![]() The transition states for these reactions typically involve the electrons of the molecules moving in continuous rings, making it a pericyclic reaction. Applications Cycloadditions Ī cycloaddition is a reaction that simultaneously forms at least two new bonds, and in doing so, converts two or more open-chain molecules into rings. It can be shown that if the total number of these systems is odd then the reaction is thermally allowed. ![]() (4q+2) s refers to the number of aromatic, suprafacial electron systems likewise, (4r) a refers to antiaromatic, antarafacial systems. "A ground-state pericyclic change is symmetry-allowed when the total number of (4q+2) s and (4r) a components is odd" In addition to providing a unified explanation of diverse aspects of chemical reactivity and selectivity, it agrees with the predictions of the Woodward–Hoffmann orbital symmetry and Dewar-Zimmerman aromatic transition state treatments of thermal pericyclic reactions, which are summarized in the following selection rule: From these observations, frontier molecular orbital (FMO) theory simplifies prediction of reactivity to analysis of the interaction between the more energetically matched HOMO-LUMO pairing of the two reactants. 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.e., smallest E r − E s value). The first and second observations correspond to taking into consideration the filled-filled interaction and Coulombic interaction terms of the equation, respectively. 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 occupied orbitals of one molecule and the unoccupied orbitals of the other (especially the HOMO and LUMO) interact with each other causing attraction.Positive charges of one molecule attract the negative charges of the other.The occupied orbitals of different molecules repel each other.This was based on three main observations of molecular orbital theory as two molecules interact: He used these interactions to better understand the conclusions of the Woodward–Hoffmann rules.įukui realized that a good approximation for reactivity could be found by looking at the frontier orbitals ( HOMO/LUMO). 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."įukui'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). 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. In chemistry, frontier molecular orbital theory is an application of MO theory describing HOMO/LUMO interactions.
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