Observations on the hydrogen peroxide-iodic acid-manganese(II)-organic species oscillating system

The effect of acidity and manganese (II) concentration on the title reaction has been examined for acetone and malonic acid as the organic species. Cerium (III) has been shown to replace manganese (II) without loss of the oscillatory behavior. Some mechanistic details are given.

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Был исследован эффект кислотности и концентрации марганца (II) в заглавной реакции на примере ацетона и малоновой кислоты, как органических компонентов. Было обнаружено, что замена марганца (II) на церий (III) не приводит к исчезновению осциллирующего поведения. Приводятся некоторые детали механистической модели.

     

Photooxidation of water by manganese(II) sulfate

The photoreduction of Mn(IV) sulfate in 6–10 M H₂SO₄ has been studied. The kinetics of the photoreaction are similar to that of the thermal oxidation of water by Mn(IV) sulfate. A strong dependence of the quantum yield on the energy of the absorbed quantum has been found. The mechanism of oxygen formation including decomposition of the “hot” dinuclear Mn complex is discussed.

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Изучена реакция фотовосстановления сульфата Mn(IV) в 6–10 М H₂SO₄ Показано, что кинетика изученной фотореакции идентична кинетике термической реакции окисления воды сульфатом Mn(IV). Обнаружена сильная зависимость квантового выхода от величины поглощенного кванта. Обсужден механизм формирования кислорода, включающий распад “горячего” биядерного комплекса марганца.

     

Solvolysis of (1-chloro-2,2-dimethylpropyl) benzene in presence and absence of mercury(II) chloride

The kinetics of solvolysis of the title compound has been studied in water and in 10 vol. % ethanol-water in the presence and absence of mercury (II) chloride. The results confirm the earlier conclusion that mercury(II) chloride is solvated in hydroxylic solvents.

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Кинетика сольволиза заглавного соединения была исследована в воде и в смеси этанола (10 об.%) с водой в присутствии и отсутствии хлористой ртути (II). Реэультаты подтверждают более раннее заключение, согласно которому хлористая ртуть (II) сольватируется в гидроксильных растворителях.

     

Reactivity studies of trans-[PtClMe(SMe2)2] towards anionic and neutral ligand substitution processes

Reaction of trans-[PtClMe(SMe₂)₂] with the mono anionic ligands azide, bromide, cyanide, iodide and thiocyanate result in substitution of the chloro ligand as the first step. In contrast the neutral ligands pyridine, 4-Me-pyridine and thiourea substitute a SMe₂ ligand in the first step as confirmed by ¹H NMR spectroscopy and the kinetic data. Detailed kinetic studies were performed in methanol as solvent by use of conventional stopped-flow spectrophotometry. All processes follow the usual two-term rate law for square-planar substitutions, k_obs = k₁ + k₂[Y] (where k₁ = k_MeOH[MeOH]), with k₁ = 0.088 ± 0.004 s⁻¹ and k₂ = 1.18 ± 0.13, 3.8 ± 0.3, 17.8 ± 1.3, 34.9 ± 1.4, 75.3 ± 1.1 mol⁻¹ dm³ s⁻¹ for Y⁻ = N₃, Br, CN, I and SCN respectively at 298 K. The reactions with the neutral ligands proceed without an appreciable intercept with k₂ = 5.1 ± 0.3, 15.3 ± 1.8 and 195 ± 3 mol⁻¹ dm³ s⁻¹ for Y = pyridine, 4-Me-pyridine and thiourea, respectively, at 298 K. Activation parameters for MeOH, , Br⁻, CN⁻, I⁻, SCN⁻, and Tu are ΔH^≠ = 47.1 ± 1.6, 49.8 ± 0.6, 39 ± 3, 32 ± 8, 39 ± 5, 34 ± 4 and 31 ± 3 kJ mol⁻¹ and ΔS^≠ = −107 ± 5, −77 ± 2, −104 ± 9,−113 ± 28, −85 ± 18, −94 ± 14 and −97 ± 10 J K⁻¹ mol⁻¹, respectively. Recalculation of k₁ to second-order units gives the following sequence of nucleophilicity: (1:13:42:57:170:200:390:840:2170) at 298 K. Variation of the leaving group in the reaction between trans-[PtXMe(SMe₂)₂] and SCN⁻ follows the same rate law as stated above with k₂ = 75.3 ± 1.1, 236 ± 4 and 442 ± 5 mol⁻¹ dm³ s⁻¹ for X⁻ = Cl, I and N₃, respectively, at 298 K. The corresponding activation parameters were determined as ΔH^≠ = 34 ± 4, 32 ± 2 and 39.3 ± 1.7 kJ mol⁻¹ and ΔS^≠ = −94 ± 14, −86 ± 8 and −68 ± 6 J K⁻¹ mol⁻¹. All the kinetic measurements indicate the usual associate mode of activation for square planar substitution reactions as supported by large negative entropies of activation, a significant dependence of the reaction rate on different entering nucleophiles and a linear free energy relationship.     

