Dynamical effects on the O(3P) + D2 reaction and its impact on the Λ-doublet population

The O(3P) + D2 → OD(2Π) + D reaction presents the peculiarity of taking place on two different potential energy surfaces (PESs) of different symmetry, 3A' and 3A'', which become degenerate for collinear configurations where the saddle-point of the reaction is located. The degeneracy is broken for non-collinear approaches with the energy on the 3A' PES rising more abruptly with the bending angle, making the frequency of this mode higher on the 3A' state. Consequently, the 3A' PES should be less reactive than the 3A'' one. Nevertheless, quantum scattering calculations show that the cross section is higher on the 3A' PES for energies close to the classical reaction threshold and rotationless reactant. It is found that the differences between the reactivity on the two PESs are greater for low values of total angular momentum, where the centrifugal barrier is lower and contribute to the higher population of the Π(A') Λ-doublet states of OD at low collision energies. At high collision energies, the Π(A') Λ-doublet state is also preferentially populated. Analysis of the differential cross sections reveals that the preponderance for the Π(A') Λ-doublet at low energies comes from backward scattering, originating from the reaction on the 3A' PES, while at high energies, it proceeds from a different mechanism that leads to sideways scattering on the 3A'' PES and that populates the Π(A') manifold.

Low-cost ceramic membrane bioreactor: Effect of backwashing, relaxation and aeration on fouling. Protozoa and bacteria removal

Membrane biological reactors (MBR) constitute an alternative to conventional wastewater treatments for improved recovery, reuse, and recycling of water. MBRs have a smaller footprint, provide better biotreatment and achieve a high-quality effluent. This work analyses the use of MBRs innovative low-cost ceramic membranes for wastewater treatment. We propose low-cost ceramic membranes as an alternative to the more expensive commercial ceramic membranes. Low-cost membranes were made of clay, calcium carbonate, potato starch, almond shell and chamotte. We synthesized two different selective layers, from clay and/or TiO₂. We characterized the membranes (pore diameter and water permeance) and their performance in a laboratory scale MBR. To mitigate membrane fouling and preserve the continued operation along time, the effect of different operating cycles was measured, considering two physical cleaning strategies: relaxation and backwashing. Cycles of 9 min of operation, 30 s of relaxation and 1 min of backwashing provided the lowest fouling rate. We investigated the effect of air scouring on fouling by operating with different air flow rates. Once experimental conditions were optimized, the overall performance of the different ceramic membranes was tested. The membrane with a TiO₂ thin layer provided the best resistance to fouling, as well as a good retention capacity of E. coli, Cryptosporidium oocysts and Giardia cysts.

Contrary effects of flow intermittence and land uses on organic matter decomposition in a Mediterranean river basin

Flow interruption in intermittent rivers (IRs) generates a mosaic of terrestrial and aquatic habitats across the river network affecting ecosystem processes, as organic matter (OM) decomposition. Water use for farming in arid and semi-arid climates intensifies the dry conditions and affects local river characteristics. In that way, flow intermittence and the distribution of land uses may affect the OM processing along the river. To understand the role of IRs in global OM dynamics and how global change affecting water flow regimes determines these dynamics, it is important to estimate OM-processing rates at a basin scale. The aim of this study was to evaluate the effect of the intensity of flow intermittence on OM processing, and how this effect was modulated by local environmental factors related to land uses across a Mediterranean river basin. To do this, wood decomposition (mass loss and fungal biomass) was selected as a functional indicator. Drying duration and frequency were measured to characterize flow intermittence in different reaches along the river, as well as local environmental factors. Linear models stablished the role of factors on decomposition. The results showed that differences in decomposition rates across the river network were negatively related to the duration of flow interruption. Dissolved inorganic nitrogen associated with agriculture counteracted the negative effect of intermittence on mass loss (increasing by up to three times); but with a higher duration of dry conditions, its effect was insignificant. An increase of 20% of canopy (higher in natural areas) resulted in increases of up to 5% of mass loss. Overall, our study is relevant to understanding the interaction between flow intermittence and land uses on OM processing, especially considering the intensification of flow intermittence and its increased distribution to other regions, which is expected to be a consequence of climate warming and human activities.

