Kinetic subtleties of nitroxide mediated polymerization

Due to the academic and industrial interest of Nitroxide Mediated Polymerization (NMP), a lot of investigations have focused on the kinetics of this process. During the last decade, although the simplified kinetic scheme--equilibrium reactions between dormant species (alkoxyamine) and active species (alkyl radicals and nitroxides), propagation reaction of the macro-alkyl radical, and termination reactions--was suitable to predict the main trends at the macromolecular level, it has become obvious that it was not sufficient to describe all the kinetic effects involved in the NMP process. Indeed, like the conventional radical polymerization, NMP should be described as a 3 stage process including initiation, propagation, and self- and cross-termination. These two types of radical polymerization differ by the occurrence during NMP of an activation/deactivation process involving the dormant species in both the initiation and propagation stages. Evidence is provided of the importance of the rate of homolysis of the initiator (alkoxyamines) and of the rate of the first alkyl radical addition onto the monomer for the success of NMP. Thus, the fundamental kinetics of the main reactions involved in NMP as well as side-reactions are also discussed in this tutorial review.     

Cryogenic Chemistry and Quantitative Non-Thermal Desorption from Pure Methanol Ices: High-Energy Electron versus X-Ray Induced Processes

X-Ray irradiation of interstellar ice analogues has recently been proven to induce desorption of molecules, thus being a potential source for the still-unexplained presence of gaseous organics in the coldest regions of the interstellar medium, especially in protoplanetary disks. The proposed desorption mechanism involves the Auger decay of excited molecules following soft X-ray absorption, known as X-ray induced electron-stimulated desorption (XESD). Aiming to quantify electron induced desorption in XESD, we irradiated pure methanol (CH₃OH) ices at 23 K with 505 eV electrons, to simulate the Auger electrons originating from the O 1s core absorption. Desorption yields of neutral fragments and the effective methanol depletion cross-section were quantitatively determined by mass spectrometry. We derived desorption yields in molecules per incident electron for CO, CO₂, CH₃OH, CH₄/O, H₂O, H₂CO, C₂H₆ and other less abundant but more complex organic products. We obtained desorption yields remarkably similar to XESD values.     

Electroactive self-assembled monolayers on gold via bipodal dithiazepane anchoring groups

Novel dithiazepane-functionalized ferrocenyl-phenylethynyl oligomers 1 and 2 have been synthesized. Self-assembled monolayers (SAMs) of these ferrocene derivatives have been studied by X-ray photoelectron spectroscopy, ellipsometry, and cyclic voltammetry. It has been shown by XPS that monolayers of the dithiazepane-anchored molecules on gold electrodes contain gold-thiolate species. Cyclic voltammetry of the SAMs were characteristic of stable electroactive monolayers even for single-component SAMs of 1 and 2, with the more ideal responses recorded for the two-component SAMs diluted with undecanethiol. The small variation in peak splittings at progressively higher scan rates in these SAMs makes dithiazepane-bridged redox species promising candidates for further studies on molecular wires with bipodal anchoring.     

Determination of phytase activity in feed: interlaboratory study

An interlaboratory study was conducted to determine the performance characteristics of a new method for the determination of phytase activity in feed samples. The method is based on the principle that inorganic phosphate is released from the substrate phytate under defined assay conditions and has been validated for its suitability to measure the enzyme activity of various phytase products. Two different experimental designs of the study were applied, allowing for the estimation of the precision of the method under repeatability, intermediate precision and reproducibility conditions, respectively. The relative standard deviation for repeatability (RSDr) ranged from 2.2 to 10.6% and the RSD for reproducibility (RSDR) ranged from 5.4 to 15%. The suitability of the validated method for the intended purpose was demonstrated. The obtained performance profile of the method validated in this study was comparable to that of similar methods that were exclusively validated for one phytase product.     

Dynamic interactive membranes with pressure-driven tunable porosity and self-healing ability

When pressure is applied to dynamic interactive membranes consisting of micelles composed of a triblock copolymer, their morphologies can be fine-tuned. Membranes with a range of porosities are accessible which can regulate and thereby control filtration performance and also display effective autonomous healing.     

