Neutral hydrolyses of carbon disulfide: An ab initio study of water catalysis

The water-catalyzed hydrolysis reaction of carbon disulfide (CS(2)) has been investigated at the levels of HF and MP2 with the basis set of 6-311++G(d,p) using the combined supramolecular/continuum models, in which up to six water molecules are involved in the hydrolysis and the effect of water bulk solvent is taken into account according to the polarizable continuum model (PCM). The activation Gibbs free energies in water solution, DeltaG(sol) (not equal) (298 K), for the rate-determining steps of one up to six water hydrolyses are 247.9, 184.2, 152.3, 141.8, 134.4, and 118.9 kJ/mol, respectively. The most favorable hydrolysis path of CS(2) involves a sort of eight-membered ring transition structure formed by six water molecules, among which three water molecules are not involved in the proton transfer, two near to the nonreactive sulfur atom, and one below the parent carbon disulfide. This suggests that the hydrolysis of CS(2) can be mediated with the water molecule(s) and be significantly facilitated by the cooperative effects of the water molecule(s) in the nonreactive region. The catalytic effects of water molecule(s) due to the alleviation of ring strain in the proton transfer process may result from the synergistic effects of rehybridization and charge reorganization from the prereaction complex to the rate-determining transition state structure induced by water molecule(s). PCM solvation models could significantly lower the rate-determining activation Gibbs free energies by 20-38 kJ/mol when two up to six explicit water molecules involved in the neutral hydrolysis of CS(2).     

Protease-catalyzed peptide synthesis on solid support

The direct enzymatic synthesis of peptides from amino acids is widely used as a useful alternative to chemical synthesis. However, good yields of such enzyme-catalyzed reactions require altered reaction conditions to overcome the bias for hydrolysis in aqueous medium. We argue that the synthesis/hydrolysis equilibrium can be shifted toward synthesis in aqueous medium by immobilizing the amine on solid support. In this report, we show the first examples of solid-phase peptide synthesis catalyzed by a protease in bulk aqueous buffer.     

Estimation of the surface sulfur content of cellulose nanocrystals prepared by sulfuric acid hydrolysis

The conditions required for the accurate measurement of the sulfur content of cellulose nanocrystals (CNCs) by conductometric titration are discussed. CNCs from sulfuric acid hydrolysis are electrostatically stabilized in aqueous suspension due to the introduction of charged sulfate ester groups onto the surface of the crystallites during reaction. The sulfur content thus largely reflects the surface charge of the crystals, and is crucial to the characterization and understanding of material properties. Conductometric titration is commonly used to quantify the sulfur content of CNCs, however, the exhaustive removal of free acid by dialysis and the necessity, type, quantity and duration of ion-exchange resin treatments are not always consistent. Here we explore the standard conditions of dialysis, ion-exchange, and the reproducibility of titration results. Extensive dialysis is found to be effective in the removal of free acid, but similar results are also achieved in shorter times and with less water using membrane ultrafiltration. It is also shown that the conditions of ion-exchange most commonly employed in the literature can lead to inaccurate sulfur contents. Finally, good agreement is obtained between the sulfur contents of different CNC batches prepared using the same hydrolysis conditions, and from titration and elemental analysis when thoroughly purified, well-dispersed samples, and appropriate resin conditions are used.     

Determination of aliphatic anhydrides and acids by reversed-phase liquid chromatography

Few chromatography methods have been reported for the determination of anhydrides in mixtures or as mixed anhydrides. The potential reactivity of anhydrides with water and other common eluent components complicates possible schemes for separation and analysis. By optimizing variables that affect hydrolysis, including the stationary phase, conditions can be found to successfully analyze anhydrides as reactive as acetic anhydride. The corresponding acids can be determined at the same time. The effect of the stationary phase on anhydride hydrolysis rates may prove to be a sensitive means of probing stationary phase chemistry.     

