Influence of the conjugate anion on the parameters of ESR spectra in radical cations of sterically hindered hydroquinones

The reactions of hindered p-quinones and hydroquinones with inorganic, organic, and Lewis acids have been studied by ESR. The conjugate anion has a strong perturbing effect on the distribution of the spin density at the reaction center. A mechanism for the reduction of p-quinones and the oxidation of hydroquinones, which includes a step involving the formation of radical-cation salts, has been proposed. The form of the ESR spectra and the composition of the final products are greatly dependent on the ratio between the reactants, the medium, and the temperature.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 6, pp. 737–741, November–December, 1986.     

Kinetics of Catalyst Poisoning during Capillary Condensation of Reactants

The influence of the capillary condensation of reactants on the poisoning of Pt/SiO₂ catalysts by thiophene is studied experimentally for p-xylene hydrogenation at T = 60 and 80°C. The poisoning kinetics is independent of a catalyst and its rate decreases with a decrease in temperature. Poisoning during capillary condensation is 1.5–6 times slower than that in the gas phase, depending on the fraction of surface platinum in the pores filled with a liquid. The poisoning of the catalyst active sites in the pores filled with a liquid requires less sulfur at the same deactivation degree. The number of sulfur atoms per one platinum atom necessary for the complete poisoning of platinum in the gas phase is higher than that in the case of capillary condensation by a factor of 1.4–1.5.     

Isolation of ternary complex precursors and partially condensed␣intermediates to macrocyclic complexes of nickel(II) and copper(II)

Reaction in unheated solutions between metal salts, bis(3–aminopropyl)amine (dpt) and 2,6–diacetylpyridine (dap) gives the ternary complexes [M(dpt)(dap)]2+ (M=Ni or Cu), in which no Schiff-base condensation has occurred. Cyclisation requires long standing or heating. For tris(3–aminopropyl)amine and nickel(II), X-ray crystallography shows that reaction in MeOH can be stopped after condensation of one carbonyl and one amine group to give a coordinated open-chain hexadentate ligand, an intermediate in the formation of the fully condensed macrocycle. With the 2–(aminoethyl)-4–(butylamino)amine (ebt), the butylamine groups are involved in intermolecular condensation with carbonyl groups from adjacent units, giving oligomeric or polymeric complexes.     

Synthesis of the amide of the C-terminal tetrapeptide of the sequence of oxytocin

The synthesis has been effected of the amide of the tetrapeptide forming the sequence 6–9 of oxytocin with the use of benzyl protection of the thiol function of cysteine by two main schemes 1+3 and 2+2. The advantageousness of performing the synthesis by the 2+2 scheme has been shown. The overall yield of tetrapeptide using the method of condensation with the formation of mixed anhydrides amounted to 51% by the scheme proposed.All-Union Scientific-Research Institute of the Technology of Blood Substitutes and Hormone Preparations, Moscow. Translated from Khimiya Prirodnykh Soedinenii, Nos. 3,4, pp. 393–400, May–August, 1992.     

Spectrophotometric determination and kinetic studies of condensation of aromatic aldehydes with 1,3-dimethylbarbituric acid

The condensation reaction of 1,3-dimethylbarbituric acid with aromatic aldehydes in ethanol has been investigated spectrophotometrically at 25–35°C. The reaction follows overall second-order kinetics, first-order each in reactant. From the dependence of the rate constants on temperature, activation parameters have been calculated. The rate of condensation increases with the presence of electron donating groups on the aromatic ring of the aldehyde. The rate-determining step involves dehydration of the aldol intermediate. The reaction was found to be catalyzed by HCl solutions. Based on this reaction, determination of 10 aromatic aldehydes in a concentration range of 0.13–70.25 μg/ml is proposed. The method was applied for determination of barbituric and 1,3-dimethylbarbituric acids also.     

