Mechanistic details of the Belousov–Zhabotinskii oscillations

A detailed chemical mechanism for the cerium catalyzed bromination and oxidation of malonic acid by bromate has been developed, and complete numerical solutions have been obtained for the temporal behavior of the concentrations of the intermediate species. The many points of similarity with the experimentally observed initial decay, long quiescence, and abrupt transition to sustained oscillation establish confidence in the essential validity of the proposed mechanism. Close examination of the time dependence of these species concentrations as well as the magnitude of the several reaction channels permits the clarification of the switching mechanism causing theoscillatory behavior of this system. The results appear to exhibit limit cycle behavior, in accordance with theoretical predictions.     

Belousov–Zhabotinskii type oscillations with amino acids or peptides as organic substrates in the presence of Mn2+ and Fe(phen)32+ as coupled catalysts

Nine amino acids, aspartic acid, glycine, serine, tyrosine, alanine, glutamic acid, threonine, cystine, phenylalanine, and two peptides, and two peptides, glycine-glycine peptide, glutamic acid-cystine-glycine peptide, give rise to damped oscillations of the Belousov–Zhabotinskii(BZ) type in a batch reactor. Both Mn²⁺ and Fe(phen)₃² are essential for most of those oscillations; and the oscillations in [Mn³⁺] and [Fe(phen)₃³⁺] are also observed. The role of two metallic ions played in the oscillations are analyzed, showing that Mn²⁺ catalyzes the oxidation of the amino acids or peptides by BrO₃⁻ to produce some intermediates which effectively reduce Br₂ to Br⁻ catalyzed by Fe(phen)₃²⁺. © 1998 John Wiley & Sons, Inc. Int. J. Chem Kinet 30: 243–247, 1998.     

A completely inorganic oscillating system of the Belousov–Zhabotinskii type

An inorganic compound, NaH₂PO₂, has been found to generate temporal chemical oscillations in Belousov–Zhabotinskii type reactions when it is substituted for the organic reducing material. Gaseous products were removed by a constant flow of nitrogen carrier gas. Periodic Br₂ evolution was measured polarographically.     

Mechanistic details of the Belousov–Zhabotinsky oscillations. II. The organic reaction subset

Our previous mechanistic model for the Belousov–Zhabotinsky reaction has been revised to include a more realistic set of reactions for the oxidation pathways of the organic intermediates. A few other rate constants have also been modified to include new information. The revised mechanism reproduces the essential experimental observations, although the periods of oscillation are somewhat too long and oscillations cease at malonic acid concentrations about ten times greater than the observed lower limit. However, the essential features of the mechanism are clearly understood.     

Monte Carlo simulation for inhomogeneous chemical kinetics: Application to the Belousov–Zhabotinskii reaction

A Monte Carlo algorithm, capable of simulating numerically the time and space dependence of chemical concentrations in a reacting system, is presented. This method is used to study the phenomenon of trigger waves in the Oregonator model of the Belousov–Zhabotinskii reaction, including the diffusion of species X and Y in one dimension. The results show that a small disturbance in a homogeneous mixture can grow into a chemical (trigger) wave propagating in space at constant velocity. The dependence of this velocity on several factors is studied, namely, initial concentrations, the diffusion of Y, and the stoichiometry of the autocatalytic step of the model. A comparison of the Monte Carlo results with a previous simulation also is discussed.     

Kinetic study of the Ce(iii)- or Mn(ii)-catalyzed Belousov–Zhabotinsky reactions with mixed organic acid/ketone substrates