Polymer-Ceramic Composite Membranes for Water Removal in Membrane Reactors

Methanol can be obtained through CO₂ hydrogenation in a membrane reactor with higher yield or lower pressure than in a conventional packed bed reactor. In this study, we explore a new kind of membrane with the potential suitability for such membrane reactors. Silicone–ceramic composite membranes are synthetized and characterized for their capability to selectively remove water from a mixture containing hydrogen, CO₂, and water at temperatures typical for methanol synthesis. We show that this membrane can achieve selective permeation of water under such harsh conditions, and thus is an alternative candidate for use in membrane reactors for processes where water is one of the products and the yield is limited by thermodynamic equilibrium.

Quantum study of reaction O(3 P) + H2 (v, j) → OH + H: OH formation in strongly UV-irradiated gas

The reaction between atomic oxygen and molecular hydrogen is an important one in astrochemistry as it regulates the abundance of the hydroxyl radical and serves to open the chemistry of oxygen in diverse astronomical environments. However, the existence of a high activation barrier in the reaction with ground state oxygen atoms limits its efficiency in cold gas. In this study we calculate the dependence of the reaction rate coefficient on the rotational and vibrational state of H2 and evaluate the impact on the abundance of OH in interstellar regions strongly irradiated by far-UV photons, where H2 can be efficiently pumped to excited vibrational states. We use a recently calculated potential energy surface and carry out time-independent quantum mechanical scattering calculations to compute rate coefficients for the reaction O(3 P) + H2 (v, j) → OH + H, with H2 in vibrational states v = 0-7 and rotational states j = 0-10. We find that the reaction becomes significantly faster with increasing vibrational quantum number of H2, although even for high vibrational states of H2 (v = 4-5) for which the reaction is barrierless, the rate coefficient does not strictly attain the collision limit and still maintains a positive dependence with temperature. We implemented the calculated state-specific rate coefficients in the Meudon PDR code to model the Orion Bar PDR and evaluate the impact on the abundance of the OH radical. We find the fractional abundance of OH is enhanced by up to one order of magnitude in regions of the cloud corresponding to A V = 1.3-2.3, compared to the use of a thermal rate coefficient for O + H2, although the impact on the column density of OH is modest, of about 60%. The calculated rate coefficients will be useful to model and interpret JWST observations of OH in strongly UV-illuminated environments.

Unveiling shape resonances in H + HF collisions at cold energies

Scattering resonances are pure quantum effects that appear whenever the collision energy matches the energy of a quasi-bound state of the intermolecular complex. Here we show that rotational quenching of HF(j = 1, 2) with H is strongly influenced by the presence of two resonance peaks, leading to up to a two-fold increase in the thermal rate coefficients at the low temperatures characteristic of the interstellar medium. Our results show that each resonance peak is formed by a cluster of shape resonances, each of them characterized by the same value of the orbital angular momentum but different values of the total angular momentum. The relative intensity of these resonances depends on the relative geometry of the incoming reactants, and our results predict that by changing the alignment of the HF rotational angular momentum it is possible to decompose the resonance peaks, disentangling the underlying resonance pattern and the contribution of different total angular momenta to the resonance.

Unexpected dynamical effects change the lambda-doublet propensity in the tunneling region for the O(3P) + H2 reaction

One of the most relevant features of the O(³P) + H₂ reaction is that it occurs on two different potential energy surfaces (PESs) of symmetries A' and A'' that correlate reactants and products. The respective saddle points, which correspond to a collinear arrangement, are the same for both PESs, whilst the barrier height rises more abruptly on the ³A' PES than on the ³A'' PES. Accordingly, the reactivity on the ³A'' PES should be always higher than on the ³A' PES. In this work, we present accurate quantum-scattering calculations showing that this is not always the case for rotationless reactants, where dynamical factors near the reaction threshold cause the ³A' PES to dominate at energies around the barrier. Further calculation of cross sections and Λ-doublet populations has allowed us to establish how the reaction mechanism changes from the deep tunneling regime to hyperthermal energies.