高硫炼焦煤化学结构及硫赋存形态对硫热变迁的影响

利用红外、拉曼、热重及XANES等技术对不同煤阶高硫炼焦煤的化学结构、原煤及焦样形态硫分布进行了准确判定,对煤中化学结构及硫赋存形态与硫的热变迁行为进行了关联分析。结果表明,高硫炼焦煤中硫的热变迁行为不仅与硫赋存形态有关,而且受化学结构不同的高硫炼焦煤热解挥发分释放特性的影响。较低煤阶高硫炼焦煤中脂肪结构热分解产生大量挥发分,且挥发分释放温区较宽,形态硫分解产生的活性硫与挥发分中富氢组分相结合,形成更多的含硫气体转移到气相中,提高了热解脱硫率,焦炭体相中噻吩硫相对含量高于表面,硫化物硫则与之相反。煤化程度升高,煤中稳定噻吩类硫含量增多,挥发分释放量减少,热解脱硫率降低,且形态硫在焦炭体相与表面的分布差异不明显。无机硫脱除率与黄铁矿硫分解程度直接相关,热解过程中也将形成部分新的无机硫滞留于焦中。煤结构及有机硫的赋存形态决定了有机硫脱除率,煤阶升高时有机硫脱除率明显降低。     

Production of a biomimetic Fe(I)-S phase on pyrite by atomic hydrogen beam surface reactive scattering

Molecular beam surface scattering and X-ray absorption spectroscopic experiments were employed to study the reaction of deuterium atoms with a pyrite, FeS(2) (100), surface and to investigate the electronic and geometric structures of the resulting Fe-S phases. Incident D atoms, produced by a radiofrequency plasma and expanded in an effusive beam, were directed at a pyrite surface held at various temperatures from ambient up to 200 °C. During exposure to the D-atom beam, D(2)S products were released with a thermal distribution of molecular speeds, indicating that the D atoms likely reacted in thermal equilibrium with the surface. The yield of D(2)S from the surface decreased approximately exponentially with exposure duration, suggesting that the surface accessible sulfur atoms were depleted, thus leaving an iron-rich surface. This conclusion is consistent with X-ray absorption measurements of the exposed surfaces, which indicated the formation of a layered structure, with elemental iron as the outermost layer on top of a formally Fe((I))-S phase as an intermediate layer and a formally Fe((II))-S(2) bulk pyrite layer at lower depths. The reduced Fe((I))-S phase is particularly remarkable because of its similarity to the catalytically active sites of small molecule metalloenzymes, such as FeFe-hydrogenases and MoFe-nitrogenases.     

Glycine at the pyrite-water interface: the role of surface defects

Ab initio molecular dynamics simulations were performed in order to study chemisorption, electronic properties, and desorption of glycine at wet pyrite surfaces focusing on the role of surface point defects. The change in the electronic structure and its influence on the chemical reactivity of the free FeS(2)(100) surface due to sulfur vacancies was studied in detail yielding several adsorption modes of glycine and water molecules. Energetically preferred adsorption modes were furthermore investigated in the presence of hot pressurized water mimicking "Iron Sulfur World" prebiotic conditions. The metadynamics Car-Parrinello technique was employed to map the free energy landscape including paths and barriers for desorption of glycine from such wet defective surfaces. The ubiquitous sulfur vacancies are found to increase the retention time of the adsorbed amino acid by many orders of magnitudes in comparison to the ideal pyrite-water interface. The importance of these findings in terms of a possible two-dimensional primordial chemistry on mineral surfaces is discussed.     

Mechanochemical reactions of elementary sulfur and iron sulfides with hydrogen,oxygen, and water

Mechanochemical reactions of elementary sulfur and iron sulfides with hydrogen, oxygen, and water were studied. Three reactions were discovered: (1) between elementary sulfur and dihydrogen, (2) between pyrite and dihydrogen, and (3) between elementary sulfur and water; these reactions are accompanied by hydrogen sulfide evolution. Mechanochemical synthesis of iron sulfide from the constituent elements in water involves hydrogen sulfide, which is generated by the reaction of elementary sulfur with water. We show that elementary sulfur is generated during pyrite oxidation by dioxygen during or after dispersion. In an oxygen-free medium, pyrite is more reactive to water than iron sulfide. Pyrite reacts with dihydrogen and water in an oxygen-free medium directly, avoiding the dissociation stage.     

