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.

 

·       It is very approximate

 

 

·       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.