New Stress Test for Ring Polymer Molecular Dynamics: Rate Coefficients of the O(3P) + HCl Reaction and Comparison with Quantum Mechanical and Quasiclassical Trajectory Results

In the past decade, ring polymer molecular dynamics (RPMD) has emerged as a very efficient method to determine thermal rate coefficients for a great variety of chemical reactions. This work presents the application of this methodology to study the O(³P) + HCl reaction, which constitutes a stringent test for any dynamical calculation due to rich resonant structure and other dynamical features. The rate coefficients, calculated on the ³A' and ³A″ potential energy surfaces (PESs) by Ramachandran and Peterson [ J. Chem. Phys. 2003 , 119 , 9590 ], using RPMD and quasiclassical trajectories (QCT) are compared with the existing experimental and the quantum mechanical (QM) results by Xie et al. [ J. Chem. Phys. 2005 122 , 014301 ]. The agreement is very good at T > 600 K, although RPMD underestimates rate coefficients by a factor between 4 and 2 in the 200-500 K interval. The origin of these discrepancies lies in the large contribution from tunneling on the ³A″ PES, which is enhanced by resonances due to quasibound states in the van der Waals wells. Although tunneling is fairly well accounted for by RPMD even below the crossover temperature, the effect of resonances, a long-time effect, is not included in the methodology. At the highest temperatures studied in this work, 2000-3300 K, the RPMD rate coefficients are somewhat larger than the QM ones, but this is shown to be due to limitations in the QM calculations and the RPMD are believed to be more reliable.

How reactant polarization can be used to change the effect of interference on reactive collisions

It is common knowledge that integral and differential cross sections (DCSs) are strongly dependent on the spatial distribution of the molecular axis of the reactants. Hence, by controlling the axis distribution, it is possible to either promote or hinder the yield of products into specific final states or scattering angles. This idea has been successfully implemented in experiments by polarizing the internuclear axis before the reaction takes place, either by manipulating the rotational angular distribution or by the Stark effect in the presence of an orienting field. When there is a dominant reaction mechanism, characterized by a set of impact parameters and angles of attack, it is expected that a preparation that helps the system to reach the transition state associated with that mechanism will promote the reaction, whilst a different preparation would generally impair the reaction. However, when two or more competing mechanisms via interference contribute to the reaction into specific scattering angles and final states, it is not evident which would be the effect of changing the axis preparation. To address this problem, throughout this article we have simulated the effect that different experimental preparations have on the DCSs for the H + D2 reaction at relatively high energies, for which it has been shown that several competing mechanisms give rise to interference that shapes the DCS. To this aim, we have extended the formulation of the polarization dependent DCS to calculate polarization dependent generalized deflection functions of ranks greater than zero. Our results show that interference is very sensitive to changes in the internuclear axis preparation, and that the shape of the DCS can be controlled exquisitely.

Angular momentum-scattering angle quantum correlation: a generalized deflection function

A natural generalization of the classical deflection function, the functional dependence of the deflection angle on the angular momentum (or the impact parameter), is the joint probability density function of these two quantities, revealing the correlation between them. It provides, at a glance, detailed information about the reaction mechanisms and how changes in the impact parameter affect the product angular distribution. It is also useful to predict the presence of quantum phenomena such as interference. However, the classical angular momentum-scattering angle correlation function has a limited use whenever quantum effects become important. Rigorously speaking, there is not a quantum equivalent of the classical joint distribution, as the differential cross section depends on the coherences between the different values of J caused by the cross terms in the expansion of partial waves. In this article, we present a simple method to calculate a quantum analog of this correlation, a generalized deflection function that can shed light onto the reaction mechanism using just quantum mechanical results. Our results show that there is a very good agreement between the quantum and classical correlation functions as long as quantum effects are not all relevant. When this is not the case, it will also be shown that the quantum correlation function is most useful to observe the extent of quantum effects such as interference among different reaction mechanisms.

The dynamics of the Hg + Br2 reaction: elucidation of the reaction mechanism for the Br exchange reaction

In spite of its importance in the Hg atmospheric chemistry, the dynamics of the Hg + Br₂ → HgBr + Br reaction is poorly understood. In this article, we have carried out a comprehensive study of the reaction mechanism of this reaction by means of quasiclassical trajectories (QCTs) on an existing ab initio potential energy surface (PES). The reaction has a non trivial dynamics, as a consequence of its large endothermicity, the presence of a deep potential well, and the competition between the Br exchange and the collision induced dissociation processes. Our calculations demonstrate that insertion is only relevant at energies just above the reaction threshold and that, at energies above 2.3 eV, HgBr formation typically takes place via a sort of frustrated dissociation. In order to compare directly with the results obtained in extensive cross molecular beam experiments for the homologous reaction with I₂, angular distributions in the laboratory frame for Hg + Br₂ have been simulated under similar experimental conditions. The lack of agreement at the highest energies considered suggests that either the two reactions have substantially different mechanisms or that calculations on a single PES cannot account for the dynamics at those energies.