In a stirred batch reactor, the Ce(III)- or Mn(II)-catalyzed Belousov–Zhabotinsky reaction with mixed organic acid/ketone substrates exhibits oscillatory behavior. The organic acids studied here are: dl-mandelic acid (MDA), dl-4-bromomandelic acid (BMDA), and dl-4-hydroxymandelic acid (HMDA), and the ketones are: acetone (Me₂CO), methyl ethyl ketone (MeCOEt), diethyl ketone (Et₂CO), acetophenone (MeCOPh), and cyclohexanone ((CH₂)₅CO). The effects of bromate ion, organic acid, ketone, metal-ion catalyst, and sulfuric acid concentrations on the oscillatory patterns are investigated. Both conventional and stopped-flow methods are applied to study the kinetics of the oxidation reactions of the above organic acids by Ce(IV) or Mn(III) ion. The order of relative reactivities of the oxidation reactions of organic acids in 1 M H₂SO₄ is Mn(III)(SINGLEBOND)HMDA reaction>Ce(IV)(SINGLEBOND)HMDA reaction>Mn(III)(SINGLEBOND)BMDA, reaction>Mn(III)(SINGLEBOND)MDA reaction>Ce(IV)(SINGLEBOND)BMDA reaction>Ce(IV)(SINGLEBOND)MDA reaction. Spectrophotometric study of the bromination reactions of the above ketones shows that these reactions are zero-order with respect to bromine and first-order with respect to ketone and that ketone enolization is the rate-determining step. The order of relative rates of bromination or enolization reactions of ketones in 1 M H₂SO₄ is (CH₂)₅CO≫(MeCOEt, Et₂CO, Me₂CO)>MeCOPh. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet:30: 595–604, 1998     

Mechanistic details of the Belousov–Zhabotinsky oscillations IV. Sensitivity analysis

The sensitivity of the period of the Belousov–Zhabotinsky oscillations to the rate constants and initial conditions of our previously proposed mechanism is computed by the technique recently developed for this purpose and shown to be valid in our analysis of the Oregonator. The principal rate-controlling steps for the determination of the period are found to be the enolization and bromination of malonic acid. Dependence of the period on initial conditions qualitatively agrees with available experimental data, except for the hydrogen ion, where a large discrepancy exists. It is suggested that this may be due to deficiencies in the organic reactions in the mechanism, or to a hydrogen-ion catalyzed bromination mechanism for malonic acid. Other components of the mechanism, which participate in characterizing the period and amplitude of the oscillations, are discussed.     

Mechanistic details of the Belousov–Zhabotinsky oscillations. III. The induction period

The oscillating Belousov–Zhabotinsky reaction exhibits an initial quiescent induction period during which the concentrations of various reactants and intermediates adjust to a quasisteady-state, which eventually switches into oscillation. The duration of this induction period is strongly dependent on the initial bromide ion concentration. The previously reported detailed mechanism for this system is used to calculate this dependence, and the results are shown to agree with recent experimental work. The factors that account for this behavior are discussed.     

A dual-phase oscillatory behavior in Belousov–Zhabotinsky system with vanillin as the substrate

Dual-phase oscillations are observed in Belousov–Zhabotinsky system with 4-hydroxy-3-methoxybenzaldehyde (vanillin) as the substrate and manganous sulphate or ammonium Ce(IV) sulphate as the catalyst. The nonoscillatory period of time between the two phases decreases with increase in the concentration of the catalyst and the substrate. Under uncatalyzed and ferroin catalyzed conditions the system exhibits single-phase oscillations. The first-phase oscillations are due to vanillin whereas the second-phase oscillations are brought about by 4-hydroxy-3-methoxybenzoic acid (vanillic acid) formed during the course of the first-phase reactions. The reactions are explained with relevant steps of the FKN mechanism. © 1998 John Wiley & Sons, Inc. Int J. Chem. Kinet 30: 201–206, 1998.     

A dual-frequency Belousov Zhabotinskii oscillating reaction with ethyl acetoacetate as organic substrate

The experimental behavior of the cerium- and manganese-catalyzed Belousov Zhabotinskii oscillating reaction with ethyl acetoacetate as organic substrate has been investigated. Under certain conditions the system displays two types of temporal oscillations. Damped highfrequency oscillations appear immediately after the addition of potassium bromate solution to complete the reaction mixture. These high-frequency oscillations may be regarded as being superimposed on an induction period of the type found in the reaction using malonic acid. After the induction period, low-frequency oscillations of the normal type are obtained. Both the high-frequency and the low-frequency oscillations can be monitored with a platinum redox or with a bromide specific ion electrode.     

