Example 1 investigates the reaction of sulfur trioxide (a Lewis acid) with water (a Lewis base). Sulfuric acid is produced in this reaction.

Diagrams

This example investigates the reaction of sulfur trioxide with water. Examination of the Lewis diagrams shows that the sulfur has no lone electron pairs, while oxygen in water has two lone electron pairs. A reasonable prediction is an interaction between the sulfur atom of sulfur trioxide and the oxygen atom of the water molecule.

Molecular Orbitals

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

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

Sulfur trioxide Water
LUMO -0.06 +4.08
HOMO -12.91 -12.30

LUMO (H₂O) - HOMO (SO₃)=
+4.08 -(-12.91) =16.99
LUMO (SO₃) - HOMO (H₂O)=
-0.06 - (-12.3) = 12.24

The lower energy difference represents the most favorable interaction. Based on the calculation shown above, we would use the LUMO of SO₃; and the HOMO of H€O.

We must now consider whether or not such an interaction is possible.

Prediction

This graphic shows the MO's with their front surfaces cut away to reveal the position of the ball and stick models for the interaction predicted. Notice that the LUMO of the sulfur trioxide molecule constructively overlaps with the HOMO of the water molecule.

We can predict that the sulfur trioxide shares an electron pair through its LUMO that was originally from the HOMO of water. This is a description of a Lewis acid/base reaction.

A generalization of this prediction is that the HOMO of a Lewis base (water in our example) donates a pair of electrons, which that are shared by the LUMO of the Lewis acid (sulfur trioxide).

Experimental data are necessary to confirm this prediction.