Calculation procedure for the kinetic analysis of a reaction with a thermal effect

A calculation procedure for the numerical analysis of a non-isothermal reaction taking place at constant pressure is described. The procedure is used to calculate the temperature dependence of the ignition delay of H₂ oxidation using a 44-reaction mechanism.

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Описывается алгоритм численного решения прямой кинетической задачи с учетом теплового эффекто реакции, протекающей при постоянном давлении. Применение метода иллюстрируется расчетом температурной зависимости задержки воспламенения водорода для кинетической схемы, включающей 44 элементарные стадии.

     

A note on the influence of water vapour in the oxidation of propene to acrylic acid on a mixed oxide catalyst

The interaction of propene and H₂ ¹⁸O in the absence of gaseous oxygen was studied on a Mo−W−Sn−Te−O catalyst prepared for the production of acrylic acid. The flow reactor working at low pressures was placed close to the ion source of the mass spectrometer. It was found that the formation of an acrolein-lattice oxygen complex is an intermediate step in the formation of acrylic acid. The acrolein type complex reacts further with oxygen from water to give acrylic acid. Evidence was found for the formation of unstable products from water on mixed oxide catalysts of molybdate type.

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Было исследовано взаимодействие пропилене и H₂O¹⁸ в отсутствии газового кислорода с катализатором предназначенным для производства акриловой кислоты. Измерения проводились при низких давлениях в проточном реакторе помещенном вблизи ионного источника масс спектрометра. Было установлено, что акролеин возникает за счет кислорода решетки катализатора. Комплекс типа акролеина окисляется в далнейшем кислородом воды в акриловую кислоту. Некоторые результаты можно объяснить возникновением нестабильных продуктов при взаимодействии воды и смешанных катализаторов молибдатного типа.

     

Определение кинетических характеристик при окислении аммиака на неплатиновом катализаторе

Развит экспериментальный метод определения кинетических параметров по критическим условиям эажигания процесса на одном эерне. Показано применение этого метода для реакции окисления аммиака на неплатиновом катализаторе. Определены энергия активации и порядок реакции по аммиаку на неплатиновом катализаторе по условиям устойчивости.

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A method has been developed for the determination of kinetic parameters on a single catalyst grain from critical conditions of burning. The application of the method is illustrated on the oxidation of ammonia over a non-platinum catalyst. The activation energy and the kinetic order with respect to ammonia have been determined from the conditions of stability.

     

Ellipsometric study of the interaction of a germanium monocrystal with bromine

Using an ellipsometric technique, the dynamics of formation of adsorption layers in the process of interaction of bromine with a germanium monocrystal surface has been studied. It is shown that the reaction begins only when the second layer of bromine molecules is formed on the germanium surface. The ellipsometric data are in agreement with the results obtained by kinetic experiments.

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Эллипсометрическим методом исследована динамика адсорбционных заполнений во время химической реакции германия с бромом. Сделан вывод о том, что химическая реакция германия с бромом становится возможной только при адсорбции второго слоя молекул брома. Получено хорошее соответствие между результатами кинетических и эллипсометрических экспериментов.

     

Dynamic properties of a heterogeneous catalytic reaction with several steady states

For the simplest catalytic mechanism (three-stage) permitting several steady states of the surface, a qualitative analysis of the properties of its kinetic model trajectories has been performed. Types of the steady states have been determined. Absence of oscillations (damped and undamped) has been shown.

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Для простейшего каталитического механизма (трехстадийного), допускающего несколько стационарных состояний поверхности, проведен качественный анализ свойств траекторий его кинетической модели. Определены типы стационарных состояний. Показано отсутствие осцилляций (затухающих и незатухающих).

