Island formation in chemisorption 2. Chemisorption kinetics

Influence of “direct” and “indirect” pathways of dissociative chemisorption on the form of kinetic dependences ϕ(MCS) and S(ϕ) was studied by the Monte Carlo method. Langmuir adsorption observed at S_indir/S_dir≤0.1 gradually changes to island-mediated adsorption with an increase of the “indirect” adsorption contribution at 0.1≤S_indir/S_dir≤1.0. At S_indir/S_dir≥1.0 the island-mediated adsorption dominates: large adsorption islands arise and gradually grow.     

Synthesis of a “memory tripeptide” (Arg—Glu—Arg,RER) and the Kabachnik—Fields reaction with di- and tripeptides as a method for the synthesis of phosphorus-containing peptide analogs

Methods for the synthesis of potential “twin-drugs” containing fragments of the glutamate receptor antagonist and cognitive function enhancing oligopeptides were developed. The “memory tripeptide” Arg—Glu—Arg (RER) containing the tripeptide sequence of a protein APP_328–330, a gB-amyloid precursor, was synthesized. A method for the synthesis of gA-aminophosphonates with oligopeptides as the amine component of the one-pot three-component Kabachnik—Fields reaction was developed. A method for the synthesis of phosphonopeptides by the introduction of gA-aminophosphonates into the peptide chain was proposed.     

Oxidation of alkanols into “symmetric” esters with the system Ce(SO<Subscript>4</Subscript>)<Subscript>2</Subscript>—LiBr

A method for the synthesis of “symmetric” esters based on the oxidation of primary straight-and iso-chain aliphatic alcohols C₆ —C₁₁ with the system Ce(SO₄)₂—LiBr in water has been suggested.     

Density of the systems (NaF/AlF<Subscript>3</Subscript>)—AlPO<Subscript>4</Subscript> and (NaF/AlF<Subscript>3</Subscript>)—NaVO<Subscript>3</Subscript>

The densities of two systems of molten mixtures (NaF/AlF₃)—NaVO₃ and (NaF/AlF3)—AlPO₄ were measured using an Archimedean method. Each molten mixture contained varying amounts of “impurity” constituents. The measurements were performed at various NaF/AlF₃ mole ratios (CR, cryolite ratio), equal to 3, 2.5, 2, and 1.5.     

Protein-like copolymers: Computer simulation

The model ofAB copolymers with a “protein-like” primary sequence was developed. This type of copolymers was obtained in a computer experiment. First, the conformation of a collapsed dense homopolymer globule was generated and then, based on this conformation, the primaryAB sequence was determied by denoting the monomeric units located near the surface of the globule as unitsA and those constituting the core of the globule as unitsB. After that, the primary structure of the chain was fixed, and different interaction potentials for theA andB units were introduced. Drawing an analogy of this model to aqueous solutions of globular proteins,A units were interpreted as hydrophilic, andB units were regarded as hydrophobic. By means of Monte Carlo simulation using the bond fluctuation model, the coli—globule transition in “protein-like”AB copolymer, induced by an increase in the attraction between the hydrophobicB units, was studied. The coil—globule transition in a copolymer with the “protein-like” primary sequence occurs at a higher temperature and has higher rate and is sharper than that in a random copolymer with the sameA/B composition and in a random block copolymer with the sameA/B composition and the same “degree of blockiness”. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 884–889, May, 1998.     

Evaluating molecular asymmetry

A method for calculating the asymmetry parameters of molecules based on Avnir’s CSM approach combined with the “dissymmetry function” method is suggested. The performance of the approach is demonstrated on various geometrical models — high-symmetry antiprisms of S₁₀ and D₅ symmetry groups, helices, and molecular objects. It is shown that the MCSM method unambiguously determines the symmetry element or estimates the degree of asymmetry for molecules from different structural classes. A. V. Bogatskii Physiocochemical Institute, Ukrainian Academy of Sciences. Odessa State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 39, No. 3, pp. 547–552, May–June, 1998.     

