Stereoelectronic effects and general trends in hyperconjugative acceptor ability of sigma bonds

A systematic study of general trends in sigma acceptor properties of C-X bonds where X is a main group element from groups IVa-IIa is presented. The acceptor ability of the C-X sigma bonds in monosubstituted ethanes increases when going to the end of a period and down a group. Enhancement of acceptor ability of C-X sigma bonds as one moves from left to right in periods parallels the increase in electronegativity of X, whereas augmentation of acceptor ability in groups is opposite to the changes in electronegativity of X and in the C-X bond polarization, following instead the decrease in the energy of sigma(C)(-)(X) orbitals when one moves from the top to the bottom within a group. This simple picture of acceptor ability of sigma bonds being controlled by electronegativity in periods and by sigma orbital energy in groups is changed in monosubstituted ethenes where the role of electronegativity of the substituent X becomes more important due to increased overlap between sigma orbitals. The combination of several effects of similar magnitude influences acceptor ability of sigma bonds in monosubstituted ethenes in a complex way. As a result, the acceptor ability of sigma bonds can be significantly modified by substitution and is conformer dependent. Stereoelectronic effects displayed by C-X bonds with X from second and third periods are highly anisotropic. For example, C-chalcogen bonds are excellent sigma acceptors at the carbon end but poor sigma acceptors at the chalcogen end. This effect can be relied upon in the design of molecular diodes with sigma bridges with unidirectional electron conductivity. While the general trends revealed in this work should be useful for the qualitative understanding of stereoelectronic effects, one should bear in mind that the magnitude of hyperconjugative effects is extremely sensitive to small variations in structure and in substitution. This advocates for the increased role of theoretical methods in analysis of stereoelectronic effects.     

Impact of humate complexation on the adsorption of REE onto Fe oxyhydroxide

Adsorption experiments of rare-earth elements (REE) onto hydrous ferric oxide (HFO) were performed to evaluate the impact of organic complexation on both REE(III) adsorption and the Ce(III) oxidation rate. Scavenging experiments were performed at pH 5.2 with NaCl and NaNO3 solutions containing either free REE (III) or REE(III)-humate complexes. The log K(d)(REE) patterns obtained from HFO suspensions exhibit a slight positive Ce anomaly and an M-type lanthanide tetrad effect, in contrast with the partitioning between REE(III)-humate complexes and HFO, which yields completely flat distribution patterns. The "organic" partitioning runs yield log K(d)(REEorganic)/log K(d)(DOC) ratios (DOC = dissolved organic carbon) close to 1.0, implying that the REE(III) and humate remain bound to each other during the adsorption experiment. The lack of any positive Ce anomaly or M-type lanthanide tetrad effect in the organic experiments seems to reflect an anionic adsorption of the REE-humate complex. Adsorption onto HFO takes place via the humate side of the REE(III)-humate complexes. The oxidation of Ce(III) by Fe(III) and the proportion of surface hydroxyl groups coordinated to REE(III) at the HFO surface are the two most commonly invoked processes for explaining the development of positive Ce anomalies and the M-type tetrad lanthanide effect. However, such processes cannot proceed since the REE are not in direct contact with the HFO suspensions, the latter being shielded by PHA. The present results further complicate the use of Ce anomalies as reliable paleoredox proxies in natural precipitates. They are also further demonstration that organic matter may inhibit the lanthanide tetrad effect in geological samples.     

Non-parametric estimation of the density of a point process

We consider the Parzen—Rosenblatt type estimator of the density of a point process. Sufficient conditions for convergence of this estimator are established. These results extend those obtained for a random sample.     

Nonlinear friction dynamics on fibrous materials, application to the characterization of surface quality. Part?II: local characterization of phase space by recurrence plots

As has been shown in the first part of this series of papers, the global analysis of phase spaces does not allow one to access topographies of attractors, generated by the singular dynamic contacts between MODALSENS and our evaluated fibrous surfaces. By using the same time series from MODALSENS, this second paper presents a local exploration of the recurrences of the phase spaces. As a complement of the results from Part I, we propose, in this part of the work, a finer analysis of the vibrations of MODALSENS. Therefore, this part of the work aims at tracing friction dynamics cartographies of fibrous surfaces with the help of Recurrence Plots. This tool allows one to obtain images of recurrences in the space portraits. Hence, by regarding passages between strong and low magnitudes of vibrations, it is possible to take into account strong heterogeneities of relief and also the various mechanical and frictional behaviors of the asperities encountered during friction. Finally, Recurrence Quantification Analysis is performed in order to discuss the relationship between expected performances of the tested surfaces and their friction dynamics behaviors.     

