Bonding of molecules

The concepts which we need for understanding the structural trends within covalently bonded solids are most easily introduced by first considering the much simpler system of diatomic molecules. They are well described within the molecular orbital (MO) framework that is based on the overlapping of atomic wave functions. This picture, therefore, makes direct contact with the properties of the individual free atoms which we discussed in the previous chapter, in particular the atomic energy levels and angular character of the valence orbitals. We will see that ubiquitous quantum mechanical concepts such as the covalent bond, overlap repulsion, hybrid orbitals, and the relative degree of covalency versus ionicity all arise naturally from solutions of the one-electron Schrodinger equation for diatomic molecules such as H2, N2, and LiH. 

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Let us consider the catalytic reaction between two molecules A and B to give a product P, see Fig. 1.1. The cycle starts with the bonding of molecules A and B to the catalyst. A and B then react within this complex to give a product P, which is also bound to the catalyst. In the final step, P separates from the catalyst, thus leaving the reaction cycle in its original state. 

The main handicap of MD is the knowledge of the function [/( ). There are some systems where reliable approximations to the true (7( r, ) are available. This is, for example, the case of ionic oxides. (7( rJ) is in such a case made of coulombic (pairwise) interactions and short-range terms. A second example is a closed-shell molecular system. In this case the interaction potentials are separated into intraatomic and interatomic parts. A third type of physical system for which suitable approaches to [/( r, ) exist are the transition metals and their alloys. To this class of models belong the glue model and the embedded atom method. Systems where chemical bonds of molecules are broken or created are much more difficult to describe, since the only way to get a proper description of a reaction all the way between reactant and products would be to solve the quantum-mechanical problem at each step of the reaction. 

Fig. 9. Some reactions obtained by breaking two bonds of molecule 8 and making two new ones (bonds broken and made are emphasized)...

Electromagnetic radiation in between KXX) and 25(X) nm is weakly absorbed by the X-H bonds of molecules causing them to stretch. The wavelength of the radiation absorbed is characteristic of the bond absorbing it. 

As in all active areas of research, there are several approaches to the development of nanotechnology. One approach involves techniques similar to sculpting, where one starts with a large piece of material and cuts away what is not needed. The problem is that a lot of the material winds up wasted on the cutting room floor. An alternative approach, described in the following section, starts at the bottom or lower end of the scale and builds up from there. An important feature of this approach is the bonding of molecules to make even larger, more complex molecules—supramolecular chemistry. 

The extension of molecular orbital theory to triatomic molecules is a major part of this chapter. It gives a very satisfactory description of the shapes and the bonding of molecules in general, and is consistent with observations of photoelectron and electronic absorption spectra. It is not possible for the VSEPR theory to explain these latter observations. 

Water has a complex structure determined by hydrogen bonding of molecules. In view of the above discussion we assume now that a dipole is not rigid. Introducing some changes in our recent works [6, 8], we represent a total dipole moment ptot as a superposition of the constant part p and of a small decaying component p(f) due to fast vibration of the H-bonded molecules ... 

Chemical surface modification may be defined as the chemical bonding of molecules or molecule fragments to a surface in order to change its chemical or physical properties in a controlled way. 

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. 

Applying the general reaction schemes contained in our program system onto the bonds of molecules allows the generation of all conceivable reactions or retro-reactions of these molecules. The task is then to select the chemically feasible reactions from the set of the mathematically possible ones. Here lie the main efforts in... 

It should be clear by now that the LE model is of great value in interpreting the structures and bonding of molecules. However, there are some problems with this model at this level of approximation. For example, since it incorrectly assumes that electrons are localized, the concept of resonance must be added. Also, the model does not deal easily with molecules containing unpaired electrons. And finally, the model in this form gives no direct information about bond energies. 

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