     

Stereochemistry of heterogeneous catalytic reactions. Dehydrogenation and dehydration of stereoisomeric cyclohexanediols on copper

Transformations of six isomeric cyclohexanediols on copper catalysts were studied. The transformation rate sequence is 1,4<1,2<1,3. 1,3-diols selectively yield cyclohexanone (and cyclohexanol). On Cu/Al, 1,2-and 1,4-diols give several products, mainly cyclohexanone and hydroxycyclohexanones. Cn Cu, these latter are formed with much higher selectivity. A correlation was found between the diol stereostructures and reaction directions.

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Изучены превращения шести изомерных циклогександиолов на медных катализаторах. Скорость превращения изменяется в ряду: 1,4<1,2<1,3. 1,3-диолы селективно преврашаются в циклогексанон (и циклогексанол). На Cu/Al-катализаторе 1,2-и 1,4-диолы образуют различные продукты, в основном циклогексанон и гидрокси-циклогексаноны. На Cu-катализаторе, последные образуются с более высокой селективностьу. Найдена корреляция между стереоструктурой диолов и направлением превращений.

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Part XLI of the series “Study of the transformations of diols and cyclic ethers”     

Quantum-chemical study on the tautomerism of primary nitramines: the mechanism of N-nitro-anion protonation excludingaci-form formation

The values of the energy barriers to intramolecular 1,3-H shift reactions, in whichaci-forms of N- and C-nitro-compounds: 1 and are capable of transforming into 2 and , respectively, have been calculated by the semi-empirical AM1 method. The barrier heights (39 and 59.4 kcal/mol) reflect the kinetic stability of both tautomeric forms of the N- and C-nitro-compounds.Ab initio calculations in the 3–21G basis set of the structure and energies of stationary points on the potential energy surface of the system MeN=NO₂ H₃O demonstrated that there exists the principal possibility of the formation of only tautomer 2 (which is more stable than1).Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No 1, pp. 100–104, January, 1993.     

Catalytic reduction of NO with hydrogen over a cobalt-alumina catalyst effect of alkali metal additives

The process was carried out over Co/Al catalysts containing oxides of alkali metals (Li, Na, Rb, Cs) as additives. The presence of alkali metals generally decreases the activity of the Co/Al catalysts, except for very low concentrations of Li, Na, Rb and Cs atoms. Activity of catalyst increases with atomic weight of alkali metal additive. Possible effect of additive on catalyst structure is briefly discussed with respect to catalyzed process.

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Реакция проводилась над катализаторами Co/Al, содержащими щелочные металлы (Li, Na, Rb, Cs) в качестве добавки. Присутствие щелочных металлов обычно уменьшает активность кчтализатора Co/Al, за исключением очень низких концентраций атомов Li, Na, Rb и Cs. Активность катализатора увеличивается с повышением атомного веса шелочного металла — добавки. Возможный эффект добавок на структуру катализатора обсуждается, исходя из особенностей катализированного процесса.

     

Oxidation of ammonia on transition metal carbides

The kinetics of low-temperature oxidation of ammonia on chromium carbide has been studied. The reaction mechanism on the carbide is similar to that on oxides and metals. The specific activity of the transition metal carbides decreases in the sequence: Mo₂C>VC>Cr₇C₃>WC>TiC>ZrC>TaC>HfC. The selectivity to N₂O generally changes in the same direction. A correlation between the activity and selectivity, and the oxygen-carbide bond energy has been found.

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Исследована кинетика низкотемпературного окисления аммиака на карбиде хрома. Механизм реакции аналогичен механизму окисления NH₃ на окислах и металлах. Удельная каталитическая активность карбидов переходных металлов уменьшается в ряду: MO₂C>VC>Cr₇C₃>WC>TiC>ZrC>TaC>NBC>HfC. Селективность по N₂O изменяется, в общем, симбатно активности. Установлена корреляция активности и селективности с энергией связи кислород-карбид.

