[Ak]

Lowest unoccupied molecular orbital...what is the point of it?...and what does it tell you? I really don't understand the point of it.  Any help on this would be appreciated

[tamim83]

It is the lowest empty orbital in a molecule. When a reaction occurs, we usually consider what happens with the HOMO and LUMO.  Lets say you are doing a Lewis acid base reaction, the electron pair of from the HOMO of the base adds to the LUMO of the Lewis acid.  The difference in energy between these two will determine if this reaction will occur or if it is favorable. 

[Shaun]

The concept of HOMOs and LUMOs important for a number of reasons.
First, in many organic reactions, the HOMO (highest occupied molecular orbital) of one molecule interacts with the LUMO of another molecule to form a new bond between them.  In order for this to occur the orbitals must overlap when they come together.  Understanding the shape of the LUMOs and HOMOs involved can tell you a lot about how a reaction works and what the products will be - or it might explain why a reaction does not work at all.

Second, the difference in energy between the HOMO and LUMO in a given molecule is (usually) the amount of energy it takes to get the molecule into an excited state.  You can also use it to predict the colors of compounds and a number of other uses.

[ArchStanton]

Dude.. you need to read ian flemming book on MO, all will become clear! HOMO and LUMO are the basis of organic chemistry! Seriously it will help no end with your general understanding. By the way the first 2 chapters in that book are quality the rest are hardcore!!!!!