Molecular orbitals benzene, antibonding

The UV spectrum of benzene is interpreted as the excitation of an electron from the highest occupied molecular orbital (HOIMO, bonding n MO) to the lowest unoccupied molecular orbital (LUIMO, antibonding n MO ). The bond order of the TT-system changes from 3 to 2. 

A molecular orbital description of benzene has three tt orbitals that are bonding and three that are antibonding Each of the bonding orbitals is fully occupied (two electrons each) and the antibonding orbitals are vacant... 

Having just seen a resonance description of benzene, let s now look at the alternative molecular orbital description. We can construct -tt molecular orbitals for benzene just as we did for 1,3-butadiene in Section 14.1. If six p atomic orbitals combine in a cyclic manner, six benzene molecular orbitals result, as shown in Figure 15.3. The three low-energy molecular orbitals, denoted bonding combinations, and the three high-energy orbitals are antibonding. 

Figure 15.3 The six benzene tt molecular orbitals. The bonding orbitals >p2 and t 3 have the same energy and are said to be degenerate, as are the antibonding orbitals tf/4 and 5. The orbitals and 4 have no tt electron density on two carbons because of a node passing through these atoms.

In marked contrast to these are the fully unsaturated aromatic compounds, epitomized by benzene, in which the carbocyclic rings formally consist of a conjugated set of alternating single and double bonds. Such systems have a specially stabilized cyclic ir-electron system in which all of the bonding molecular orbitals are completely filled and the antibonding orbitals are all empty. The concept of... 

Another array of orbitals with lhe same sort of linear combinations is that of the rr system of cydopolyencs (benzene, naphthalene) with which you are probably already Familiar The molecular Orbitals constructed from sets of parallel p orbitals are both bonding and antibonding at various energies. The system b particularly stable if Huckd s rule 2m + 2 electrons, see also Chapter 15) e obeyed In the case of an mb rate array of hythogen atoms (Is1), the situation is unstable (as you should have questioned immcdiniclvl it reverts to an array of H2 mdeoics ... 

Energy diagram of the molecular orbitals of benzene. Benzene s six 77 electrons fill the three bonding orbitals, leaving the antibonding orbitals vacant. 

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