Lumo

LUMO stands for the lowest unoccupied molecular orbital, the lowest energy orbital that is empty in the ground state of a molecule. It is the unoccupied counterpart to the HOMO, the highest occupied molecular orbital. In many organic molecules the LUMO is a pi* antibonding orbital, especially in conjugated systems.

In frontier molecular orbital theory, chemical reactions often involve interactions between a nucleophile’s HOMO and an electrophile’s LUMO. The energy difference between the HOMO and LUMO, the HOMO–LUMO gap, correlates with reactivity, stability, and the wavelength of light absorbed by the molecule. A smaller gap generally indicates higher reactivity and often a longer-wavelength (redder) absorption.

Factors that influence LUMO energy include substituents and conjugation. Electron-withdrawing groups tend to lower the LUMO energy, making a molecule a better electron acceptor, while extended conjugation can also lower or stabilize the LUMO. Conversely, electron-donating groups or reduced conjugation can raise LUMO energy. The LUMO energy, together with the HOMO energy, helps predict sites of electrophilic attack and the overall electronic character of a molecule.

Applications and significance span spectroscopy, where HOMO→LUMO transitions determine color, to materials science and drug design, where LUMO energy informs electron-accepting ability and interactions with nucleophiles. LUMO energies are estimated using quantum chemical methods such as density functional theory or Hartree–Fock calculations, and are understood within the context of a particular molecular geometry and charge state.