加压热解气氛对碳酸钾催化煤热解过程中硫迁移的影响

在加压热解装置上,考察了碳酸钾及热解气氛对煤热解过程中硫分布及其形态的影响。结果表明,碳酸钾通过捕获H_2S增加了半焦硫含量,同时可将煤焦表面活化,导致煤中有机质与黄铁矿分解产生的活泼硫结合形成新的有机硫。氢气能促进煤中硫的脱除,但是碳酸钾存在下热解释放的硫一部分以K_2S的形式固定于半焦中。水蒸气可显著促进煤中黄铁矿的分解,同时可与煤焦中的K_2S反应,降低半焦中的硫含量。两段床催化气化炉中,碳酸钾催化剂经热解后不影响其对煤焦的催化性能。     

Analysis of some anhydrides as their corresponding acids by capillary electrophoresis

Summary The potential of capillary electrophoresis for the indirect determination of anhydrides was demonstrated by the analysis of five closely-related anhydrides as their corresponding acids. Direct analysis of such substances is generally difficult because of their susceptibility to hydrolysis. In this work anhydrides were first hydrolyzed in water, hydrolysis being monitored using micellar electrokinetic chromatography (MEKC) which separated neutral anhydrides and anions of corresponding acids. After hydrolysis, separation of the acids was quickly achieved within two minutes by reversed electroosmotic-flow, capillary electrophoresis (REF-CE), with UV detection at 210 nm.     

On the nature of organic catalysis "on water"

A molecular origin of the striking rate increase observed in a reaction on water is studied theoretically. A key aspect of the on-water rate phenomenon is the chemistry between water and reactants that occurs at an oil-water phase boundary. In particular, the structure of water at the oil-water interface of an oil emulsion, in which approximately one in every four interfacial water molecules has a free ("dangling") OH group that protrudes into the organic phase, plays a key role in catalyzing reactions via the formation of hydrogen bonds. Catalysis is expected when these OH's form stronger hydrogen bonds with the transition state than with the reactants. In experiments more than a 5 orders of magnitude enhancement in rate constant was found in a chosen reaction. The structural arrangement at the "oil-water" interface is in contrast to the structure of water molecules around a small hydrophobic solute in homogeneous solution, where the water molecules are tangentially oriented. The latter implies that a breaking of an existing hydrogen-bond network in homogeneous solution is needed in order to permit a catalytic effect of hydrogen bonds, but not for the on-water reaction. Thereby, the reaction in homogeneous aqueous solution is intrinsically slower than the surface reaction, as observed experimentally. The proposed mechanism of rate acceleration is discussed in light of other on-water reactions that showed smaller accelerations in rates. To interpret the results in different media, a method is given for comparing the rate constants of different rate processes, homogeneous, neat and on-water, all of which have different units, by introducing models that reduce them to the same units. The observed deuterium kinetic isotope effect is discussed briefly, and some experiments are suggested that can test the present interpretation and increase our understanding of the on-water catalysis.     

Surfactant Scavenging and Surface Deposition by Rising Bubbles

When bubbles rise through a liquid they are known to scavenge dissolved surface-active materials (surfactants). Small bubbles in the size range of tens of micrometers quickly become covered with surfactants in any but the cleanest conditions. This has the effect of immobilizing the bubble surface and affecting the drag and therefore the bubble rise speed. A large number of bubbles rising as a cloud toward a free surface will populate the bulk surface with surfactants at a richness that far exceeds that which would occur in the absence of bubbling. However, in addition to the increased deposition of surfactants on the bulk surface, the random and agitated motions of the rising bubbles induce mixing of the liquid. In a companion paper (R. L. Stefan and A. J. Szeri, submitted for publication) the mixing properties of a bubble cloud rising toward a free surface were determined. In the present work, a model for the uptake of surfactants by bubbles and subsequent deposition on the bulk surface is developed including the crucial feature of bubble-induced fluid mixing. It is found that the mixing of desorbed surfactant down into the bulk is key to predicting what will be the enrichment of the bulk surface. Copyright 1999 Academic Press.     

