Reaction graphs and a construction of reaction networks

Summary A general definition of reaction graphs is presented. For a pair of isomeric molecular graphs and , related by a chemical transformation , the reaction graph is determined using a maximal common subgraph defined for vertex mapping . A binary operation defined for graphs constructed over the same vertex set enables us to decompose the reaction graph into the sum of prototype reaction graphs. A decomposition of an overall reaction graph can be advantageously used for the construction of a reaction network. An oriented path in this network beginning at and ending at corresponds to a breakdown of the transformation into a sequence of intermediates.     

The effectiveness of solvents as hydrogen bond donors

Taft and Kamlet's -scale of solvent hydrogen bond donation ability is reexamined with regard to its correlations with three widely used polarity scales: Dimroth and Reichardt's E _T (30), Kosower's Z and Mayer's A _N , as well as with the _m values of the solvents when present as monomeric solutes. The correlation with E _T serves to extend the solvent -scale according to the expression:

Effect of solvent on the partial molal volumes and heat capacities of nonelectrolytes

Group contributions to in seven solvents and to in three solvents have been tabulated. The variation of group parameters is discussed in terms of the solvent compressibility coefficient, _T. The scaled particle theory (SPT) is used to calculate cavity contributions to and C _p2 ^o . Interaction contributions are obtained from the cavity terms and and values estimated through the additivity schemes. values are more sensitive to solute-solvent interactions than in water and less sensitive in methanol. The SPT results for heat capacities support the concept of structural promotion by hydrophobic solutes in water.     

Kinetics and mechanism of oxidation of bis(2,2′,6′,2″-terpyridine) iron(II) by manganese(IV)

A study has been made on the oxidation of bis(2,2,6, 2-terpyridine)-iron(II), Fe(tpy) ₂ ²⁺ by manganese (IV) using stopped-flow spectrophotometry in H₂SO₄–H₃PO₄ mixtures. The reaction is first order in each the substrate and the oxidant. The rate of the reaction increases with hydrogen ion concentration. A plausible mechanism is proposed considering the protonated forms of manganese(IV) as reactive oxidizing species. The reaction obeys the rate law

A generalized self‐consistent‐field procedure in the improved BCS theory

The basic ideas of the Improved Bardeen–Cooper–Schrieffer (IBCS) approach to the first and secondorder Reduced Density Matrices (1 and 2RDM) are briefly reviewed. The molecular orbital occupations are expressed by means of new quantities , which, satisfying a trigonometric relation, guarantee the nonidempontent condition. Thus, a variational method is introduced to determine , involving only an unconstrained minimization which may be performed using a conjugate gradient technique. A new effective Hamiltonian which is composed of the Coulomb, exchange and exchangetime inversion operators is also presented. It leads exactly to equations of Hartree–Fock type, however, the electronic field includes now an arbitrary number of orbitals and fractional occupation numbers. Accordingly, a generalized selfconsistentfield method is proposed: the iterative procedure is repeated until convergence is reached for the actual density matrix.     

Two Algorithms for Computing the Randles–Sevcik Function from Electrochemistry

We derive two expansions of the Randles–Sevcik function : an asymptotic expansion of for x and its Taylor expansion at any x ₀ . These expansions are accompanied by error bounds for the remainder at any order of the approximation.     

Oligomeric proanthocyanidin glycosides ofClementsia semenovii. II

The structures of proanthocyanidins CS-3 and CS-4, isolated from the roots ofClementsia semenovii have been established on the basis of chemical and spectral studies. CS-3 is 7-O-(6-O-galloyl--D-Glcp O--D-Glcp O--D-Glcp O--D-Glcp O--D-Glcp)-(+)-catechin-(4—8)-(–)-epigallocatechin-(4—8)-(+)-catechin-(4—8)-(–)-epigaLLocatechin-(4—8)-(–)-epigallocatechin-(4—8)-(–)-epigallocatechin, and CS-4 is 3-O-galloyl-7-O-[6-O-galloyl--D-Glcp O--D-Glcp O--D-Glcp-(+)-gallocatechin-(4—8)-[(+)-catechin-(4—8)-(3-O-galloyl-(–)-epigallocatechin]₂-(4—8)-(–)-epicatechin.Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 50–58, January–February, 1999.     

