Chemistry Reference
In-Depth Information
graphic, as the spheres used to mark the H atom positions are largely outside of the orbital
contour. The next level, 1
b
2
, does contribute to bonding, being formed from the 2p
y
and
the
b
2
H(1s) SALC, which are much closer to one another. Similarly, the 3
a
1
level contains
significant contributions from both the O(2p
z
) orbital and H(1s)
a
1
SALC.
The HOMO is the nonbonding 1
b
1
level. This is the O(2p
x
) AO, which is perpendicu-
lar to the plane of the molecule. The unoccupied orbitals are the antibonding 4
a
1
and 2
b
2
levels.
The weak involvement of the H(1s) orbitals in the 2
a
1
state leads to a bond order
estimate of 2 split evenly over the two O H bonds so that each is a single bond.
Water is an essential component of biological systems and has many interesting phys-
ical properties. For example, the density of ice at 0
◦
C is actually less than that of the
liquid state, and so the solid floats. This is unusual, since for most other molecules
the intermolecular forces cause a denser packing in the solid state. However, the
intermolecular bonding for H
2
O is directional in nature due to hydrogen bonding. The
electronegativity of O is higher than H, and so the OH bonds are polarized to give posi-
tively charged H atoms and a negatively charged O. In addition, the nonbonding electrons
on O lead to an anisotropic distribution of electron density at O, commonly referred to as
the O lone pairs. To obtain optimal intermolecular H bonding the structure that ice adopts
is quite open, leading to a low density compared with the liquid state.
Problem 7.10:
In Figure 7.25 we compare the energy levels for NH
3
in
C
3v
and
D
3h
geometries. Here, H
2
O is taken to be
C
2v
, whereas we have seen that BeH
2
is a linear
D
∞
h
molecule with the MO diagram given in Figure 7.16. Draw a diagram linking the
O atomic states in the alternative geometries and so demonstrate that the H
2
O structure
is preferred when the 3
a
1
state is occupied.
7.5 The Second-Row Diatomic Molecules
7.5.1 Homonuclear Diatomics
All the molecules considered so far have had H as one of the participants in the bonding
scheme. In this section we will start to consider the MOs produced for interactions between
heavier atoms, beginning with homonuclear diatomics, A
2
, of second-row elements. Gen-
eral MO diagrams for these diatomics will be constructed and then used to discuss the
relative stability of those that are observed experimentally: N
2
,O
2
and F
2
and those that
are not found under normal circumstances, Li
2
,Be
2
,B
2
and C
2
. In common with H
2
, these
molecules all belong to the
D
∞
h
point group and so the 2s valence orbitals will be linked
together as
σ
g
+
and
σ
u
+
SALCs:
1
√
2
(
s
1
+
σ
g
+
=
s
2
)
(7.60)
1
√
2
σ
u
+
=
(
s
1
−
s
2
)
(7.61)
