Geoscience Reference
In-Depth Information
1.2.2.2 Pi Bonds (π Bonds)
Pi bonds (
π
bonds) occur where an orbital occurs in two regions above and below the bond
axis between the two atom nuclei. Pi bonding is a much weaker interaction than sigma
bonding and involves only electrons from
p
or
d
orbitals, never
s
orbitals. A pair of atomic
nuclei may be connected by pi bonds only if a
σ
bond also exists between them. The forces
involved in
σ
bonding are much stronger than for
π
bonding, which is explained by less
overlap between the component
p
-orbitals due to their parallel orientation. The distribution
of electronic charge through
π
bonding is concentrated outside of the bond axis, and there-
fore
π
electrons are more able to move between the atoms. This mobility of
π
electrons
means that in certain situations, where multiple atoms are connected by a series of
σ
bonds
along with the correct geometry of
p
and
d
orbitals, a system of delocalized
π
bonds can be
formed that spreads over many atoms. The interaction of one
p
-orbital with another across
an intervening sigma bond can also lead to conjugation. In reality, this manifests itself by
the formation of a molecular entity whose structure may be represented as a system of
alternating single and multiple bonds where the delocalized
σ
electrons do not belong to a
single bond or atom, but rather to a group of atoms or the entire molecule. As long as each
contiguous atom in a chain has an available
p
-orbital, the system can be considered to be
conjugated. Examples of
π
and
σ
bonding are shown in
Figure 1.1
.
1.2.2.3 Antibonding Orbitals
If we consider a linear homonuclear diatomic molecule such as H
2
we can represent the
interaction of both 1s molecular orbitals (wave function) associated with each hydrogen
atom as
ΨΨ
=
±
Ψ
(1.2)
As
1
Bs
1
This equation tells us that an electron can be found with equal probability in either orbital A
or B. More explicitly, there are two possible wave functions for this arrangement and these
are shown in
Eqs. (1.3)
and
(1.4)
:
ΨΨ Ψ
As
=
+
1
(1.3)
1
Bs
or
ΨΨ Ψ
=
−
(1.4)
As
1
Bs
1
Equation (1.3)
represents a sigma orbital (1
σ
), just like the ones described in
Section
1.2.2.1
. Both of these electron orbitals behave like waves, and therefore, according to wave
theory, can interact constructively in the internuclear region. This constructive interference
intensifies the amplitude of the wavefunction, and this in turn, increases the probability of
finding the electron between the two nuclei. Any electron located between the two hydro-
gen nuclei will interact strongly with both of them. As a result, the overall energy of the