Ion–molecule reactions of methoxymethyl cations with some organic substrates studied by ion cyclotron resonance spectrometry

The reactions of methoxymethyl cations generated from dimethyl ether with propene, butene-2, vinyl methyl ether, acetaldehyde and acetone have been studied. The collision complexes, generated with the olefines, may eliminate an olefine, a methanol and a formaldehyde molecule as shown by double resonance experiments. From deuterium labelling it is found, that in the cases of propene and butene-2 the elimination of an olefine is accompanied by an extensive H/D interchange in the collision complexes, which has been shown not to occur in the long-lived reactant methoxymethyl cations if the internal energy of the methoxymethyl cations is less than 2.3 eV. The H/D interchange in these collision complexes is reduced in the elimination of methanol and is almost completely suppressed in the elimination of formaldehyde. In reactions with vinyl methyl ether, however, the eliminations of methanol and formaldehyde from the corresponding collision complexes appear to proceed with extensive H/D interchange. These observations point to acyclic collision complexes rather than 4-membered ring complexes. The collision complexes generated with acetaldehyde and acetone decompose by elimination of formaldehyde only. Deuterium labelling has shown that the formaldehyde molecule contains the original methylene group of the reactant methoxymethyl cations. In addition, ¹⁸O labelling in acetone has shown that the original oxygen atom of the methoxymethyl cations is retained completely in the eliminated formaldehyde. These observations exclude any formation of 4-membered ring collision complexes and can only be explained by acyclic complexes. Possible mechanisms of all reactions mentioned are discussed in the light of these results.     

Expedient copper-catalyzed borylation reactions using amino acids as ligands

Amino acids were found to be as good ligands for copper-catalyzed borylation reactions of primary and secondary alkyl halides,and the B2pin2 acted as bi-boron source for borylation.The high reaction efficiency and mild conditions make the new catalyst system a useful alternative to the recently developed methods for the preparation of alkylboronic esters.     

Site specificity in the H–D exchange reactions of gas-phase protonated amino acids with CH3OD

H–D exchange reactions of methanol-d₁ with protonated amino acids were performed in an external-source Fourier transform mass spectrometer. Absolute rate constants were determined for the group which included glycine, alanine, valine, leucine, isoleucine and proline. By comparing reactivities with selected methyl esters, it was found that exchange on the carboxylic acid occurs 3–10 times faster than exchange on the amino group. No simple correlation is observed between the rates of H–D exchange on the acid group and the size of the alkyl group. However, the rates of exchange on the amine decrease with increasing gas-phase basicity. Glycine, the least basic amino acid, exchanges its amine hydrogens the fastest. These results are useful for determining the interaction of methanol with protonated amino acids and can provide insight into the H–D exchange reactions observed with polyprotonated proteins produced by electrospray ionization.     

Alumina catalyzed reactions of amino acids

Thermal reactions of glycine (Gly), alanine (Ala), leucine (Leu), valine (Val) and proline (Pro) adsorbed on activated alumina were studied by means of thermal analysis. In the absence of alumina, decomposition of amino acids was detected as a sharp endotherm above 200°C, whereas no thermal effects were detectable by differential thermal analysis (DTA) and differential scanning calorimetry (DSC) for amino acid/alumina mixtures. This could be explained by a continuous amino acid condensation to peptides and simultaneous absorption of formed water by alumina, the latter being gradually released at higher temperatures. Thermogravimetry (TG) and differential thermogravimetry (DTG) measurements revealed that the reactions of the amino acids adsorbed on alumina surface were spread over a wide range of temperatures. The catalysis of peptide bond formation on alumina surface at 85°C was proven directly by the identification of the reaction products, mainly dipeptides and cyclic anhydrides. This revised version was published online in August 2006 with corrections to the Cover Date.