Transition metal clusters physical properties

L.7 Transition metal clusters physical properties [gas-phase]... 

Ames Laboratory (Iowa State University, USA) investigating new solid state phases based on reduced rare earth halides. Since 1993, she has held a position at the University Jaume 1 of Castello (Spain) and became Associate Professor of Physical Chemistry in 1995. During the second semester of 2005, she held a visiting professor position at the Laboratory of Chemistry, Molecular Engineering and Materials of the CNRS-Universtity of Angers (France). Her research has been focussed on the chemistry of transition metal clusters with special interest in multifunctional molecular materials and the relationship between the molecular and electronic structures of these systems with their properties. She is currently coauthor of around 80 research papers on this and related topics. 

Gas phase transition metal cluster chemistry lies along critical connecting paths between different fields of chemistry and physics. For example, from the physicist s point of view, studies of clusters as they grow into metals will present new tests of the theory of metals. Questions like How itinerant are the bonding electrons in these systems and Is there a metal to non-metal phase transition as a function of size are frequently addressed. On the other hand from a chemist point of view very similar questions are asked but using different terminology How localized is the surface chemical bond and What is the difference between surface chemistry and small cluster chemistry Cluster science is filling the void between these different perspectives with a new set of materials and measurements of physical and chemical properties. 

Cluster compounds are fascinating research objects for many scientists in the fields of complex chemistry, solid-state chemistry, physical chemistry and physics. Syntheses, structures, bond-types, physical and chemical properties make transition metal clusters a unique class of chemical compounds. Although fantastic developments in all areas of cluster research could be observed in the course of the last 10-15 years, it looks as if we could still be at the beginning of an evolution, the end of which is not to be foreseen. 

What are transition metal clusters and why are they interesting For example what are the properties of a 5-atom platinum cluster Is it similar to bulk platinum or does it behave more like the atom What we are discovering is that below a certain size each small n-atom duster has unique chemical and electronic properties, it behaves neither iike the atom, the bulk, nor even 1i ke other clusters of the same metal. Thus there are opportunities to exploit their novel properties by making it possible to create new materials with potential applications in such diverse areas as solid state physics, electronics, chemistry and catalysis. 

Furthermore, since the 2-integration in Equation 4 extracts only the spherical average of the hole function with respect to the reference point 1, details of its angular dependence are unimportant. Spherically symmetric hole-function models are therefore perfectly justified. These constraints, among others discussed elsewhere (4), are satisfied in any many-electron system, whether a helium atom, a transition-metal cluster, a uniform electron gas, etc.. To the extent that properties such as these contain the essential physics of exchange and correlation phenomena, hole-function models provide a simple and convenient alternative to traditional ab initio technology. 

The close structural similarity between metal clusters and elemental metals leads one to wonder at what size do metal clusters possess physicochemical properties generally associated with metals. Furthermore, given the fact that metal surfaces are important in catalysis, there is considerable interest in determining whether large transition metal clusters will be good models for chemical and physical phenomena at metal surfaces. The essential question, stated imprecisely, is how will increasing the metal-core size affect the electronic structure and reactivity patterns of transition metal cluster compounds ... 

The focus of this paper is experimental work on the chemical properties of neutral transition metal clusters. The outline is as follows. We first discuss in some detail the techniques used to generate neutral gas-phase clusters. Next the known physical properties of metal clusters are summarized. This is followed by a discussion of the definition of chemical reactivity in the context of the cluster experiments. Finally, several examples of specific reactions are presented and an electronic model is proposed which can explain many of the more striking observations. Results from recent cluster ion reaction studies... 

Apart from their interesting structures, molecular clusters are intensively studied because of their physical properties. It has been shown for transition-metal clusters that small uniform particles of a certain size show interesting electronic, size-dependent effects (quantum size effects). In Au55(PR3)i2Cl6 a few electrons are trapped in a metallic state and single electrons tunnel between the cluster units. (SET, single electron tunneling).Such effects have yet to be observed in main-... 

Evidence that transition metal clusters can display highly unusual physical properties was recently provided by... 

Fthenakis, Z., Andriotis, A. N., 8c Menon, M. (2003). Temperature evolution of structural and magnetic properties of transition metal clusters. Journal of Chemical Physics, 119, 10911. 

The underlying motivation of the work presented in this paper is to provide a theoretical understanding of basic physical and chemical properties and processes of relevance in photoelectrochemical devices based on nanostructured transition metal oxides. In this context, fundamental problems concerning the binding of adsorbed molecules to complex surfaces, electron transfer between adsorbate and solid, effects of intercalated ions and defects on electronic and geometric structure, etc., must be addressed, as well as methodological aspects, such as efficiency and reliability of different computational schemes, cluster models versus periodic ones, etc.. 

Polynuclear clusters fill the gap between mononuclear and extended solid transition metal vibronic systems. The applications of the theory of vibronic interaction allow to describe physical and chemical properties of these systems, sometimes directly linked to their application. The Jahn-Teller distortion found for the rhenium clusters defines the architecture of hybrid inorganic-organic materials and, as a result, their electric and magnetic properties. The application of the vibronic theory to the decatungstate cluster elucidates the details of its reactivity in the photocatalytic reaction. The modern DFT methods give a key to the calculations of key parameters of the vibronic theory. In future, we will assist at the combination of these methods with phenomenological approaches leading to the description of vibronic effects in physical and chemical properties of polynuclear clusters from first principles. 

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