UV photodesorption of interstellar CO ice analogues: from subsurface excitation to surface desorption

Carbon monoxide is after H(2) the most abundant molecule identified in the interstellar medium (ISM), and is used as a major tracer for the gas phase physical conditions. Accreted at the surface of water-rich icy grains, CO is considered to be the starting point of a complex organic--presumably prebiotic--chemistry. Non-thermal desorption processes, and especially photodesorption by UV photons, are seen as the main cause that drives the gas-to-ice CO balance in the colder parts of the ISM. The process is known to be efficient and wavelength-dependent, but, the underlying mechanism and the physical-chemical parameters governing the photodesorption are still largely unknown. Using monochromatized photons from a synchrotron beamline, we reveal that the molecular mechanism responsible for CO photoejection is an indirect, (sub)surface-located process. The local environment of the molecules plays a key role in the photodesorption efficiency, and is quenched by at least an order of magnitude for CO interacting with a water ice surface.     

Scaling tests of the cross section for deeply virtual Compton scattering

We present the first measurements of the e[over -->]p-->epgamma cross section in the deeply virtual Compton scattering (DVCS) regime and the valence quark region. The Q(2) dependence (from 1.5 to 2.3 GeV(2)) of the helicity-dependent cross section indicates the twist-2 dominance of DVCS, proving that generalized parton distributions (GPDs) are accessible to experiment at moderate Q(2). The helicity-independent cross section is also measured at Q(2)=2.3 GeV(2). We present the first model-independent measurement of linear combinations of GPDs and GPD integrals up to the twist-3 approximation.     

Recycling of distillery effluents in alcoholic fermentation: role in inhibition of 10 organic molecules

In beet distilleries, condensates arising from stillage concentration could be recycled as dilution water for the fermentation step, thus preserving groundwater resources and ensuring a quality-controlled water supply. However, the recycling of condensates has been found to cause a significant reduction in fermentation activity. This study aimed to verify that condensates are toxic to alcoholic fermentation. Ten compounds found in condensates (formic, acetic, propionic, butyric, valeric, and hexanoic acids; 2,3-butanediol, furfuryl alcohol, furfural, and 2-phenyl-ethyl-alcohol) were tested. With the exception of 2,3-butanediol, they all proved to be inhibitors. At the same molar concentration, the longer the carbonaceous chain, the stronger the inhibition by fatty acids. An experimental design was used to study the inhibitory characteristics of the 10 compounds at the concentrations found in condensates. Synergistic effects were also confirmed. In real effluents, acetic acid was so highly concentrated that it became the strongest inhibitor. It is therefore necessary to eliminate it before recycling, as well as less concentrated compounds that may accumulate, as illustrated by the simulation.     

Ab initio multireference configuration-interaction study of hydrogen molecule activation by Cs-promoted Pt clusters

The adsorption of the H2 molecule on CsnPt(5-n) bcc (111) clusters for Cs/Pt rates of 20%, 40%, and 80% is studied using ab initio multiconfigurational self-consistent field plus multireference configuration-interaction variational and perturbative calculations. The H2 interaction with the clusters is studied in ground and excited states with geometry optimization, where the hydrogen adsorption takes place by a Pt atom. These calculations are compared with those of H2 adsorption on Pt4. The most stable configurations of Cs/Pt4 and Cs2Pt3 clusters (Cs/Pt rates of 20% and 40%) are a doublet and a closed-shell singlet, respectively. Both clusters capture and activate the hydrogen molecule and their behaviors resemble Pt4. The H2 capture distances are, respectively, similar and smaller than Pt4 capture distances, while the H-H bond dissociation distances are similar and bigger than those of Pt4; however, none of them presents activation barriers. The most stable Cs4Pt cluster (Cs/Pt rate of 80%) is also a closed-shell singlet; it also captures and activates the hydrogen molecule and shows a different behavior as compared with Cs/Pt4, Cs2Pt3, and Pt4 clusters. The capture distance is quite smaller and is obtained after surmounting an activation barrier. For all clusters studied here, no hydrogen absorption was observed, only the adsorption of H2.     

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