Synthesis, structures, and spectral properties of biomimetic azomethine metal chelates with chromophores CuN2S2, CuN2O2, and CuN2Se2. Crystal structure of bis[4-(benzyl)aldimino-3-methyl-1-phenyl-5-pyrazolothiolato]copper(II)

Copper(II) chelates of composition CuL₂ were synthesized based on 4-aminomethylene derivatives of 5-thiopyrazoles (LH). The complexes were studied by UV, IR, ESR, and EXAFS spectroscopy, magnetochemistry, and X-ray diffraction analysis. The coordination polyhedra in the complexes are pseudotetrahedra or octahedra of the types CuN₂S₂ or CuN₄S₂, respectively, which are distorted due to the Jahn—Teller effect. The UV and ESR spectra of copper chelates with a six-coordinate metallocycle formed by the N and S atoms of the azomethine ligand and the nitrogen atom of the quinoline substituent (R) of the C=N−R fragment are most similar to the spectra observed for metals involved in the active centers of natural metalloenzymes (“blue” copper proteins). Published inIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1891–1896, November, 2000.     

Review of the molar heats of chemisorption and chemabsorption by crystalline oxides and the surface “homogeneity versus heterogeneity” problem

To obtain a solution of the surface “homogeneity versus heterogeneity” problem, the results of microcalorimetric measurements of the dependences of the molar heats of chemisorption and chemabsorption of different gases on the amounts of chemisorbed or chemabsorbed gases in more than 20 gas/metal-oxide systems, in which the molar chemisorption heats are coverage-independent over rather wide ranges of the surface coverages, are presented. In order to approach the states of the metal oxide samples to those in real catalytic processes catalyzed by these oxides, the coverage dependences of the heats of chemisorption of gases at the samples were measured for a number of gas/metal-oxide systems against the chemisorbed amounts of not only the gas under study but also of another gas chemisorbed previously. The calorimetric data-set is supplemented with data obtained by other methods capable of helping to solve the surface “homogeneity versus heterogeneity” problem. These data are discussed together with the data on chemisorption in more than 40 gas/metal systems for which homogeneity of the surfaces was stated in our previous review. The entire set of the measurements was published for several decades by about 40 different composite authors. The chemisorption and chemabsorption mechanisms are discussed. It is concluded that thermally stabilized powder metal and metal oxide surfaces are homogeneous relative to the chemical ability of their atoms in chemisorption and catalytic processes in line with Langmuir’s opinion and the band theory of solids.     

Standardization of pH measurements based on the ion interaction approach

The Pitzer method was used to calculate the pH values on the conventional and “true” scales for the TRIS—TRIS·HCl−NaCl−H₂O buffer system in the 0–40 °C temperature region and 0–4 NaCl molality interval. This buffer can be used as a standard for pH measurements in a wide range of ionic strengths. The conventional scale is used in cells without a salt bridge. The “true” scale is recommended for pH measurements using cells with a salt bridge. At the same concentrations of the buffer solution, the “true” scale is essentially transformed into the scale of the National Bureau of Standards (NBS) of the USA. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 676–680, April, 2000.     

Quantum-chemical investigation of the mechanisms of oxidation of dimethyl sulfide by hydrogen peroxide and peroxoborates

It was established by the DFT method in the B3LYP/6-311G-d,p approximation that the oxidation of dimethyl sulfide (Me₂S) by peroxides (XOOH) can take place by two mechanisms depending on the nature of X. In the reaction of Me₂S with hydrogen peroxide (X = H) the direct reagent is the HOOH molecule while in the reactions with monoperoxoborate [X = B(OH)₃] and diperoxoborate [X = B(OH)₂OOH] it is a reagent containing the “water oxide” fragment X—(⁺OH)—O^–.     

Internal and external eigenvalue problems of Hermitian operators and their use in electronic structure theory

Within the fragment resolution of molecular systems the conceptual and interpretative advantages of using the separate eigenvalue problems for the internal and external part of the Hermitian matrix representing a physical quantity in quantum mechanics are examined. By definition, these two parts accordingly combine only the diagonal and off-diagonal subsystem-resolved blocks of matrix elements. These two partial eigenvalue problems bring about the matrix internal or external decouplings, respectively, which have recently been used in several interpretations of the molecular electronic structure. A character and structure of the external eigensolutions is examined in some detail and their recent applications in the Charge Sensitivity Analysis—to extract the most important electron-transfer effects between constituent atoms of model chemisorption systems, and in the Molecular-Orbital theory—to precisely identify the inter-orbital flows of electrons, are summarized and commented upon. The grouping relation, for combining the external/internal eigensolutions into those for the whole matrix, is derived in the context of the complementary “rotations” of the basis set vectors.     