     

Enthalpy change and mechanism of oxidation of o-phenylenediamine by hydrogen peroxide catalyzed by horseradish peroxidase

The hydrogen peroxide-oxidation of o-phenylenediamine (OPD) catalyzed by horseradish peroxidase (HRP) at 37 °C in 50 mM phosphate buffer (pH 7.0) was studied by calorimetry. The apparent molar reaction enthalpy with respect to OPD and hydrogen peroxide were −447 ± 8 kJ mol⁻¹ and −298 ± 9 kJ mol⁻¹, respectively. Oxidation of OPD by H₂O₂ catalyzed by HRP (1.25 nM) at pH 7.0 and 37 °C follows a ping-pong mechanism. The maximum rate V_max (0.91 ± 0.05 μM s⁻¹), Michaelis constant for OPD K_m,S (51 ± 3 μM), Michaelis constant for hydrogen peroxide Km,H₂O₂ (136 ± 8 μM), the catalytic constant k_cat (364 ± 18 s⁻¹) and the second-order rate constants k₊₁ = (2.7 ± 0.3) × 10⁶ M⁻¹ s⁻¹ and k₊₅ = (7.1 ± 0.8) × 10⁶ M⁻¹ s⁻¹ were obtained by the initial rate method.     

Fast sequential multi-element determination of Ca,Mg, K,Cu, Fe,Mn and Zn for foliar diagnosis using high-resolution continuum source flame atomic absorption spectrometry: Feasibility of secondary lines,side pixel registration and least-squares background correction

The fast sequential multi-element determination of Ca, Mg, K, Cu, Fe, Mn and Zn in plant tissues by high-resolution continuum source flame atomic absorption spectrometry is proposed. For this, the main lines for Cu (324.754 nm), Fe (248.327 nm), Mn (279.482 nm) and Zn (213.857 nm) were selected, and the secondary lines for Ca (239.856 nm), Mg (202.582 nm) and K (404.414 nm) were evaluated. The side pixel registration approach was studied to reduce sensitivity and extend the linear working range for Mg by measuring at wings (202.576 nm; 202.577 nm; 202.578 nm; 202.580 nm; 202.585 nm; 202.586 nm; 202.587 nm; 202.588 nm) of the secondary line. The interference caused by NO bands on Zn at 213.857 nm was removed using the least-squares background correction. Using the main lines for Cu, Fe, Mn and Zn, secondary lines for Ca and K, and line wing at 202.588 nm for Mg, and 5 mL min^− 1 sample flow-rate, calibration curves in the 0.1–0.5 mg L^− 1 Cu, 0.5–4.0 mg L^− 1 Fe, 0.5–4.0 mg L^− 1 Mn, 0.2–1.0 mg L^− 1 Zn, 10.0–100.0 mg L^− 1 Ca, 5.0–40.0 mg L^− 1 Mg and 50.0–250.0 mg L^− 1 K ranges were consistently obtained. Accuracy and precision were evaluated after analysis of five plant standard reference materials. Results were in agreement at a 95% confidence level (paired t-test) with certified values. The proposed method was applied to digests of sugar-cane leaves and results were close to those obtained by line-source flame atomic absorption spectrometry. Recoveries of Ca, Mg, K, Cu, Fe, Mn and Zn in the 89–103%, 84–107%, 87–103%, 85–105%, 92–106%, 91–114%, 96–114% intervals, respectively, were obtained. The limits of detection were 0.6 mg L^− 1 Ca, 0.4 mg L^− 1 Mg, 0.4 mg L^− 1 K, 7.7 µg L^− 1 Cu, 7.7 µg L^− 1 Fe, 1.5 µg L^− 1 Mn and 5.9 µg L^− 1 Zn.     

Sorptive potential of sunflower stem for Cr(III) ions from aqueous solutions and its kinetic and thermodynamic profile

