A potential source of free radicals in iodine-based chemical oscillators

The iodide-peroxide system in an acidic medium was investigated as a potential source of free radicals in iodine-based chemical oscillators. The radicals were detected by EPR spin-trapping using spin-trap 5-(tert-butoxycarbonyl)-5-methyl-1-pyrroline N-oxide (BMPO), which forms stable spin-adducts with oxygen-centered radicals. The iodide-peroxide system is introduced as an easily available laboratory source of free radicals.     

Coupling of identical chemical oscillators

Identical BZ oscillators, in a CSTR, modeled by the Field-Körös-Noyes (FKN) mechanism, are coupled in a diffusion-like manner. In addition to the obvious symmetric solutions, i.e. solutions in which both CSTRs are oscillating in unison, or are in the same stable steady state, unsymmetric, broken symmetry, solutions may coexist under the same set of constraints. Thus, depending on constraints and initial conditions, the combined system can be in the following states: a) stable symmetric steady state. b) symmetric oscillations, when both cells oscillate in phase. c) coexistence of symmetric and unsymmetric steady states. d) coexistence of symmetric oscillations and unsymmetric stable steady state (broken symmetry). e) coexistence of symmetric and unsymmetric oscillations. The latter differ from the former in phase, in amplitude and in period. On the other hand, no unsymmetric oscillations were found to coexist with the symmetric steady state. All the initial conditions tried ended in either of the two, possible, stable states. The change of periods and amplitude of both types of oscillations are examined as a function of the system constraints namely, concentrations and coupling rate.     

Pulse-coupled chemical oscillators with time delay

Finger on the pulse: in a system of two pulse-coupled Belousov-Zhabotinsky oscillators, introducing a time delay or increasing the coupling strength brings about novel dynamic features (see picture, the two oscillators are shown in different colors), such as reversal of the roles of excitatory and inhibitory coupling or fast anti-phase oscillation. These features are not observed in diffusively coupled systems, and shed light on how such pulse coupling occurs at synapses.     

Internal stochastic resonance in two coupled chemical oscillators

The dynamics of two coupled chemical oscillators was investigated numerically when the first subsystem was subjected to external parametric noise. The signal-to-noise ratio (SNR) of the response of each subsystem to external noise shows internal stochastic resonance (SR). In addition, the SNR also shows resonance behavior with the variation of coupling strength.     

Novel phase-fitted symmetric splitting methods for chemical oscillators

Splitting strategy is considered for the numerical solution of oscillatory chemical reaction systems. When applied to the harmonic oscillator, traditional splitting methods with constant coefficients are shown to be have some order of phase lag though they are zero-dissipative. Phase-fitted symmetric splitting methods of order two and order four are constructed. The result of the numerical experiment on the Lotka–Volterra system shows that the new phase-fitted symmetric splitting methods are more effective than their prototype splitting methods and can preserve the invariant of the system in long-term compared with the classical Runge–Kutta method.     

Experimental and theoretical studies of coupled chemical oscillators

Chemical oscillators may be coupled together in a variety of ways. Two of the most important forms of coupling are physical (via transport) and chemical (via common species). Such coupling can result in new phenomena. Here we focus on rhythmogenesis, the onset of oscillations when two steady state systems are coupled, and oscillator death, the cessation of oscillations when two oscillatory systems are coupled. We also discuss briefly a biological example, the crustacean stomatogastric ganglion, and the important role of delay, which may be brought on by coupling, in chemical oscillation.     

Nature of sulfur centered radicals formed during pulse radiolysis of 2-mercaptopyridine in aqueous solutions