Electron transfer in condensed media with mobile defects

A study has been made of electron transfer in a condensed polar medium containing two types of dynamically changing intrinsic parameters — static polarization and mobile defects, interacting locally with electronic states of the reactants. An expression has been obtained for the rate constant of a homogeneous, nonadiabatic, charge transfer reaction with allowance for large-scale and local fluctuations of the intrinsic parameters of the medium. It is shown that, along with fluctuations of the static polarization, Brownian motion of defects close to the reactants may lead to a significant variance of the electronic levels of the donor and acceptor.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 406–413, July–August, 1990.     

Mechanism of the interactions of NO with CO on oxide catalysts

The rate of the reduction of nitrogen monoxide by carbon monoxide on transition metal oxides and mixed catalysts based on them and the rates of the individual interactions of the reactants with the catalysts at different extends of reduction of the surface have been studied. These data, as well as the results of an investigation of the adsorbed molecules by IF diffuse reflectance spectroscopy attest to a concerted mechanism of catalysis, which includes a step resulting in the formation of a nitrosyl complex. It has been concluded that catalysts for this reaction must contain complex-forming eléments, which may be oxides of iron, cobalt, and nickel among the oxides and may be rhodium, ruthenium, and palladium among the metals, in their composition.Azerbaidzhan Institute of Petroleum and Chemistry, Baku. Translated from Teoreticheskaya i ÉksperimentaI'naya Khimiya, Vol. 27, No. 5, pp. 567–573, September–October, 1991. Original article submitted July 23, 1991.     

Condensation of phthalonitrile with weakly basic diamines

Conclusions It has been found that, in contrast to the condensation of phthalonitrile with diamines with pK_a > 2.7, the principal products from which are macroheterocycles, reaction of phthalonitrile with diamines with pK_a 2.7 gives linear condensation products of the reactants.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2145–2148, September, 1988.     

Activation Parameters for the Formation and Decay of Partial-Charge-Transfer Exciplexes of 9-Cyanoanthracene, 1,12-Benzperylene, and Pyrene

The factors affecting the rate of formation and decay of exciplexes with partial charge transfer, which form in the kinetic region of photoinduced electron transfer (G ^* _et > –0.2 eV), were studied. The rate of formation of exciplexes is controlled mainly by the diffusion of reactants and the low steric factor (0.15–1.0). The activation enthalpy and entropy for the exciplex formation (9–13 kJ mol^–1 and –(12–28) J mol^–1 K^–1) are close to the activation enthalpy and entropy of diffusion, respectively. Charge transfer in an exciplex and polarization of the medium generally occur after passing the transition state. In contrast, the activation enthalpy of exciplex decay (its conversion into the reaction products) is close to zero (±6 kJ mol^–1) and the activation entropy is strongly negative –(80–130) J mol^–1 K^–1.     

Compensation effect in the theory of nonadiabatic reactions

It has been shown in the framework of the theory of nonadiabatic transitions that consideration of the exponential dependence of the electronic matrix element of a transition on the distance between the reactants leads to a compensation effect. An expression for the isokinetic temperature in terms of the activation energy and the characteristic vibrational frequency has been obtained for reactions taking place in the normal region in nonpolar media. The conditions for the observation of the compensation effect and the conditions determining the sign of the isokinetic potential have been discussed.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 2, pp. 223–226, March–April, 1986.     

Kinetics and mechanism of ligand interchange in the [RuIII(edta)L] complexes; L=Cysteine and related thiolates