The sorptive potential of sunflower stem (180-300 μm) for Cr(III) ions has been investigated in detail. The maximum sorption (≥85%) of Cr(III) ions (70.2 μM) has been accomplished using 30 mg of high density sunflower stem in 10 min from 0.001 M nitric and 0.0001 M hydrochloric acid solutions. The accumulation of Cr(III) ions on the sorbent follows Dubinin-Radushkevich (D-R), Freundlich and Langmuir isotherms. The isotherm yields D-R saturation capacity X_m = 1.60 ± 0.23 mmol g⁻¹, β = −0.00654 ± 0.00017 kJ² mol⁻², mean free energy E = 8.74 ± 0.12 kJ mol⁻¹, Freundlich sorption capacity K_F = 0.24 ± 0.11 mol g⁻¹, 1/n = 0.90 ± 0.04 and of Langmuir constant K_L = 6800 ± 600 dm³ mol⁻¹ and C_m = 120 ± 18 μmol g⁻¹. The variation of sorption with temperature (283-323 K) gives ΔH = −23.3 ± 0.8 kJ mol⁻¹, ΔS = −64.0 ± 2.7 J mol⁻¹ K⁻¹ and ΔG_298k = −4.04 ± 0.09 kJ mol⁻¹. The negative enthalpy and free energy envisage exothermic and spontaneous nature of sorption, respectively. Bisulphate, Fe(III), molybdate, citrate, Fe(II), Y(III) suppress the sorption significantly. The selectivity studies indicate that Cr(III), Eu(III) and Tb(III) ions can be separated from Tc(VII) and I(I). Sunflower stem can be used for the preconcentration and removal of Cr(III) ions from aqueous medium. This cheaper and novel sorbent has potential applications in analytical and environmental chemistry, in water decontamination, industrial waste treatment and in pollution abatement. A possible mechanism of biosorption of Cr(III) ions onto the sunflower stem has been proposed.     

Total reflection X-ray fluorescence spectrometric determination of elements in water hyacinth from the Lerma River

The Lerma River is one of the most polluted body water in Mexico. For this reason, only the highly resistant organisms such as water hyacinth are able to reproduce in this river. The aim of this work was to evaluate the concentration of K, S, Fe, Ca, Mn, Ti, Zn, Sr, Rb, Cu, Cr, Ni, Pb and Br in roots of water hyacinth (Eichhornia crassipes) from the Lerma River. The samples were collected from five sites in the river and analyzed in triplicate using a TXRF Spectrometer ‘TX-2000 Ital Structures’ with a Si(Li) detector and a resolution of 140 eV (FWHM) at Mn Kα. A Mo tube (40 kV, 30 mA) with 17.4 KeV excitation energy was used for a counting time of 500 s. Results show that the average metal concentration in the water hyacinth roots decrease in the following order: K (9698.2 µg/g) > S (7593.3 µg/g) > Fe (4406.6 µg/g) > Ca (2601.8 µg/g) > Mn (604.2 µg/g) > Ti (230.7 µg/g) > Zn (51.65 µg/g) > Sr (43.55 µg/g) > Rb (18.61 µg/g) > Cu (12.78 µg/g) > Cr (6.45 µg/g) > Ni (4.68 µg/g) > Pb (4.32 µg/g) > Br (4.31 µg/g) and the bioconcentration factors in the water hyacinth decrease in the sequence: Ti > Fe > Mn > Cu > Ni > Zn > S > Pb > Rb > K > Cr > Sr > Br > Ca. The concentrations in roots of water hyacinth reflect the high pollution level of the river.     

Novel and efficient syntheses of 3′,5′-diamino derivatives of 2′,3′,5′-trideoxycytidine and 2′,3′,5′-trideoxyadenosine. Protonation behavior of 3′,5′-diaminonucleosides

High yielding synthetic routes to 3′,5′-diamino-2′,3′,5′-trideoxycytidine and 3′,5′-diamino-2′,3′,5′-trideoxyadenosine are described. In addition, the protonation behavior of 3′,5′-diamino-2′,3′,5′-trideoxycytidine, 3′,5′-diamino-2′,3′,5′-trideoxyadenosine, 3′,5′-diamino-3′,5′-dideoxythymidine, and 3′,5′-diamino-2′,3′,5′-trideoxyuridine has been studied by means of pH-metric measurements and NMR spectroscopy. The ionization constants and the sequence of protonation sites have been determined.     

Thermodynamic properties of the gaseous barium silicates BaSiO2 and BaSiO3

In the present study, the stability of gaseous barium silicates was confirmed by the high temperature mass spectrometry. On the basis of equilibrium constants measured for gas-phase reactions, the standard formation enthalpies were determined for gaseous barium silicates as (−510 ± 15) kJ · mol⁻¹ and (−884 ± 18) kJ · mol⁻¹ at 298 K; standard atomization enthalpies as (1637 ± 17) kJ · mol⁻¹ and (2318 ± 20) kJ · mol⁻¹ at 298 K for BaSiO₂ and BaSiO₃, respectively. Based on the results obtained the critical analysis of the literature data was carried out.