Molecular dynamics simulation of water in a contact with an iron pyrite FeS2 surface

A strong adsorption of the water molecules to the pyrite surface is shown by a molecular dynamic simulation of the water-iron pyrite FeS2 interface. Water molecules closest to the pyrite surface are bound by an electrostatic interaction to the iron atoms in grooves running parallel to one of the crystal axes. The grooves are about two atoms wide and are directed along 010 for the (001) surface. The position of the water-surface potential minimum and the energy of adsorption were determined by optimization for a single water molecule at the interface. At room temperature and normal density there are altogether three distinguishable layers of water above the surface. One is associated with the groove: one with H bonding to the sulphur atoms comprising the ridges separating the grooves, and the third with the soft wall boundary between the absorbed water layers and bulk region of water. Simulations were also used to explore the effect of a temperature range significant for geophysical studies.     

Preparation and spectroscopic properties of 3-acyl-1,3,4-oxadiazolines

1,3,4-Oxadiazolines I , in which substituent groups R₁ through R₄ may be varied through appropriate choice of precursor reactants, can be prepared in good yields and purity by convenient cyclizations of acyl-hydrazones with anhydrides. The reactions can be effectively monitored by salient changes in the infrared spectra. The oxadiazolines are stable materials and thus serve as suitable derivatives for the chemical characterization of such tuberculostatic Schiff bases as IIa-c.     

The origin of thermodynamic stability of microemulsions

The adsorption of surfactant and cosurfactant on the surface of the globules decreases the interfacial tension between oil and water to very low values. In addition, the decrease of the bulk concentrations of the surfactant and cosurfactant decreases their chemical potential both in the bulk and at the interface, thus decreasing the free energy of the system (dilution effect). The thermodynamic stability of microemulsions is due to the fact that the total free energy change caused by these effects can become negative. The theory can explain the occurence of stable microemulsions for both non-ionic and ionic surfactants.     

A review of the fundamental studies of the copper activation mechanisms for selective flotation of the sulfide minerals, sphalerite and pyrite

A review of the considerable, but often contradictory, literature examining the specific surface reactions associated with copper adsorption onto the common metal sulfide minerals sphalerite, (Zn,Fe)S, and pyrite (FeS(2)), and the effect of the co-location of the two minerals is presented. Copper "activation", involving the surface adsorption of copper species from solution onto mineral surfaces to activate the surface for hydrophobic collector attachment, is an important step in the flotation and separation of minerals in an ore. Due to the complexity of metal sulfide mineral containing systems this activation process and the emergence of activation products on the mineral surfaces are not fully understood for most sulfide minerals even after decades of research. Factors such as copper concentration, activation time, pH, surface charge, extent of pre-oxidation, water and surface contaminants, pulp potential and galvanic interactions are important factors affecting copper activation of sphalerite and pyrite. A high pH, the correct reagent concentration and activation time and a short time delay between reagent additions is favourable for separation of sphalerite from pyrite. Sufficient oxidation potential is also needed (through O(2) conditioning) to maintain effective galvanic interactions between sphalerite and pyrite. This ensures pyrite is sufficiently depressed while sphalerite floats. Good water quality with low concentrations of contaminant ions, such as Pb(2+)and Fe(2+), is also needed to limit inadvertent activation and flotation of pyrite into zinc concentrates. Selectivity can further be increased and reagent use minimised by opting for inert grinding and by carefully choosing selective pyrite depressants such as sulfoxy or cyanide reagents. Studies that approximate plant conditions are essential for the development of better separation techniques and methodologies. Improved experimental approaches and surface sensitive techniques with high spatial resolution are needed to precisely verify surface structures formed after copper activation. Sphalerite and pyrite surfaces are characterised by varying amounts of steps and defects, and this heterogeneity suggests co-existence of more than one copper-sulfide structure after activation.     

