Letter. On the activation of [CrCl3{R-SN(H)S-R}] catalysts for selective trimerization of ethene: a freeze-quench Cr K-edge XAFS study

Homogeneous chromium catalysts for the selective conversion of ethene to hex-1-ene are formed from Cr(III) reagents, amino-thioether ligands of the type HN(CH2CH2SR)2, and aluminum reagents. In this study the early activation steps are investigated by EPR, UV-visible and Cr K-edge XAFS spectroscopy; rapid stopped-flow mixing and a freeze-quench allows good quality EXAFS analysis of a species formed in ~ 1 second of reaction. This is shown to involve reduction to Cr(II) and deprotonation of a NH group of the auxiliary ligand. This 4-coordinate metal-center may act as precursor for the coordination of ethene and subsequent selective oligomerization

Activation of [CrCl3{PPh2N(iPr)PPh2}] for the selective oligomerisation of ethene: a Cr K-edge XAFS study

The activation of the ethene tetramerisation catalyst system based upon [CrCl3(THF)3] and N(iPr)(PPh2)2 has been investigated in situ via the reaction of [CrCl3{PPh2N(R)PPh2}(THF)] 1a (R=iPr) with excess AlMe3 in toluene. The Cr K-edge XAFS spectrum of the solution freeze quenched after 1 min reaction time indicated monomethylation of the metal with the resultant product being [CrClMe(ClAlCl3){PPh2N(R)PPh2}(THF)] 4a (R=iPr). After 5 minutes reaction time the XAFS spectra indicate that ~50% of 4a had been converted to a Cr(II) species, with the central core being high spin [CrCl2{PPh2N(R)PPh2}] 7a (R=iPr); a similar species, [CrClMe{PPh2N(R)PPh2}] 9a (R=iPr) was observed as its adduct with AlMe3 (10a) (R=iPr) when spectra were recorded on samples maintained a room temperature. Detailed analysis (EXAFS and XANES) indicated that 7a and 9a are stabilised by adduct formation of a Cr-Cl bond to the Lewis acids B(C6F5)3 and AlMe3, respectiveley. Modelling with DFT methods indicated that five-coordination was achieved, respectively by Cr-F (11a) and Cr-C (10a) interactions. In the presence of [Ph3C][Al{OC(tBuF)3}4], the Cr XAFS of the room temperature solution was inconsistent with the maintenance of a phosphine complex, but could be modelled with a site like [Cr2Me8]4- {Cr-Cr 2.01(2), Cr-C 2.14(4)}, thus demonstrating considerable variation in the effects of differing Lewis acids

Neurology

The question of the long-term safety of pregnancy is a major concern in patients with multiple sclerosis (MS), but its study is biased by reverse causation (women with higher disability are less likely to experience pregnancy). Using a causal inference approach, we aimed to estimate the unbiased long-term effects of pregnancy on disability and relapse risk in patients with MS and secondarily the short-term effects (during the perpartum and postpartum years) and delayed effects (occurring beyond 1 year after delivery). We conducted an observational cohort study with data from patients with MS followed in the Observatoire Français de la Sclérose en Plaques registry between 1990 and 2020. We included female patients with MS aged 18-45 years at MS onset, clinically followed up for more than 2 years, and with ≥3 Expanded Disease Status Scale (EDSS) measurements. Outcomes were the mean EDSS score at the end of follow-up and the annual probability of relapse during follow-up. Counterfactual outcomes were predicted using the longitudinal targeted maximum likelihood estimator in the entire study population. The patients exposed to at least 1 pregnancy during their follow-up were compared with the counterfactual situation in which, contrary to what was observed, they would not have been exposed to any pregnancy. Short-term and delayed effects were analyzed from the first pregnancy of early-exposed patients (who experienced it during their first 3 years of follow-up). We included 9,100 patients, with a median follow-up duration of 7.8 years, of whom 2,125 (23.4%) patients were exposed to at least 1 pregnancy. Pregnancy had no significant long-term causal effect on the mean EDSS score at 9 years (causal mean difference [95% CI] = 0.00 [-0.16 to 0.15]) or on the annual probability of relapse (causal risk ratio [95% CI] = 0.95 [0.93-1.38]). For the 1,253 early-exposed patients, pregnancy significantly decreased the probability of relapse during the perpartum year and significantly increased it during the postpartum year, but no significant delayed effect was found on the EDSS and relapse rate. Using a causal inference approach, we found no evidence of significantly deleterious or beneficial long-term effects of pregnancy on disability. The beneficial effects found in other studies were probably related to a reverse causation bias.Observatoire Français de la Sclérose en Plaque

