Chemistry Reference
In-Depth Information
4
a
1
e
a
1
3
a
1
2p
x
2p
y
2p
z
h
2
(
a
1
)
e
h
1
(
a
1
)
2s (
a
1
)
2
a
1
N
a
1
, hybrids
NH
3
H, SALCs
Figure 7.24
The use of a
1
hybrid orbitals to understand the origin of the three NH
3
MOs of
a
1
symmetry.
is dominated by the p-orbital. In this illustration we have arbitrarily taken
c
1
0.1; in
general, the greater the difference in energy between the two AOs; the smaller
c
1
will be.
With such unequal weightings; each hybrid closely resembles the atomic state to which it
is nearest in energy.
The two hybrids for NH
3
are shown to the right of the original N AOs in Figure 7.24,
where their use in constructing the
a
1
symmetry MOs for NH
3
is illustrated. The 2s-2p
separation for N is quite large and so the hybrids follow Figure 7.19b. The difference
between the hybrid states is that the lobe of
h
1
(
a
1
) is skewed downward toward the H
atom plane, and so this orbital has strong overlap with the
a
1
H(1s) SALC, forming the
bonding 2
a
1
and antibonding 4
a
1
MOs. In contrast, the larger lobe of
h
2
(
a
1
) is orientated
away from the H atoms, resulting in a 3
a
1
MO that is practically nonbonding, through poor
orbital overlap with the H(1s)
a
1
SALC.
The use of hybrids of symmetry-matched AOs in this way is really just a change of
the choice of basis set. It does not change the resulting electron density calculated for the
molecule, as the sum over all occupied states (Equation (7.1)) will give the same result
irrespective of the way the SALCs are formed.
The hybrid scheme is clearer for estimating the bond order for NH
3
. Since the 3
a
1
level
is nonbonding it does not contribute to our calculation of bond order (Equation (7.24)).
So, we are left with three doubly occupied bonding orbitals (2
a
1
and the pair of 1
e
MOs).
There are also three N H bonds, so that each bond is single order.
From the MO diagram, with or without the use of hybrids, the highest occupied state
(3
a
1
)inNH
3
has a large lobe above the N atom and away from the plane of the H atoms.
This orbital is available to interact with other molecules and gives NH
3
its Lewis basic
=
