III. Unusual AO Energy

From WikidChem

Iii._unusual_ao_energy

As the previous slide discusses, there are four main factors that can contribute to creating an unusually high HOMO or unusually low LUMO: Unmixed Valence Shell Atomic Orbitals, Poor Overlap, Charged Molecules, and Unusual Atomic Orbital Energies. This wiki page is dedicated to the last situation. To establish what "unusual" energies are, we will compare to the standard C-H or C-C bond. (essentially the overlap of bonds that start with energies like that of 1s H orbitals and sp3 hybridized C orbitals) These work as a good standards because the atomic orbitals from both atoms are very close in energy and we make molecular orbitals from good energy match. Therefore, the LUMOs and HOMOs formed by C-H and C-C bonds are "usual" molecular orbitals.

Unusually low LUMOs and high HOMOs then result from unusually low or high energies in the AOs that combine to form them. The slide shows this with an AO that is lower in energy than C (F) and an AO that is higher in energy than C (B). They are lower and higher respectively because of their nuclear charges. F has 9 protons and B has 5, compared to C which has 6. F's supply of protons stabilizes its AO and lowers its energy becuase its nuclear charge pulls its electons in closer to the nucleus. B is more losely held because of its fewer protons.

When a "usual" energy AO combines with an "unusual" energy AO, either the HOMO or LUMO created will be "unusual" in its energy. In the C-F instance, while the HOMO is at about the same energy as the "usual" despite the mismatch in energy, the LUMO is unusual because the molecular orbital diagram is shifted down becuase of the low energy of the F's AO. Similarly, in H-B, the HOMO is unusual becuase the diagram is shifted up by B's high energy AO.

It might be important to note that the diagram is merely a qualitative picture of the splitting of the bonding and anti-bonding orbitals. In reality, the energy mismatch of both the C-F and the H-B should produce a larger splitting than the C-H, creating more of a difference between the energies of the bonding and anti-bonding orbitals. However, we would still retain the unusually high HOMO of B-H and the unusually low LUMO of C-F

last modified by Connie Wang (cyw4) on Oct 30, 2006 9:57:15 AM