Highest occupied molecular orbital photochemical HOMO

If the reverse back reaction is prevented or is forbidden by other considerations, the energy remains stored in the photoproducts. Some simple photorearrangement reactions which are governed by Woodward-Hoffman rules have been found useful. These rules provide the stereochemical course of photochemical rearrangement based on symmetry properties of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the molecule (Section 8.6). A reaction which is photochemically allowed may be thermally forbidden. Front the principle of microscopic reversibility, the same will be true for the reverse reaction also. Thermally forbidden back reaction will produce. ble - photoproducts. Such electrocyclic rearrangements are given in . ..ure... 

These theories assert that the pathway of a chemical reaction accessible to a compound is controlled by its highest occupied molecular orbital (HOMO). For the thermal reaction of butadiene, which is commonly called ground-state chemistry, the HOMO is 2 and lowest unoccupied molecular orbital (LUMO) is photochemical reaction of butadiene, which is known to be excited-state chemistry, the HOMO is 1//3 (Fig. 3.5.6). 

An acronym for highest occupied molecular orbital. In a photochemically excited state, this orbital is represented as HOMO, (p. 693)... 

The easiest explanation is based on the frontier orbitals—the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied orbital (LUMO) of the other. Thus if we compare a [2 + 2] cycloaddition 6.133 with a [4 + 2] cycloaddition 6.134 and 6.135, we see that the former has frontier orbitals that do not match in sign at both ends, whereas the latter do, whichever way round, 6.134 or 6.135, we take the frontier orbitals. In the [2 + 2] reaction 6.133, the lobes on C-2 and C-2 are opposite in sign and represent a repulsion—an antibonding interaction. There is no barrier to formation of the bond between C-l and C-l, making stepwise reactions possible the barrier is only there if both bonds are trying to form at the same time. The [4 + 4] and [6 + 6] cycloadditions have the same problem, but the [4 + 2], [8 + 2] and [6 + 4] do not. Frontier orbitals also explain why the rules change so completely for photochemical reactions, as we shall see in Chapter 8. 

Although the chemistry of a compound is determined by all its molecular orbitals, we can learn a great deal about the chemistry of a compound by looking at only the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). These two molecular orbitals are known as the frontier orbitals. We will now see that simply by evaluating one of the frontier molecular orbitals of the reactant(s) in a pericyclic reaction, we can predict the conditions under which the reaction will occur (thermal or photochemical, or both) and the products that will be formed. 

The excitation of a molecule promotes an electron from a filled molecular orbital to a vacant molecular orbital. The highest occupied molecular orbital (HOMO) is the filled nonbonding n molecular orbital of a saturated carbonyl compound. For an aUcene, it is the bonding k molecular orbital, whereas for a diene, it is the bonding /2 molecular orbital for thermal and ii molecular orbital for photochemical reaction. The lowest unoccupied molecular orbital (LUMO) is the antibonding n molecular orbital of a saturated carbonyl compound or alkene. For a diene, it is and /4 molecular orbital, for thermal and photochemical reaction, respectively. 

Within each class, we will consider the number of electrons in n molecular orbitals in the transition state. Pericyclic reactions may involve either An or 4 + 2 electrons, where is an integer. This distinction is important because the symmetry of the molecular orbitals in thermal or photochemical reactions depends on the number of n electrons in the transition state. The stereochemistry of the products of pericychc reactions depends on the symmetry of these molecular orbitals. We will see that the stereochemistry of a product of a pericychc reaction derived from a 4 7i electron system differs from that of a 4 + 2 7t electron system. Thus, the concepts of orbital symmetry and Hiickle systems that we considered in Chapter 11 and in particular the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) will play a central role in our discussions of pericychc reactions. 

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