Between Molecule and Solid

To give more breadth to our discussion of the connections between molecules and solids we now consider examples of systems in which one can see transition-metal complexes. They are of pedagogical interest because their simple electronic structures can be used to good effect in elementary treatments of the electronic structure of coordination compounds. 

Figure 3.4 Schematic representation of forces acting between molecules, and between molecules and solid wall (adapted from Kerkhof [22]).

It is often said that the band description of one-electron states is in terms of itinerant electrons and is mainly useful for solids, while the bond description looks at localized electrons and is appropriate for molecules. Since our subject concerns interactions between molecules and solid surfaces, we need to establish our vocabulary clearly. We will consider an electron as localized if it cannot participate in (electrical) transport phenomena otherwise it is itinerant. This is not the same as describing the one-electron orbitals by localized functions (such as the Wannier functions, introduced below) respectively by extended functions (such as the Bloch functions, see below). Nor is it simply a distinction between tight-binding orbitals constructed from (so-called localized) J-orbitals as opposed to those derived from (so-called extended) -orbitals. 

The transition between molecules and solids has become a fascinating area of research. Such compounds have been termed clusters, and their size ranges from a few atoms up to thousands of atoms. With growing cluster size, the properties of these compounds gradually change towards solid-state properties. It is therefore essential to understand the growth mechanism of clusters in order to design compounds with particular useful properties. 

Its characteristic is low interaction between molecules and solid surfaces. The resulting forces are of the same order of the van der Waals forces, and the enthalpy of adsorption is in the same range of the condensation enthalpy or evaporation gases... 

Different from the physisorption, the chemisorption is irreversible and occurs at higher temperatures than the condensation temperature, and since the interaction is specific between molecules and solids, the adsorbed molecules form a monolayer. 

Table 4.3 Value of m Determined from Type of Binding Between Molecule and Solid Surface...

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