Surface orbital fragments

It is important to explicitly account for the possibility that the metal surface atoms coordinated to the adsorbate can also be metal surface nearest-neighbor atoms. It appears to be essential to incorporate this feature in the chemisorption model, and we will discuss it for the particular case of the threefold coordination. If the adsorbate orbital o has a s-symmetry, it will coordinate with a surface fragment orbital that is a symmetric combination of surface s-atomic orbitals 1, 2 and 3, located on the coordinating surface atoms. If the adsorbate orbital has p-symmetry, it will coordinate with an antisymmetric fragment orbital. The surface orbital fragment... 

The PDOSs of the jr-symmetric surface orbital fragments show also a stronger interaction in the threefold adsorption site. The bonding density of states is shifted to lower energies, which will give a larger contribution to the attractive interaction. 

In the chemisorbed state the adsorbate surface-orbital fragments are made up of a mix of adsorbate a and metal valence-electron molecular orbitals. The cr-electron occupation of the adsorbate, which was originally 2 electrons, decreases upon adsorption. This type of interaction is therefore referred to as an electron-donative interaction. 

The theoretical chemical application of surface chemical bonding theory, highlighted next, is related to formal chemisorption theory as developed in surface physics, but concentrates on quantum chemical concepts as the electron distribution over bonding and antibonding orbital fragments [5, 6]. It will be seen that both approaches complement each other. The notion of a surface molecule relates to the surface physicists concept of surface state. 

When the d valence electron band is nearly completely filled, interaction with the doubly occupied CO 5o orbital, leading to a significant fraction of occupied antibonding orbital fragments a between adsorbate and surface atoms, will be repulsive. This Pauli repulsion is proportional to the number of surface atom neighbours and hence is a minimum in atop coordination. This counteracts the... 

The covalent interaction with the adatom 2p orbitals increases with decreasing surface d valence electron occupation, because fewer antibonding orbital fragments then become occupied. The adsorption energy of atoms varies much more strongly with d valence electron occupation than that of molecules because compensating effects occur in the surface chemical bond of molecules. As we discussed above for CO, variation in the interaction with the 5a orbital is partially compensated for by changes in the interaction with the 2jc orbital. 

Because of the similarity in symmetry the quantum-chemical behavior of the H-atomic orbital and the 5important difference as far as the bond strength is concerned. Before bonding with the metal surface, the 5anti bonding orbital fragments with the surface metal orbitals, 5ir electron density is transferred to the metal. Overall donation of electrons from the 5a orbital to the metal surface occurs. The smaller the difference in energy between the unoccupied surface orbitals and the CO 5a orbital, the larger this electron donation and the stronger the 5a CO metal bond. 

The metal-metal interatomic interaction is generally weaker than the strong interaction, with the adsorbates chemisorbed to the surface. Therefore, adatoms or admolecules tend to localize electrons into the surface complex fragment orbitals formed between the adsorbate and surface atoms. 

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