Progress in Reaction Kinetics and Mechanism

In this paper, different methods of obtaining the two parameters controlling the rate of electron transfer processes (reaction and reorganization free energies, Λ and ΔG⁰’, respectively) under restricted geometry conditions are considered. The main difficulty of accomplishing this comes from lack of knowledge of the properties in the interfacial region, where the reaction occurs. A general method has been presented and illustrated with the study of intermolecular processes in micelles. This method is optimized when the free energies for (at least) the three reactions required are quite different. For excited state electron transfer, the general approach is based on the appearance of the so-called Marcus inverted region: at the starting point of this region the value of ΔG^0’ gives the value of Λ directly. These reaction free energies also present some uncertainties because in their calculation it is necessary to know the value of the local dielectric constant. Finally, it should be mentioned that some authors have suggested that the treatments for electron transfer reactions could not be applicable under restricted conditions. However, experiments do seem to show the applicability of the Marcus-Hush treatment under these conditions.

Isothermal decomposition of unirradiated and γ-irradiated gallium acetylacetonate Ga(acac)₃ with 10³ kGy total γ-ray dose was carried out in static air. The isothermal operating temperatures were 160, 170, 180 and 190°C. The kinetics of decomposition were followed using both model-fitting and model-free approaches. The results of model fitting application on the investigated data showed that the decomposition behaviour was best described by phase-boundary controlled reaction (R₂). Kinetic parameters of the decomposition process were calculated and evaluated. Analysis of the data using model free approach signifies the dependency of E_a on extent of conversion (α). Pre-γ-irradiation of gallium acetylacetonate Ga(acac)₃ with 10³ kGy total γ-ray dose has almost no effect on the kinetic parameters.

The oxidation of 34 sulfides by benzyltrimethylammonium chlorobromate (BTMACB) resulted in formation of the corresponding sulfoxides. The reaction is first order with respect to both BTMACB and the sulfide. The solvent composition effect indicated that the rate of reaction increases with increase in the polarity of the medium. The reaction failed to induce the polymerization of acrylonitrile. There is no effect of added benzyltrimethylammonium chloride or potassium bromide. Chlorobromate ion is postulated as the reactive oxidizing species. The rates of oxidation of meta- and para-substituted phenyl methyl sulfides were correlated with Charton's LDR equation. The rates of oxidation of the ortho-compounds showed excellent correlation with the LDRS equation. The oxidation of meta-compounds exhibited a greater dependence on the field effect. The reaction is subject to steric inhibition when an ortho-substituent is present. The oxidation of alkyl phenyl sulfides is subject to both polar and steric effects of the alkyl groups. The polar reaction constants are negative, indicating an electron-deficient sulfur centre in the rate-determining step. A mechanism has been proposed involving formation of a halogenosulfonium cation in the slow step.

Theoretical study of the thermal decomposition kinetics of 2,2-dimethyloxetane has been carried out at the B3LYP/6-311 + G**, B3PW91/6-311 + G**, and MPW1PW91/6-311 + G** levels. The MPW1PW91/6-311 + G** method was found to give a reasonably good agreement with the experimental kinetics and thermodynamic parameters. Two parallel unimolecular reactions occur to give either isobutene and formaldehyde [reaction (1)] or ethene and acetone [reaction (2)]. The calculated data demonstrate that the polarization of the O₁-C₂ bond in reaction (1) and C₄-O₁ bond in reaction (2), are rate determining. The analysis of bond order and NBO charges, bond indices, synchronicity parameters, and IRC calculations suggest reactions (1) and (2) occur through a concerted and asynchronous four-membered cyclic transition state type of mechanism.

The kinetics and mechanism of formation of [Fe(CN)₅INH]^3- by a ligand substitution reaction between aquapentacyanoferrate(II) ion and isoniazid, (pyridine-4-carboxylic acid hydrazide) (INH) have been investigated spectrophotometrically in acidic media at 435 nm (Λ_max of product), under pseudo-first order conditions, using excess of INH over [Fe(CN)₅H₂O]^3-. The reaction was studied as a function of [Fe(CN)₅H₂O^3-], [INH], pH, ionic strength and temperature, by varying only one of these variables while other variables were held constant. The reaction exhibited first-order kinetics in [INH] while it was independent of [Fe(CN)₅H₂O^3-] in the chosen concentration range. The effect of temperature on the reaction rate yielded the activation parameters. The most plausible mechanism consistent with the experimental results has been proposed.

For the first time, a kinetic investigation of the interchange process in the equilibrium between the Z- and E-isomers of a stable phosphorus ylide involving a 2-indolinone has been undertaken by the ¹H NMR technique. Activation parameters such as k_-1, k₁, k_total, E_a1, E_a-1, E_atotal, ΔS^≠, ΔH^≠ and ΔG^≠ along with thermodynamic parameters such as K_e, ΔH°, ΔG° and ΔS° were obtained from experimental data for the forward and reverse reactions.

The electrochemical oxidation of two flavonoids, quercetin and catechin, has been studied in the absence and presence of nitrite ion. A reaction mechanism has been proposed, based on cyclic voltammetry. Variation in the cathodic peak, due to the competition between the intramolecular Michael addition of hydroxy group and the intermolecular addition of nitrite ion, was studied as a critical parameter. The results indicate an EC mechanism for both electrode reactions. The observed homogeneous rate constant (k_obs) for the Michael addition was estimated by comparing the experimental cyclic voltammetric responses with the digital simulated results.