Molecular orbitals describe the location and density of electrons in a molecule. They can be sensitive indicators of molecular properties, such as nucleo/electrophilicity, UV-stability, redox potential, and much more. Molecular orbitals are formed from combinations of atomic orbitals and come in bonding, non-bonding, and anti-bonding forms. Orbitals can be spread across a whole molecule or localized to individual atoms or fragments.
Frontier orbitals (i.e. the highest energy occupied orbitals and lowest energy unoccupied orbitals) contribute the most to chemical reactivity and photophysical properties. The location of frontier orbitals can indicate where a reaction will take place, while the energy levels indicate the relative willingness to accept or donate an electron. The Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) are indicators of the reduction potential and oxidation potential. The gap between them (often referred to as the HOMO–LUMO gap) can be a sensitive indicator of the UV-stability of a molecule, and modifications to the molecule to increase this gap can lead to significant increases in lifespan of the molecule. The overlap of orbitals between molecules can predict the flow of electrons and holes in OLEDs and other electronics, allowing one to finely tune the efficiency of this transfer.
Core orbitals have less of an effect on reactivity, but they are fundamental to any high-energy spectroscopy techniques. X-ray absorption spectroscopy measures the energy of the core electrons by ejecting them with high-energy x-rays, giving a sensitive indicator of the elements in a material. Bonding information can be derived from the fine structure of the resulting spectra via x-ray near edge spectroscopy (XANES) and x-ray absorption fine structure (EXAFS), with the core and frontier orbital energies being fundamental to understanding the results. Computing the energies of these core orbitals and how orbitals are perturbed when bonded to other elements can help identify materials, the arrangement of atoms in a molecule, and the oxidation state of metals.
