 
Summary: A4: Theoretical techniques and ideas
Ali Alavi
Part II Chemistry, Michaelmas 2009
I. RECAP OF H ¨UCKEL THEORY
We begin with a brief recap of H¨uckel theory, taking initially the practical viewpoint of
`howtodo' calculations, rather than the `meaning' of the subject. We will later return to
this latter aspect. This approach, I hope, will have the advantage of getting you started on
familiar ground. Actually, H¨uckel theory is an archetype of more general molecularorbital
theory, and even more widely, of many quantum mechanical principles. Therefore, a good
practical grasp of it proves invaluable in many areas of chemistry.
Let us start with the benzene molecule, the H¨uckel theory of which is very instructive. The
steps are as follows. To begin with, we suppose we can treat separately the electrons from
the electrons. For planar molecules such as benzene, this assumption is strictly justifiable
on symmetry grounds, but we will nevertheless assume it to be true for nonplanar molecules
as well. Next, we set up a "basis" of 6 pz (or, as we shall sometimes interchangeably refer to
them, as p) atomic orbitals, one for each carbon in the appropriate geometry. We will label
these atomic orbitals 1, ..., 6.
1
2 3
4