Identification of a founder BRCA1 mutation in the Moroccan population

Breast cancer (BC) is the most frequent cancer among women in Morocco. However, the role of the most prevalent BC-predisposing genes, BRCA1 and BRCA2, has been largely unexplored. To help define the role of BRCA1 in BC in Morocco, we characterized the first potential BRCA1 founder mutation in this population. Genetic testing of BRCA1 and BRCA2 in BC high-risk families identified mutation BRCA1 c.5309G>T, p.(Gly1770Val) or G1770V in five independent families from Morocco, suggesting a founder effect. To confirm this hypothesis, haplotype construction was performed using seven intragenic and flanking BRCA1 microsatellite markers. Clinical data were also compiled. Clinical data from carriers of mutation G1770V correspond to data from carriers of BRCA1 pathogenic mutations. Microsatellite analysis showed a common haplotype for the five families in a region comprising 1.54 Mb, confirming G1770V as the first specific founder BRCA1 mutation in the Moroccan population. Our findings contribute to a better understanding of BC genetics in the Moroccan population. Nevertheless, comprehensive studies of mutation G1770V in large series of BC patients from Morocco are needed to assess the real prevalence of this mutation and to improve genetic testing and risk assessment in this population.

Recycling of a fine, heavy fluff automobile shredder residue by density and differential fragmentation

A compilation of the physical properties of materials which might typically occur in automobile shredder residue and an analysis of their suitability for the separation of materials in fine (<15mm) heavy fluff ASR (fhf-ASR) is presented. Differences in density and resistance to crushing of fhf-ASR materials were identified as potentially the most suitable low cost, technologically simple means for the separating this waste into its three principal components - metals, minerals (glass/stones) and organics (plastics). Results presented of laboratory scale tests demonstrate that fhf-ASR can in large part be separated into three principal components. Tests were conducted with 0.63-2.0mm and 2-10mm fractions. Recovery of plastics by density separations were conducted with water only jigs for the 2-10mm fraction and shaker tables for the 0.63-2mm fraction. Comparisons are presented of the separations of glass and stones from metals obtained by linear screening and vibratory screening of roller mill and impact mill crushing products of the high density 2-10mm fraction. Equipment used for these tests are of a laboratory or demonstrative scale. It is reasonable to anticipate that industrial scale processing would produce significantly better results. The 2-15mm fraction was found to constitute 91.6% of the fhf-ASR sampled. The metals content of the 2-10mm portion of this fraction was upgraded from 2.5% to 31% and 76.9% with recoveries varying inversely with grade from 91.9% to 40.1%. From 63.6% to 17.1% with a recovery of 93.5% of the organic materials. A residual product of fine sand of crushed glass/stones of 99.4% purity recovered 71.3% of these.

The Cl + O3 reaction: a detailed QCT simulation of molecular beam experiments

QCT calculations have been carried out to determine angle–velocity differential cross-sections to simulate the results of molecular beam experiments.

The effect of the reactant internal excitation on the dynamics of the C(+) + H2 reaction

We have performed a dynamical study of the endothermic and barrierless C(+) + H2((1)Σg(+)) → CH(+)((1)Σg(+)) + H reaction for different initial rotational states of the H2(v = 0) and H2(v = 1) manifolds. The calculations have been carried out using quasiclassical trajectories and the Gaussian binning methodology on a recent potential energy surface [R. Warmbier and R. Schneider, Phys. Chem. Chem. Phys., 2011, 13, 10285]. Both state-selected integral cross sections as a function of the collision energy and rate coefficients, kv,j(T), have been determined. We show that rotational excitation of the reactants is as effective as vibrational excitation when it comes to increasing the reactivity, and that both types of excitation could contribute to explain the unexpectedly high abundance of CH(+) in the interstellar media. Such an increase in reactivity takes place by suppressing the reaction threshold when the internal energy is sufficient to overcome the endothermicity. Whenever this is the case, the excitation functions at collision energies Ecoll ≤ 0.1 eV display a ∝E(-1/2)coll dependence. However, the absolute values of the state selected kv=1(T) are one order of magnitude below the Langevin model predictions. The disagreement between the approximately derived experimental rate coefficients for v = 1 and those calculated by this and previous theoretical treatments is due to the neglect of the effect of the rotational excitation in the derivation of the former. In spite of the deep well present in the potential energy surface, the reaction does not show a statistical behaviour.

