Electrocatalytic oxygen evolution from water on a Mn(III-V) dimer model catalyst--a DFT perspective

A complete water oxidation and oxygen evolution reaction (OER) cycle is monitored by means of density functional theory (DFT). A biomimetic model catalyst, comprising a μ-OH bridged Mn(III-V) dimer truncated by acetylacetonate ligand analogs and hydroxides is employed. The reaction cycle is divided into four electrochemical hydrogen abstraction steps followed by a series of chemical steps. The former employ the tyrosine/tyrosyl redox couple acting as electron and proton sink, thus determining the reference potential. Stripping hydrogen from water leads to the formation of two highly unstable Mn(V)=O/Mn(IV)-O˙ moieties, which subsequently combine to form a μ-peroxy O-O bond. O(2) evolution results from subsequent consecutive replacement of the remaining Mn-O bonds by water. A Zener "spintronic" type mechanism for virtually barrierless O(2) evolution is found. The applicability of DFT is discussed and extended to include the rate-limiting steps in the OER. Rather than attempting to compute transition states where KS-DFT is unreliable, an upper bound for the activation barrier of the O-O bond formation step is estimated from the hessians of the relevant intermediates.     

Temperature-induced changes in the composition of floatglass surfaces

Floatglass is an important kind of commercial glass, comprising the main body of modern flat glass used in buildings and vehicles. The stoichiometry of the surface layer differs from that of the bulk and will usually change due to subsequent high-temperature process steps or attack by water or humidity. Glass samples have been investigated by means of ion beam analysis. Using resonant nuclear reaction analysis (¹⁵N technique), hydrogen concentration profiles have been determined. Profiles of the heavier glass constituents, especially sodium and tin, have been obtained by RBS. Changes in the composition of the floatglass surfaces caused by subsequent temperature treatment (up to 700^°C) and by controlled hydration treatment are reported. Possible mechanisms of hydrogen uptake and release are discussed.     

多壁碳纳米管的纯化

用900℃高温氢气处理以及5 mol/L盐酸回流的方法，纯化了一种由甲烷在Ni- Mg-O催化剂上裂解生长的多壁碳纳米管（MWCNTs），考察了不同纯化阶段MWCNTs的 吸水率、比表面积、Ni和Mg残留量以及在不同温度下苯、正己烷、乙醇、丙酮四种 化合物在MWCNTs填充色谱柱上的脱附率的变化，并用透射电镜观察了MWCNTs的形态 。结果表明，高温氢气处理可去除MWCNTs的无定形碳和表面极性基团，使其比表面 积和吸水率减小，同时可打开MWCNTs端口。高温氢气处理后，再用盐酸回流即容易 去除MWCNTs中单用盐酸回流方法无法去除的Ni.经过纯化的MWCNTs的吸水率远小于 活性碳，比Carbopack B稍大，比表面积和Carbopack B相近。苯、正己烷、己醇、 丙酮四种化合物在纯化的MWCNTs填充色谱柱上的脱附率和Carbopack B的相同。经 纯化的MWCNTs的Ni残留量为30 μg/g，Mg残留量低于检测限（10 μg/g）。由于管 腔的存在，纯化的MWCNTs对有机物的保留能力大于Carbopack B。它可以作为气相 色谱固定相和吸附挥发性有机物的吸附剂。     

Low-frequency vibrational spectrum of water in the hydration layer of a protein: a molecular dynamics simulation study

An atomistic molecular dynamics simulation has been carried out to understand the low-frequency intermolecular vibrational spectrum of water present in the hydration layer of the protein villin headpiece subdomain or HP-36. An attempt is made to explore how the heterogeneous rigidity of the hydration layers of different segments (three alpha helices) of the protein, strength of the protein-water hydrogen bonds, and their differential relaxation behavior influence the distribution of the intermolecular vibrational density of states of water in the hydration layers. The calculations revealed that compared to bulk water these bands are nonuniformly blue-shifted for water near the helices, the extent of shifts being more pronounced for water molecules hydrogen bonded to the protein residues. It is further noticed that the larger blue shift observed for the water molecules hydrogen bonded to helix 2 residues correlates excellently with the slowest structural relaxation of these hydrogen bonds. These results can be verified by suitable experimental measurements.     

