![]() Therefore, there is increased electron density between the nuclei in the molecular orbital – this is why it is a bonding orbital. In the bonding Ψ 1 orbital, the two (+) lobes of the 2p z orbitals interact constructively with each other, as do the two (-) lobes. These are sometimes denoted, in MO diagrams like the one below, with the Greek letter psi ( Ψ) instead of π. Molecular orbital interaction between frontier orbitals.Īccording to MO theory discussed in Section 1-11, when a double bond is non-conjugated, the two atomic 2 p z orbitals combine to form two pi ( π) molecular orbitals, one a low-energy π bonding orbital and one a high-energy π-star ( π*) anti-bonding molecular orbital. The electrons would be donated, in turn, to the lowest empty energy level on the other species, called the lowest unoccupied molecular orbital (LUMO). In this level, called the highest occupied molecular orbital (HOMO), the electrons are further from the nucleus and therefore less tightly held by the protons in the nucleus. This idea says that if one species is going to donate electrons to another in order to form a new bond, then the donated electrons are most likely going to come from the highest occupied energy level. One common way of thinking about reactions in this way is through the concept of frontier orbitals. HOMO and LUMO are often referred to as frontier orbitals and their energy difference is termed the HOMO–LUMO gap. A detailed analysis of three reaction types is provided in the subsequent sections of this chapter. Their analysis of cycloadditions, electrocyclic reactions, and sigmatropic rearrangements is commonly referred to as the Woodward-Hoffmann Rules. In 1965 Robert Burns Woodward and Roald Hoffmann used Frontier Molecular Orbital Theory, initially proposed by Kenichi Fukui, to develop their Theory of Conservation of Orbital Symmetry where outcomes of pericyclic reactions are explained by examining the Highest Occupied Molecular Orbital (HOMO) or Lowest Unoccupied Molecular Orbital (LUMO) of the reacting system. Prior to 1965, pericyclic reactions were known as "no mechanism reactions" since no one could adequately explain why reaction outcomes changed depending on whether reactants were exposed to heat or light. ![]()
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