All molecules have a HOMO (highest occupied molecular orbital) and a LUMO (lowest unoccupied molecular orbital).

It is often through overlap of the HOMO of one molecule with the LUMO of another that new bonds are formed during chemical reactions.

In order for bonds to form, the overlap of the orbitals must be constructive. In the regions of large overlap the orbitals (wave functions) must have the same sign.

In this program, color codes for the wave function signs are arbitrary. Constructive overlap is possible between the blue region of an occupied orbital and the yellow region of an unoccupied orbital or between the green region of an occupied and the red region of an unoccupied orbital.

If two constructive interactions are possible, the one with the lower energy difference between the HOMO and LUMO is more likely to occur.

In order to compare the LUMO of one molecule with the HOMO of another, select M.O. Reaction Predict from the Optional Views menu. Then, by selecting the LUMO/HOMO option in the Compare mode, the database will display the LUMO of the currently selected molecule with the HOMO of the molecule chosen from the molecule comparison list.

Molecular Orbitals

The graphic shows the MO's as seen using the Compare feature in the database.

A check for the smaller energy differences of these orbitals is needed. Find the energy difference between LUMO of one and the HOMO of the other and vice versa.

Carbon Dioxide Hydroxide Ion
LUMO +1.24 +14.99
HOMO -12.77 -1.04

The smaller energy difference is between the carbon dioxide LUMO and the hydroxide ion HOMO. A visual inspection of the orbitals shows that if the oxygen atom of the hydroxide ion were to approach the carbon atom in carbon dioxide, there would be green-red and/or blue-yellow overlap depending on the direction of approach.

Reaction Prediction

This graphic shows the LUMO and HOMO with their front surfaces cut away to reveal the positions of the ball and stick models. Notice that the LUMO of the carbon dioxide molecule overlaps constructively (red-green overlap) with the HOMO of the hydroxide ion.

Thus the predicted reaction is a sharing of electrons from the HOMO of the hydroxide ion with the LUMO of carbon dioxide. This is a description of a Lewis acid/base reaction. Hydroxide ion, a Lewis base, shares electrons from its HOMO with the LUMO of carbon dioxide, a Lewis acid, forming bicarbonate ion, HCO‹–.