Analytical solutions to mathematical models of the surface and bulk erosion of solid polymers

The process by which polymeric materials hydrolyze and disappear into their environments is often called erosion. Two types of erosion have been defined according to how the hydrolysis takes place. If hydrolysis occurs throughout the entire specimen at the same time, it is called bulk erosion. If the hydrolysis is mainly confined to a region near the surface of the specimen and the surface continuously degrades by moving inward, it is termed surface erosion. In this article, a kinetic relationship for bulk erosion is developed. This relationship provides a method for estimating the hydrolysis kinetic constants for bulk‐eroding polymers. This same relationship is also applicable to surface erosion at a microscopic level. Through its combination with a diffusion–reaction equation and the provision of moving boundary conditions, an analytical solution to the steady‐state surface‐erosion problem is obtained. The erosion rate, erosion front width, and induction time can all be expressed as simple functions of the rate of polymer bond hydrolysis, water diffusivity, and solubility, plus other parameters that can be experimentally determined. The erosion front width is the product of the induction time and the erosion rate. The ratio of the erosion front width to the polymer specimen thickness is a parameter that determines whether the specimen undergoes surface or bulk erosion. Theoretical results are compared with experimental observations from the literature, and agreement is found. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 383–397, 2005     

Effects of chromium ion on sulfur removal during pyrolysis and hydropyrolysis of coal

The effects of impregnated Cr³⁺ on sulfur removal during pyrolysis and hydropyrolysis of coal were investigated by loading CrCl₃ into raw, demineralized and pyrite removed coal, respectively. The results indicate that Cr has no effect on the removal of pyrite. Cr affects the removal of total sulfur by forming Cr₇S₈ and affecting the removal of organic sulfur. Cr acts as the sulfur removing agent by promoting the decomposition of the unstable organic sulfur at low temperature. However, it behaves to be sulfur fixing agent between 400 and 700 °C so as to inhibit the evolution of H₂S, even in hydropyrolysis. With the increase of temperature from 700 to 1050 °C, a certain ratio of Cr₇S₈ is converted into organic sulfur during pyrolysis; however, almost all the Cr₇S₈ is reduced into Cr at 1050 °C during hydropyrolysis. And Cr significantly promotes the removal of organic sulfur at high temperature within reducing atmosphere. The XPS results indicate that the sulfur is enriched on coke surface by Cr, which is attributable to the formation of Cr₇S₈ as well as the transfer of organic sulfur from bulk to surface during pyrolysis and hydropyrolysis.     

The Synthesis of a Novel Phosphorus Containing Antigen

The hydrolysis of peptide is one of the most important chemical processes in life chemistry. It is of great significance to study catalytic antibody, which is capable of catalyzing the hydrolysis on a specific peptide bond. In recent years, we have successfully synthesized some tetrahedral geometry analogues mimicking that of the hippuryl phenylalanine 1 hydrolyzing transition state catalyzed by carboxypeptidase A(CPA)1,2, which is specific for cleavage of the C-terminal amino acid from an o…     

Surface-assisted assembly of an ionic-complementary peptide: controllable growth of nanofibers

Numerous studies have shown that a surface can direct and regulate molecular assembly. In this study, the nanofiber growth of an ionic-complementary peptide, EAK16-II, on a mica surface was investigated under various solution conditions via in situ atomic force microscopy. In comparison to the assembly in bulk solution, nanofiber growth of EAK16-II on mica is surface-assisted and involves two steps: (1) adsorption of nanofibers and fiber clusters (from the bulk solution) on the surface, serving as the "seeds"; (2) fiber elongation of the "seeds" from their active ends. The nanofiber growth can be controlled by adjusting the solution pH since it modulates the adsorption of the "seeds" on mica and their growth rates. The amount of the adsorbed "seeds" decreases with increasing solution pH, while the growth rate under different solution conditions is found to follow the order pure water > 1 mM HCl > 1 mM NaOH > 10 mM HCl approximately 10 mM NaOH approximately 0. The pH-dependent nanofiber growth is due to the surface charge of the peptides and peptide assemblies in various solutions as indicated by zeta-potential measurements. A simple model was proposed to describe surface-assisted nanofiber growth. This study provides insights into the assembly of peptide/protein on a surface, which is essential to understand such physiological protein aggregation systems as amyloid fibrillogenesis. In addition, the potential of this finding to construct biocompatible electrodes for biomolecular sensing is also discussed.