Frontier molecular orbital theory

Frontier molecular orbital theory explains chemical reactivity through interactions between the highest occupied molecular orbital and the lowest unoccupied molecular orbital of reacting species.

Frontier molecular orbitals: The highest occupied and lowest unoccupied molecular orbitals that dominate many chemical interactions. Their overlap and energy gap provide the theory’s central account of reactivity.
Diels–Alder reaction: A concerted cycloaddition between a conjugated diene and a dienophile that forms a six-membered ring. Its favored interaction pairs the diene’s HOMO with the dienophile’s LUMO.
Kenichi Fukui: A Japanese chemist who developed frontier-orbital theory and shared the 1981 Nobel Prize in Chemistry. His calculations connected frontier-electron distributions with reaction sites and reactivity.
Hard and soft acids and bases: A chemical principle that predicts preferences between acids and bases based on their hardness or softness. It classifies reactive partners by polarizability rather than directly comparing frontier orbital interactions.
Highest occupied molecular orbital: The highest-energy molecular orbital occupied by electrons in a molecule. It commonly acts as the electron donor in a frontier-orbital interaction.
Electrocyclic reaction: A pericyclic reaction in which a conjugated system closes or opens a ring by forming or breaking one sigma bond. Frontier orbital phases predict its conrotatory or disrotatory mode.
Molecular orbital theory: A theory of chemical bonding that describes electrons in orbitals spread over molecules. Frontier-orbital theory selects particular molecular orbitals from this broader framework.
Transition state theory: A theory relating reaction rates to the properties of a high-energy transition-state configuration. It predicts rates through activation barriers, while frontier-orbital theory emphasizes orbital interactions and selectivity.
Lowest unoccupied molecular orbital: The lowest-energy molecular orbital with no electrons in a molecule’s ground state. It commonly acts as the electron acceptor in a frontier-orbital interaction.
1,3-Dipolar cycloaddition: A cycloaddition joining a 1,3-dipole and a dipolarophile to form a five-membered ring. Frontier orbital coefficients help rationalize regioselectivity in these cycloadditions.

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