Answer Happy

P9E.6 Electronic excitation of a molecule may weaken or strengthen some bonds because bonding and antibonding characteristics differ between the HOMO and the LUMO. For example, a carbon-carbon bond in a linear polyene may have bonding character in the HOMO and antibonding character in the LUMO. Therefore, promotion of an electron from the HOMO to the LUMO weakens this carbon-carbon bond in the excited electronic state, relative to the ground electronic state. Consult Figs. 9E.2 and 9E.4 and discuss in detail any changes in bond order that accompany the t* tita ultraviolet absorptions in butadiene and benzene.

41 2 – 1.62B + + - 31 X - 0.62B + - 1 + C2p Energy 21 & +0.62B + + :- 11 + + & + 1.62B + + Figure 9E.2 The Hückel molecular orbital energy levels of butadiene and the top view of the corresponding n orbitals. The four p electrons (one supplied by each C) occupy the two lower it orbitals. Note that all the orbitals are delocalized.

b. 2g е. 2u Energy 19 + + + a 2u Figure 9E.4 The Hückel orbitals of benzene and the corresponding energy levels. The orbital labels are explained in Topic 10B. The bonding and antibonding character of the delocalized orbitals reflects the numbers of nodes between the atoms. In the ground state, only the bonding orbitals are occupied.