Measurements and Simulations of the Acidity Dependence of the Kinetics of the Iron-Catalyzed Belousov–Zhabotinsky Reaction: Proton-Catalysis in the Electron Transfer Reaction Involving the [Fe(phen)₃]³⁺ Species

Abstract

The acidity dependence of the iron-catalyzed bromate-malonic acid Belousov-Zhabotinsky reaction was studied in the range 0.36 M < [HSO] < 1.20 M, and the temporal evolutions of the oscillation patterns were analyzed. The experimental results show that the period times PT decrease exponentially with increasing acidity and that the period times parallel the decrease of the reduction times RT with increasing acidity. Simulations using the reactions of the commonly accepted core reaction mechanism failed to match the measurements even in a qualitative fashion. However, we found that compelling agreement between the experiments and the simulations can be achieved over the entire range with the inclusion of second-order proton-catalysis of the oxidation of bromomalonic acid (BrMA) by the [Fe(phen)] species in the reaction identified in this paper as reaction 9 (R9), and this [H] dependence is informative about the species involved in the outer sphere electron transfer reaction. The trication [Fe(phen)] species is stabilized by ion pairing and solvation, and one may anticipate the presence of [Fe(phen)(HSO) (HO) ] species ( n = 0-3). Our results suggest that the removal of aggregating HSO ions by protonation creates a better oxidant and facilitates the approach of the reductant BrMA, and the second-order [H] dependence further suggests that BrMA is primarily oxidized by a doubly charged [Fe(phen)(HSO)(L) ] species. Considering the complexity of the BZ system and the uncertainties in the many reaction rate constants, we were somewhat surprised to find this high level of agreement by (just) the replacement of R9 by R9'. In fact, the near-quantitative agreement presents a powerful corroboration of the core reaction mechanism of the BrMA-rich BZ reaction, and the replacement of R9 by R9' extends the validity of this core reaction mechanism to acidities above and below the typical acidity of BZ reactions ([H] ≈ 1 M).

Origin of the Second-Order Proton Catalysis of Ferriin Reduction in Belousov–Zhabotinsky Reactions: Density Functional Studies of Ferroin and Ferriin Aggregates with Outer Sphere Ligands Sulfate, Bisulfate, and Sulfuric Acid

“…20 higher oxidization state catalyst (R8 in FKN mechanism). 21 In varying the sulfuric acid concentration in the iron-catalyzed BZR, we found a strong inverse dependence of period time on acidity. In simulations of the kinetics of the iron-catalyzed reaction, strong agreement with experiment was found by including second-order proton catalysis of the electron transfer reaction (reaction R9 in ref 21, shown below) between [Fe(phen) 3 ] 3+ and bromomalonic acid.…”

“…We have found in our previous experimental work that this ferriin reduction is proton-catalyzed. 21 This finding suggested that the VEA values of OSL complexes of ferriin should increase upon OSL protonation. Indeed, this expectation is borne out in the computed data.…”

“…In parallel, we developed methods for the video analysis of the kinetics of oscillating reactions, , and we developed software for the evaluation of multiequilibria reactions using the dynamical approach. − Studies by Foersterling and Varga discovered a pH dependence of this disproportionation (R4 in FKN mechanism), and the rate constant for this disproportionation was further studied by Agreda and Field . Our experimental work and the associated simulations eventually shifted our focus to the pH dependence of the reduction reaction of the higher oxidization state catalyst (R8 in FKN mechanism) . In varying the sulfuric acid concentration in the iron-catalyzed BZR, we found a strong inverse dependence of period time on acidity.…”

Systematic Application of the Principle of Detailed Balancing to Complex Homogeneous Chemical Reaction Mechanisms

“…An example of a loop that is illegal because it contains an unnecessary process is provided by a mechanism recently proposed for the reaction of bromate with malonic acid as catalyzed by [Fe­(phen) 3 ] 3+ . This 14-step mechanism includes the following case 2 loop The loop is obtained by combining the steps so that R4b = R4 + A1.…”

“…An example of a loop that is illegal because it contains an unnecessary process is provided by a mechanism recently proposed for the reaction of bromate with malonic acid as catalyzed by [Fe(phen) 3 ] 3+ . 45 (R4b)…”