Solution of the generalized eigenvalue equation perturbed by a generalized low rank perturbation

Summary The problem of finding eigenvalues and eigenstates of the generalized perturbed eigenvalue equation = g3(+) is considered. The eigenvalues and the eigenstates of the unperturbed eigenvalue equation = are assumed to be known. Matrices , and can be arbitrary, except for the requirement that be nonsingular and that the eigenstates of the unperturbed equation be complete. It is shown that the eigenvalues and the eigenstates of the perturbed equation can be easily obtained if the rank of the generalized perturbation , is small. A special case of low rank perturbations are piecewise local perturbations which are common in physics and chemistry. If the perturbation is piecewise local with fixed localizability, the operation count for the derivation of a single eigenvalue and/or a single eigenstate is (n). If the perturbation has a fixed rank, the operation count for the derivation of all eigenvalues and/or all eigenstates is (n ²).Research supported by the Welch Foundation of Houston, Texas, and by the Yugoslav Ministry for Development (Grant P-96)     

Cluster Oxalate Complexes [W3(μ3-Q)(μ2-Q)3 (C2O4)3(H2O)3]2? (Q=S, Se): Mechanochemical Synthesis and Crystal Structure

Mechanochemical reaction of cluster coordination polymers (Q=S, Se) with solid leads to the cluster core excision with the formation of anionic complexes . Extraction of the reaction mixture with water followed by crystallization gives crystalline (main product) and (1) (minor product). In the case of the Se cluster, the complex could not be isolated, and the treatment of the aqueous extract with PPh₃ gave (2) in a low yield. Alternatively, it was obtained from and in high yield. Both 1 and 2 were characterized by X-ray structure analysis. Dedicated to Academician I. I. Moiseev on the occasion of his 75th birthday and in recognition of his outstanding contribution to cluster chemistry.     

Configuration and conformation of substituted 1,3,2-dioxaarsenanes

The configuration and conformations often 2-, 4-, and 5-substituted 1,3,2-dioxaarsenanes were studied from their PMR spectra. Inversion of the shielding constants of protons in the 4, 6, and 5 positions and of methyl groups in the 5 position was established, and the axial orientation of the As-Cl and As-OR bonds, the equatorial orientation of the 4-CH₃ group, and the chair conformation of the six-membered heteroring were proved. The anisotropies of the diamagnetic susceptibility were estimated for the first time: and (dipole approximation); and (nondipole approximation). The cyclic torsion angle (= 58°) as found for 2-chloro-1,3,2-dioxaarsenane by the R-factor method. Conclusions regardiwng the conformation of the ring and substituents were confirmed by a study of the specific effect of an aromatic solvent on the position of the resonance lines.Communication I from the series Investigation of the Stereochemistry of Organic Arsenic Compounds by NMR Spectroscopy.Deceased.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 4, pp. 457–463, April, 1973.     

Solvent effects in the reactivity of phenylethyl hydroperoxide, II. Reactivity and β-scission of α-phenylethoxy radicals in the solutions of α-phenylethylhydroperoxide

Efficient cleavage of -phenylethoxy radicals (HRO^.) was observed in CH₃CN at 304 K. From measurements of the quantum yields of product formation, it was calculated that      

Phase behavior of ethylene oxide-dimethylsiloxane PEO-PDMS-PEO triblock copolymers with water

Linear ethylene oxide-dimethylsiloxane PEO-PDMS-PEO triblock copolymers have been synthesized by hydrosilation of ,-dihydropoly(dimethylsiloxane) ) and -methyl--propargylpoly(ethylene oxide) . Studies by optical microscopy, complementary small-angle x-ray scattering (SAXS), and differential scanning calorimetry (DSC) have shown that the copolymers mixed with water are characterized by lyotropic liquid crystalline phases according to composition and temperature. The binary phase diagrams with varying copolymer composition are reported.     

Radiolysis of p-benzoquinone solutions

The products of the continuous radiolysis of p-benzoquinone (Q) at different concentrations of H₂SO₄, Q and Cl^– are p-hydroquinone (H₂Q) and 2-hydroxy-p-benzoquinone (2-Q-OH). In the presence of some alcohols, a carbonyl compound is produced in addition to H₂Q and 2-Q-OH. The dependence of G values of the products on these factors is described. The material balance between G(-Q) and G(H₂Q)+G(2-Q-OH) is maintained. The experimental results indicate the occurrence of the following reaction: . By competition studies, it was possible to evaluate the rate coefficients for the following reactions: .     

