Frontier Molecular Orbitals

We can focus further on two very important types of molecular orbitals: the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), also referred to collectively as the frontier molecular orbitals (Figure 1.1 “Frontier molecular orbitals HOMO and LUMO”). As their names imply, the HOMO is the molecular orbital that has the highest energy and contains electrons, while the LUMO is the lowest energy molecular orbital that does not contain electrons.

Figure 1.1. Frontier molecular orbitals HOMO and LUMO.

When molecules absorb energy, it is typical for a HOMO electron to use this energy to transition from the ground HOMO orbital to the LUMO excited-state orbital. This type of transition can be observed in ultraviolet-visible (UV-Vis) radiation spectroscopy experiments. As well, in many chemical reactions, one reactant molecule may donate HOMO electrons to the LUMO of another reactant (Figure 1.2 “Formation of a new bonding molecular orbital by combining reactant HOMO and LUMO”). Therefore, understanding frontier molecular orbital energy levels can provide chemists with a great deal of insight in the areas of molecular spectroscopy and reactivity.

Figure 1.2. Formation of a new bonding molecular orbital by combining reactant HOMO and LUMO.