Coordination of gold(III) in the chemisorption by cadmium diisobutyldithiocarbamate: Structure and thermal properties of the polynuclear gold-cadmium complex [Au“S2CN(iso-C4H9)2”2]2n[CdCl4]n

Gold(III) is coordinated by binuclear cadmium diisobutyldithiocarbamate (Dtc) via chemisorption to give a heteropolynuclear Au(III)-Cd complex of the formula [Au“S₂CN(iso-C₄H₉)₂”₂]_2n [CdCl₄] _n (I). According to X-ray diffraction data, structure I contains three structurally nonequivalent complex cations [Au“S₂CN(iso-C₄H₉)₂”₂]⁺. These cations are conformers. Relatively weak nonvalence interactions produce zigzag polymer chains of the type (...C...A...B...A...) _n with alternating nonequivalent cations A, B, and C in a ratio of 2: 1: 1. The anions [CdCl₄]²⁻ are localized at the side. The calculated theoretical chemisorption capacity of cadmium Dtc with respect to [AuCl₄]⁻ is 378.0 mg of Au³⁺ per gram of the sorbent. To optimize the conditions for isolation of sorbed gold, the thermal properties of complex I were studied by simultaneous thermal analysis. The two-step thermolysis of complex I involves (1) thermal decomposition of the dithiocarbamate part and [CdCl₄]²⁻ with liberation of metallic gold and cadmium dichloride and (2) evaporation of CdCl₂. The final thermolysis product of complex I is reduced metallic gold.     

The study of alanine oscillating reaction catalyzed by tetraazamacrocyclic nickel(II) complex

A closed oscillation system comprised of alanine, KBrO₃, H₂SO₄ and acetone catalyzed by tetraazamacrocyclic nickel(II) complex is introduced, and quantitatively characterized with kinetic parameters, namely the rate constant (k _in, k _p), the apparent activation energy (E _in, E _p) and pre-exponential constant (A _in, A _p) and thermodynamic functions (ΔH _in, ΔG _in, ΔS _in and ΔH _p, ΔG _p, ΔS _p), where indexes “in” and “p” mean “induction period” and “oscillation period,” respectively. The results indicate that tetraazamacrocyclic nickel(II) complex can catalyze alanine oscillating reaction and the reaction corresponds exactly to the feature of irreversible thermodynamics as the entropy of system is negative.     

Hydrogen-induced metal-oxide interaction studied on noble metal model catalysts

Summary The effect of hydrogen reduction on the structure and catalytic properties of “thin film”and “inverse”model systems for supported metal catalysts is discussed. Thin film model catalysts were obtained by epitaxial growth of Pt and Rh nanoparticles on NaCl(001), which were coated with amorphous or crystalline supports of alumina, silica, titania, ceria and vanadia. Structural and morphological changes upon hydrogen reduction between 473 and 973 K were examined by high resolution electron microscopy. Metal-oxide interaction sets in at a specific reduction temperature and is characterized by an initial “wetting”stage, followed by alloy formation at increasing temperature, in the order VO_x< TiO_x< SiO₂< CeO_x< Al₂O₃. “Inverse”model systems were prepared by deposition of oxides on a metal substrate, e.g. VO_x/Rh and VO_x/Pd. Reduction of inverse systems at elevated temperature induces subsurface alloy formation. In contrast to common bimetallic surfaces, the stable subsurface alloys of V/Rh and V/Pd have a purely noble metal-terminated surface, with V positioned in near-surface layers. The uniform composition of the metallic surface layer excludes catalytic ensemble effects in favor of ligand effects. Activity and selectivity, e.g. for CO and CO₂methanation and for partial oxidation of ethene, are mainly controlled by the temperature of annealing or reduction. Reduction above 573 K turned out to be beneficial for the catalytic activity of the subsurface alloys, but not for the corresponding thin film systems which tend to deactivate viaparticle encapsulation.</o:p>     

Finding the possible mechanisms for a given type of overall reaction

A systematic way to derive all thea priori possible mechanisms classified according to the numbers, ϱ, of elementary or molecular reaction steps was presented recently. The method is now applied to overall reactions of the type A +B ⇒ C+D. The “laminar” (elementary steps' stoichiometric coefficients unity) mechanisms that turn out possible are derived and listed for the important ϱ = 2 case. The ϱ = 3 three step mechanisms were reported in another paper. Many examples from actual chemical, biochemical mechanisms, such as S_E1, S_N1, S_E2, S_N2, activated complex theory of abstraction reactions, free radical chain propagation and biochemical electron transport chains are given. By far most mechanisms usually encountered are of the laminar type, although the method also gives the “turbulent” ones, i.e. with some stoichiometric coefficients in elementary steps larger than one or some species occurring in many of the steps. Alexander von Humboldt Senior Scientist Awardee, West Germany. Predoctoral Research Assistant, Yale University 1973–74. Then at the Roswell Park Memorial Institute, Department of Experimental Therapeutics, Buffalo, New York 14203.     

