Chemistry - A Molecular Science

p* n p

LUMO HOMO

s

s

o

o

o

o

+

+

s

o

o

abc

Figure 6.25 MO’s for the

π system of SO

(^2)
(a) The p orbitals from the side of the molecule, so the ‘figure 8’ shape is apparent (b) The p orbita
ls viewed from the top of the
molecule, so only the tops of the lobes are visible (c) The relative MO energies
LUMO HOMO
CC
CC
H HH
H H
H
CH
46
p^4 p
(^2) p^1
p^3
Figure 6.26
π MOs of C
H 4
(^6)
Atomic orbitals viewed from the side (left) and top (right)
to construct the
molecular orbitals, then only three π
molecular orbitals are produced. The
Lewis structures show that there are four electrons in the
system: two from the π
bond π
and two from the lone pair that appears on both oxygen atoms. Using the Rules for MO construction of simple multi-atom systems,
we obtain the three MO’s shown in Figure
6.25. The lowest energy orbital contains no nodal planes perpendicular to the bonding axis, so both interactions between adja
cent atoms are bonding. The number of bonding
interactions (2) is greater than the number of antibonding interactions (0), so it is a bonding MO. The orbital is delocalized over all three atoms (both bonds), so it is a delocalized
bond. The next highest energy MO must contain one nodal plane that must π
be situated in the center of the molecule, which is on the central atom in this case. Placing a nodal plane on the central atom means that there is no electron density on the atom in this MO. The phases of the other orbitals must be opposite because they must change at the nodal plane. There are no interactions between adjacent
atoms, so this is a non-bonding
MO that is delocalized over both oxygen atoms. The highest energy MO has nodal planes between each pair of atoms, so both inter
actions are antibonding. The number of bonding
interactions (0) is less than the number of
antibonding interactions (2), so it is the

• π
  antibonding MO. Two electrons occupy the
  bonding MO, and two the nonbonding (n) π
  MO. The


• antibonding orbital is unoccupied. π
  We now consider the example of the
  systemπ
  of butadiene (C
  H 4
  ), which is a four- 6
  orbital system constructed from the four carbon
  p orbitals that are perpendicular to the
  plane of the molecule (we will refer to these as the p
  orbitals). There are four atomic z
  orbitals, so four molecular orbitals must
  be produced. Each carbon p orbital contains one
  electron, so four electrons occupy the molecular orbitals. We again use the rules for MO construction to construct the four molecular
  orbitals shown in Figure 6.26. The lowest
  energy MO contains no nodal planes, and has three bonding interactions, so it is a
  MO π
  that is delocalized over all four atoms. The nodal plane in the next highest energy MO must be placed in the center of the molecu
  le. The MO contains two bonding interactions
  and one antibonding interaction, so it is still a bonding
  MO. The third highest MO π
  contains two nodal planes that must be placed symmetrically. However,
  phase changes
  must occur at nodal planes
  , so they cannot be placed on adjacent atoms. Therefore, the
  two nodal planes must be placed between the atoms as shown. There are two antibonding interactions and one bonding interaction, so this is an antibonding


• MO. The highest π
  energy MO contains nodal planes between each
  pair of atoms, so it is an antibonding


• π
  MO. The four electrons are placed into the two lowest energy levels (
  π^1
  and
  π^2
  ) as shown,
  Chapter 6 Molecular Structure & Bonding
  © by
  North
  Carolina
  State
  University