Kinetic study of hydrogen peroxide reaction with hydroxonitrilotri(methylenephosphonato)iron(III) complex

The decomposition of hydrogen peroxide in the presence of hydroxonitrilotri(methylenephosphonato)iron(III), [Fe(NTMP)(OH)^4–], was studied in nitrate media (=0.10–0.26 M) over the 0.2–0.5 mM concentration range for the iron complex and the temperature range 26–40°C. The rate law;

Reactions of alkoxy and hydroxy radicals generated photochemically from hydroperoxide molecules

In the present work as well as HRO^. radicals were generated in the photochemical interaction of 1,2-benzanthracene with -ethyl phenyl hydroperoxide /HROOH/ in C₆H₆ and CCl₄ at 304 K. radicals were trapped by C₆H₆. The main reaction of HRO^. radicals is hydrogen abstraction from the hydroperoxide group of HROOH. Although OH radicals are less selective, the hydrogen abstraction is the main process during their interaction with aromatics in contrast to reactions in aqueous solutions, where addition to the benzene ring is the rate-determining process in CCl₄:

A generalized separation reaction scheme for assessment of the radioanalytical method of concentration dependent distribution

For the assessment of the analytical error of concentration dependent distribution (CDD), complex-forming separation reaction was proposed in a generalized form of equilibrium , where n is the effective stoichiometric coefficient, i.e. the difference of mean ligand numbers and <n> of a mixture of complexes of analyte M with reagent L in the respective groups (distinguished by bars above the symbols) of the separation system. Calibration curve is derived from measurement of gross activity of complexes, A=A(ML_<n>) and . Theoretical relative error is expressed as a product of three terms, x/x=f₁f₂f₃. The first term f₁ depends on the degree of isotopic dilution, and the recommended ratio of amounts of nonradioactive (x) and radioactive (y) substance M is x/y(1;4). The second term f₂ depends first of all on the slope of distribution ratio (yield of separation ) vs. the analyte; reagent ratio, n(Z+1)/T. The form of slope is analyzed on the basis of the generalized separation reaction. Optimal conditions were discussed from this point of view and the ideal case is at f₂=1. The third term f₃ depends on the activities A and , i.e. on the distribution ratio, sample volumes, and the manner of counting. The ideal ratio of sample activities is A= and the optimal interval (0.2;0.8) is suggested     

Formation constants in the ternary systems: CuII-dipeptideamino acids

Summary A potentiometric titration technique has been used to determine formation constants for the various complexes of Cu^II with amino acids (L) and dipeptides (B) at 298 K and I-0.1 M (NaClO₄). The structure and mode of bonding in binary and ternary complexes are discussed. Values of log are evaluated. The plots of pK of amino acidsversus log K for the present and the earlier known systems gave an empirical relation: pK=–5.3 log K+10.5.     

Integral capacity of buffer

Summary The buffering power is the most important characteristic of the buffer solution. This paper presents a new concept integral capacity of buffer which is defined as pH. The generalized expressions relating the integral capacity with the acidity or with buffer ratio have been derived for HA-A systems. The calculations involved and brief discussions are given.

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Integrale Pufferkapazität

Zusammenfassung Die Pufferwirkung ist das wichtigste Charakteristikum einer Pufferlösung. Die vorliegende Arbeit beschreibt die integrale Pufferkapazität, definiert als pH. Mit Hilfe allgemeingültiger Ausdrücke wurde die Abhängigkeit der integralen Pufferkapazität von der Acidität bzw. von dem Pufferverhältnis in HA-A-Systemen abgeleitet. Die entsprechenden Gleichungen und deren kurze Diskussion wurden angegeben.

     

Catalytic Mechanism of the “Noncatalytic” Autooxidation of Sulfite

Iron ions are shown to play a special role among transition metal ions in the oxidation of sulfite by oxygen. The thermodynamically favorable formation of chain carriers S : FeOH²⁺+ HSO₃ ^– Fe²⁺+ H₂O + , H _{r 298} ⁰ –250 kJ/mol accompanied by the regeneration of the active Fe(III) form in the reactions of Fe(II) with and HSO₅ ^–provides the efficient catalytic mechanism for sulfite consumption even at [Fe]₀ 10^–8mol/l. Any aqueous solution contains iron ions in this amounts. Thus, the noncatalytic oxidation of sulfite is in fact the catalytic reaction involving unavoidable microadmixtures of iron ions. Other transition metal ions (Mn²⁺, Co²⁺, etc.) can only enhance the catalytic effect of iron admixture.     

Kinetics of the oxidation of [N-(2-hydroxyethyl)-ethylene-diamine-N,N′,N′-triacetato]cobalt(II) by N-bromosuccinimide

The kinetics of the oxidation of [N-(2-hydroxyethyl)-ethylene-diamine-N,N,N-triacetato] cobalt(II), [Co^II-(HEDTA)]^–, by N-bromosuccinimide, NBS, have been studied in aqueous solutions and water-methanol solvent mixtures under various conditions. The reaction stoichiometry indicates that one mole of NBS reacts with one mole of [Co^II(HEDTA)]^–. In aqueous solutions the reaction obeys the following rate law: