Question

identify and justify the approximations used in the Huckel theory of hydrocarbons.

Answer 1

1) The atomic orbitals contributing to the π-bonding in a planar molecule (e.g. the so-called pπ orbitals in a molecule such as benzene) are antisymmetric with respect to reflection in the molecular plane; they are therefore of a different symmetry to the atomic orbitals contributing to the σ-bonding and may be treated independently

2) The Coulomb integrals for all the carbon atoms are assumed to be identical.

3) All resonance integrals between directly-bonded atoms are assumed to be the same; whilst those between atoms that are not directly bonded are neglected.

4) All overlap integrals representing the overlap of atomic orbitals centred on different atoms are neglected.