate linear fit of E_1/2^red1 vs. the DFT LUMO energy suggests a notably higher value (?0.30 V/SCE) for a carbo‐quaterphenyl congener (n=3). Increase with n of the effective π‐conjugation is also evidenced by a red shift of two of the three main visible light absorption bands, all being assigned to TDDFT‐calculated excited states, one of them restricting to a HOMO→LUMO main one‐electron transition.     

Resonance-enhanced,multiphoton-ionization spectroscopy of NH3

The multiphoton-ionization spectroscopy of NH₃ has been explored using three-photon ionization through a two-photon resonant intermediate state. This method provides greatly increased sensitivity when compared to three-photon excitation to the intermediate state.     

Smoothing out positively curved metric cones by Ricci expanders

We investigate the possibility of desingularizing a positively curved metric cone by an expanding gradient Ricci soliton with positive curvature operator. This amounts to study the deformation of such geometric structures. As a consequence, we prove that the moduli space of conical positively curved gradient Ricci expanders is connected.     

Solid-phase extraction using molecularly imprinted polymers for selective extraction of a mycotoxin in cereals

The aim of this work was to develop a method for the clean-up of a mycotoxin, i.e. Ochratoxin A (OTA), from cereal extracts employing a new molecularly imprinted polymer (MIP) as selective sorbent for solid-phase extraction (SPE) and to compare with an immunoaffinity column. A first series of experiments was carried out in pure solvents to estimate the potential of the imprinted sorbent in terms of selectivity studying the retention of OTA on the MIP and on a non-imprinted polymer using conventional crushed monolith. The selectivity of the MIP was also checked by its application to wheat extracts. Then, after this feasibility study, two different formats of MIP: crushed monolith and micro-beads were evaluated and compared. Therefore an optimization procedure was applied to the selective extraction from wheat using the MIP beads. The whole procedure was validated by applying it to wheat extract spiked by OTA at different concentration levels and then to a certified contaminated wheat sample. Recoveries close to 100% were obtained. The high selectivity brought by the MIP was compared to the selectivity by an immunoaffinity cartridge for the clean-up of the same wheat sample. The study of capacity of both showed a significant higher capacity of the MIP.     

Visible light/Ir(III) photocatalytic initiation of xanthate-based radical-chain reactions: Xanthate group transfer and oxidative addition to aromatic systems

A photocatalyzed redox generation of radicals from O-ethyl xanthates to generate electrophilic radicals under photoredox catalysis, using Ir(ppy)₃ and blue LEDs irradiation is described. The protocol can be used in classical xanthate-based inter- and intra-molecular group transfer reactions and oxidative radical addition to several heteroaromatic systems. The process does not require high temperature and reactions are cleaner compared with the traditional peroxide initiation. In the oxidative addition to aromatic systems, the oxidation process is part of the catalytic cycle and does not require a stoichiometric oxidant such as DLP which is particularly difficult to separate from the product.     

Formation of tubular silicon carbide from a carbon–silica material by using a reactive replica technique: infra-red characterisation

Thermal treatment of carbon fibres covered with silica prepared by a sol–gel technique leads to the formation of a tubular silicon carbide material keeping the morphology of the carbon source. The conversion of silica into silicon carbide, its kinetics and the structure of the final material were mainly determined by infra-red (IR) spectroscopy. The analytical results were crossed examined with those obtained by gravimetric and microscopic methods. In particular, it is shown, here, that IR-spectroscopy is a suitable technique to quantitatively determine the amount of SiC formed during the heat treatment of a C/SiO₂ material.     

Stability of non compact steady and expanding gradient Ricci solitons

We study the stability of non compact steady and expanding gradient Ricci solitons. We first show that linear stability implies dynamical stability. Then we give various sufficient geometric conditions ensuring the linear stability of such gradient Ricci solitons.     

The two-machine open-shop problem with unit-time operations and time delays to minimize the makespan

This paper addresses the problem of scheduling unit-time operations with integral and non-negative time delay considerations on a two-machine open-shop environment. The criterion to minimize is the makespan. Two well solvable cases and two approximation algorithms, with their worst-case analyses, are presented.     

Bioactive molecules in Kalanchoe pinnata leaves: extraction, purification, and identification

Kalanchoe pinnata (Lam.) Pers. (syn. Bryophyllum pinnatum; family Crassulaceae) is a popular plant used in traditional medicine in many temperate regions of the world and particularly in South America. In Guyana, the leaves are traditionally used as an anti-inflammatory and antiseptic to treat coughs, ulcers, and sores. The purpose of this study was to implement a method for targeting and identifying molecules with antimicrobial activity, which could replace chemical preservatives in cosmetic applications. The leaves were extracted by a method based on pressurized liquid extraction (PLE), using different solvents. A study of antimicrobial activity and cytotoxicity tests were performed to select the most interesting extract. To isolate one or more active molecules, the selected crude extract was fractionated by centrifugal partition chromatography (CPC) and then antimicrobial activity and cytotoxicity of each fraction were tested under the same procedure. The last step consisted of identifying the main compounds in the most active fraction by LC-MS/MS.