The energy-level diagram in Figure 9.36 shows that the sideways overlap of a pair of p orbitals produces two molecular orbitals, one bonding and one antibonding. In ethylene there is a pair of electrons in the bonding π orbital between the two carbons. Absorption of a photon of the appropriate wavelength can result in promotion of one of the bonding electrons from the p2p to the p*2p molecular orbital. (a) Assuming this electronic transition corresponds to the HOMO-LUMO transition, what is the HOMO in ethylene?

Key Concepts

Molecular Orbitals

Molecular orbitals (MOs) are formed when atomic orbitals combine during the bonding process. In ethylene, the sideways overlap of p orbitals creates bonding (π) and antibonding (π*) molecular orbitals. The bonding orbital is lower in energy and stabilizes the molecule, while the antibonding orbital is higher in energy and can destabilize it if occupied.

HOMO and LUMO

The Highest Occupied Molecular Orbital (HOMO) is the molecular orbital that contains the highest energy electrons in a molecule, while the Lowest Unoccupied Molecular Orbital (LUMO) is the lowest energy orbital that is unoccupied. In the context of ethylene, the transition of an electron from the HOMO to the LUMO upon photon absorption is crucial for understanding its electronic properties and reactivity.

Electronic Transitions

Electronic transitions occur when an electron absorbs energy, typically from a photon, and moves from a lower energy state (HOMO) to a higher energy state (LUMO). In ethylene, this transition is significant for understanding its absorption spectrum and the wavelengths of light it can absorb, which is essential for applications in photochemistry and materials science.