How does organic matter constrain the nature, size and availability of Fe nanoparticles for biological reduction?

Few studies have so far examined the kinetics and extent of the formation of Fe-colloids in the presence of natural organic ligands. The present study used an experimental approach to investigate the rate and amount of colloidal Fe formed in presence of humic substances, by gradually oxidizing Fe(II) at pH 6.5 with or without humic substances (HS) (in this case, humic acid--HA and fulvic acid--FA). Without HS, micronic aggregates (0.1-1 μm diameter) of nano-lepidocrocite is obtained, whereas, in a humic-rich medium (HA and FA suspensions at 60 and 55 ppm of DOC respectively), nanometer-sized Fe particles are formed trapped in an organic matrix. A proportion of iron is not found to contribute to the formation of nanoparticles since iron is complexed to HS as Fe(II) or Fe(III). Humic substances tend to (i) decrease the Fe oxidation and hydrolysis, and (ii) promote nanometer-sized Fe oxide formation by both inhibiting the development of hydroxide nuclei and reducing the aggregation of Fe nanoparticles. Bioreduction experiments demonstrate that bacteria (Shewanella putrefaciens CIP 80.40 T) are able to use Fe nanoparticles associated with organic matter about eight times faster than in the case of nano-lepidocrocite. This increase in bioreduction rate appears to be related to the presence of humic acids that (i) indirectly control the size, shape and density of oxyhydroxides and (ii) directly enhance biological reduction of nanoparticles by electron shuttling and Fe complexation. These results suggest that, in wetlands but also elsewhere where mixed organic matter-Fe colloids occur, Fe nanoparticles closely associated with organic matter represent a bioavailable Fe source much more accessible for microfauna than do crystallized Fe oxyhydroxides.     

Nanoscale organization of thiol and arylsulfonic acid on silica leads to a highly active and selective bifunctional, heterogeneous catalyst

Ordered mesoporous silicas functionalized with alkylsulfonic acid and thiol group pairs have been shown to catalyze the synthesis of bisphenols from the condensation of phenol and various ketones, with activity and selectivity highly dependent on the distance between the acid and thiol. Here, a new route to thiol/sulfonic acid paired catalysts is reported. A bis-silane precursor molecule containing both a disulfide and a sulfonate ester bond is grafted onto the surface of ordered mesoporous silica, SBA-15, followed by simultaneous disulfide reduction and sulfonate ester hydrolysis. The resulting catalyst, containing organized pairs of arylsulfonic acid and thiol groups, is significantly more active than the alkylsulfonic acid/thiol paired catalyst in the synthesis of bisphenol A and Z, and this increase in activity does not lead to a loss of regioselectivity. The paired catalyst has activity similar to that of a randomly bifunctionalized arylsulfonic acid/thiol catalyst in the bisphenol A reaction but exhibits greater activity and selectivity than the randomly bifunctionalized catalyst in the bisphenol Z reaction.     

Cooperative catalysis by silica-supported organic functional groups

Hybrid inorganic-organic materials comprising organic functional groups tethered from silica surfaces are versatile, heterogeneous catalysts. Recent advances have led to the preparation of silica materials containing multiple, different functional groups that can show cooperative catalysis; that is, these functional groups can act together to provide catalytic activity and selectivity superior to what can be obtained from either monofunctional materials or homogeneous catalysts. This tutorial review discusses cooperative catalysis of silica-based catalytic materials, focusing on the cooperative action of acid-base, acid-thiol, amine-urea, and imidazole-alcohol-carboxylate groups. Particular attention is given to the effect of the spatial arrangement of these organic groups and recent developments in the spatial organization of multiple groups on the silica surface.