Aquaperoxovanadium(V) complexes: Quantum-chemical study

The addition of a water molecule to mono-, di-, and triperoxovanadium(V) complexes has been studied at the density functional theory (B3LYP/6-31G**) and Møller-Plesset perturbation theory (MP2/6-31G**) levels. It has been demonstrated that the H₂O…V donor-acceptor interaction cannot compete with hydrogen bonds and becomes weaker with an increase in the number of peroxo groups in the complex. In the most stable isomers of aquaperoxo complexes, water is mainly held by intermediate hydrogen bonds. The energy of addition of a water molecule to peroxovanadate is lower than or close to the heat of evaporation of water; i.e., the formation of stable aquaperoxovanadium complexes in aqueous solutions is improbable. This conclusion is consistent with the mass spectra of aqueous solutions of peroxovanadates, which show that the concentration of water-free peroxo complexes considerably exceeds the concentration of complexes with a coordinated water molecule. The coordination of the water molecule through the V…OH₂ donor-acceptor interaction is prevented by the cis effect of the peroxo group, which has the HOMO orbital of symmetry suitable for interaction with the LUMO orbital of the VO group.     

On the mechanism of hydrogen evolution at GaAs electrodes

The hydrogen evolution reaction at n- and p-GaAs electrodes has been reinvestigated. As in the case of metal electrodes, hydrogen evolution can occur in two ways: at ?0.5 V (SCE) hydronium ions are reduced, at ?1.25 V (SCE) reduction of water molecules takes place. It is confirmed that in both cases conduction band electrons are responsible for the two reduction steps, forming adsorbed hydrogen atoms in the first and hydrogen molecules in the second step. Hole injection can occur only to a negligible extent, although it appears energetically feasible.     

From Isocyanides via Four-Component Condensations to Antibiotic Syntheses

The four-component condensation of amines and carbonyl compounds with isocyanides

1 Previously, isocyanides were almost always referred to as isonitriles. However, according to IUPAC Rule C-833.1 only the name “isocyanides” is permissible.

and suitable acid components (water, thiosulfuric acid, hydrogen selenide, hydrogen azide, cyanic acid, thiocyanic acid, carboxylic acids, methyl hydrogen carbonate) to form α-amino acid derivatives was discovered in 1959. This reaction principle shares some features with the Strecker synthesis and the Passerini reaction. The four-component condensation affords easy and effective one-pot synthesis of complex molecules from simple building blocks. Only in recent years, however, have the preparative advantages of the four-component condensation been exploited by numerous authors in the synthesis of diverse natural products and related compounds, although many of the possibilities opened by this principle were recognized more than two decades ago. In this progress report some instructive syntheses of various antibiotics are reviewed. The design of each of these syntheses involves a four-component condensation as key step, by means of which lengthy sequences of reactions are avoided, which otherwise would be required to achieve the synthetic goal.     

The Role of the Reactor Wall in Hydrothermal Biomass Conversions

The processing of renewable feedstocks to platform chemicals and, to a lesser degree, fuels is a key part of sustainable development. In particular, the combination of lignocellulosic biomass with hydrothermal upgrading (HTU), using high temperature and pressure water (HTPW), is experiencing a renaissance. One of the many steps in this complicated process is the in‐situ hydrogenation of intermediate compounds. As formic acid and related low‐molecular‐weight oxygenates are among the species generated, it is conceivable that they act as a hydrogen source. Such hydrogenations have been suggested to be catalyzed by water, by bases like NaOH, and/or to involve “reactive/nascent hydrogen”. To achieve the temperatures and pressures required for HTU, it is necessary to conduct the reactions in high‐pressure vessels. Metals are typical components of their walls and/or internal fittings. Here, using cyclohexanone as a model compound for more complex biomass‐derived molecules, iron in the wall of high‐pressure stainless steel reactors is shown to be responsible for the hydrogenation of ketones with low‐molecular‐weight oxygenates acting as a hydrogen source in combination with water.     

Synthesis of fluorescent long-chain thiols/disulfides as building-blocks for self-assembled monolayers preparation

New symmetrical disulfides together with the corresponding thiols bearing fluorescent end-groups have been synthesized as building-blocks for self-assembled monolayers (SAMs). The synthesis has been accomplished starting from aromatic nitrogen heterocycles in three steps. The conversion of the tosylated intermediate into the final disulfide is accomplished by use of sodium hydrogen sulfide (NaSH). Both products (thiols and disulfides) were isolated and characterized.      

