Metal surface molecular orbital description

We describe here some of our initial efforts concerned with the use of dispersed metals as catalysts for organometallic reactions and the development of a Frontier Molecular Orbital description of the reactions taking place on the metal surface. 

Obviously, chemisorption on d-metals needs a different description than chemisorption on a jellium metal. With the d-metals we must think in terms of a surface molecule with new molecular orbitals made up from d-levels of the metal and the orbitals of the adsorbate. These new levels interact with the s-band of the metal, similarly to the resonant level model. We start with the adsorption of an atom, in which only one atomic orbital is involved in chemisorption. Once the principle is clear, it is not difficult to invoke more orbitals. 

Recently, we have proposed a methodology [71] similar to the last approach, but using semiempirical molecular orbital methods instead of TDHE methods, to calculate the frequency-dependent polarizability properties of the molecule-surface complex. Although this is a lower level description of the electronic structure, the use of semiempirical methods allows us to describe more complex molecules than has been considered in earlier studies. In the following, we present some recent results for the pyridine-copper tetramer system, and we examine the influence of molecule-metal distance on the Raman intensities for this model. There are two components to the calculation of the Raman intensity (1) calculation of the ground state structure and normal coordinates and (2) calculation of the derivative of the frequency-dependent... 

A simple description of electrons in a solid is the model of a free electron gas in the lattice of the ions as developed for the description of metals and metal clusters. The interaction of electrons and ions is restricted to Coulomb forces. This model is called a jellium model. Despite its simplicity, the model explains qualitatively several phenomena observed in the bulk and on the surface of metals. For a further development of the description of electrons in solids, the free electron gas can be treated by the rules of quantum mechanics. This treatment leads to the band model. Despite the complexity of the band model, Hoffmann presented a simple description of bands in solids based on the molecular orbital theory of organic molecules that will also be discussed below. 

Chemisorption of H on simple metals and on transition metals has been studied theoretically using approaches such as molecular orbit theory, valence bond theory, density functional theory, cluster calculations (a detailed description up to 1980 can he found in Smith, 1980), and effective medium theory (N rskov, 1984). An extension of the effective medium theory, the so-called embedding atom method - originally developed to study the embrittlement problem - was shown to yield very valuable results for H on metals, particularly for the surface relaxation and the H adsorption sites including subsurface sites (see Pd) (Daw and Baskes, 1984). 

In the theory of adsorption, in addition to the methods of quantum chemistry, widely used the method of model Hamiltonians [3]. In the study of the adsorption of atoms and molecules on metals used primarily molecular orbital approach-self-consistent field, as this takes into accoimt the delocalization of electrons in the metal. Under this approach, the most commonly used model Hamiltonian Anderson [4, 5], originally proposed for the description of the electronic states of impurity atoms in the metal alloys. The model has been successfully applied to study the adsorption of atoms on the surface of metals... 

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