Ethylene is the simplest molecule that has a double
bond. As we
saw from the valence bond model, we should find the presence of a
σ-bond framework, and a π-bond between carbons.
(Calculated at the B3LYP/cc-pvdz level using Jaguar, version 7.8, Schrödinger, LLC, New York, NY, 2011.)
Let's
start with the π bond: click on MO 6 in the list below. You should
clearly see the bonding interaction that forms between two pure p
atomic orbitals, with a node in the molecular plane.
Next, let's click through the remaining bonding MOs (1-5). We will see
the same pattern as in methane: the lowest energy MO has the
electron density spread out along the sigma gramework, and higher
energy MOs have electron density concentrated between atoms but still
delocalized over the entire molecule.
Note carefully that there are a total of 6 bonding MOs: the pi
system, plus combinations of 4 C-H and one C-C sigma bond.
One of the crucial peatures of the pi bond is that it restricts bond rotation. Note that ethane (with only a C-C sigma bond) rotates freely; we will see in the next chapter that the barrier to rotation is low--2.9 kcal/mol (12 kJ/mol)--meaning that at room temperature, the bond spins at approximately 50 billion times per second!
For ethene, the double bond increases this barrier to 65 kcal/mol (260 kJ/mol). At room temperature, this means that a molecule will rotate on average once every 1027years.