Synthesis and properties of cationic surfactants with tuned hydrophylicity

A series of pyridinium-based cationic surfactants has been synthesised and their amphiphilic properties have been studied by conductivity and surface tension measurements. The modification of the substitution pattern on the pyridinium ring by hydrophobic moieties (methyl vs. hydrogen and presence or not of condensed benzene ring) gave the opportunity to investigate structure–activity relationships. Characterization by conductivity and surface tension measurements shed light on the behaviour at the air/water interface and in the micellar environment. In particular, the tendency to form ion pairs at very low concentration was evidenced for all the surfactants substituted on the ring, but not for the simple pyridinium ones. The formation of ion pairs affects both the conductivity and the surface tension plots, showing that a series of steps is involved during the adsorption to the air/water surface. An attempt was made to qualify the single steps in the adsorption at the surface layer. Those steps were attributed to different chemical species (free surfactant ions or ion pairs) and to different arrangements of the surfactant. This work also represents a contribution of investigation at very low surfactant concentrations and high surface tension values.     

Aliphatic Ketones as Photoinitiators for Photografting

Summary: The photografting of methacrylic acid (MAA) onto high‐density polyethylene (HDPE) initiated by butanone, pentan‐3‐one, and heptan‐3‐one is presented. These aliphatic ketones can act as photoinitiators for photografting when they are a component of a suitable solvent mixture with water and ethanol. It is possible that the photoinitiation effect of the aliphatic ketone is induced by the hydrogen bond formed between it and water.

The grafting of 2 M MAA in different ketone/water/ethanol (30/30/40) solvents.     

On the mechanism of low-temperature water gas shift reaction on copper

Periodic, self-consistent density functional theory (DFT-GGA) calculations are used to investigate the water gas shift reaction (WGSR) mechanism on Cu(111). The thermochemistry and activation energy barriers for all the elementary steps of the commonly accepted redox mechanism, involving complete water activation to atomic oxygen, are presented. Through our calculations, we identify carboxyl, a new reactive intermediate, which plays a central role in WGSR on Cu(111). The thermochemistry and activation energy barriers of the elementary steps of a new reaction path, involving carboxyl, are studied. A detailed DFT-based microkinetic model of experimental reaction rates, accounting for both the previous and the new WGSR mechanism show that, under relevant experimental conditions, (1) the carboxyl-mediated route is the dominant path, and (2) the initial hydrogen abstraction from water is the rate-limiting step. Formate is a stable "spectator" species, formed predominantly through CO2 hydrogenation. In addition, the microkinetic model allows for predictions of (i) surface coverage of intermediates, (ii) WGSR apparent activation energy, and (iii) reaction orders with respect to CO, H2O, CO2, and H2.     

Mikrobestimmung von monosubstituierten Acetylenverbindungen durch Bestimmung des aktiven Wasserstoffs mit Alanat

1. The determination of monosubstituted acetylenic compounds was investigated at various temperatures (?60° to +95°C) by the alanate method described byMerz.
2. At 90°C all active hydrogen atoms were determined with sufficient accuracy and good precision, but at 0° C the active hydrogen atom of the ethinyl group in monosubstituted acetylenes did not react. This is the basis of a method for determining the acid hydrogen in the ethinyl group: the active hydrogen reactive at 0° C is first determined, then the temperature is raised to 90° C, when all the remaining active hydrogen atoms are determined.
3. The Merz method for determining active hydrogen is in our opinion the most suitable chemical method for determining ethinylcarbinols.
4. It has been shown that active hydrogen atoms react faster with alanate (sodium diethyldihydridoaluminate) than with Grignard reagent. The active hydrogen atoms in certain compounds do not react quantitatively with Grignard reagent within the normal tune. The alanate method is thus more generally applicable than the Grignard method.

     

Chemical effects induced by gamma-irradiated alkali halides in organic emulsions

Dissolution of -irradiated alkali halides in the emulsions of aromatic hydrocarbon and water results in the formation of halogen charge transfer complexes and hydrogen. These have been identified by spectrophotometry. Further, their formation has been verified by studying the absorption of a chemical model involving hydrogen incorporated in halogen complex. These products are correlated to the F and hole centers of the irradiated salts.     

