Leone Spiccia (Monash University) gave a really interesting Chemistry seminar today concerning bioinspired manganese clusters for the photocatalytic oxidation of water. This chemical reaction is a key component of producing hydrogen from water for a hydrogen economy. The associated paper is here. The bioinspired material is centred around a cubic [Mn4O4] cluster. A similar (but not structurally identical) cluster is present in Photosystem II (PSII) and is the only known natural system that is able to oxidize water using visible light. These metal-oxide clusters are particularly interesting from the point of view of a strongly correlated electron system. In both the natural and artificial systems there appears to be still uncertainty about basic questions such as:
What is the oxidation and spin state of each of the four manganese ions at each of the stages of the photocatalytic cycle?
Are the electrons localised or delocalised over the manganese cluster?
Modelling of electron spin resonance experiments on the S2 oxidation state (this is just one of the five charge states the cluster takes during the cycle) in terms of Heisenberg model Hamiltonians has provided some constraints on the geometry, valence states, and magnetic interactions. Two papers I found informative are here and here. The second paper found a d electron configuration quite different to that found in similar synthetic systems.
A beautiful review has considered the interplay of electron magnetic exchange and electron transfer in protein metal complexes in terms of the same type of double exchange Hamiltonians that are relevant to colossal magnetoresistance materials.
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