Hückel Theory

Approximate treatment of Conjugated p-electron systems.

see: P.W. Atkins, Physical Chemistry, 6^th edn., pages 412-416.

Consider the p-MOs of ethene, y_i, which we shall assume can be constructed as linear combinations of the carbon 2p atomic orbitals perpendicular to the molecular plane.

In this equation i labels the particular MO, p the individual carbon atoms. f₁ and f₂ are the carbon 2p atomic orbitals (AOs) as shown in the figure.Note that in this case there are only two AOs (p=1,2) and consequently only two MOs (i=1 and 2).The c_ip are the MO coefficients which are determined by minimizing the energy (i.e., using the variation principle) - we now discuss this in detail.

The pi MOs of ethene