What is the HOMO LUMO concept
Molecular Orbital Theory (MO Theory)
Construction of molecular orbitals
With the help of Molecular orbital theory orbitals can be described within molecules. This creates binding and antibonding molecular orbitals. There are just as many molecular orbitals as the atoms involved have atomic orbitals. The orbitals are formed using linear combinations (LCAO method). The more nodes there are in the linear combination, the higher the energetic level of the orbitals.
Highest Occupied Molecule Orbital (HOMO) / Lowest Unoccupied Molecule Orbital (LUMO)
The highest occupied orbital (HOMO - Highest Occupied Molecule Orbital) and the lowest unoccupied orbital (LUMO - Lowest Unoccupied Molecule Orbital).
Example hydrogen molecule
Molecular orbitals of different bond types
The energies that are released during the bond are basically composed of two parts. Once the ionic part and once the covalent part . The total energy is the energy that arises from the lowering of the atomic orbitals to the molecular orbital of both binding partners.
For one covalent binding the molecular orbital looks like this (compare above with the hydrogen molecule):
For ethane the binding energy is approx .
For one polar binding the molecular orbital looks like this:
For methanol the binding energy is composed of . It is as mentioned above the ionic part of the bond.
For one ionic binding the molecular orbital looks like this:
For sodium fluoride, the binding energy consists almost entirely of .
Spin pairing energy
The above values are only approximately correct. Indeed, there are other factors to consider, such as the Spin pairing energy.
If an electron is in an orbital with another, the energy is slightly higher than if it were there alone. This energy is called the spin mating energy.
Interactions and higher order interactions
The antibonding molecular orbital requires a higher energy than the binding molecular orbital supplies. Therefore, interactions in which the binding and the antibonding orbital are occupied are of an antibonding nature:
If, on the other hand, only one orbital, namely the binding molecular orbital, is occupied, a binding interaction occurs between the HOMO and the LUMO:
However, a second order interaction can also occur between a deep atomic orbital and a very high unoccupied atomic orbital:
However, these second-order interactions are not so strong, which is why the HOMO and the LUMO contribute significantly to the stability of the resulting molecule.
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