A História da Alimentação: balizas historiográficas

Os M. pretenderam traçar um quadro da História da Alimentação, não como um novo ramo epistemológico da disciplina, mas como um campo em desenvolvimento de práticas e atividades especializadas, incluindo pesquisa, formação, publicações, associações, encontros acadêmicos, etc. Um breve relato das condições em que tal campo se assentou faz-se preceder de um panorama dos estudos de alimentação e temas correia tos, em geral, segundo cinco abardagens Ia biológica, a econômica, a social, a cultural e a filosófica!, assim como da identificação das contribuições mais relevantes da Antropologia, Arqueologia, Sociologia e Geografia. A fim de comentar a multiforme e volumosa bibliografia histórica, foi ela organizada segundo critérios morfológicos. A seguir, alguns tópicos importantes mereceram tratamento à parte: a fome, o alimento e o domínio religioso, as descobertas européias e a difusão mundial de alimentos, gosto e gastronomia. O artigo se encerra com um rápido balanço crítico da historiografia brasileira sobre o tema

X-ray absorption spectroscopy for the study of a homogeneous catalysis system based upon chromium

The selective trimerisation and tetramerisation of ethane to 1-hexene and 1-octene is of great interest due to the importance of these co-monomers in the production of linear low-density polyethylene.  A route based upon a cyclic trimerisation and using a chromium catalyst has achieved extremely high selectivity for a trimer.  This mechanism is thought to be the basis of newer reactions based upon Cr(III) precursors, using methylaluminoxane (MAO) as a co-catalyst. The body of this work was commenced by investigating the nature of the active species when the chromium(III) complex [CrCl3{HN(CH2CH2SC10H2}], known to trimerise ethylene to 1-hexene, is treated with an aluminium reagent.  Studies were carried out via a wide range of techniques, such as UV/visible and EPR spectroscopy as well as X-ray absorption spectroscopy, to validate or otherwise the first species of the catalytic cycle.  Similar investigations were carried out on the chromium(III) complex [CrCl3{PNP}(THF)], known to tetramerise ethylene to 1-octene. The work was then focused on the synthesis of novel S-, N- and O-donor tridentate ligands with three geometries; the open-chains S(CH2CH2SC10H21)2, O(CH2CH2SC10H21)2, the tripods MeC(CH2SC4H9)3, MeC(CH2SC10H21)3 and the macrocycles (C10H21)[9]aneN3, (C10H21)3[9]aneN3.  Their coordination chemistry with Cr(III) is investigated via spectroscopic methods and the ability of these complexes to trimerise, oligomerise and polymerise is studied with catalytic testings.  Partial structure of the active species is proposed after analysis of EXAFS data. The coordination chemistry of N- and S-donor ligands S(CH2CH2SCH3)2, MeC(CH2SCH3)3, [9]aneN3, Me3[9]aneN3, Me3[9]aneN3 and (isoPr)2[9]aneN3 with Cr(III) is investigated.</p

CHARACTERIZATION (TWO-DIMENSIONAL - THREE-DIMENSIONAL) OF CERAMIC MICROFILTRATION MEMBRANE BY SYNCHROTRON RADIATION: NEW AND ABRADED MEMBRANES

International audienceMembranes are used in many industrial fields and, when performances of these processes decrease, the issue of modifying the porous structure is often raised. Many optical or microscopic techniques allow us to perfectly characterize the membrane's surface but very few allow the characterization of its depth. Through the comparison between a new and an abraded membrane, this paper presents the post-processing of images obtained by radiation synchrotron and associated dimensions that can be obtained by three-dimensional (3D) reconstruction. Thus, the whole thickness of the membrane is obtained from the skin to the permeate exit and a morphological analysis of the solid and pore phase is proposed at the heart of the membrane. The two-dimensional characterization allows one to perfectly define the mapping of the pores and to quantify by different comparisons the modification of the skin of a membrane after usage. The 3D characterization by X-ray tomography at the scale of the thickness of the membrane allows us to obtain the granulometric distribution of the different phases of the porous matrix. This ability to characterize both the solid and the pores is relevant to the study of membranes, showing not only the modification of the solid matrix but also highlighting pore fouling

Cr K-edge XANES spectroscopy: ligand and oxidation state dependence - what is oxidation state?

