Antibonding orbital (Ψ 4 *)
C C C C
C C C C
Antibonding orbital (Ψ 3 *)
Atomic orbitals (φ 1 )
Atomic orbitals (φ 2 )
C C C C
C C C C
C C C C
Bonding orbital (Ψ 1 )
C C C C
C C C C
Bonding orbital (Ψ 2 )
π π*
Figure 5.19 A partial MO
energy-level diagram for butadiene emphasizing the π
bonding (Ψ 1 and Ψ 2 ) and
antibonding (Ψ 3 * and Ψ 4 *)
MOs. MOs are constructed by
the linear combination of the
AOs of butadiene, in this illustration the unhybridized porbitals on each carbon
atom grouped together and
described as ϕ 1 and ϕ 2 . The
four electrons from each porbital are placed in the
bonding MOs (Ψ 1 and Ψ 2 ). The
MOs due to σ-bonding are not
shown.
Ψ 2
*
Ψ 4
*
Ψ 3
*
Ψ 3
Ψ 3
Ψ 4
*
Ψ 4
Ψ 5
*
Ψ 6
*
Ψ 8
*
Ψ 7
*
Ψ 6
*
Ψ 5
*
Ψ 2
Ψ 2
Ψ 2
Ψ 1
Ψ 1
Ψ 1
Ψ 1
Energy
Ethylene
Hexatriene
Octatetraene
Conjugation length (number of C atoms)
2
Butadiene
8
6
4
Figure 5.20 The MO energylevel diagram for a series of
linear conjugated hydrocarbons. The figure shows how
ΔE for the π ! π* transition
changes as the conjugation
length increases.
CHAPTER 5: Intermolecular Interactions and Self-Assembly
172