Gyula Rábai

Oscillatory symmetry breaking in the Soai reaction

A kinetic model of spontaneous amplification of enantiomeric excess in the autocatalytic addition of diisopropylzinc to prochiral pyrimidine carbaldehydes is extended by a negative feedback process. Simulations based on the extended model result in large-amplitude oscillations both in a continuous-flow stirred tank reactor (CSTR) and in a semibatch configuration under optimized initial conditions. When sustained oscillations are maintained in a CSTR, no enantiomeric product distribution could be observed in the calculated series; the system keeps its initial enantiomeric ratio endlessly. During damped oscillations, or steady-state conditions, however, chiral amplification from a very small initial enantiomeric excess to more than 99% occurs in a semibatch configuration. Calculations indicated spontaneous enantiomeric product enrichment (i.e., accumulation of one of the enantiomers at the cost of the other one) from strictly achiral starting conditions in a semibatch configuration due to the inherent numerical error of the integrator method, which can be regarded as a model of the statistical fluctuation in the numbers of enantiomeric molecules.

Temperature compensation in the oscillatory hydrogen peroxide–thiosulfate–sulfite flow system

A simple controlling mechanism assuming reasonable activation energies for the reaction steps is used to simulate temperature compensation of the period length of the large amplitude sustained pH oscillations observed experimentally in a H2O2–S2O32––SO32––H+ aqueous flow reaction system under optimized conditions.

Mechanistic studies of oscillatory copper(II) catalyzed oxidation reactions of sulfur compounds

Abstract Trace amounts of copper ion catalyst induce exotic phenomena in the oxidation of several inorganic sulfur compounds by peroxides in aqueous solution. Simple and complex oscillations and several kinds of bistability are observed when the copper(II)-catalyzed oxidation of S 2 O 3 2− by either H 2 O 2 or S 2 O 8 2− is carried out in a CSTR and when SCN − ions are oxidized with H 2 O 2 in the presence of copper ions under either batch or flow conditions. For the S 2 O 3 2− –H 2 O 2 –Cu(II) reaction, a four-step model is proposed, in which formation of the intermediate HOS 2 O 3 − and attack on that species by S 2 O 3 2− and H 2 O 2 play key roles. When this core of reactions is extended with additional steps, computer simulations yield good agreement between the experimentally observed and calculated pH oscillations, bistability and batch behavior. In the oscillatory S 2 O 3 2− –S 2 O 8 2− –Cu(II) flow reaction, Cu(I), Cu(II) and Cu(III) species as well as SO ⋅ − 4 and S 2 O ⋅ − 3 are postulated to participate in a free radical mechanism, which successfully simulates the oscillations. To model the experimentally observed oscillations and bistability in the H 2 O 2 –SCN − –Cu(II) system, we have proposed a complex mechanism involving 30 reactions and 26 independent species.

Model for the oscillatory reaction between hydrogen peroxide and thiosulfate catalysed by copper(II) ions

A mechanistic model is presented for the CuII-catalysed oxidation of thiosulfate by hydrogen peroxide, which gives oscillatory behaviour in a flow reactor. A simple four-step model, in which formation of the intermediate HOS2O3– and attack on that species by H2O2 and S2O32– play key roles, suffices to give oscillatory behaviour. By extending this core set of reactions with additional steps that describe acid–base equilibria and reactions between H2O2 and sulfur species, we obtain better agreement with the observed oscillatory behaviour as well as the ability to simulate the observed batch behaviour and the bistability and complex multi-peaked oscillations found in flow systems. Our results suggest that mechanistic descriptions of sulfur-containing hydrogen peroxide oscillators should emphasize the capability of the sulfur substrate to be partially oxidized to a relatively stable intermediate prior to its total oxidation to sulfate.

Temperature-compensation in pH-oscillators

Temperature independent period length (temperature-compensation) in pH-oscillators has been simulated with a simple general model. Opposing effects of the composite reactions on the period length with changing temperature have been shown to be responsible for this peculiar phenomenon. Experiments have shown that temperature-compensation exists in the oscillatory hydrogen peroxide–sulfite ion–thiosulfate ion flow system in a narrow range of conditions. A simple mechanism with estimated activation energies of the steps was used successfully to simulate the phenomenon.

