A way to predict a reaction pathway using molecular orbitals is described below. This method works especially well for reactions between Lewis acids and bases. A Lewis base contains an atom with a lone pair of electrons that will interact with an atom in the Lewis acid that needs electrons. This method should be applied with extreme caution to other types of chemical reactions.

Caution:

There are many seemingly simple reactions between diatomic molecules that appear to be good candidates for this method. Examples include hydrogen with nitrogen, oxygen, halogens; and oxygen with nitrogen. Application of this method will indicate that the most probable interaction is highly improbable, suggesting the molecules do not react with one another. The problem is that the reactions are not simple bimolecular processes. Other mechanisms including dissociation of molecules or the use of catalysts are far more likely, and are in fact the reactions observed in the laboratory. Consult the kinetics chapter of a general chemistry text for examples of mechanisms of reactions.

Two examples are illustrated by clicking the buttons to the left. For these examples the Molecular Orbitals view shows both MO pairs and energies as seen in the database using the Compare feature. The Prediction view shows a possible orientation of MO's for a reaction.

Procedure

1. Use ball and stick models or Lewis diagrams of the molecules to propose a specific interaction of atoms in the two molecules that might lead to a chemical reaction.

2. Examine and compare the HOMO/LUMO and the LUMO/HOMO pairs for both molecules. If the molecule is a Lewis base (electron pair donor), it uses a HOMO. If the molecule is a Lewis acid (electron pair acceptor), it uses a LUMO.

• Select a molecule from the molecule database

• Select M.O. Reaction Predict from the Optional Views menu at the top of the screen.

• Click the Compare button. First select a set of molecules such as "All Molecules", and then select a second molecule.

• Click the Property title field and then select HOMO/LUMO or LUMO/HOMO.

3. Two conditions must be met to make a possible reaction prediction using molecular orbitals:

• The HOMO of one of the molecules can overlap with the LUMO of the other. Visualize rotations and translations of the M.O. views given in the database for possible overlaps. According to this model, the electrons from the HOMO of one molecule must be able to go into the LUMO of the other molecule.

• The interaction having the lowest energy difference will be favored. Subtract the HOMO energy of one from the LUMO energy of the other. Then subtract the LUMO energy of the first from the HOMO energy of the second.