Rate coefficients from quantum and quasi-classical cumulative reaction probabilities for the S(1D) + H2 reaction

Cumulative reaction probabilities (CRPs) at various total angular momenta have been calculated for the barrierless reaction S((1)D) + H(2) → SH + H at total energies up to 1.2 eV using three different theoretical approaches: time-independent quantum mechanics (QM), quasiclassical trajectories (QCT), and statistical quasiclassical trajectories (SQCT). The calculations have been carried out on the widely used potential energy surface (PES) by Ho et al. [J. Chem. Phys. 116, 4124 (2002)] as well as on the recent PES developed by Song et al. [J. Phys. Chem. A 113, 9213 (2009)]. The results show that the differences between these two PES are relatively minor and mostly related to the different topologies of the well. In addition, the agreement between the three theoretical methodologies is good, even for the highest total angular momenta and energies. In particular, the good accordance between the CRPs obtained with dynamical methods (QM and QCT) and the statistical model (SQCT) indicates that the reaction can be considered statistical in the whole range of energies in contrast with the findings for other prototypical barrierless reactions. In addition, total CRPs and rate coefficients in the range of 20-1000 K have been calculated using the QCT and SQCT methods and have been found somewhat smaller than the experimental total removal rates of S((1)D).

SPG7 mutational screening in spastic paraplegia patients supports a dominant effect for some mutations and a pathogenic role for p.A510V

Mutations in the SPG7 gene were initially reported in patients with autosomal recessive hereditary spastic paraplegia (HSP). Recent works suggested a dominant effect for some SPG7 mutations. To characterize the SPG7 mutational spectrum in a large cohort of Spanish HSP patients, we sequenced the whole SPG7 gene in a total of 285 Spastic Paraplegia patients. Large gene rearrangements were also ascertained in some patients. We found a total of 14 SPG7 mutations (12 new) in 14 patients; 2 were large deletions. All the mutation carriers had an adult onset age but only five (35%) had a complicated phenotype. We identified a single mutation in 13 patients. Familial analysis suggested a dominant inheritance for one (p.Leu78*) of these mutations. Carriers of the rare p.A510V variant were significantly more frequent in patients vs healthy controls (3% vs 1%), suggesting a pathogenic role for this SPG7 variant. We reported a high frequency of patients with only one SPG7 mutation, and a putative pathogenic role for the p.A510V variant.

MICRODISPERSION OF SILICA IN TIRE TREAD COMPOUNDS ABOVE THE PERCOLATION THRESHOLD BY TEM IMAGE MEASUREMENTS

Dispersion values in rubber are usually subjective values or the result of a comparison against empirical standards. we present a method to measure the filler dispersion in rubber compounds reinforced beyond the percolation threshold. Applying this method to an actual technical formulation with high filler loading, the dispersability of different silica grades, as well as the effect of coupling agent concentration in the dispersion has been showed. To carry out the measurements, the finished rubber is sliced in very thin films and then examined with transmission electron microscopy (TEM). Those images are processed to obtain information on the filler aggregation properties. As expected, the highly dispersible silica presents a smaller aggregate size and more contact surface with the rubber than the conventional and easily dispersible grades. In addition, increasing the amount of silane does diminish the aggregate size but with a lesser effect than that of the silica grade.

The dynamics of the O(1D) + HCl --> OH + Cl reaction at a 0.26 eV collision energy: a comparison between theory and experiment

The dynamics of the O((1)D) + HCl(v = 0, j = 0) --> Cl + OH reaction at a 0.26 eV collision energy has been investigated by means of a quasiclassical trajectory (QCT) and statistical quantum and quasiclassical methods. State-resolved cross sections and Cl atom velocity distributions have been calculated on two different potential energy surfaces (PESs): the H2 surface (Martinez et al. Phys. Chem. Chem. Phys. 2000, 2, 589) and the latest surface by Peterson, Bowman, and co-workers (PSB2) (J. Chem. Phys. 2000, 113, 6186). The comparison with recent experimental results reveals that the PSB2 PES manages to describe correctly differential cross sections and the velocity distributions of the departing Cl atom. The calculations on the H2 PES seem to overestimate the OH scattering in the forward direction and the fraction of Cl at high recoil velocities. Although the comparison of the corresponding angular distributions is not bad, significant deviations with a statistical description are found, thus ruling out a complex-forming mechanism as the dominant reaction pathway. However, for the ClO + H product channel, the QCT and statistical predictions are found to be in good agreement.