Water oxidation complex model system

T[[dotb]he nature of the initial attack by the water (eq. 10) is a matter of some controversy (205,206). Stereochemical and kinetic studies of model systems have been reported that support trans addition of external water (207,208) or internal addition of cis-coordinated water (209), depending on the particular model system under study. Other paHadium-cataly2ed oxidations of olefins ia various oxygen donor solvents produce a variety of products including aldehydes (qv), ketones (qv), vinyl acetate, acetals, and vinyl ethers (204). However the product mixtures are complex and very sensitive to conditions. 

The search for inorganic compounds that can act as model systems of the tetranuclear manganese centre of photosystem II, responsible for the oxidation of water, has led to the characterization of a number of complexes of diverse nature and geometry. 

Another model compound, the tris(2,2 -bipyridine)ruthenium(II) complex, has prompted considerable interest because its water-splitting photoreactivity has been demonstrated in various types of photochemical systems (77,99,100,101). Memming and Schroppel (102) have attempted to deposit a monolayer of a surfactant Ru(II) complex on a Sn02 OTE. In aqueous solution, an anodic photocurrent attributable to water oxidation by the excited triplet Ru complex was observed. A maximum quantum efficiency of 15% was obtained in alkaline solution. 

The bulk of our knowledge regarding thermal oxidation has been derived from studies with model systems of fatty acids and their derivatives, or with individual natural oils (2,3,6,12,13,14,15,16). However, in biological systems as complex as food, lipids usually exist in a complicated environment markedly different from that of the single phase model system. In cell membranes, for example, the lipid molecules are highly ordered, relatively restricted in distance and mobility, and closely associated with different neighboring molecules, e.g., other lipids, protein, cholesterol, water, pro- and antioxidants. What influence does such an environment have on the oxidation of the lipids at elevated temperature Even in less organized systems, e.g., depot fat from animal or plant, the lipids... 

In this paper we examine the assumptions of our previous modeling approach and present new model calculations which consider alternative assumptions. In addition, we discuss the physicochemical factors which affect the formation of surface complexes at the oxide/water interface, in particular the effect of decreasing dielectric strength of the solvent. Finally, to demonstrate the general applicability of the model we present modeling results for a complex electrolyte system, where adsorption of a metal-ligand complex must be considered. 

Models for photosynthetic oxygen evolving center have been reviewed especially in relevant to metal complexes incorporated in a polymer membrane. Polymer membranes provide active and stabile catalytic site they often convert inactive metal complexes to active structure, and stabilize the active form as well. Water oxidation is the primary and the most important process to provide electrons to the whole photosynthetic system. This is a key reaction in order to develop an artificial photosynthetic system which is one of the most promising candidates for the 21st century s new energy resource. The use of polymeric materials is a crucial point for this approach. 

The work of Darensbourg et al. has superseded these early model systems. A series of model compounds with the core unit Fe(CO)2(CN) or Fe(CO)(CN)2 were found to reproduce the unique IR absorption spectra of [FeNi]-hydrogenases very well. The IR spectrum of the [FeNi]-hydrogenase enzyme from D. gigas in the A state exhibits bands at 1947, 2093, and 2083 cm . The IR spectrum of the iron(II) model complex K [CpFe(CO)(CN)2] in acetonitrile exhibits absorption bands at 1949, 2094, and 2088 cm which are assigned to the vco, the symmetric vcn and the asymmetric voj, respectively. The energies and peak-widths at half-maximum of this absorption are sensitive to the oxidation state of the iron center, to the medium and to the counter-ion. Polar media produce broad bands with peak width at half-maximum of the vco band of 17 cm in water. The use of non-polar solvents is required to achieve the narrow (4 cm ) peak width at half-maximum observed for the enzyme. 

For the ruthenium complexes in System 1, the potential of the Ru(II)/Ru(III) process (which was the initial model reaction for the definition of the F scale) shows very little sensitivity to the nature of MPhj—however, in all cases F for the complex with the AsPhj ligand is lower than (or equal to) for the complex with the PPhj ligand as predicted by the E values. For System 1 the data obtained in water are for combined electron and proton transfer processes, showing pH dependence , and the Ru(IV) oxo complexes resulting from the second oxidation process are potent catalysts for electro-catalytic oxidation of organic substrates . ... 

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