The Reactivity of Subcolloidal Iron Clusters

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The enormous advances in methods to synthesize nanometer-sized Fe(III)-hydroxide clusters can answer questions about the Fe(III)-solid surfaces that havepuzzled geochemists for decades. These questions include: (i) Which bridging hydroxylsexchange oxygen isotopes with bulk solution via what rate laws? (ii) Where do electronexchangesproceed? (iii) Can our computer models predict this chemistry? (iv) Whatlevel of accuracy, that is, expense, is needed?We propose a set of projects involving molecular clusters that have many Fe(III)or Al(III) atoms linked together with sets of hydroxyl bridges. The family ofhomologous molecules (Fe17, Fe19, Al13 and Al15) have a similar structural core butvarying numbers of Al(III) or Fe(III) metal centers. Clusters can be chosen that have, ordo not have, bound water molecules. The fact that Fe(III) and Al(III) clusters can besynthesized that are structurally similar is an enormous experimental advantage. TheAl13 and Al15 clusters are diamagnetic and allow NMR experiments, including 17O, 27Al,13C and 1H. Even the Fe(III) clusters allow probing via the methods of isotope-exchangeand paramagnetic NMR.The projects that we propose are to: (i) Determine the reactivity of differentoxygens in the clusters via isotope-exchange experiments; (ii) Evaluate the dynamics inthe 'adsorbed' organic ligand that makes the clusters stable in water, including theresidence times in the bound state; and particularly: (iii) Evaluate the tools needed topredict reactivities in Fe(III)-hydroxide solids and molecules. This last point can berestated: 'What level of quantum sophistication is required to obtain a structure accurateto better than 0.05 A? Better than 0.01 A'? In previous studies, we found that a change inbond length of 0.03 A changes the adjacent hydroxide oxygen-isotope exchange rates bya factor of i-103. 'How sophisticated must our methods of treating electron correlationbe?' 'What constitutes an adequate basis for wave-function representation?' and: 'Whatare the most promising types of computationally inexpensive methods that can yieldhidden reaction pathways in systems that are sufficiently large to be geochemicallyrelevant?' How does one construct a useful reduced representation of electronicstructure? The molecules allow a fairly dense progression from small to large(monomers, dimers, tetramers, Al13, Al15, Fe17, and Fe19 clusters) and this series providesa way to systematically extrapolate accurate calculations that are possible on smallmolecules to more approximate methods needed for extended solids surfaces andgeochemistry.The implications of this research extend well beyond Earth science. This researchwill be used by many disciplines, including colloid chemistry, nanoscience, and medicine(metalloproteins such as ferritin resemble these clusters). Also, enrollment at UCD drawsdisproportionately from the high population of new immigrants to the United States, sothese funds bring nontraditional but highly talented students into the Earth Sciences. Wetrain a diverse group of students in the latest techniques of quantitative geochemistry andwe blur the distinction between Earth science and inorganic chemistry, which isimportant for the training of fearless young geochemists.