Effect of grinding on dehydration of crystal water of theophylline

The effect of grinding on the dehydration of crystal water of theophylline has been studied. It was observed that the water content of theophylline hydrate decreased with increased grinding time. As the grinding time proceeded, the results of differential scanning calorimetry (DSC) indicated that crystal water of ground theophylline hydrate dehydrated in three steps at ca. 58, 44, and 17 degrees C, respectively. Powder X-ray diffraction study revealed that the crystal lattice of theophylline monohydrate collapsed by grinding, and part of the theophylline molecules subsequently rearranged the collapsed lattice to form theophylline anhydrate. The result of Fourier transformed infrared spectroscopy demonstrated that the hydrogen bonds between crystal water molecules and theophylline molecules were weakened or destroyed to some extent by grinding. It was supposed that crystal water in the ground theophylline hydrate might exist at least in three molecular states of different hydrogen-bonding. From DSC study, it was suggested that the ruptured hydrogen bonds of water molecules in the ground theophylline hydrate were strengthened after storage under 96.5% relative humidity at 30 degrees C.     

Artificial photosynthesis: from molecular catalysts for light-driven water splitting to photoelectrochemical cells

Photosynthesis has been for many years a fascinating source of inspiration for the development of model systems able to achieve efficient light-to-chemical energetic transduction. This field of research, called "artificial photosynthesis," is currently the subject of intense interest, driven by the aim of converting solar energy into the carbon-free fuel hydrogen through the light-driven water splitting. In this review, we highlight the recent achievements on light-driven water oxidation and hydrogen production by molecular catalysts and we shed light on the perspectives in terms of implementation into water splitting technological devices.     

Automatic isotope gas analysis of tritium labelled organic materials

An analytical procedure and an automatic apparatus are described for the determination of tritium in organic compounds by gas counting. The sample is pyrolysed in hydrogen atmosphere at 1000°C, then, with hydrogen, the decomposition products are rinsed through a column of molecular sieve-5A heated to 550°C. Tritium in water, hydrogen sulphide, ammonia and hydrogen cyanide is transferred into the hydrogen stream by isotope exchange completed on the column. The inactive water vapor, hydrogen sulphide, ammonia and hydrogen cyanide as well as carbon dioxide are removed from the gas stream by appropriate absorbents, and the radioactive hydrogen together with tritiated methane, carbon monoxide and nitrogen included in the pyrolytic products is led into an internal proportional counter tube for radioactivity measurement. The method provides quantitative recovery, it is free of memory effect and suitable for the rapid assay of a wide variety of organic compounds containing C, H, N, O, S in addition to tritium.     

Interactions between organic molecules and water: rotational spectrum of the 1:1 oxetane-water complex

The 1:1 molecular complex between oxetane and water has been investigated by using free-jet millimeter-wave spectroscopy. The rotational spectra of five isotopomers (with H(2)O, D(2)O, DOH, HOD and H(2) (18)O) have been assigned. Partial r(0) and r(s) structures of the complex have been derived. The water moiety lies in the plane of symmetry of oxetane, with the "free" hydrogen E with respect to the ring. The oxetane ring appears to be slightly nonplanar, with the C(beta) carbon tilted on the opposite side of the water unity. The three atoms involved in the hydrogen bond adopt a linear arrangement with an O(ring).H distance of about 1.86 A, and the angle between the COC bisector and the O(ring).H bond being congruent with 106 degrees. Additionally, quantum-chemical calculations for the complex were performed and were found to be in agreement with the experimental results.     

Gas-phase Diels-Alder cycloaddition of benzyne to an aromatic hydrocarbon bay region: Groundwork for the selective solvent-free growth of armchair carbon nanotubes

Benzyne, generated in the gas phase by pyrolysis of phthalic anhydride, has been shown to undergo Diels-Alder cycloaddition to the bay region of perylene, a typical polycyclic aromatic hydrocarbon, under solvent-free conditions in a high temperature flow system. The initial cycloadduct spontaneously loses two hydrogen atoms, thereby rearomatizing to give naphtho[1,2,3,4-ghi]perylene. Analogous Diels-Alder cycloadditions of benzyne to bay regions on the rims of suitable cylindrical hydrocarbon templates, when followed by rearomatizations and thermal cyclodehydrogenations to join adjacent benzo groups, are proposed as key steps for a directed chemical synthesis of uniform diameter armchair carbon nanotubes.