Spectral, redox and kinetic properties of the transient species formed by the reaction of 2- mercaptopyridine (2-MPy) with oxidants such as OH, Br¯₂ ^. ; N ^. ₃ and Cl¯₂ ^. radicals and reductants such as e¯_aq, H-atoms and (CH₃)₂ ^. COH radicals have been studied by pulse radiolysis technique. Reaction of one-electron oxidants with 2-MPy at pH 11.5 led to the formation of 2-pyridyl thiyl radical. The reduction potential for the couple C₅H₄NS ^. /C₅H₄NS¯ was estimated to be 0.84 V vs NHE from the equilibrium studies with I¯₂ ^. /2I¯ couple. At pH 6.8, the reaction of N ^. ₃ radical with 2-MPy gave a cation radical derived from 2-MPy. At pH 6.8 and 11.5, OH radicals react with 2-MPy by addition pathway. Reaction of e¯_aq with 2-MPy was found to give a reducing radical capable of transferring electron to methyl viologen. At acidic pH, the reaction of (CH₃)₂ ^. COH radicals and H-atoms with 2-MPy gave transient species identical to those produced by the reaction of oxidising radicals, namely, OH radicals, Cl¯₂ ^. and Br¯₂ ^. radicals.     

Oscillations death revisited; coupling of identical chemical oscillators

The coupling of identical reactors containing chemical oscillators is discussed. The coupling is executed by means of transferring chemical species from one cell (reactor) to the other in a diffusion like manner i.e. in proportion to the concentration difference between the cells. The coupling rate constant, however, is the same for all species. The individual, uncoupled, cells may be oscillating or in a stable steady state (the same for all reactors). In both cases, depending on the initial conditions, the symmetry breaks, and the cells may end up-contrary to intuition-in a stable steady state in which the final concentrations are not equal in the various reactors. The Brusselator and Oregonator mechanisms are examined and they behave in the manner described. On the other hand, the Lotka-Volterra mechanism, being conservative, keeps, when coupled, only the homogeneous solutions.     

Dissipative structures in systems of diffusion-bonded chemical nano- and micro oscillators

In spatially extended classical Belousov-Zhabotinsky (BZ) reaction, trigger spiral or concentric waves usually occur. If the BZ reaction is dispersed into nanodroplets of water-in-oil emulsion, new patterns are observed such as standing waves, anti-spirals, oscillons, dash-waves, jumping waves, Turing patterns, and other. If the size of water droplets is increased up to tens of micrometers, coupled micro-oscillators produce new stationary and oscillatory discrete dissipative patterns. In the review, comparative analysis of these patterns is done and a possibility of creating a chemical computer on the basis of dissipative patterns is discussed.     

Spin trapping reactions with nitric oxides. III. Alkoxyalkylnitroxides and new nitrogen centered radicals

Subsequent trapping of photochemically generated alkyl and alkoxy radicals by nitric oxide yields alkoxyalkylnitroxide radicals. New types of nitrogen centered radicals have been detected and tentatively explained by reactions of nitrogen oxides N₂O, NO, NO₂, N₂O₃, N₂O₄ with alkyl and peroxide radicals.     

Oxidation of alcohols by iodine in the presence of nitroxyl radicals generated electrochemically

Oxidation of primary and secondary alcohols in the two-phase system of methylene chloride-aqueous solution of sodium hydrocarbonate in the presence of the mediator system of potassium iodide-nitroxyl radical was studied. It is supposed that under these conditions the iodonium ion generated on a platinum electrode is the primary oxidizing agent.     

Spin-trapping of oxygen free radicals in chemical and biological systems: new traps, radicals and possibilities

The choice of the spin-trap that is to be applied in any EPR study represents the crossroad between a comprehensive investigation and an "ordinary" quantification of production of radicals. So, the scope of our study was to compare the performance of different spin-traps for qualitative analysis of radical-generating systems, and their ability to recognize previously unnoticed radicals. In addition, we present a brief account of the difficulties involved in the detection of oxygen-centered radicals in chemical and biological systems accompanied by the rationale for using the EPR spin-trapping technique in quantitative studies of such reactive species. Certain technical aspects of EPR experiments related to efficient trapping of free radicals in biochemical systems are also discussed. As an example we present here results obtained using EPR spectroscopy and the spin-trap DEPMPO, which show that the Fenton reaction, as well as various biological systems generate a previously unappreciated hydrogen (*H) atom.     

Quantum chemical study of the rearrangement of phenoxyl-hydroxyphenyl radicals

The absorption spectra and decomposition kinetics of intermediates formed upon the photolysis of p-iodophenol are studied via flash photolysis. The extinction coefficient of the p-iodophenoxyl radical is calculated. It is found that p-iodophenol acts as an inhibitor of light-independent liquid-phase oxidation reactions.