Complexes of the [RuIII(edta)SR]n series, with SR–= deprotonated cysteine, N- acetylcysteine, 2–mercaptoethanol, glutathione and penicilamine, were prepared from [Ru(edta)H2O]– and the corresponding RSH thiols, at pH=5.5. The complexes exhibit intense visible absorption bands at ca. 520nm (3500M–1 cm–1), associated with LMCT from the sulfur ligands bound to RuIII. The kinetics of the formation reactions were first order in [RuIII(edta)H2O]– and thiol reactants, with k1 values ca. 1–5×102 M–1s–1 (25°C) for all the sulfur ligands except penicilamine, which reacted slower by a factor of 10. Activation parameters suggest an associative mechanism, as for the coordination of other S- and N-bound ligands to [RuIII(edta)H2O]–. A reactivity decrease is apparent at low and high pH's (ranges 1–3 and 8–10, respectively), associated with acid-base equilibria involving the less reactive [RuIII(Hedta)H2O] and [RuIII(edta)OH]2– species. A significant rate increase was found for cysteine and penicilamine at ca. pH=8.0, because the thiol reactants deprotonate. The equilibrium constants for all the ligands showed that robust complexes were formed, with K=ca. 1×105 M–1 (25°C). The dissociation rate constants, k–1, were in the 10–3–10–4 s–1 range. The influence of nucleophilic and steric effects increasing and decreasing the formation rates, respectively, is discussed for the thiolate ligands, with adequate comparisons with other L species bound to [RuIII(edta)H2O]–.     

The mechanism of the formation of quinonic acids in the oxidation of the bark of the Siberian larch. II

Summary 1. It has been shown by chemical and physicochemical methods of analysis that the oxidation of bark lignin by atmospheric oxygen in nitric acid is accompanied by oxidative-condensation and oxidative-hydrolytic processes.2. Oxidative condensation leads to the formation of a stable intermediate product with a quinoid system of bonds. This complex fragments under the oxidative-hydrolytic action of the medium. The oligomers obtained undergo oxidative-condensation transformations more readily. Such reactions probably take place successively as far as the formation of relatively simple compounds.Siberian Technological Institute, Kransoyarsk. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 409–412, May–June, 1977.     

Principles of molecular design of binuclear polymethine systems

The electronic nature of reaction centers of reactants in the formation of polymethine compounds has been analyzed, and it has been established that the polymethine structures, depending on the combination of parity of the external polymethine chain and the signs of the electron-donor strength of the terminal heterocyclic nuclei, can be divided into two classes: classical polymethine dyes and generalized Hünig systems. In the generalized Hünig systems, the dependence of the charge distribution and bond orders in the polymethine chain on the difference in basicities of the terminal nuclei should be the opposite of the known relationships in a number of classical polymethine dyes.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 3, pp. 307–314, May–June, 1990.     

Interpretation of the Brønsted dependence in a quantum-mechanical model of proton transfer from CH acids to bases

The question of the interpretation of the Brønsted dependence has been examined in the case of processs involving proton transfer from CH acids to bases on the basis of the quantum theory of elemental atom-molecular conversions with consideration of the repulsive interaction between the reactants and products of the elementary chemical act. It has been shown that the linear character of this dependence observed in certain regions may be caused by the quasi-linear form of the term of the intermolecular interaction. Estimates of the principal energy parameters of the elementary act have been given.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 1, pp. 32–39, January–February, 1990.     

Quantum-chemical study of the mechanism of the influence of cations of d metals on the rate of the oxidation of phosphine by Cu(II) halides

A hypothesis concerning the formation of associates between intermediate complexes and cations has been introduced in order to account for the experimental dependence of the rate constant of the oxidation of phosphine by Cu(II) halides on the nature of the cation in the salt MCl₂ (M = Mn, Co, Ni). The quantum-chemical characteristics of the systems formed have been calculated by the Mulliken—Wolfsberg—Helmholz-SCCC method. It has been shown that the correct correlation between the electronic properties of the intermediates and the rate constant of the reaction upon the variation of M can be obtained only for a specific mechanism for the addition of M²⁺ to the complex.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 6, pp. 718–722, November–December, 1985.     

