Chemistry Reference
In-Depth Information
2
e
4
a
1
e
a
1
2p
x
2p
y
2p
z
3
a
1
a
1
e
1
e
2s (
a
1
)
2
a
1
NH
3
N
H, SALCs
Figure 7.22
The MO diagram for the C
3v
molecule NH
3
. For clarity, the dotted lines for the
a
1
orbitals are drawn in a different style to those for the others.
N(2p
x
) and N(2p
y
) with their corresponding H(1s) SALCs. The final occupied orbital, 3
a
1
,
has a large N(2p
z
) character with only weak interaction with the H(1s)
a
1
SALC.
The unoccupied orbitals begin at 4
a
1
, which has a larger contribution from the H(1s)
a
1
SALC than the earlier
a
1
MOs. Finally, the 2
e
antibonding doubly degenerate levels are
the highest energy MOs.
In the case of NH
3
, a hybridization picture can be useful in understanding the bonding
scheme. We have found three
a
1
valence MOs formed from linear combinations of the
functions: N(2s), N(2p
z
) and the
a
1
SALC of the H(1s) AOs. We can view these linear
combinations as the result of first mixing the N-centred AOs, to form sp hybrids, and
then dealing with the interaction between these hybrids and the H atom SALC. This is
consistent with the irreducible representations on the N atoms, as it will produce hybrid
orbitals that have the same
a
1
symmetry. The linear combinations forming the hybrids
have to be orthogonal to one another and normalized. This can be achieved by linking the
coefficients
c
1
and
c
2
used in the hybrids so that
h
1
(
a
1
)
=
N
h
(
c
1
s
+
c
2
p
z
)
(7.57)
and
h
2
(
a
1
)
=
N
h
(
c
2
s
−
c
1
p
z
)
(7.58)
