Summary: A4: Theoretical techniques and ideas
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
`how-to-do' 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 molecular-orbital
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 non-planar 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.