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Concept
Frontier molecular orbital theory
Summary
In chemistry, frontier molecular orbital theory is an application of MO theory describing HOMO/LUMO interactions.
In 1952, Kenichi Fukui published a paper in the Journal of Chemical Physics titled "A molecular theory of reactivity in aromatic hydrocarbons." Though widely criticized at the time, he later shared the Nobel Prize in Chemistry with Roald Hoffmann for his work on reaction mechanisms. Hoffman's work focused on creating a set of four pericyclic reactions in organic chemistry, based on orbital symmetry, which he coauthored with Robert Burns Woodward, entitled "The Conservation of Orbital Symmetry."
Fukui's own work looked at the frontier orbitals, and in particular the effects of the Highest Occupied Molecular Orbital (HOMO) and the Lowest Unoccupied Molecular Orbital (LUMO) on reaction mechanisms, which led to it being called Frontier Molecular Orbital Theory (FMO Theory). He used these interactions to better understand the conclusions of the Woodward–Hoffmann rules.
Fukui realized that a good approximation for reactivity could be found by looking at the frontier orbitals (HOMO/LUMO). This was based on three main observations of molecular orbital theory as two molecules interact:
The occupied orbitals of different molecules repel each other.
Positive charges of one molecule attract the negative charges of the other.
The occupied orbitals of one molecule and the unoccupied orbitals of the other (especially the HOMO and LUMO) interact with each other causing attraction.
In general, the total energy change of the reactants on approach of the transition state is described by the Klopman-Salem equation, derived from perturbational MO theory. The first and second observations correspond to taking into consideration the filled-filled interaction and Coulombic interaction terms of the equation, respectively. With respect to the third observation, primary consideration of the HOMO-LUMO interaction is justified by the fact that the largest contribution in the filled-unfilled interaction term of the Klopman-Salem equation comes from molecular orbitals r and s that are closest in energy (i.
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