Chemical origin of the sustained-like pattern formation observed in the bromate — Dual substrate — Dual catalyst oscillatory batch system

The BrO ₃ ⁻ — BrAc — Ru(bpy) ₃ ²⁺ subsystem is shown to represent the core oscillator that serves as source of the long lasting temporal and spatial periodic behaviors observed in the BrO ₃ ⁻ — H₂PO ₂ ⁻ — acetone — Mn²⁺ — Ru(bpy) ₃ ²⁺ — acid “double substrate-double catalyst” oscillatory batch system. The BrAc — the substrate of the core oscillator — is formed and accumulated in the reactions taking place in the six-component system. BrAc was produced in a separate experiment with bromide, acetone, acid and excess bromate and the mixture was used for bringing about patterns in the thin solution layer after adding the Ru(bpy) ₃ ²⁺ catalyst. The two-dimensional reaction-diffusion patterns that appear in the subsystem and its parent system are very similar in wave speed, wavelength, color and in the duration of the pattern evolution, therefore a common chemical origin is supposed to exist in their formation. The role that the BrAc may play in the mechanism of the BrO ₃ ⁻ — reductant — acetone — catalyst type oscillators (∼ 30 variants) is also pointed out.     

Effect of phosphate ions in the properties of titania sol-gel

Sol-gel titania was phosphated in two different ways: i) “in situ” phosphation using phosphoric acid as hydrolysis catalyst in the titanium butoxide gelling system, and ii) gelling with nitric acid and impregnation with ammonium phosphate solution. In calcined samples at 600°C a positive effect on the specific surface area for the “in situ” phosphated sample was found. X-ray diffraction patterns showed that the presence of phosphates ions stabilized the anatase phase. FTIR-pyridine adsorption identified only Lewis acid sites in phosphated samples. In the isopropanol decomposition for “in situ” phosphated titania, the activity was six times higher than that obtained for titania phosphated by impregnation, showing strong acidity for the in situ phosphated TiO₂ catalyst.     

Effect of metallic cobalt particles size on occurrence of CO disproportionation. Role of fluidized metallic cobalt-carbon solution in carbon nanotube formation

The behavior of Co−MgO catalysts in the reaction of CO disproportionation was studied in the temperature range up to 800°C. Two temperature regions that differ by the morphology of graphite produced (“egg-shells” or nanotubes) were detected. The experimental data obtained were analyzed in terms of the phase equilibria “amorphous carbon”—“carbon solution in metallic Co” and “graphite”— “carbon solution in metallic Co”. Under certain assumptions, the Co° particles are considered to dissolve amorphous carbon under the reaction conditions and transform to oversaturated fluidized carbon-cobalt solution, these fluidized particles being responsible for the formation of carbon nanotubes.     

The SML pump of carbon cycles in oceans

1 Introduction Recently the concepts and models of “pump” are used frequently in studying global cycles and varieties of elements in marine chemistry and marine biogeo-chemistry. For example, there are many “pump” con-cepts including the concepts of s…     

Harmonium as a laboratory for mathematical chemistry

Thanks to an algebraic duality property of reduced states, the Schmidt best approximation theorems have important corollaries in the rigorous theory of two-electron moleculae. In turn, the “harmonium model” or “Moshinsky atom” constitutes a non-trivial laboratory bench for energy functionals proposed over the years (1964–today), purporting to recover the full ground state of the system from knowledge of the reduced 1-body matrix. That model is usually regarded as solvable; however, some important aspects of it, in particular the exact energy and full state functionals—unraveling the “phase dilemma” for the system—had not been calculated heretofore. The solution is given here, made plain by working with Wigner quasiprobabilities on phase space. It allows in principle for thorough discussions of the merits of several approximate functionals popular in the theoretical chemical physics literature; in this respect, at the end we focus on Gill’s “Wigner intracule” method for the correlation energy.