A series of Cr complexes varying in oxidation state, ligand and geometry were studied with Cr K-edge XANES. The main absorption edge energy shift for an oxidation state change from Cr° to Cr6+ is found to be similar to that for a series of Cr3+ complexes with different ligands. Theoretical XANES and density of states calculations using FEFF8.0 provided detailed insights in the origin of the XANES features for the series of distorted octahedral CrCl3L complexes. The geometry of the CrCl3L complex governs the position of the main absorption edge. Hard versus soft donor effects are overruled by the chlorine ligand for complexes with a facial geometry, whereas the chlorine ligand does not play a significant role in meridional geometry. The combined results call for a redefinition of generally used concepts like oxidation state

New membrane characterisation from the support to the skin layer applied to SiC membranes

International audienc

Characterisation of alluvial plains by remote sensing: Cases studies and current stakes

Recent progress in remote sensing is promising significant advances in fluvial sciences particularly in the study of small and medium rivers. Indeed, sensor resolutions are markedly Improving Malthus Allowing for narrower channels to be imaged and acquisition frequencies for airborne methods aussi Have Improved Malthus Allowing for multi-annual surveys. Indeed, sensor resolutions are improving markedly thus allowing for narrower channels to be imaged and acquisition frequencies for airborne methods have also improved thus allowing for multi-annual surveys. Furthermore, satellite data can now be complemented by catchment scale high resolution airborne imagery Orthorectified Often Collected by national agencies: such as the IGN. Furthermore, satellite data can now be complemented by catchment scale high resolution airborne orthorectified imagery often collected by national agencies such as the IGN. Data acquisition can now be Planned on a multi-annual BASIS and ranks of Acquired from a standard survey platforms: such as aircraft, helicopters, ultralight aircraft or unmanned aerial vehicles (UAVs). Data acquisition can now be planned on a multi-annual basis and acquired from a range of platforms such as standard survey aircraft, helicopters, ultralight aircraft or unmanned aerial vehicles (UAVs). The aim of this paper is to Illustrate progress in this field with a series of examples. The aim of this paper is to illustrate progress in this field with a series of examples. Using Both satellite data (SPOT, Landsat and QuickBird) and UAV data, the paper examines a Will ranks of temporal and spatial scales from river Reaches Exceeding ten kilometers to smaller Reaches Where human impacts Have Been Diagnosed. Using both satellite data (SPOT, Landsat and QuickBird) and UAV data, the paper will examine a range of temporal and spatial scales from river reaches exceeding ten kilometres to smaller reaches where human impacts have been diagnosed

Quantifying the effect of tissue deformation on diffusion-weighted MRI: a mathematical model and an efficient simulation framework applied to cardiac diffusion imaging

International audienceCardiac motion presents a major challenge in diffusion weighted MRI, often leading to large signal losses that necessitate repeated measurements. The diffusion process in the myocardium is difficult to investigate because of the unqualified sensitivity of diffusion measurements to cardiac motion. A rigorous mathematical formalism is introduced to quantify the effect of tissue motion in diffusion imaging. The presented mathematical model, based on the Bloch–Torrey equations, takes into account deformations according to the laws of continuum mechanics. Approximating this mathematical model by using finite elements method, numerical simulations can predict the sensitivity of the diffusion signal to cardiac motion. Different diffusion encoding schemes are considered and the diffusion weighted MR signals, computed numerically, are compared to available results in literature. Our numerical model can identify the existence of two time points in the cardiac cycle, at which the diffusion is unaffected by myocardial strain and cardiac motion. Of course, these time points depend on the type of diffusion encoding scheme. Our numerical results also show that the motion sensitivity of the diffusion sequence can be reduced by using either spin echo technique with acceleration motion compensation diffusion gradients or stimulated echo acquisition mode with unipolar and bipolar diffusion gradients