106 CHAPTER 29 Pericyclic Reactions
molecular orbitals energy levels
ground
state
excited
state
energy of the
p atomic orbitals
Energy
4
3
2
1
node
Figure 29.2
Four patomic orbitals interact to give the four molecular orbitals of 1,3-butadiene.p
If you examine the interacting orbitals in Figure 29.2, you will see that in-phase
orbitals interact to give a bonding interaction and out-of-phase orbitals interact to
create a node. Recall that a node is a place in which there is zero probability of find-
ing an electron (Section 1.5). You will also see that as the energy of the molecular
orbital increases, the number of bonding interactions decreases and the number of
nodes betweenthe nuclei increases. For example, has three bonding interactions
and zero nodes between the nuclei, has two bonding interactions and one node
between the nuclei, has one bonding interaction and two nodes between the nu-
clei, and has zero bonding interactions and three nodes between the nuclei.
Notice that a molecular orbital is bonding if the number of bonding interactions is
greater than the number of nodes between the nuclei, and a molecular orbital is an-
tibonding if the number of bonding interactions is fewer than the number of nodes
between the nuclei.
The normal electronic configuration of a molecule is known as its ground state. In
the ground state of 1,3-butadiene, the highest occupied molecular orbital (HOMO)is
and the lowest unoccupied molecular orbital (LUMO)is If a molecule
absorbs light of an appropriate wavelength, the light will promote an electron from its
ground-state HOMO to its LUMO (from to ). The molecule is then in an
excited state.In the excited state, the HOMO is and the LUMO is In a ther-
mal reaction the reactant is in its ground state; in a photochemical reaction the reac-
tant is in an excited state.
Some molecular orbitals are symmetricand some are asymmetric(also called
dissymetric), and they are easy to distinguish. If the porbitals at the ends of the mo-
lecular orbital are in-phase (both have blue lobes on the top and green lobes on the
bottom), the molecular orbital is symmetric. If the two end porbitals are out-of-
phase, the molecular orbital is asymmetric. In Figure 29.2, and are symmetric
molecular orbitalsand and are asymmetric molecular orbitals. Notice that
as the molecular orbitals increase in energy, they alternate in being symmetric and
c 2 c 4
c 1 c 3
c 3 c 4.
c 2 c 3
c 2 , c 3.
c 4
c 3
c 2
c 1
Tutorial:
pMolecular orbitals