Conjugated Multiple Bonds

When each adjacent atom in a row of have a p orbital with the same orientation, a px orbital for example, the p orbitals will combine to make pi orbitals that extend over all those atoms. The atoms in such a set have a conjugated pi system.

For a conjugated set of n atoms:
  1. n atomic p orbitals forms a set of n pi symmetry molecular orbitals

  2. average energy of the pi molecular orbitals equals the average energy of the p orbitals

  3. as n increases, the energy difference between the pi molecular orbitals decreases

Why is this important? When molecules contain pi bonding orbitals, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are typically pi symmetry molecular orbitals.

Molecules are colored when they absorb visible light. In unsaturated molecules, an electron is promoted from one pi orbital to another. The difference in energy between these orbitals determines the wavelength of light that the molecule can absorb.

The molecules in the table above are all colorless. The energy between the HOMO and LUMO pi orbitals is above the visible light region. They absorb light in the shorter wavelength ultraviolet region of the spectrum.

The beta-carotene molecule, with 26 interacting p orbitals, does absorb visible light. You've seen that ozone acts as a bleach, converting the orange molecule to a colorless one. This is because it breaks the pi system. Instead of 26 interacting p orbitals with an energy gap in the visible region, there is a 14 p orbital unit and a 10 p orbital unit. These pi systems have a HOMO-LUMO energy gap in the ultraviolet.

Professor Patricia Shapley, University of Illinois, 2012