A study of solvates of copper(II) chloride in mixed solvents by a kinetic method

The equilibrium reactions of the formation of solvates of CuCl₂ with dimethyl sulfoxide, N,N-dimethylformamide, and triethyl phosphate have been studied on the basis of the kinetic results of the oxidation of N,N-dimethylaniline (DMA) with copper(II) chloride in mixed solvents. The solvates investigated do not take part in the reaction with DMA because of their greater stability as compared with the complex between copper(II) and DMA. The nature of the primary complex between the reactants is discussed.Translated from Teoreticheskaya i Éksperimental-naya Khimiya, Vol. 21, No. 5, pp. 608–611, September–October, 1985.     

Heterofunctional condensation of chlorosilanes with tetra- and hexaphenylsiloxanediols

A number of new chloro- and hydrochlorosiloxanes with cyclic and linear structures were synthesized by the condensation of tetraphenyldisiloxane-1,3-diol and hexaphenyltrisiloxane-1,5-diol with silicon tetrachloride and trichlorosilane. The effects of the starting ratio of the reactants and dilution on the reaction rates and yields of cyclosiloxanes were studied.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 893–896, July, 1972.     

Effect of Aging on Nonhydrolytic Alumina Xerogels

Nonhydrolytic sol-gel route is a relatively recent process which enables production of complex, multicomponent oxide materials. This process has some advantages over the conventional hydrolytic sol-gel route due to the ability to produce low-shrinkage, homogeneous, multicomponent gels. The objective of this work was to determine the effects of aging of nonhydrolytic gels on the composition, yield, phase transformations and morphology. Xerogels were prepared from aluminum chloride and isopropyl ether. Properties were studied using AgNO₃ titrations, TGA/DTA, XRD, and BET analysis. We have found that the gels contain significant amount of chlorine where the Cl/Al atomic ratio ranges from 1.1–0.6 depending on the aging time. The crystallization temperature and enthalpy of crystallization decreased with aging time. The decrease of the surface area near the crystallization temperature correlates well with the decrease of the enthalpy of crystallization as a function of aging time. A closed pore phenomenon has been observed in the nonhydrolytic alumina system. Finally, analysis of the condensation degree (CD) yielding Al–O–Al bonds suggests that the rate determining step before the gel point is the alkoxy groups formation. However, during aging of the gels, the CD remains constant since the condensation of chloride with isopropoxy groups is stericly inhibited. Surface areas in the 300–650 m²/g range were obtained depending on the aging time.     

Diels-Alder reaction volumes in the solid state and solution

The densities of anthracene, tetracyanoethylene, maleic anhydride, N-phenylmaleineimide, trans, trans-1,4-diphenylbuta-1,3-diene, and their Diels-Alder adducts were measured in the solid state and in solution at 25 °C. The reaction volumes in the solid state were calculated from the difference in molar volumes. They turned out to be low, close to each other (–4 to –11 cm³ mol^–1), and slightly different from the reaction volumes (–8±1 cm³ mol^–1) calculated from the van der Waals radii. The reaction volumes in solutions (–15 to –32 cm³ mol^–1) were found from the difference in partial molar volumes of the reactants in dioxane, acetonitrile, and 1,2-dichloroethane, The experimental Diels-Alder reaction volumes in solution are determined not only by the formation of new bonds in an adduct: a considerably higher contribution (to 75%) is made by a change in the volume of intermolecular empty spaces in solution on going from reactants to adducts.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2386–2390, November, 2004.     

Kinetics and mechanism of oxidation of thallium(I) by 12-tungstocobaltate(III)in aqueous hydrochloric acid

The reaction between Tl^I and [Co^IIIW₁₂O₄₀]^5– proceeds in two one-electron steps, involving formation of unstable Tl^II in a slow first step followed by reaction with oxidant in a fast step. The reaction rate is unaffected by the [H⁺] as protonation equilibria are not involved with either reactant, whereas the accelerating effect of chloride ion is due to the formation of an active chloro-complex of the reductant, TlCl₃ ^2–. Increasing the ionic strength and decreasing the relative permittivity of the medium increases the rate of the reaction which is attributed to the formation of an outer sphere complex between the reactants. The activation parameters were also determined and the values support the proposed mechanism.