p-bond orders from
Hückel Theory agree quite well with experimental bond lengths for aromatic
hydrocarbons (shorter bonds have higher bond orders), and also correlate well
with experimental vibrational frequencies (higher bond orders give higher
frequencies).
Hückel Theory can be extended
to treat other types of atoms in conjugated molecules (e.g. nitrogen and oxygen
– see the Class). Also, it can be extended to treat not only p orbitals, but
s-orbitals as well. Extended Hückel Theory is still used today, particularly to
provide simple models to understand chemical behaviour. However, it is highly
approximate and qualitative at best.
We should remember the limitations of Hückel Theory.
· For example, the repulsion between electrons is not calculated.
Example of the limitations of Hückel Theory: Non-alternant
hydrocarbons
Hückel Theory correctly predicts that non-alternant hydrocarbons are polar, in agreement with experiment. However, it greatly overestimates their dipole moments:
Compound |
Experimentally observed (D) |
Hückel calculation (D) |
Fulvene |
1.2 |
4.7 |
Azulene |
1.0 |
6.9 |
This is because electron repulsion is not allowed for, and also because some atoms have higher charges than others, so the approximation that Hii and Hij are the same for all the atoms fails.
Hückel Theory is best used to provide models for understanding chemistry.
For detailed understanding of chemical
behaviour, ‘ab initio’ molecular orbital methods can provide a good approach.
One area where Hückel MOs have proved extremely valuable is in understanding pericyclic reactions, as we will see in the next lectures.