The strength of the interaction is determined by:

• the energy difference between the two orbitals involved (the smaller the better).
• the amount of overlap between the orbitals (the larger the better).

These effects can account for the subtle differences that are found in facial selectivity, caused by

• steric effects which hinder overlap, or by
• asymmetry of a pi orbital, which favours overlap on that side where the orbital is larger.

Example: Diels-Alder reaction of fluorocyclopentadiene, which takes place on the side where the F resides. Sterically the other side would be preferred, so an electronic effect must play a role. We can investigate the orbital concerned, the HOMO of fluorocyclopentadiene, and other cyclopentadienes.