Electron transfer reactions stoichiometry determination

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

The RDE technique has found widespread use in analytical electrochemistry because of an excellent signal-to-noise ratio resulting from the enhanced mass transport. The RDE method has also been employed for monitoring concentrations in kinetic applications [59], as described for ultramicroelectrodes [60] and in the determination of the stoichiometry for electron-transfer reactions by means of redox titration [61]. The latter procedure will be described next. 

EXPERIMENT 5.4 ELECTRON TRANSFER REACTION BETWEEN [CO(en)3]2+ AND [Co(ox)2(en)]-—STOICHIOMETRY DETERMINATION USING ION-EXCHANGE CHROMATOGRAPHY11... 

Results Summary for the Stoichiometry Determination of the Electron Transfer Reaction between [Co(en)3]2+ and [Co(ox)2(en)] ... 

Cyclic voltammetry (CV) can be used to determine if a material will undergo a redox reaction, and test whether the reaction is reversible (cyclic). A material is tested using an applied voltage, the voltage source supplies electrons, so it can be used to test the oxidation and reduction properties of a material. Then the current potential can be reversed, and the material can be tested again to measure what potential is required for the reverse reaction to occur. Because it can test the dynamics of electron transfer reactions, it can be applied to understand catalytic reactions, to analyze stoichiometry of complex compounds, and can determine the bandgap of photovoltaic materials. 

In cyclic voltammetry, the current-potential curves are completely irreversible whatever the scan rate, since the electron transfer/bond-breaking reaction is itself totally irreversible. In most cases, dissociative electron transfers are followed by immediate reduction of R, as discussed in Section 2.6, giving rise to a two-electron stoichiometry. The rate-determining step remains the first dissociative electron transfer, which allows one to derive its kinetic characteristics from the cyclic voltammetric response, ignoring the second transfer step aside from the doubling of the current. 

A further requirement for the development of a multi-enzyme oxidizing process would be the determination of the kinetic parameters of the enzymes and hence development of a model of the intended reaction system in terms of the relative productivities of the enzymes with respect to substrate conversion rates as well as electron transfer stoichiometry. 

The stoichiometry of a reaction which involves electron transfer is related to the electrical quantities determined by Faraday s law which states... 

As already mentioned, there are two established sites of proton uptake coupled to electron transfer in the thylakoid membrane in the absence of added electron carriers one at the water oxidation reaction and the second at the plastoquinone to iron-sulfur protein reaction. This would predict that the H /e stoichiometry measured during electron transport should be 2. However, designing an unambiguous experiment to determine this exact ratio in isolated thylakoids turned out to be more difficult than it seemed at first. The literature contains, therefore, numerous values for this ratio [12], some of which are indeed close to 2. 

In general, many kinetics data are accumulated prior to proposing a reaction mechanism. In our case, we will simply use the stoichiometry information obtained in Experiment 5.4 along with intuition based on past work in the field. The following is an interactive pre-lab exercise for proposing the rate law for the electron transfer between [Co(en)3)]2+ and [Co(ox)2(en)]. The kinetics will then be investigated using conventional visible spectroscopy. Experimental data, in combination with the rate law, will be used to determine the outer-sphere electron rate constant. 

As depicted in Figure 2.3, electrons are transferred from the oxidation step to the reduction step of the redox reaction. The number of electrons exchanged is the fundamental basis for establishing the stoichiometry of the redox process. This fact is crucial when establishing a mass balance, as will be done by modeling sewer processes (cf. Chapters 5 and 6). The OX value is, by definition, a key element in determination of this number. 

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