Oxidation of thiourea by iodate: a new type of oligo-oscillatory reaction

In the oxidation of thiourea by iodate in weakly acidic solution the concentration of iodide may exhibit several extrema. The number of extrema mainly depends on the initial ratio of the concentrations of thiourea and iodate, and is at most four. The first step of the reaction results in the formation of iodide which then reacts with iodate to give iodine. The latter oxidizes thiourea in several steps, the end products being sulphate ions, ammonium ions, and carbon dioxide. Taking into account the independently determined rate constants for the sub-systems, the change in the concentrations of iodide and iodine with time can be calculated. There is good agreement between the experimental and theoretical curves.

pH-oscillations in a closed chemical system of CaSO3-H2O2-HCO3(-).

Long-lasting large amplitude periodic change of the pH is measured in an aqueous suspension of CaSO(3)-H(2)O(2)-HCO(3)(-) at 2.0-10.0 °C in a closed reactor. The amplitude can be as large as 2 pH units between pH 5 and 7. The observed phenomenon is explained and simulated by taking into account a slow dissolution of CaSO(3), which serves as a continuous supply of HSO(3)(-) for a H(+)-producing autocatalytic composite reaction between H(2)O(2) and HSO(3)(-). Protonation of HCO(3)(-) to form CO(2) in a reversible reaction provides for the necessary negative feedback in [H(+)].

Quantitative description of the oscillatory behavior of the iodate-sulfite-thiourea system in CSTR

In a limited range of concentration of the reactants, oscillatory changes of pH and iodide ion concentration are observed in the iodate — sulfite — thiourea system in a CSTR. These changes can be quantitatively described by the simultaneous solution of four empirical rate laws of the corresponding sub-systems (iodate-hydrogen sulfite, iodate-iodide, iodine-hydrogen sulfite, and iodine-thiourea), considering two equilibria (sulfite-hydrogen ion and iodine-iodide).AbstractВ ограниченнчм интервале концентраций реагентов наблюдали колебательные изменения pH и концентраций иодидного иона в системе иодат — сульфит — тиомочевина в CSTR. Эти изменения количественно описаны с помощью одновременного решения четырех эмпирических уравнений скорости соответ-ствующих подсистем (иодат — сульфит водорода, иодат — иодид, иод — сульфит водорода и иод — тиомочевина), принимая во внимание два равновесия (сульфит — ион водорода и иод — иодид).

Reversible adsorption-desorption oscillations of nanoparticles on a patterned hydrogel surface induced by a pH oscillator in a closed chemical system

Oscillatory adsorption-desorption of Ag nanoparticles on a pH-responsive hydrogel surface was induced by a pH oscillator in a closed reaction system. The hydrogel surface was prepared as a honeycomb-patterned film using a honeycomb-patterned polystyrene film as a template to speed up the response time in the stimuli-responsive hydrogel. The surface morphology and hydrophobic interaction of the patterned hydrogel surface were significantly altered by the pH change of the aqueous solution that came into contact with the gel. The surface of the hydrogel became hydrophobic for adsorption in a lower-pH solution but became hydrophilic with decreased adsorptivity at higher pH conditions. A closed system chemical pH oscillator composed of CaSO3-H2O2-NaHCO3-H2SO4 was applied to force the periodic adsorption-desorption of Ag nanoparticles on the gel surface. The experimental conditions for the chemical oscillator were optimized to obtain long-lasting high-amplitude pH oscillations in a closed reactor. The periodic adsorption-desorption was proved to be induced by the periodic pH change in the solution, although the two phenomena were not completely synchronized. That is, the periodic time was longer and the number of oscillations was less for the adsorption-desorption compared with the pH oscillations that occurred in the solution state. However, the heterogeneous oscillations obtained in this study clearly suggested that the hydrophobic interaction was reversibly changed in the patterned pH-responsive hydrogel surface, similar to various biological systems in nature.

Temperature-Induced Route to Chaos in the H2O2-HSO3--S2O32-Flow Reaction System

Low-frequency, high-amplitude pH-oscillations observed experimentally in the H2O2-HSO3(-)-S2O3(-) flow reaction system at 21.0 degrees C undergo period-doubling cascades to chemical chaos upon decreasing the temperature to 19.0 degrees C in small steps. Period-4 oscillations are observed at 20.0 degrees C and can be calculated on the basis of a simple model. A reverse transition from chaos to high-frequency limit cycle oscillations is also observable in the reaction system upon decreasing further the temperature step by step to 15.0 degrees C. Period-2 oscillations are measured at 18.0 degrees C. Such a temperature-change-induced transition between periodic and chaotic oscillatory states can be understood by taking into account the different effects of temperature on the rates of composite reactions in the oscillatory system. Small differences in the activation energies of the composite reactions are responsible for the observed transitions. Temperature-change-induced period doubling is suggested as a simple tool for determining whether an experimentally observed random behavior in chemical systems is of deterministic origin or due to experimental noise.