It has been found that Hückel Theory can give very good agreement
with experiments, and so make useful predictions.
- However, the values of b that come from
comparing with different types of experiments on different types of
molecules are very different. (For example, plotting the energy of p ®p* transitions in
conjugated hydrocarbons against the Hückel HOMO-LUMO energy difference
gives a value of b = –253.1 kJ/mol).
- The real value of b is different in different types of molecules,
and may be quite different between different atoms (e.g. the bond lengths
in benzene are all the same but they are not in butadiene, so the amount
of conjugation is different in the two cases). We should not expect b to be a constant –
this is an approximation. It can be relatively constant for a series of
similar molecules, however.
We can use Hückel Theory to calculate other molecular
properties too. Two of the most important properties we are interested in
chemically are how strong bonds are, and if/where charge is concentrated.
Next: p-electron charge