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 π*←π ultraviolet absorptions in butadiene and benzene. 4tX – 1.62ß3na - 0.62BC2p2nx + 0.62ß + +1nlX + 1.62B + +Figure 9E.2 The Hückel molecular orbital energy levels ofbutadiene and the top view of the corresponding t orbitals. Thefour p electrons (one supplied by each C) occupy the two lower torbitals. Note that all the orbitals are delocalized.Energy+ 1ga2uFigure 9E.4 The Hückel orbitals of benzene and the correspondingenergy levels. The orbital labels are explained in Topic 10B. Thebonding and antibonding character of the delocalized orbitalsreflects the numbers of nodes between the atoms. In the groundstate, only the bonding orbitals are occupied.Energy

The number of e-s in butadiene is 4 and the number of e-s in benzene is 6. The HOMO of butadiene…

Answered: 4t X – 1.62ß 3n a - 0.62B C2p 2n x +…

Chemistry & Chemical Reactivity

10th Edition

ISBN:9781337399074

Author:John C. Kotz, Paul M. Treichel, John Townsend, David Treichel

Publisher:John C. Kotz, Paul M. Treichel, John Townsend, David Treichel

Chapter9: Bonding And Molecular Structure: Orbital Hybridization And Molecular Orbitals

Section: Chapter Questions

Problem 65SCQ: Three of the four molecular orbitals for cyclobutadiene are pictured here. Place them in order of...

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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 π*←π ultraviolet absorptions in butadiene and benzene.

4t
X – 1.62ß
3n
a - 0.62B
C2p
2n
x + 0.62ß + +
1n
lX + 1.62B + +
Figure 9E.2 The Hückel molecular orbital energy levels of
butadiene and the top view of the corresponding t orbitals. The
four p electrons (one supplied by each C) occupy the two lower t
orbitals. Note that all the orbitals are delocalized.
Energy
+

1g
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.
Energy

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